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DOI： 10.1103/PhysRevB.104

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Spin-triplet superconductors are novel materials capable of harboring Majorana bound states that can be used in topological quantum computing. However, such bulk materials are still rare. Here we review the results that established K₂Cr₃As₃ as a spin-triplet superconductor with transition temperature T_c as high as 6.5 K. We focus on the multiple-phases feature, and its exquisite distance to a ferromagnetic quantum critical point which is likely responsible for the high T_c. We touch on the topological aspects of the superconducting state, and suggest that it is a new route to the technical implementation using a topological spin-triplet superconductor at the highest temperature ever.

DOI： 10.1088/1742-6596/2545/1/012001

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.92.064711

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Abstract

Clarifying the interplay between charge-density waves (CDWs) and superconductivity is important in the kagome metal CsV₃Sb₅, and pressure (P) can play a crucial role. Here, we present ^121/123Sb nuclear quadrupole resonance (NQR) measurements under hydrostatic pressures up to 2.43 GPa in CsV₃Sb₅ single crystals. We demonstrate that the CDW gradually changes from a commensurate modulation with a star-of-David (SoD) pattern to an incommensurate one with a superimposed SoD and Tri-hexagonal (TrH) pattern stacking along the c-axis. Moreover, the linewidth δν of ^121/123Sb-NQR spectra increases with cooling down to T_CDW, indicating the appearance of a short-range CDW order due to CDW fluctuations pinned by quenched disorders. The δν shows a Curie–Weiss temperature dependence and tends to diverge at P_c ~ 1.9 GPa, suggesting that a CDW quantum critical point (QCP) exists at P_c where T_c shows the maximum. For P > P_c, spin fluctuations are enhanced when the CDW is suppressed. Our results suggest that the maximal T_c at P_c ~ 1.9 GPa is related to the CDW QCP, and the presence of spin fluctuations prevents the T_c from a rapid decrease otherwise, after the CDW is completely suppressed.

DOI： 10.1038/s41535-023-00555-w

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.107.l100505

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.104.224508

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The specific heat C of the cuprate superconductors La2-xSrxCuO4 and Bi2+ySr2-x-yLaxCuO6+δ was measured at low temperatures (down to 0.5 K) for dopings p close to p∗, the critical doping for the onset of the pseudogap phase. A magnetic field up to 35 T was applied to suppress superconductivity, giving direct access to the normal state at low temperatures, and enabling a determination of Ce, the electronic contribution to the normal-state specific heat at T→0. In La2-xSrxCuO4 at x=p=0.22, 0.24 and 0.25, Ce/T=15to16mJmol-1K-2 at T=2K, values that are twice as large as those measured at higher doping (p>0.3) and lower doping (p<0.15). This confirms the presence of a broad peak in the doping dependence of Ce at p∗≃0.19 as previously reported for samples in which superconductivity was destroyed by Zn impurities. Moreover, at those three dopings, we find a logarithmic growth as T→0 such that Ce/T∼Bln(T0/T). The peak versus p and the logarithmic dependence versus T are the two typical thermodynamic signatures of quantum criticality. In the very different cuprate Bi2+ySr2-x-yLaxCuO6+δ, we again find that Ce/T∼Bln(T0/T) at p≃p∗, strong evidence that this ln(1/T) dependence of the electronic specific heat - first discovered in the cuprates La1.8-xEu0.2SrxCuO4 and La1.6-xNd0.4SrxCuO4 - is a universal property of the pseudogap critical point.

DOI： 10.1103/PhysRevB.103.214506

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

A fundamental problem posed from the study of correlated electron compounds, of which heavy-fermion systems are prototypes, is the need to understand the physics of states near a quantum critical point (QCP). At a QCP, magnetic order is suppressed continuously to zero temperature and unconventional superconductivity often appears. Here, we report pressure T-c. (P)-dependent In-115 nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh0.5Ir0.5In5. These experiments reveal an antiferromagnetic (AF) QCP at P-c(AF) = 1.2 GPa where a dome of superconductivity reaches a maximum transition temperature Tc. Preceding P-c(AF), however, the NQR frequency nu(Q) undergoes an abrupt increase at P-c* = 0.8 GPa in the zero-temperature limit, indicating a change from localized to itinerant character of cerium's f-electron and associated small-to-large change in the Fermi surface. At P-c(AF) where T-c is optimized, there is an unusually large fraction of gapless excitations well below T-c that implicates spin-singlet, odd-frequency pairing symmetry.

DOI： 10.1038/s42005-020-00418-x

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The electronic and superconducting properties associated with the topologically nontrivial bands in Weyl semimetals have recently attracted much attention. We report the microscopic properties of the type-I Weyl semimetal TaAs measured by As-75 nuclear magnetic (quadrupole) resonance under zero and elevated magnetic fields over a wide temperature range up to 500 K. The magnetic susceptibility measured by the Knight shift K is found to be negative at low magnetic fields and has a strong field (B) dependence as In B at T = 1.56 K. Such nonlinear field-dependent magnetization can be well accounted for by Landau diamagnetism arising from the three-dimensional linearly dispersed bands, and thus is a fingerprint of topological semimetals. We further study the low-energy excitations by the spin-lattice relaxation rate 1/T-1. At zero field and 30 <= T <= 250 K, 1/T1T shows a T-2 variation due to Weyl-node excitations. At B similar to 13 T, 1/T1T exhibits the same T dependence but with a smaller value, scaling with K-2 alpha T-2, which indicates that the Korringa relation also holds for a Weyl semimetal. Analysis of the Korringa ratio reveals that the energy range of the linear bands is about 250 K in TaAs.

DOI： 10.1103/physrevb.101.241110

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We report As-75-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As (2), KCa2Fe4As4F2, and A(2)Cr(3)As(3). In the superconducting state of LaRu2As2, a Hebel-Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T-1 just below T-c, which indicates that LaRu2As2 is a full-gap superperconducor. For KCa2Fe4As4F2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF=T1c/T1ab<i is small and temperature independent, implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa2Fe4As4F2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A(2)Cr(3)As(3) (A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the As-75-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.

DOI： 10.1088/1674-1056/ab892d

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Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing CuxBi2Se3 single crystals with unprecedented high doping levels. For samples with x = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by the Knight shift, the in-plane upper critical field H-c2 shows a two-fold symmetry. However, the angle at which the H-c2 becomes minimal is different by 90 degrees among them, which indicates that the d-vector direction is different for each crystal likely due to a different local environment. The carrier density for x = 0.46 and 0.54 increases substantially compared to x <= 0.37. Surprisingly, the in-plane H-c2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases.

DOI： 10.1038/s41467-019-14126-w

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.2.0_1987

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.2.0_1074

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DOI： 10.1007/s11433-018-9240-0

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Authorship：Last author,　Corresponding author  

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We report the Pb-207 nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe2 with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate 1 / T-1) shows a suppressed coherence peak below the superconducting transition temperature T-c = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below T = 0.7 T-c. The Knight shift K decreases below T-c. These results suggest spin-singlet superconductivity with a fully opened gap 2 Delta = 3.5 k(B)T(c) in PbTaSe2.

DOI： 10.1103/physrevb.97.184510

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Language：Japanese   Publisher：The Japan Society of Vacuum and Surface Science  

DOI： 10.14886/sssj2008.2018.0_298

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DOI： 10.11316/jpsgaiyo.73.1.0_1332

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DOI： 10.1038/s41467-017-01465-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The In-doped topological crystalline insulator Sn1-x In-x Te is a candidate for a topological superconductor, where a pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of Sn1-x In-x Te, we perform Te-125-nuclear magnetic resonance (NMR) measurements in polycrystalline samples with 0 <= x <= 0.15. The penetration depth calculated from the NMR line width is T independent below half the superconducting transition temperature (T-c) in polycrystalline Sn0.96In0.04Te, which indicates a fully opened superconducting gap. In this sample, the spin susceptibility measured by the spin Knight shift (K-s) at an external magnetic field of H-mu 0(0) = 0.0872 T decreases below T-c, and K-s(T = 0)/K-s(T = T-c) reaches 0.36 +/- 0.10, which is far below the limiting value 2/3 expected for a spin-triplet state for a cubic crystal structure. Our result indicates that polycrystalline Sn0.96In0.04Te is a spin-singlet superconductor.

DOI： 10.1103/PhysRevB.96.104502

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We report a new kagome quantum spin liquid candidate Cu3Zn(OH)(6)FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (similar to 200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from F-19 nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.

DOI： 10.1088/0256-307X/34/7/077502

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Nematic order, a self-organized state with rotational symmetry broken, has been observed in both copperoxide and iron-pnictide high temperature superconductors. However, its origin is still a mystery in the iron pnictides although it is considered as a key to understand the mechanism of superconductivity. Here, we report a systemic nuclear magnetic resonance (NMR) study on NaFe(1-x)C(o)xAs (0 &lt;= x &lt;= 0.042) that an orbital order, accompanied by an instant spin nematicity, occurs at at a temperature T* far above structural transition temperature Ts in the tetragonal phase. We show that the observed NMR spectra splitting and its evolution is due to an incommensurate orbital order that sets in below T* and becomes commensurate below T-s. We show that the electric field gradient asymmetry parameter is a good measure for the orbital order parameter which undergoes a Landau-like 2nd-order phase transition. We further show that the spin nematicity is well accounted for by the observed orbital order.

DOI： 10.1007/s10948-017-3997-5

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The In-doped topological crystalline insulator Sn1-xInxTe is a promising candidate for a topological superconductor, where it is theoretically suggested that In creates an impurity state responsible for superconductivity. We synthesized high purity Sn1-xInxTe samples and performed Te-125-nuclear magnetic resonance (NMR) measurements. The NMR spectra under a magnetic field of H-0 = 5T show a broadening characteristic due to a localized impurity state. The spin-lattice relaxation rate (1/T-1) divided by temperature shows a Curie-Weiss like temperature-dependence under H-0 = 0.1 T but is temperature-independent under H-0 = 5T. These results indicate the existence of quasi-localized impurity states due to In doping.

DOI： 10.7566/JPSJ.86.024702

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The noncentrosymmetric superconductor Re6Zr has attracted much interest for its possible unconventional superconducting state with broken time-reversal symmetry. Here we report Re-185/187 nuclear quadrupole resonance measurements on Re6Zr (T-c = 6.72 K) and the isostructural compounds Re27Zr5 (T-c = 6.53 K) and Re24Zr5 (T-c = 5.00 K). The nuclear spin-lattice relaxation rate 1/T-1 shows a coherence peak below T-c and decreases exponentially at low temperatures in all three samples. The superconducting gap Delta derived from the 1/T-1 data is 2 Delta = 3.58k(B)T(c), 3.55k(B)T(c), and 3.51k(B)T(c) for Re6Zr, Re27Zr5, and Re24Zr5, respectively, which is close to the value of 3.53k(B)T(c) expected for weak-coupling superconductivity. These data suggest conventional s-wave superconductivity with a fully opened gap in this series of compounds.

DOI： 10.1103/PhysRevB.94.214513

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We report Te-125 nuclear magnetic resonance and Ta-181 nuclear quadrupole resonance studies on single-crystal Ta4Pd3Te16, which has a quasi-one-dimensional structure and superconducts below T-c = 4.3 K. Ta-181 with spin I = 7/2 is sensitive to quadrupole interactions, while Te-125 with spin I = 1/2 can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate (1/T-1) of Ta-181 and Te-125, we found that electric-field-gradient (EFG) fluctuations develop below 80 K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge densitywave that sets in at T-CDW = 20 K, below which the spectra are broadened and 1/T1T drops sharply. In the superconducting state, 1/T-1 shows a Hebel-Slichter coherence peak just below Tc for Te-125, indicating that Ta4Pd3Te16 is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the 1/T-1 of Ta-181 due to the strong EFG fluctuations.

DOI： 10.1103/PhysRevB.94.174511

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We report on the discovery and novel physics of a new superconductivity dome in LaFeAsO1-xFx with high-doping rate (0.25 &lt;= x &lt;= 0.75) synthesized by using the high-pressure technique. The maximal critical temperature T-c = 30 K peaked at x(opt) = 0.5 similar to 0.55, which is even higher than that at x &lt;= 0.2. By nuclear magnetic resonance (NMR), we find that the new superconducting dome is far away from a magnetically ordered phase without low-energy magnetic fluctuations. Instead, NMR and transmission electron microscopy measurements indicate that a C4 rotation symmetry-breaking structural transition takes place for x &gt; 0.5 above T-c. The electrical resistivity shows a temperature-linear behavior around the doping level where the crystal transition temperature extrapolate to zero and T-c is the maximal, suggesting the importance of quantum fluctuations associated with the structural transition. Our results point to a new paradigm of high temperature superconductivity.

DOI： 10.1007/s10751-016-1355-9

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Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break further symmetries. In particular, spin-rotational symmetry is expected to be broken in spin triplet superconductors. However, experimental evidence for such symmetry breaking has not been conclusively obtained so far in any candidate compounds. Here, using Se-77 nuclear magnetic resonance measurements, we show that spin-rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature T-c = 3.4 K. Our results not only establish spin-triplet superconductivity in this compound, but may also serve to lay a foundation for the research of topological superconductivity.

DOI： 10.1038/NPHYS3781

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In copper-oxide and iron-based high-temperature (high-T-c) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron pnictides, a tetragonal-to-orthorhombic structural transition temperature T-s coincides with the magnetic transition temperature T-N, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where T-s = 54 K is well separated from T-N = 42 K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on NaFe1-xCoxAs (0 &lt;= x &lt;= 0.042) that revealed orbital and spin nematicity occurring at a temperature T* far above T-s in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below T* and becomes commensurate below T-s, which brings about the observed spin nematicity.

DOI： 10.1103/PhysRevB.93.060502

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2071

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DOI： 10.11316/jpsgaiyo.71.1.0_1985

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In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here, we show that antiferromagnetism in Ca1-xLaxFeAs2 is robust against and even enhanced by doping. Using As75-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the Néel temperature TN=62 K at a high doping concentration (x=0.15) where superconductivity sets in at the transition temperature Tc=35 K. In the superconducting state coexisting with antiferromagnetism, the nuclear-spin-lattice relaxation rate 1/T1 becomes proportional to T, indicating gapless excitations. Unexpectedly, TN is enhanced with increasing doping, rising up to TN=70 K at x=0.24. The obtained phase diagram of this system enriches the physics of iron-based high-Tc superconductors.

DOI： 10.1103/PhysRevB.92.180508

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In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here, we show that antiferromagnetism in Ca1-xLaxFeAs2 is robust against and even enhanced by doping. Using As-75-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the Neel temperature T-N = 62 K at a high doping concentration (x = 0.15) where superconductivity sets in at the transition temperature T-c = 35 K. In the superconducting state coexisting with antiferromagnetism, the nuclear-spin-lattice relaxation rate 1/T-1 becomes proportional to T, indicating gapless excitations. Unexpectedly, TN is enhanced with increasing doping, rising up to T-N = 70 K at x = 0.24. The obtained phase diagram of this system enriches the physics of iron-based high-T-c superconductors.

DOI： 10.1103/PhysRevB.92.180508

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High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1-xFx (x &lt;= 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1-xFx with 0.25 &lt;= x &lt;= 0.75, where the maximal critical temperature T-c at x(opt) = 0.5-0.55 is even higher than that at x &lt;= 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x &gt; 0.5 above T-c. Our results point to a new paradigm of high temperature superconductivity.

DOI： 10.1088/0256-307X/32/10/107401

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We report As-75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb2Cr3As3 with a quasi-one-dimensional crystal structure. Below T similar to 100 K, the spin-lattice relaxation rate (1/T-1) divided by temperature, 1/T1T, increases upon cooling down to T-c = 4.8 K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/T-1 decreases rapidly below T-c without a Hebel-Slichter peak, and follows a T-5 variation below T similar to 3 K, which points to unconventional superconductivity with point nodes in the gap function.

DOI： 10.1103/PhysRevLett.115.147002

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In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. LaPt2Ge2 undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at Ts=394 K, accompanying a double period modulation in the a-axis direction, and superconducts at Tc=0.41 K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesting. A reduction in the density of states in the monoclinic phase was found in the calculation and confirmed by Pt195-NMR. We suggest a CDW as a possible cause for the SPT. By changing the stoichiometry between Pt and Ge, we succeeded in suppressing Ts and increasing Tc in LaPt2-xGe2+x. Comparison of La139- and Pt195-NMR data reveals moderate fluctuations associated with SPT. From La139-NQR measurements at zero field, we found that an isotropic superconducting gap is realized in LaPt2-xGe2+x (x=0.20). We discuss the relationship between superconductivity and the SPT order/fluctuations.

DOI： 10.1103/PhysRevB.91.174516

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In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. LaPt2Ge2 undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at T-s = 394 K, accompanying a double period modulation in the a-axis direction, and superconducts at T-c = 0.41 K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesting. A reduction in the density of states in the monoclinic phase was found in the calculation and confirmed by Pt-195-NMR. We suggest a CDW as a possible cause for the SPT. By changing the stoichiometry between Pt and Ge, we succeeded in suppressing T-s and increasing T-c in LaPt2-xGe2+x. Comparison of La-139- and Pt-195-NMR data reveals moderate fluctuations associated with SPT. From La-139-NQR measurements at zero field, we found that an isotropic superconducting gap is realized in LaPt2-xGe2+x (x = 0.20). We discuss the relationship between superconductivity and the SPT order/fluctuations.

DOI： 10.1103/PhysRevB.91.174516

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The relationship between charge-density wave (CDW) orders and superconductivity in arsenide superconductor SrPt2As2 with T-c = 5.2K which crystallizes in the CaBe2Ge2-type structure was studied by As-75 nuclear magnetic resonance (NMR) measurements up to 520 K, and As-75 nuclear quadrupole resonance (NQR) and Pt-195-NMR measurements down to 1.5 K. At high temperature, As-75-NMR spectrum and nuclear-spin-relaxation rate (1/T-1) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below T-CDW(As(1)) = 410 K and the other in the Pt-As-Pt layer below T-CDW(As(2)) = 255 K. The 1/T-1 measured by As-75-NQR shows a clear Hebel-Slichter peak just below Tc and decreases exponentially well below T-c. Concomitantly, Pt-195 Knight shift decreases below T-c. Our results indicate that superconductivity in SrPt2As2 is in the spin-singlet state with an s-wave gap and is robust under the two distinct CDW orders in different layers.

DOI： 10.1103/PhysRevB.91.060510

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DOI： 10.11316/jpsgaiyo.70.2.0_1704

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DOI： 10.11316/jpsgaiyo.70.1.0_1989

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DOI： 10.11316/jpsgaiyo.70.1.0_2059

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DOI： 10.11316/jpsgaiyo.70.1.0_2244

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DOI： 10.11316/jpsgaiyo.70.1.0_2485

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We report As-75 and V-51 nuclear magnetic resonance (NMR) measurements on the iron-based superconductor Sr2VFeAsO3 with alternating stacks structure. We find that the As-75 nuclear spin-spin relaxation rate (1/T-2) shows a pronounced peak at T-N = 165 K, below which the resonance peak shifts to a higher frequency due to the onset of an internal magnetic field. The V-51 spectrum does not shift, but is broadened below T-N. We conclude that the Fe electrons order antiferromagnetically below T-N with a magnetic moment m(Fe) similar to 0.4 mu B. Application of external pressure up to 2.4 GPa reduces T-N at a rate of -40 K/GPa, and enhances the superconducting transition temperature T-c at a rate of 2 K/GPa. The pressure-temperature phase diagram for Sr2VFeAsO3 shows that superconductivity coexists with antiferromagnetism over a wide pressure range with an unprecedented high T-c up to 36.5 K.

DOI： 10.1103/PhysRevB.89.184506

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We report Pt-195-NMR and As-75 nuclear quadrupole resonance measurements for the locally noncentrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin-lattice relaxation rate 1/T-1 shows a well-defined coherence peak below T-c and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below Tc down to T < T-c/6. These data, together with the penetration depth obtained from the NMR spectra, can be consistently explained by a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-symmetry breaking is not large enough to bring about an exotic superconducting state, or the interlayer hopping interaction is larger than the spin-orbit coupling.

DOI： 10.1103/PhysRevB.89.140504

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Superconductors with noncentrosymmetric crystal structures such as Li2T3B(T:Pd,Pt) have been the focus of in-depth research with their parity mixing nature. In this study, we focused our research on non-magnetic impurity effect in Li2T3B(T:Pd,Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We prepared different quality samples of Li2Pd3B and Li2Pt3B by changing conditions in synthesizing, and sample qualities were estimated by residual resistivity. Spin singlet dominant superconductor Li2Pd3B exhibits the weak T-c suppression attributed by nonmagnetic impurity and defects, while H-c2 (0) value increased. This behavior is similar in ordinary s-wave superconductor. On the other hand, for the spin triplet dominant superconductor Li2Pt3B, it was suggested that the Cooper pair was broken and superconducting gap was decreased by non-magnetic impurities and defects. Li2Pt3B is similar to unconventional superconducting state. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2013.04.059

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Language：Japanese   Publisher：(公社)日本生化学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We report the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements for non-centrosymmetric superconductors Re7B3, LaBiPt, and BiPd containing heavy elements. For all three compounds, the spin-lattice relaxation rate 1/T-1 shows a coherence peak just below T-c and decreases exponentially at low temperatures, which indicates that an isotropic superconducting gap is dominant in these compounds. In BiPd, the height of the coherence peak just below T-c is much suppressed, which suggests that there exists a substantial component of gap with nodes in this compound. Our results indicate that heavy element is not the only factor, but the extent of inversion symmetry breaking is also important to induce a large spin-orbit coupling and an unconventional superconducting state.

DOI： 10.7566/JPSJ.82.084711

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

A quantum critical point is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently discovered iron-pnictide superconductors, we report transport and NMR measurements on BaFe2 - xNixAs2 (0 &lt;= x &lt;= 0.17). We find two critical points at x(c1) = 0.10 and x(c2) = 0.14. The electrical resistivity follows rho = rho(0) + AT(n), with n = 1 around x(c1) and another minimal n = 1.1 at x(c2). By NMR measurements, we identity x(c1) to be a magnetic quantum critical point and suggest that x(c2) is a new type of quantum critical point associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.

DOI： 10.1038/ncomms3265

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

DOI： 10.7566/JPSJ.82.065002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report the systematic investigation of the specific heat of the noncentrosymmetric superconductor Li-2(Pd1-xPtx)(3)B as a function of x. There is a large deviation of the phononic specific heat from the conventional Debye specific heat for Pt-rich samples. In contrast with the fully gapped conventional behavior for small x, a power-law temperature dependence of the electronic specific heat is observed even at x = 0.5. These results manifest a strongly anisotropic or nodal superconducting gap even at x = 0.5 and a nodal superconducting gap for x greater than or similar to 0.9.

DOI： 10.1103/PhysRevB.87.161203

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Over the past 20 years, fullerides have been studied as the source of high-transition-temperature (T-c) superconductivity except for copper oxides. The recent finding of the Mott insulating state right beside superconductivity in Cs3C60 has suggested that magnetism helps raise T-c even in fullerides as in heavy-fermion compounds, high-T-c copper oxides, two-dimensional organic conductors, and iron pnictides. Namely, one tends to think that the link between Mott insulator and superconductivity takes place in fullerides, which can give rise to the mechanism beyond the Bardeen-Cooper-Schrieffer framework. However, the relationship between the Mott state and the superconductivity in Cs3C60 is still under debate. By nuclear magnetic resonance measurements under pressure, we find that the magnetism and superconductivity in Cs3C60 are competing orders. Different from previous reports, the phase separation of Cs3C60 crystals into the Mott and metallic states allows us to systematically study the evolution of the ground state under pressure. Our careful experiments have found that the prevention of a magnetic order is rather essential for the superconductivity in face-centred cubic Cs3C60, which presents a basic strategy for finding still higher T-c in this system.

DOI： 10.7566/JPSJ.82.014709

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Solution O-17-NMR application to biological macromolecules is extremely limited. We describe here O-17-NMR observation of the O-17(2)-oxidized cysteine side chain of human Cu,Zn-superoxide dismutase in solution using selective O-17(2) oxidation. O-17-NMR with the aid of O-17-labeling has wide potential to probe the environment and dynamics of oxidizable functionalities in proteins.

DOI： 10.1039/c2cc36918d

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We report B-11 nuclear magnetic resonance (NMR) studies of the impurity effect in noncentrosymmetric superconductors Li2Pt3B and Li2Pd3B. We substitute B with Al, and find that the full width of half maximum (FWHM) of the NMR spectrum increases with increasing Al content. The superconducting transition temperature (T-c) of Li2Pt3 B1-xAlx decreases drastically with increasing non-magnetic impurity (Al) content, while it is hardly affected in Li2Pd3 B(1-x)A(l)x. These results provide new evidence for superconducting gap with line nodes in Li2Pt3B.

DOI： 10.1088/1742-6596/449/1/012034

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We have performed nuclear magnetic resonance (NMR) measurements on an underdoped single-crystal Ba0. 77K0. 23Fe2As2 with Tc = 16. 5 K. Below TN = 46 K, an internal magnetic field splits the NMR peaks of H ∥ c and shifts those of H ∥ a to higher frequencies. The nuclear spin-lattice relaxation rate 1/T1 measured at the central peak with H ∥ a shows a distinct decrease below Tc(μ0H = 12 T) = 16 K. Our results clearly show that antiferromagnetic order and superconductivity coexist at a microscopical length scale. © 2013 Springer Science+Business Media Dordrecht.

DOI： 10.1007/s10751-013-0862-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report As-75 nuclear magnetic resonance studies on an underdoped single-crystal Ba0.77K0.23Fe2As2 with T-c = 16.5 K. Below T-N = 46 K, the NMR peaks for H parallel to c split and those for H parallel to a shift to higher frequencies, which indicates that an internal magnetic field along the c axis develops below T-N. The spin-lattice relaxation rate 1/T-1 measured at the shifted peak with H parallel to a, which experiences the internal field, shows a distinct decrease below T-c(mu H-0 = 12 T) = 16 K, following a T-3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

DOI： 10.1103/PhysRevB.86.180501

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present results for the pseudogap ground state and its doping evolution in single-layered copper-oxide Bi2Sr2-x La (x) CuO6+delta . We apply very high magnetic fields up to 44 T to remove the superconducting state and reveal the hidden low temperature (T) normal state. Through Cu-63-NMR Knight shift and spin-lattice relaxation rate measurements, we find that there remains a finite density of states (DOS) at the Fermi level in the zero-T limit when the superconductivity is removed, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x), but remains quite large even at the vicinity of the magnetically ordered phase of xa parts per thousand yen 0.8. The result indicates that the superconductivity emerges from the remaining Fermi surface and coexists with the pseudogap.

DOI： 10.1007/s10948-012-1575-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

This article reviews the NMR and NQR studies on iron-based high-temperature superconductors by the IOP/Okayama group. It was found that the electron pairs in the superconducting state are in the spin-singlet state with multiple fully-opened energy gaps. The antiferromagnetic spin fluctuations in the normal state are found to be closely correlated with the superconductivity. Also, the antiferromagnetic spin fluctuations are anisotropic in the spin space, which is different from the case in copper oxide superconductors. This anisotropy originates from the spin-orbit coupling and is an important reflection of the multiple-bands nature of this new class of superconductors.

DOI： 10.1142/S0217984912300086

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Based on the NMR measurements on Bi2Sr2-xLaxCuO6+delta (La-Bi2201) in strong magnetic fields, we identify the nonsuperconducting pseudogap phase in the cuprates as a Luttinger-volume violating Fermi liquid (LvvFL). This state is a zero-temperature quantum liquid that does not break translational symmetry, and yet, the Fermi surface encloses a volume smaller than the large one given by the Luttinger theorem. The particle number enclosed by the small Fermi surface in the LvvFL equals the doping level p, not the total electron number n(e) = 1 - p. Both the phase string theory and the dopon theory are introduced to describe the LvvFL. For the dopon theory, we can obtain a semiquantitative agreement with the NMR experiments.

DOI： 10.1103/PhysRevB.85.134519

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report a systematic study by As-75 nuclear-quadrupole resonance in LaFeAsO1-xFx. The antiferro-magnetic spin fluctuation found above the magnetic ordering temperature T-N = 58 K for x = 0.03 persists in the regime 0.04 &lt;= x &lt;= 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T-c is found, with the highest T-c = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T-c. These features resemble closely the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T-1) below T-c decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T1 below T-c is rendered nonexponential for other x by impurity scattering.

DOI： 10.1103/PhysRevLett.108.047001

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We report the results of (63,65) Cu-NMR measurements on single-layered copper-oxide Bi2Sr1.6La0.4CuO6+delta (T-c = 32 K) conducted under very high magnetic fields up to 45 T. The high magnetic field suppresses superconductivity completely, and the pseudogap ground state is revealed. The Cu-63-NMR Knight shift shows that there remains a finite density of states at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a fi nite volume of Fermi surface.

DOI： 10.1088/1742-6596/400/2/022052

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The spin singlet and spin triplet superconducting states can coexist in noncentrosymmetric crystal structures like Li2T3B (T: Pd, Pt), while it is prohibited in ordinary crystal structures. In this study, we focused our research on non-magnetic impurity effect in Li2T3B(T: Pd, Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We investigated the effect of non-magnetic impurity on s-wave dominant parity mixing superconductor Li2Pd3B by substituting B with Al. In this substitution process, we controlled the quality of the samples and showed their quality deteriorations through the measurement of residual resistivity at T-c. Furthermore, we checked the relations of residual resistivity and T-c as well as its superconducting phase diagram. Our results showed that s-wave dominant parity mixing superconductor Li2Pd3B exhibits the small T-c suppression attributed by the non-magnetic impurities and defects, while the H-c2 value clearly increased about 1.5 times in the poor quality samples. Such behaviours are similarly observed in ordinary s-wave superconductors.

DOI： 10.1088/1742-6596/391/1/012084

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We report on the pairing symmetry and the spin correlations in the Fe(Ni)-based superconductors RFe(Ni)AsO1-xFx (R=Pr, La), LiFeAs and Ba1-xKxFe2As2 single crystals, based on our extensive nuclear magnetic resonance and nuclear quadrupole resonance measurements. The spin susceptibility measured by the Knight shift decreases below T-c along all crystal directions, which indicates spin-singlet pairing. Evidence for multiple gaps is given. We find that the antiferromagnetic spin fluctuation is anisotropic in the spin space due to spin-orbit coupling, but becomes isotropic in the temperature limit, which points to spin-singlet superconductivity with nodeless gap.

DOI： 10.1088/1742-6596/400/2/022145

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The H-T phase diagram and several superconducting parameters for the noncentrosymmetric system Li-2(Pd1-xPtx)(3)B have been determined as a function of cation substitution x. Notably, the coherence length may be linear in platinum concentration. Despite the superconducting pairing state and band structure apparently changing fundamentally, the H-T phase diagram is essentially unchanged. Unusual aspects of the shape of the H-T phase diagram are discussed. The upper critical field H-c2(0) is not anomalously high for any Pt content, likely due to an absence of high carrier masses-in such a case, the value of H-c2(0) would not serve as a probe for novel physics.

DOI： 10.1103/PhysRevB.84.054521

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

As-75-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T-1) measured at tetragonal phase show no coherence peak just below T-c and decrease with decreasing temperature. The superconductivity is of gapless at P = 4.7 kbar but evolves to multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We present extensive (75)As NMR and NQR data on the superconducting arsenides PrFeAs(0.89)F(0.11) (T(c) = 45 K), LaFeAsO(0.92)F(0.08) (T(c) = 27 K), LiFeAs (T(c) = 17 K) and Ba(0.72)K(0.28)Fe(2)As(2) (T(c) = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO(0.9)F(0.1) (T(c) = 4.0 K). In contrast to LaNiAsO(0.9)F(0.1) where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T(1) in the Fe-arsenides decreases below T(c) with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T(c) and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO(0.9)F(0.1). We will discuss the implications of these results and highlight the importance of the Fermi surface topology. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.038

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

As-75-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P=4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T-1) measured at tetragonal phase show no coherence peak just below T-c and decrease with decreasing temperature. The superconductivity is of gapless at P=4.7 kbar but evolves to multiple gaps at P=10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We report As-75 nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO1-xFx (x=0.08, 0.15) and LaNiAsO1-xFx (x=0, 0.06, 0.10, 0.12). In LaFeAsO0.92F0.08 (T-c=23 K), nuclear spin-lattice relaxation rate 1/T-1 shows no coherence peak just below T-c and decreases with decreasing temperature accompanied by a hump structure at T similar to 0.4T(c), which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T1T increases with decreasing temperature to T-c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO1-xFx shows a clear Hebel-Slichter (coherence) peak just below T-c, evidencing that the LaNiAsO1-xFx is a BCS superconductor. In the normal state, 1/T-1 T is constant in the temperature range 4 &lt;= T &lt;= 10 K for all LaNiAsO1-xFx, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO0.92F0.08 and LaNiAsO1-xFx between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report As-75 nuclear magnetic resonance studies on a very clean hole-doped single-crystal Ba0.68K0.32Fe2As2 (T-c = 38.5 K). The spin-lattice relaxation rate 1/T-1 shows an exponential decrease below T similar or equal to 0.45T(c) down to T similar or equal to 0.11T(c), which indicates a fully opened energy gap. From the ratio (T-1)(c)/(T-1)(a), where a and c denote the crystal directions, we find that the antiferromagnetic spin fluctuation is anisotropic in the spin space above T-c. The anisotropy decreases below T-c and disappears at T -&gt; 0. We argue that the anisotropy stems from spin-orbit coupling whose effect vanishes when spin-singlet electron pairs form with a nodeless gap.

DOI： 10.1103/PhysRevB.83.140506

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report the results of the Knight shift by Cu-63,Cu-65-NMR measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely, and the pseudogap ground state is revealed. The Cu-63-NMR Knight shift shows that there remains a finite density of states at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual density of states in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x &gt;= 0.8, which suggests that the density of states plunges to zero upon approaching the Mott insulating phase.

DOI： 10.1103/PhysRevLett.105.137002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We report the As-75-NQR and NMR studies on the iron arsenide superconductor LiFeAs with T-c similar to 17 K. The spin lattice relaxation rate, 1/T-1, decreases below T-c without a coherence peak, and can be fitted by gaps with s(+/-)-wave symmetry in the presence of impurity scattering. In the normal state, both 1/T1T and the Knight shift decrease with decreasing temperature but become constant below T &lt;= 50 K. Estimate of the Korringa ratio shows that the spin correlations are weaker than those in other families of iron arsenides, which may account for the lower T-c in this material.

DOI： 10.1143/JPSJ.79.083702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present extensive As-75-NMR data on the electron-doped pnictides PrFeAsO0.89F0.11 (T-c = 45 K), LaFeAsO0.92F0.08 (T-c = 23 K), and the hole-doped Ba0.72K0.28Fe2As2 (T-c = 31.5 K) single crystal. We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1 / T-1 decreases below T-c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T-c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.

DOI： 10.1007/s10948-010-0749-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

As-75-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependencies of nuclear-spin-lattice relaxation rate (1/T-1) measured in the tetragonal phase show no coherence peak just below T-c(P) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar &lt;= P &lt;= 10.8 kbar. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.

DOI： 10.1088/0953-2048/23/5/054004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report systematic As-75-NQR and La-139-NMR studies on nickel pnictide superconductors LaNiAsO1-xFx (x=0, 0.06, 0.10, and 0.12). The spin-lattice relaxation rate 1/T-1 decreases below T-c with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T1T is constant in the temperature range T-c similar to 4 K &lt; T &lt; 10 K for all compounds and up to T=250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron pnictides. We also provide a possible interpretation for the "pseudogap-like" behavior in the normal state which is also seen in iron pnictides.

DOI： 10.1103/PhysRevB.81.140509

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DOI： 10.1007/s10909-009-0130-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We have performed As-75 nuclear magnetic resonance measurements on a single crystal of the hole-doped superconductor Ba0.72K0.28Fe2As2 (T-c = 31.5 K). The spin-lattice relaxation rate 1/T-1 decreases below T-c with no coherence peak and shows a step-wise variation at low temperatures, which is indicative of multiple superconducting gaps. In the normal state, we find that Fe antiferromagnetic spin fluctuations are anisotropic. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.174

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DOI： 10.1143/JPSJ.78.095001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report B-11 NMR measurements in noncentrosymmetric superconductors Mg9.3Ir19B16.7 (T-c=5.8 K) and Mg10.5Ir19B17.1 (T-c=4.8 K). The spin-lattice relaxation rate and the Knight shift indicate that the Cooper pairs are predominantly in the spin-singlet state with an isotropic gap. However, Mg10.5Ir19B17.1 is found to have more defects and the spin susceptibility remains finite even in the zero-temperature limit. We interpret this result as that the defects enhance the spin-orbit coupling and bring about more spin-triplet component.

DOI： 10.1103/PhysRevB.80.060503

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We report the As-75-NMR study on a single crystal of the hole-doped iron-pnictide superconductor Ba0.72K0.28Fe2As2 (T-c = 31.5 K). We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T-1 decreases below T-c with no coherence peak and shows a stepwise variation at low temperatures, which is indicative of multiple superconducting gaps, as in the electron-doped Pr(La) FeAsO1-xFx. Furthermore, no evidence was obtained for a microscopic coexistence of a long-range magnetic order and superconductivity. Copyright (C) EPLA, 2009

DOI： 10.1209/0295-5075/87/27012

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CoO2 is the parent compound for the superconductor NaxCoO2 center dot 1.3H(2)O and was widely believed to be a Mott insulator. We performed Co-59 nuclear magnetic resonance and nuclear quadrupole resonance studies on LixCoO2 (x=0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical deintercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li content (x), the end member, CoO2, is a noncorrelated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x -&gt; 0 for A(x)CoO(2) (A=Li,Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two-dimensional structure of A(x)CoO(2).

DOI： 10.1103/PhysRevB.79.220514

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We report the results of first photoemission spectroscopy on a newly discovered noncentrosymmetric superconductor Mg10Ir19B16. We measured the valence band structure of Mg10Ir19B16 using soft X-ray photoemission technique and found that the first principal calculation explains our observation well. In the bulk-sensitive laser-excited photoemission study, we observed that the intensity at the Fermi level decreases as we lower the sample temperature, indicative of the opening of superconducting energy gap. Using Dynes' function, we performed fitting analysis on the spectra of superconducting state.

DOI： 10.1143/JPSJ.78.034705

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DOI： 10.1016/j.physc.2009.05.175

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We report As-75 nuclear quadrupole resonance studies on superconducting oxypnictide LaFeAsO0.92F0.08 (T-c=23 K). The temperature dependence of the spin-lattice relaxation rate (1/T-1) decreases below T-c without a coherence (Hebel-Slichter) peak and shows a temperature dependence that is not simple power law nor exponential. We show that the result can be understood in terms of two superconducting gaps of either d- or +/- s-wave symmetry, with the larger gap Delta(1)similar to 4k(B)T(c) and the smaller one Delta(2)similar to 1.5k(B)T(c). Our result suggests that the multiple-gap feature is universal in the oxypnictide superconductors, which is probably associated with the multiple electronic band structures in this class of materials. We also find that 1/T1T above T-c increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.

DOI： 10.1103/PhysRevB.78.220506

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We report Co-59 NMR studies in single crystals of the cobalt oxide superconductor Na0.42CoO2 center dot 1.3H(2)O (T-c = 4.25 K) and its parent compound Na0.42CoO2. We find that both the magnitude and the temperature (T) dependence of the Knight shifts are identical in the two compounds above T-c. The spin-lattice relaxation rate (1/T-1) is also identical above T-0 similar to 60K for both compounds. Below T-0, the unhydrated sample is found to be a non-correlated metal that well conforms to the Fermi liquid theory, while spin fluctuations develop in the superconductor. These results indicate that water intercalation does not change the density of states at the Fermi level or the carrier density but its primary role is to bring about spin fluctuations. Our result shows that, in the hydrated superconducting compound, the electron correlation is anisotropic. Namely, the spin fluctuation around the finite wave vector is much stronger along the a-axis direction than that along the c-axis direction. Copyright (C) EPLA, 2008

DOI： 10.1209/0295-5075/84/57010

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We report (123)Sb nuclear-quadrupole-resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs(4)Sb(12) under high pressure. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/T(1) indicate that the crystal-electric-field splitting Delta(CEF) between the ground-state Gamma(1) singlet and the first-excited-state Gamma((2))(4) triplet decreases with increasing pressure. ac-susceptibility measurements indicate that the superconducting transition temperature (T(c)) also decreases with increasing pressure. However, above P similar to 2 GPa, both Delta(CEF) and T(c) do not depend on external pressure up to P =3.82 GPa. These pressure dependences of Delta(CEF) and T(c) suggest an intimate relationship between quadrupole excitations associated with the Gamma((2))(4) level and unconventional superconductivity in PrOs(4)Sb(12). In the superconducting state, 1/T(1) below T(c)= 1.55 and 1.57 K at P= 1.91 and 2.63 GPa shows a power-law temperature variations and is proportional to T(5) at temperatures considerably below T(c). These data can be well fitted by the gap model Delta(theta) = Delta(0) sin theta, with Delta(0)= 3.08k(B)T(c) and 3.04k(B)T(c) for P= 1.91 and 2.63 GPa, respectively. The results indicate that there exist point nodes in the gap function.

DOI： 10.1103/PhysRevB.78.064510

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In order to obtain microscopic information about the metal-insulator (M-I) transition in SmRu4P12, P-31-nuclear magnetic resonance (NMR) measurements using a single crystal have been carried out. Each P-31-NMR line splits into two lines below the M-I transition temperature, consistent with the antiferro-type ordered structure with a wave vector q = (1, 0, 0) proposed by the previous Ru-101-NQR experiments. We also found a remarkable kink in the temperature dependence of the resonance frequencies at 12.5K, which may be related to an anomaly in the temperature dependence of the order parameter.

DOI： 10.1143/JPSJS.77SA.206

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We report As-75 and F-19 NMR studies on the superconducting properties of the newly discovered compound PrFeAsO0.89F0.11 (T-c = 45 K). The Knight shift in the direction of H parallel to ab-plane decreases below T-c to almost zero, which indicates spin-singlet pairing. The temperature dependencies of both the Knight shift and the spin lattice relaxation rate indicate that there are two gaps opening below T-c, with nodes in the gap function. Copyright (c) EPLA, 2008.

DOI： 10.1209/0295-5075/83/57001

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DOI： 10.1016/j.physb.2007.10.206

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DOI： 10.1016/j.physb.2007.10.205

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We report the spin susceptibility in the superconducting state of the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O, via the (CO)-C-59 Knight shift measurements. Along both a- and c-axis directions, the shift decreases below T-c. At T = 1.7 K, the shift along the a-axis, K-a increases with increasing the strength of magnetic field (H), following a root H variation. Together with the T-3 dependence of the spin-lattice relaxation rate, 1/T-1, these results indicate unambiguously that the Cooper pairs are in the spin-singlet form and that there exist nodes in the gap function. The data are most consistent with a d-wave symmetry. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.251

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We report T-c and Co-59 nuclear quadrupole resonance measurements on the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O (T-c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T-c decreases with increasing pressure at an average rate of -0.49 +/- 0.09 K/GPa. At low pressures P <= 0.49 GPa, the decrease of T-c is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of T-c is mainly due to a reduction of the DOS. These results indicate that the electronic and magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T-1 at P=0.49 GPa shows a T-3 variation below T-c down to T similar to 0.12T(c), which provides compelling evidence for the presence of line nodes in the superconducting gap function.

DOI： 10.1103/PhysRevB.76.100506

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The surprise discovery of superconductivity below 5 K in sodium cobalt oxides when hydrated with water has caught the attention of experimentalists and theorists alike. Most explanations for its occurence have focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, but direct attempts to look for them have instead cast serious doubts over their existence. Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.

DOI： 10.1103/PhysRevB.76.052509

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We report the results of NQR (nuclear quadrupole resonance) study on SmRu4P12, one of a few candidates for materials showing octupolar ordering. In this study, we firstly succeeded in observing the Ru-101-NQR signal in filled skutterudite compounds RRu4P12 (R: rare earth). A single Ru-101-NQR signal splits into two resonance lines, having intensity ratio of 3 : 1, just below the metal-insulator (M-I) transition temperature of 16.5 K. This result gives convincing evidence for symmetry lowering in the ordered state of this compound, namely the cubic Im(3) over bar space group for the filled skutterudite compounds is lowered to the trigonal R(3) over bar. The model of the ordered structure characterized by a wave vector q = (1, 0, 0) satisfactorily explains the observed phenomena in the present study and the M-I transition in this compound.

DOI： 10.1143/JPSJ.76.043714

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In order to investigate the microscopic origin of the transitions at T-MI similar to 16.5K and T* similar to 14K in filled skutterudite compound SmRu4P12, we have carried out P-31-NMR measurements using a single crystal sample. The drastic change in the NMR line-shape below TMI was observed, which is attributed to the occurrence of some long-range magnetic ordering. The order is probably associated with octupole ordering as indicated from the NMR measurements on the powder sample of SmRu4P12. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.128

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We present a La-139 NMR study on the colossal magneto-resistance (CMR) in double-layered manganese oxide La1.2Sr1.8Mn2O7. Above the Curie temperature TC = 126 K, applying a magnetic field is found to induce a ferromagnetic order that persists up to T = 330 K. The critical field at which the induced magnetic moment is saturated agrees well with the field at which the CMR effect reaches to a maximum. Our results indicate that the CMR observed above TC is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction. (C) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.914

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We report on the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu2Si2, CeRhIn5 and CeIn3, on the verge of antiferromagnetism (AFM) through nuclear-quadrupole-resonance (NQR) measurements under pressure (P). The exotic SC in a homogeneous CeCu2Si2 (T-c = 0.7 K) revealed antiferromagnetic critical fluctuations at the border to AFM or a marginal AFM. Remarkably, it has been found that the application of magnetic field induces a spin-density-wave (SDW) transition by suppressing the SC near the upper critical field. Furthermore, the uniform mixed phase of SC and AFM in CeCu2(Si1-xGex)(2) emerges on a microscopic level, once a tiny amount of 1% Ge (x = 0.01) is substituted for Si to expand its lattice. The application of minute pressure (P similar to 0.19 GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than T-c and T-N is ascribed to the uniform mixed phase of SC and AFM.
Likewise, the P-induced HF superconductor CeRhIn5 coexists with AFM on a microscopic level in P = 1.5-1.9 GPa. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below the onset temperature, presumably associated with an amplitude fluctuation of superconducting order parameter. The unconventional gapless nature of SC in the low-lying excitation spectrum emerges due to the uniform mixed phase of AFM and SC.
By contrast, in CeIn3, the P-induced phase separation of AFM and paramagnetism (PM) takes place without any trace for a quantum phase transition. The outstanding finding is that SC sets in at both the phases magnetically separated into AFM and PM in P = 2.28-2.5 GPa. A new type of SC forms the uniform mixed phase with AFM and the HF SC occurs in PM. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from AFM to PM might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM.

DOI： 10.1088/0953-8984/19/12/125202

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We report Sb-123 nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs4Sb12 under high pressures of 1.91 and 2.34GPa. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/T-1 indicate that the crystal-electricfield splitting Delta(CEF) between the ground state Gamma(1) singlet and the first excited state Gamma((2))(4) triplet decreases with increasing pressure. The 1/T-1 below T-c = 1.55 K at P = 1.91 GPa shows a power-law temperature variation and is proportional to T-5 at temperatures considerably below T,, which indicates the existence of Point nodes in the superconducting gap function. The data can be well fitted by the gap model Delta(theta) = Delta(0) sin theta with Delta(0) = 3.08k-B T-c. The relation between the superconductivity and the quadrupole fluctuations associated with the 1,(2) state is discussed.

DOI： 10.1143/JPSJ.76.023701

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We report B-11 and Pt-195 NMR measurements in noncentrosymmetric superconductor Li2Pt3B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature T-c. With decreasing temperature (T) below T-c, the spin-lattice relaxation rate 1/T-1 decreases with no coherence peak and is in proportion to T-3. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural Li2Pd3B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.

DOI： 10.1103/PhysRevLett.98.047002

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We report intensive studies on the cobalt oxide superconductors NaxCoO2 center dot 1.3H(2)O via Co-59 NMR and NQR measurements. In the sample with x = 0.26 and T-c = 4.6, we find that the spin-lattice relaxation rate 1/T-1 shows a T-3 variation below T-c and down to very low temperatures, which indicates the presence of line nodes in the superconducting (SC) gap function. We have also studied the spin susceptibility below T-c via the Knight shift measurement. The spin part of the Knight shift decreases below T-c in both the a- and c-directions indicating singlet spin state of the Cooper pairs. Our results strongly suggest that the superconducting state is a d-wave state. (c) 2006 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.531

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We report a Co-59 Knight-shift measurement in a single crystal of the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O (T-c=4.25 K). We find that the shift due to the spin susceptibility, K-s, is substantially large and anisotropic, with the spin shift along the a-axis K-a(s) being two times that along the c-axis K-c(s). The shift decreases with decreasing temperature (T) down to T approximate to 100 K, then becomes a constant until superconductivity sets in. Both K-a(s) and K-c(s) decrease below T-c. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.

DOI： 10.1103/PhysRevB.73.180503

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We report NQR measurements in the filled-skutterudite compound superconductors Pr(Os1-xRux)(4)Sb-12 (x = 0.1,0.2). Upon replacing Os with Ru, the spin-lattice relaxation rate 1/T-1 becomes proportional to temperature (T) at low temperatures far below T-c, and the magnitude of (1/T1T)(low-T) increases with increasing Ru content. These results indicate that a finite density of states are induced at the Fermi level by the impurity, suggesting that there exist nodes in the gap function of PrOs4Sb12. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.072

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We report measurements on the cobalt oxide superconductors NaxCoO2 (.) 1.3H(2)O over a wide Na content range x = 0.25-0.34, using X-ray diffraction, susceptibility and Co-59 nuclear quadrupole resonance (NQR) techniques. We find that T, increases with decreasing x but reaches to a plateau for x <= 0.28, which correlates closely with the NQR frequency that measures the occupancy of the Co 3d-orbitals. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.480

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We have carried out P-31-NMR (nuclear magnetic resonance) studies on Sm-based filled skutterudite compounds SmRu4P12. The measurements of NMR spectrum at various fields (H's) give evidence that spontaneous internal field occurs below the metal-insulator transition temperature T-M1 similar to 16.5 K. Concomitantly H-induced local field was observed between two successive transition temperatures, T* (similar to 14K) &lt; T &lt; T-M1, suggesting that some multipole ordering coexists with dipole ordering in this compound. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.083

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In order to gain insights into the origin of colossal magnetoresistance (CMR) in manganese oxides, we performed a La-139 NMR study in the double-layered compound La1.2Sr1.8Mn2O7. We find that above the Curie temperature T-C = 126 K, applying a magnetic field induces a long-range ferromagnetic order that persists up to T = 330 K. The critical field at which the induced magnetic moment is saturated coincides with the field at which the CMR effect reaches a maximum. Our results therefore indicate that the CMR observed above T-C in this compound is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction.

DOI： 10.1103/PhysRevLett.96.057203

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We report systematic measurements using the Co-59 nuclear quadrupole resonance (NQR) technique on the cobalt oxide superconductors NaxCoO2.1.3H(2)O over a wide Na content range x = 0.25-0.34. We find that T, increases with decreasing x but reaches a plateau for x <= 0.28. In the sample with x similar to 0.26, the spin-lattice relaxation rate 1/T-1 shows a T-3 variation below T-c and down h, to T similar to T-c/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x, 1/T-1 deviates from the T-3 variation below T similar to 2 K even though the T-c (similar to 4.7 K) is similar, which suggests an unusual evolution of the SC state. In the normal state, the spin correlations at a finite wavevector become stronger upon decreasing x, and the density of states at the Fermi level increases with decreasing x, which can be understood in terms of a single-orbital picture suggested on the basis of LDA calculation.

DOI： 10.1088/0953-8984/18/5/L02

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We report the novel phase diagram of superconductivity (SC) and anti ferromagnetism (AFM) in heavy-fermion (HF) pressure(P)-induced superconductors CeRhIn5 and CeIn5 through nuclear-quadrupole-resonance (NQR) measurements under P. The P-induced superconductivity in CeRhIn5 coexists with AFM on a microscopic level in P = 1.5-1.9 GPa. By contrast, in CeIn3, the P-induced phase separation of AFM and paramagnetism (PM) takes place without any trace of a quantum phase transition. The outstanding finding is that SC sets in at both the phases magnetically separated into AFM and PM in P=2.28-2.5 GPa. (c) 2005 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.10.043

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We have carried out nuclear quadrupole resonance (NQR) measurements in the filled skutterudite compounds Pr(Os1-x,Ru,)(4)Sb-12 (x = 0. 1, 0.2), in order to gain insights into the symmetry of the superconducting gap function. Upon replacing Os with Ru, the spin-lattice relaxation rate 1/T-1 becomes proportional to temperature (T) at low T far below T-c, and the magnitude of (1/T1T1)(low-T) increases with increasing Ru content. These results indicate that a finite density of states is induced at the Fermi level by the impurity, and thus suggest that there exist nodes in the gap function of PrOs4Sb12.

DOI： 10.1143/JPSJ.74.1938

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We report on a novel phase transition at T = 0.9 K in the Ce-based filled-skutterudite compound CeOs4Sb12 via measurements of the nuclear-spin lattice relaxation rate 1/T-1 and nuclear quadrupole resonance (NQR) spectrum of Sb nuclei. The temperature (T) dependence of]IT, behaves as if approaching closely an antiferromagnetic (AFM) quantum critical point (QCP), following the relation 1/T-1 proportional to T/(T - T-N)(1/2) with T-N = 0.06K in the range of T = 1.3-25K. The onset of either the spin-density-wave (SDW) or charge-density-wave (CDW) order at To = 0.9 K, that is, of the first order, is evidenced by a broadening of the NQR spectrum and a marked reduction in 1/T-1 just below T-0. The f-electron-derived correlated band realized in CeOs4Sb12 is demonstrated to give rise to the novel phase transition on the verge of AFM QCP.

DOI： 10.1143/JPSJ.74.1950

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We report on a novel phase transition at T = 0.9 K in the Ce-based filled-skutterudite compound CeOs4Sb12 via measurements of the nuclear-spin lattice relaxation rate 1/T-1 and nuclear quadrupole resonance (NQR) spectrum of Sb nuclei. The temperature (T) dependence of]IT, behaves as if approaching closely an antiferromagnetic (AFM) quantum critical point (QCP), following the relation 1/T-1 proportional to T/(T - T-N)(1/2) with T-N = 0.06K in the range of T = 1.3-25K. The onset of either the spin-density-wave (SDW) or charge-density-wave (CDW) order at To = 0.9 K, that is, of the first order, is evidenced by a broadening of the NQR spectrum and a marked reduction in 1/T-1 just below T-0. The f-electron-derived correlated band realized in CeOs4Sb12 is demonstrated to give rise to the novel phase transition on the verge of AFM QCP.

DOI： 10.1143/JPSJ.74.1950

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We report a B-11 NMR measurement on the recently discovered superconductor Li2Pd3B. The nuclear spin lattice relaxation rate 1/T-1 shows a well-defined coherence peak just below T-c(H=1.46 T)=5.7 K, and the spin susceptibility measured by the Knight shift also decreases below T-c. These results indicate that the superconductivity is of conventional nature, with an isotropic gap. Our results also suggest that the p-electrons of boron and the d-electrons of palladium that hybridize with boron p-electrons are primarily responsible for the superconductivity.

DOI： 10.1103/PhysRevB.71.220505

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We report on a systematic evolution of the superconducting (SC) characteristics of CeCoIn5 via nuclear-quadrupole-resonance (NQR) measurement under pressure (P). The application of P significantly suppresses the nuclear spin-lattice relaxation rate 1/T-1 that is dominated by antiferromagnetic (AFM) spin fluctuations (SFs). The marked suppression of AFM SFs leads to the reduction in the size of SC energy gap. Nevertheless, T-c increases with the pressure due to the increase in HF bandwidth. This is expected to make the lifetime of quasi-particles sufficiently long. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.080

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We report the discovery of novel phases in CeIn3 on the verge of antiferromagnetism (AFM) through the In-nuclear quadrupole resonance (NQR) measurements. We have found that even though a pressure (P)-induced phase separation into AFM and paramagnetism (PM) takes place, a novel superconductivity (SC), which is induced without strong spin fluctuations, has been found to coexist with AFM on a microscopic level. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.083

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Sb-121 nuclear magnetic resonance (NMR) measurements have been carried out on a single-crystalline PrOs4Sb12 for magnetic field (H) up to 15 T. A well-split Sb-121-NMR spectrum was observed for H parallel to &lt; 0 0 1 &gt;. We calculated the electric field gradient (EFG) of Sb site by a FLAPW method based on LDA. We have identified the observed NMR lines corresponding to three different Sb sites with asymmetric EFG tensors. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.254

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We report on the pressure-induced novel phases of ferromagnetism and superconductivity (SC) in the itinerant ferromagnet UGe2 via the Ge-73-NQR measurements under pressure (P). The NQR spectrum has revealed that the ferromagnetic phases are separated into weakly and strongly polarized phases around a critical value of P-c(*) similar to 1.2 GPa, pointing to a first-order transition around P-c(*). Here we present further evidence for the phase separation into ferromagnetic and paramagnetic phases around a critical pressure P-c similar to 1.6GPa. The measurements of nuclear spin-lattice relaxation rate 1/T-1 have probed that SC sets only in the ferromagnetic phase at T-sc similar to 0.2 K, but it does not in the paramagnetic phase. (c) 2005 Published by Elsevier B.V.

DOI： 10.1016/j.physb.2005.01.283

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We report on the pressure (P) versus temperature (T) phase diagram of the P-induced heavy fermion (HF) superconductor CeRhIn5 via nuclear quadrupole resonance (NQR) measurements under P. At P = 2.1 GPa, the T dependence of the nuclear spin-lattice relaxation rate 1/T-1 revealed antiferromagnetic critical fluctuations due to the closeness of antiferromagnetism. As P decreases slightly down to 1.9 GPa, a pseudogap emerges above T-c similar to 2 K, suggesting the appearance of superconducting fluctuations associated with a strong coupling effect among quasiparticles.

DOI： 10.1088/0953-8984/17/11/019

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We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by Cu-63 nuclear spin-lattice relaxation rate (1/T-1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at xsimilar to0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (T-c=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.

DOI： 10.1103/PhysRevLett.94.047006

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We report the systematic evolution of the superconducting (SC) characteristics of the heavy-fermion (HF) superconductor CeCoIn5 via nuclear-quadrupole-resonance (NQR) measurement under pressure (P). The application of P significantly suppresses the nuclear spin-lattice relaxation rate 1/T-1 that is dominated by antiferromagnetic (AFM) spin fluctuations (SFs) specific to a quantum critical point (QCP). It is demonstrated that the marked suppression of AFM SFs leads to a reduction in the SC energy gap or in the coupling strength of the Cooper pair. T-c,T- nevertheless, increases with increasing P due to the increase in HF bandwidth. This is expected to make the lifetime of quasi-particles sufficiently long.

DOI： 10.1143/JPSJ.73.2073

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We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa2Ca4Cu5Oy (Hg-1245) with T-c = 108 K, which is composed of two types of CuO2 planes in a unit cell, three inner planes (IP) and two outer planes (OP). The Cu-NMR study has revealed that the OP undergoes a superconducting (SC) transition at T-c = 108 K, whereas three underdoped IPs do an antiferromagnetic (AF) transition below T-N similar to 60 K with the Cu moments of similar to 0.3-0.4 mu(B). The metallic behavior observed at the IP well below T-N points to a possibility that SC and AF order parameters coexist in a single CuO2 plane. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.03.081

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We report the discovery of new superconducting and novel magnetic phases in CeIn3 on the verge of antiferromagnetism (AFM) under pressure (P) through the In-nuclear quadrupole resonance (NQR) measurements. We have found a P-induced phase separation of AFM and paramagnetism (PM) without any trace of a quantum phase transition in CeIn3. A new type of superconductivity (SC) was found, under P = 2.28-2.5 GPa, to coexist with AFM. It is magnetically separated from PM where the heavy fermion SC takes place. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs.

DOI： 10.1143/JPSJ.73.1647

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We present a systematic In-115 NQR study on the heavy-fermion compounds CeRh1-xIrxIn5 (x=0.25, 0.35, 0.45, 0.5, 0.55, and 0.75). The results provide strong evidence for the microscopic coexistence of antiferromagnetic (AF) order and superconductivity (SC) in the range of 0.35less than or equal toxless than or equal to0.55. Specifically, for x=0.5, T-N is observed at 3 K with a subsequent onset of superconductivity at T-c=0.9 K. T-c reaches a maximum (0.94 K) at x=0.45 where T-N is found to be the highest (4.0 K). Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF order. The nuclear spin-lattice relaxation rate 1/T-1 shows a broad peak at T-N and follows a T-3 variation below T-c, the latter property indicating unconventional SC as in CeIrIn5 (T-c=0.4 K). We further find that, in the coexistence region, the T-3 dependence of 1/T-1 is replaced by a T-linear variation below Tsimilar to0.4 K, with the value (T-1)(Tc)/(T-1)(low T) increasing with decreasing x, likely due to low-lying magnetic excitations associated with the coexisting magnetism.

DOI： 10.1103/PhysRevB.70.014511

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We present a systematic 115In NQR study on the heavy-fermion compounds CeRh1-xIrxIn5 (x=0.25, 0.35, 0.45, 0.5, 0.55, and 0.75). The results provide strong evidence for the microscopic coexistence of antiferromagnetic (AF) order and superconductivity (SC) in the range of 0.35≤x≤0.55. Specifically, for x=0.5, TN is observed at 3 K with a subsequent onset of superconductivity at Tc=0.9 K. Tc reaches a maximum (0.94 K) at x=0.45 where TN is found to be the highest (4.0 K). Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF order. The nuclear spin-lattice relaxation rate 1/T1 shows a broad peak at TN and follows a T3 variation below Tc, the latter property indicating unconventional SC as in CeIrIn5 (Tc=0.4 K). We further find that, in the coexistence region, the T3 dependence of 1/T1 is replaced by a T-linear variation below T ∼0.4 K, with the value (T1)Tc/(T1)low T increasing with decreasing x, likely due to low-lying magnetic excitations associated with the coexisting magnetism.

DOI： 10.1103/PhysRevB.70.014511

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We report on experimental studies of the phase state and the character of phase transitions in the quasi-one-dimensional organic compound (TMTSF)(2)PF6 in the close vicinity of the borders between the metal (M), antiferromagnetic (AF) insulator, and superconducting (SC) states. In order to drive the system precisely through the phase border P-0(T-0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P-0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for smooth and precise tuning deltaP=P-P-0 [thanks to a monotonic P-0(B) dependence] and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured magnetoresistance R(B) and its temperature dependence R(B,T) in the pressure range 0-1 GPa. Our studies focus on the features of the magnetoresistance at the phase transitions between the M and AF phases and in the close vicinity to the superconducting transition at Tapproximate to1 K. We found pronounced history effects arising when the AF/M phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the M to AF phase, driven by increasing magnetic field, the features of the M phase extend well into the AF phase. At the opposite transition from the AF to M phase, the features of the AF phase are observed in the M phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase, spatially separated from the majority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the phase boundary.

DOI： 10.1103/PhysRevB.69.224404

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We report on experimental studies of the phase state and the character of phase transitions in the quasi-one-dimensional organic compound (TMTSF) 2PF6 in the close vicinity of the borders between the metal (M), antiferromagnetic (AF) insulator, and superconducting (SC) states. In order to drive the system precisely through the phase border P 0(T0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for smooth and precise tuning δP = P - P0 [thanks to a monotonic P0(B) dependence] and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured magnetoresistance R(B) and its temperature dependence R(B,T) in the pressure range 0-1 GPa. Our studies focus on the features of the magnetoresistance at the phase transitions between the M and AF phases and in the close vicinity to the superconducting transition at T ≈ 1 K. We found pronounced history effects arising when the AF/M phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the M to AF phase, driven by increasing magnetic field, the features of the M phase extend well into the AF phase. At the opposite transition from the AF to M phase, the features of the AF phase are observed in the M phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase, spatially separated from the majority phase. When the system is driven away from the transition, the homogeneous state is restored
upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the phase boundary.

DOI： 10.1103/PhysRevB.69.224404

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We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in CeRhIn5 under pressure P = 1.75 GPa: The onset of the magnetic order is evidenced from a clear split of In-115-NQR spectrum due to the spontaneous internal field below T-N = 2.5 K: Simultaneously, bulk SC is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.122

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We report novel phase transition at similar to 0.9 K in the Ce-based filled-skutterudite compound CeOs4Sb12 via the measurements of nuclear-spin-lattice-relaxation-rate 1/T-1 and Sb-123-NQR spectrum. The temperature (T) dependence of 1/T-1 follows 1/T-1 proportional to (T-theta)(1/2) with theta = 0.06 K in the range of 1.3 K &lt; T &lt; 25 K, suggesting that the system is close to an antiferromagnetic critical point. The onset of spin-density-wave (SDW) order at T-SDW similar to 0.9 K is evidenced by the broadening of NQR spectrum and marked reduction in 1/T-1 just below TSDW: (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.12.448

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We report a pressure (P) effect on CeCu2(Si0.98Ge0.02)(2) where an antiferromagnetic (AFM) order at T-N similar to 0.75 K coexists with superconductivity below T-c similar to 0.4 K. At pressures exceeding P = 0.19 GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at P = 0 GPa is found to evolve into a typical heavy-fermion one with a line-node gap above P = 0.91 GPa. We highlight that the anomalous enhancement in nuclear spin-lattice relaxation rate 1/T-1, that follows a T1T = const. behavior well below T-c at P = 0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity.

DOI： 10.1143/JPSJ.73.194

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We report a careful Co-59 nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T-1 follows a Curie-Weiss type temperature (T) variation, 1/T1T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T-c=3.9 K, 1/T-1 decreases with no coherence peak and follows a T-n dependence with nsimilar or equal to2.2 down to similar to2.0 K but crosses over to a 1/T(1)proportional toT variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

DOI： 10.1103/PhysRevLett.92.047004

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DOI： 10.1016/j.physc.2004.02.122

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We report In-NQR and Co-NMR experiments of CeCoIn5 that undergoes a superconducting transition with a record high T-c = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, an anomalous temperature (T) dependence of nuclear spin-lattice relaxation rate 1/T-1 of In-115 is explained by the relation 1/T-1 proportional to T (.) chi(Q)(T)(3/4) based on the anisotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of 1/T-1 similar to T-1/4 over a wide T range of T-c &lt; T &lt; 40 K arises because the staggered susceptibility almost follows the Curie law chi(Q)(T) proportional to 1/(T + theta) with theta = 0.6 K and hence 1/T-1 proportional to T/(T + 0.6)(3/4) similar to T-1/4 for theta &lt; T. We highlight that the behavior 1/T-1 similar to T-1/4 is due to the proximity to the anisotropic AFM QCP relevant with. its layered structure, and is not associated with the AFM QCP for isotropic 3D systems. We have also found that the AFM spin fluctuations in CeCoIn5 are suppressed by small magnetic field so that theta = 0.6 K at H = 0 increases to theta = 2.5 K at H = 1.1 T, reinforcing that CeCoIn5 is closely located at the QCP.

DOI： 10.1143/JPSJ.72.2308

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DOI： 10.1103/PhysRevLett.91.109903

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

We a describe a measurement of the spin Knight shift (K-s) around the middle point between two vortices as a function of magnetic field (H) up to 28 T in a high-T-c superconductor TlSr2CaCu2O6.8. At low temperatures, K-s increases substantially with increasing field, which indicates that the quasiparticle states with an ungapped Spectrum are extended outside the vortex cores in a d-wave superconductor. The density of such states is found to be similar to0.7N(0root)/H/H-c2, with N-0 being the normal-state density of states and H,2 being the upper critical field.

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We report Ge-73 nuclear magnetic resonance/nuclear quadrupole resonance (NMR/NQR) measurements on the itinerant ferromagnetic superconductor UGe2 at ambient pressure (P = 0) and P = 1.3 GPa. Measurements of the nuclear spin-lattice relaxation rate 1/T-1 of the Ge-73 NMR at P = 0 have revealed a T1T = constant behaviour well below T-Curie = 52 K, evidencing the presence of a non-zero density of states at the Fermi level for the up-and down-spin bands in the ferromagnetic state. At P = 1.3 GPa, where the ferromagnetic transition is reduced to T-Curie = 26 K, the dependence on temperature (T) of NQR 1/T-1 at zero field has demonstrated the onset of the superconducting transition at T, = 0.55 K. The lack of a coherence peak in (1/T-1) just below T,, followed by a T-3-like behaviour, provide compelling evidence for the unconventional nature of the superconducting state that coexists with the ferromagnetic state on a microscopic scale in UGe2.

DOI： 10.1088/0953-8984/15/28/323

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We have studied the chemical-pressure effect on the Neel temperature (T-N) due to the substitution of Ir for Rh in the layered heavy fermion compound CeRhIn5 by nuclear quadrupolar resonance (NQR) technique. We find that T-N increases slightly upon replacing Ir for Rh up to the Ir content of 0.45. This feature resembles that in hydrostatically pressurized CeRhIn5 below 1.0 GPa where T-N shows a similar dependence against pressure. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02733-8

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We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor PrRu4Sb12 with T-c=1.3 K via the measurements of nuclear-quadrupole-resonance frequency nu(Q) and nuclear-spin-lattice-relaxation time T-1 of Sb nuclei. The temperature dependence of nu(Q) has revealed the energy scheme of Pr3+ crystal electric field that is consistent with an energy separation Delta(CEF)similar to70 K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T1T)(Pr)=const is valid, with [(1/T1T)(Pr)/(1/T1T)(La)](1/2)similar to1.44, where (1/T1T)(La) is for LaRu4Sb12. These results are understood in terms of a conventional Fermi-liquid picture in which the Pr-4f(2) state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T-1 shows a distinct coherence peak just below T-c, followed by an exponential decrease with a value of 2Delta/k(B)T(c)=3.1. These results demonstrate that PrRu4Sb12 is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs4Sb12 that is in an unconventional strong coupling regime. The present study on PrRu4Sb12 highlights that the Pr-4f(2)-derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs4Sb12.

DOI： 10.1103/PhysRevB.67.180501

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We have studied the ground state of the overdoped single-layered cuprate superconductor Bi2.1Sr1.9CuO6+delta (T-c = 8 K) by applying a strong magnetic field of 15.3 T to suppress superconductivity. In the normal state down to T = 1.6 K induced by the field Cu-63 nuclear spin-lattice relaxation rate (1/T-1) is found to obey a T1T = const. relation, indicating that the ground state in the overdoped regime is a Fermi liquid state. Our result is at variance with a recent study by scanning tunneling microscopy (STM) that finds a pseudogap in a similar sample. The implication on the phase diagram is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02410-3

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Nuclear quadrupolar resonance measurements find that, under hydrostatic or chemical pressure, the heavy fermion antiferromagnet CeRhIn5 becomes more itinerant and the Neel temperature T-N increases slightly at low pressures but decreases at large pressures. In CeRh0.5Ir0.5In5, unconventional superconductivity sets in well below T-N, with enhanced T(c)approximate to1 K. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02001-X

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We report a coexistence of superconductivity and antiferromagnetism in HgBa2Ca4Cu5Oy with T-c 108 K trough Cu-NMR measurements. This compound is composed of two kinds Of CuO2 planes in a unit cell, three inner planes (IP) and two outer planes (OP). The OP is optimally-doped, undergoing a superconducting transition at T-c = 108 K, whereas the IP is strongly underdoped, doing an antiferromagnetic transition below T-N similar to 60 K. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02400-0

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We report evidences for bulk nature and unconventional superconductivity in CeIn3 at P = 2.65 GPa revealed by the measurements of nuclear-spin-lattice-relaxation time (T-1) and ac-susceptibility (ac-chi). Both the measurements of ac-chi and T, have pointed to a superconducting transition at T = 95 mK, which is very much lower than an onset temperature T-onset = 0.15 K at zero resistance. The temperature dependence of 1/T-1 shows no coherence peak just below T-c, indicative of an unconventional nature for the superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02732-6

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We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr0.91LaCe0.09CuO4-y (T-c=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and appears to contrast with that in p-type copper oxides with a similar doping level where high-T-c superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-node gap model.

DOI： 10.1103/PhysRevLett.90.197005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS  

The In-115 nuclear spin-lattice relaxation rate (1/T-1) measurements are reported for the heavy fermion (HF) compounds Ce(Ir,Rh)In-5 along with their La analogs La(Ir,Rh)In-5. 1/T-1 for Ce(Ir,Rh)In-5 is enhanced by one order of magnitude over that in La(Ir,Rh)In-5, indicating strong magnetic fluctuations in these compounds. It is evidenced that CeIrIn5 is located near a quantum critical point, with quasi-2D spin fluctuations. Also in CeIrIn5, 1/T-1 follows a T-3 variation below T-c = 0.40 K, indicating unconventional superconductivity with line-node gap. These aspects are reminiscent of the high-T-c copper oxides and suggest the importance of the magnetic fluctuations for the occurrence of the unconventional superconductivity in these HF compounds.

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We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn5 under pressure P=1.75 GPa. The onset of the magnetic order is evidenced from a clear split of In-115 nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Neel temperature T-N=2.5 K. Simultaneously, bulk SC below T-c=2.0 K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.

DOI： 10.1103/PhysRevLett.90.077004

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We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in PrOs4Sb12 with T-c=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate, 1/T-1, and NQR frequency unravel a low-lying crystal-electric-field splitting below T(0)similar to10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T-1 shows neither a coherence peak just below T-c K nor a T-3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T-c below T(*)similar to2.3 K. It is surprising that PrOs4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.

DOI： 10.1103/PhysRevLett.90.027001

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Phase transitions occuring in a quasi-one-dimensional organic compound (TMTSF)(2)PF6 near the boundaries between the paramagnetic metallic (PM), antiferromagnetic insulator (AFI), and superconducting (SC) states were studied experimentally. A controlled transition through the phase boundary was achieved by maintaining the sample at fixed temperature T and pressure P, while the critical pressure was tuned by varying a magnetic field B. When the PM/AFI phase boundary was crossed due to the variation of a magnetic field, history effects were observed: the resistance was found to depend on the trajectory described by the system before arriving at a given point (P-B-T) of the phase space. The results of the experiment give evidence for the formation of a macroscopically inhomogeneous state characterized by the inclusions of a minor phase that is spatially separated from the major phase. Away from the phase boundary, the homogeneous state is restored. After this, upon approaching the phase boundary in the back direction, the system exhibits no features of the minor phase up to the very boundary. (C) 2003 MAIK "Nauka/Interperiodica".

DOI： 10.1134/1.1609570

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We report systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ([Formula presented]) on the pressure ([Formula presented])-induced heavy-fermion superconductor [Formula presented]. The temperature ([Formula presented]) dependence of [Formula presented] at [Formula presented] has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at [Formula presented] and [Formula presented]. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below [Formula presented], but [Formula presented], followed by a [Formula presented] law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in the low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below [Formula presented]. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevLett.91.137001

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We report nuclear magnetic resonance studies on the low-doped [Formula presented]-type copper-oxide [Formula presented] ([Formula presented]) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and appears to contrast with that in [Formula presented]-type copper oxides with a similar doping level where high-[Formula presented] superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-node gap model. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevLett.90.197005

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We report evidence for unconventional superconductivity in CeIn3 at a pressure P=2.65 GPa above the critical pressure (Pc∼2.5 GPa) revealed by measurements of the nuclear-spin-lattice-relaxation time (T1) and ac susceptibility (ac-x). Both measurements of T1 and ac-x have pointed to a superconducting transition at Tc=95 mK, which is much lower than the onset temperature Tonset=0.15 K at zero resistance. The temperature dependence of 1/T1 shows no coherence peak just below Tc, indicative of an unconventional nature for the superconductivity induced in CeIn3.

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We report the novel pressure (P)-temperature (T) phase diagrams of antiferromagnetism (AFM) and superconductivity (SC) in CeRhIn5, CeIn3, and CeCu2Si2 revealed by the nuclear quadrupole resonance measurement. In the itinerant helical magnet CeRhIn5, we found that the Neel temperature T-N is reduced at P greater than or equal to 1.23 GPa with an emergent pseudogap behavior. The coexistence of AFM and SC is found in a narrow P range of 1.63-1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1-5 GPa. In CeIn3, the localized magnetic character is robust against the application of pressure up to P similar to 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu2Si2, the SC and AFM coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal AFM state is due to collective magnetic excitations in the superconducting state in CeCu2Si2. An interplay between AFM and SC is discussed on the SO(5) scenario that unifies AFM and SC. We suggest that the SC and AFM in CeCu2Si2 have a common mechanism. (C) 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(02)00133-6

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We report Cu-63 and Tl-205 nuclear magnetic resonance (NMR) measurements on trilayered high-T-c compounds TlBa2Ca2Cu3O10-y (Tl-1223) that were synthesized at a high pressure and exhibits a high value of superconducting (SC) transition temperature T-c&gt;130 K. In the SC state, the nuclear spin-lattice relaxation rate 1/T-1 reveals a T-3 behavior below T-c, followed by a T1T=const behavior at very low temperatures. The results are accounted for by the d-wave pairing model where the presence of the residual density of the states N-res(E-F) is incorporated at the Fermi level to be consistent with the experiments. The important finding is that the size [in N-res(E-F)] is much smaller for the present compounds of Tl-1223 with T-c&gt;130 K than for the previous Tl-2223. Correspondingly, it is shown that an NMR intensity at a satellite peak in the Tl-NMR spectrum, which originates from a possible substitution of Tl for Ca, is much smaller for the former than for the latter. The synthesis technique at a high pressure is found to be a good way to remove such a crystal imperfection due to Tl/Ca intersubstitution. We have demonstrated from a microscopic point of view that as a result of decreasing the degree of disorder effect, the T-c in Tl-1223 has revealed a value higher than 130 K.

DOI： 10.1103/PhysRevB.65.184504

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We report a pressure effect on CeCu2(Si0.98Ge0.02)(2) where an anti ferromagnetic order below T-N similar to 0.75 K coexists with superconductivity below T-c similar to0.4 K. At pressures exceeding 0.56 GPa, the magnetic order is suppressed and concomitantly the unconventional superconducting state evolves into a typical heavy-fermion one with a line-node gap. This result demonstrates that the magnetic order is due to the lattice expansion caused by doping Ge. We remark that the characteristic feature of exotic coexisting state at ambient pressure is the persistence of low-lying magnetic excitations even in the superconducting state. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01531-9

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We report 115In nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field at the In site is substantially reduced from 1.75 kOe at P = 0 to 0.39 kOe at P = 1.23 GPa, while TN slightly changes with increasing P. This suggests that either the size or the direction of the ordered moment changes as P increases. In the SC state at P = 2.1 GPa, the nuclear spin-lattice relaxation rate has revealed a T3 dependence without the coherence peak just below Tc, giving evidence for the unconventional superconductivity. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01157-7

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We report In-115 nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal held at the In site is substantially reduced from 1.75 kOe at P = 0 to 0.39 kOe at P = 1.23 GPa, while T-N slightly changes with increasing P. This suggests that either the size or the direction of the ordered moment changes as P increases. In the SC state at P = 2.1 GPa, the nuclear spin-lattice relaxation rate has revealed a T-3 dependence without the coherence peak just below T-c, giving evidence for the unconventional superconductivity. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01157-7

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We report the spin Knight shift (K-s) and the nuclear spin-lattice relaxation rate (1/T-1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-T-c superconductor TlSr2CaCu2O6.8 (T-c = 68 K). At low temperatures well below T-c, both K-s and 1/T-1 measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be similar tokappaN(o)rootH/H-c2, with kappa = 0.5-0.7 and N-o being the normal-state DOS.

DOI： 10.1103/PhysRevLett.88.077003

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We report the pressure (P)-temperature (T) phase diagram of antiferromagnetism and superconductivity in CeRhIn5 and CeIn3 revealed by the 115In nuclear-spin-lattice-relaxation (T1) measurement. In the itinerant magnet CeRhIn5, we found that the Néel temperature TN is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. In CeIn3, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

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Language：English   Publisher：The Physical Society of Japan  

We report a novel pressure(<italic>P</italic>)-induced evolution on the magnetism in the <italic>P</italic>-induced superconductor CeRhIn₅revealed by measurements of the ¹¹⁵In nuclear–spin–lattice-relaxation rate (¹¹⁵1/<italic>T</italic>₁). In the itinerant antiferromagnet CeRhIn₅with the Néel temperature <italic>T</italic>_N=3.8 K at <italic>P</italic>=0, <italic>T</italic>_Nbegins to be reduced to <italic>T</italic>_N=3.6 K at <italic>P</italic>=1.23 GPa that is close to the border of the superconducting phase. An emergent behavior is the observation of a pseudogap behavior that is revealed by a broad peak in the temperature dependence of 1/<italic>T</italic>₁<italic>T</italic>at <italic>P</italic>=1.23 GPa. With further increasing <italic>P</italic>up to <italic>P</italic>=2.1 GPa where its bulk superconductivity is fully established, the pseudogap behavior is not seen in the normal state. The insensitive <italic>P</italic>dependence of <italic>T</italic>_cin CeRhIn₅is suggested to be related to that of the characteristic temperature <italic>T</italic>^*which marks a localized to itinerant magnetic crossover.

DOI： 10.1143/JPSJS.71S.282

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We report the pressure (P)-temperature (T) phase diagram of antiferromagnetism and superconductivity in CeRhIn5 and CeIn3 revealed by the In-115 nuclear-spin-lattice-relaxation (T-1) measurement. In the itinerant magnet CeRhIn5, we found that the Neel temperature T-N is reduced at P greater than or equal to 1.23 GPa with an emergent pseudogap behavior. In CeIn3, the localized magnetic character is robust against the application of pressure up to Psimilar to1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

DOI： 10.1103/PhysRevB.65.020504

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We report magnetic field (H) effect on the unconventional magnetism. in LiNiO9 that conSists of a spin-1/2 triangular (TM) lattice through the tLi nuclear spin lattice relaxation (1/T-1) and NMR intensity measurements. Instead of a ferromagnetic (FM) ground state expected from. the FM-Weiss constant, a marginal ordered state emerges below T-M -&gt; 20 K, evidenced from the wipe-out effect of Li-7-NMR signal and the divergent behavior of 1/T-1 towards T-M. The existence of the strong antiferromagnetic (.,NIF) interaction along with the FM one is concluded to lead to a magnetically frustrated state at low temperatures and its AF exchange constant J(AF) is estimated to be about 20 K. This marginal ordered, state due to the frustration of the antiferro-orbital order is found to he suppressed by applying H exceeding 10 T, revealing an orbital exchange constant J(0) similar to -10 K. In the paramagnetic regime at high temperatures, the system behaves like an assembly of local moments with an exchange frequency omega(ex) similar to 11 K. consistent with the energy scale of vertical bar J(0)vertical bar similar to 10 K. The present experiments have revealed that vertical bar J(0)vertical bar similar to 10 K is lower than spin exchange constants J(FM) similar to 30 K vertical bar J(AF)vertical bar similar to 20 K.

DOI： 10.1143/JPSJS.71S.169

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LaNiO3 is a transition-metal oxide with Perovskite crystal structure where Ni is in the low spin state of r(2g)(6)e(g)(1),. It is a metal and shows enhanced susceptibility x with a strong temperature (T) dependence at low T. We have carried out a La-139 NMR study in the polycrystal sample in the temperature range between 1,4 K and 300 K. We find that the uniform spin susceptibility is T-independent which suggests that the low-T upturn in x is an extrinsic property of the metallic state. The results of the Knight shift and the nuclear spin relaxation rate indicate that the spin susceptibility is almost wave-vector (q) independent, implying that the electron correlation is " dominantly due to a band narrowing, MASS enhancement, effect near a metal-insulator transition, rather than to a ferromagnetic interaction peaked at q=0,

DOI： 10.1143/JPSJS.71S.166

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We report the 'In nuclear spin-lattice relaxation rate (1/T-1) measurements down to 90 mK in the new heavy fermion (HF) compound CeIrIn5. At 0.4 K &lt;= T &lt;= 100 K, 1/T-1 is strongly T-dependent, which indicates that the 4f electrons are much more itinerant than in known Ce-based HFs. From the unique temperature variation of 1/T1T, we find that this compound is located near a quantum critical point, with anisotropic spin fluctuations due to a layered crystal structure, Below the hulk superconductivity temperature 0.40 K, 1/T-1, follows a T-3 variation without a coherence peak, which indicates unconventional superconductivity with line-node gap. These aspects are reminiscent of the high-T-c copper oxides and suggest the importance of the magnetic fluctuations for the occurrence of the unconventional superconductivity

DOI： 10.1143/JPSJS.71S.279

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We report evidence for unconventional superconductivity in (formula presented) at a pressure (formula presented) above the critical pressure (formula presented) revealed by measurements of the nuclear-spin-lattice-relaxation time (formula presented) and ac susceptibility (ac- (formula presented) Both measurements of (formula presented) and ac- (formula presented) have pointed to a superconducting transition at (formula presented) which is much lower than the onset temperature (formula presented) at zero resistance. The temperature dependence of (formula presented) shows no coherence peak just below (formula presented) indicative of an unconventional nature for the superconductivity induced in (formula presented). © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.66.054521

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We report the pressure (formula presented)-temperature (formula presented) phase diagram of antiferromagnetism and superconductivity in (formula presented) and (formula presented) revealed by the (formula presented) nuclear-spin-lattice-relaxation (formula presented) measurement. In the itinerant magnet (formula presented) we found that the Néel temperature (formula presented) is reduced at (formula presented) with an emergent pseudogap behavior. In (formula presented) the localized magnetic character is robust against the application of pressure up to (formula presented) beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevB.65.020504

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We report (formula presented) and (formula presented) nuclear magnetic resonance (NMR) measurements on trilayered high-(formula presented) compounds (formula presented) (Tl-1223) that were synthesized at a high pressure and exhibits a high value of superconducting (SC) transition temperature (formula presented) In the SC state, the nuclear spin-lattice relaxation rate (formula presented) reveals a (formula presented) behavior below (formula presented) followed by a (formula presented) behavior at very low temperatures. The results are accounted for by the d-wave pairing model where the presence of the residual density of the states (formula presented) is incorporated at the Fermi level to be consistent with the experiments. The important finding is that the size [in (formula presented) is much smaller for the present compounds of Tl-1223 with (formula presented) than for the previous Tl-2223. Correspondingly, it is shown that an NMR intensity at a satellite peak in the Tl-NMR spectrum, which originates from a possible substitution of Tl for Ca, is much smaller for the former than for the latter. The synthesis technique at a high pressure is found to be a good way to remove such a crystal imperfection due to Tl/Ca intersubstitution. We have demonstrated from a microscopic point of view that as a result of decreasing the degree of disorder effect, the (formula presented) in Tl-1223 has revealed a value higher than 130 K. © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.184504

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We report the spin Knight shift ([Formula presented]) and the nuclear spin-lattice relaxation rate ([Formula presented]) in the vortex state as a function of magnetic field ([Formula presented]) up to 28 T in the high- [Formula presented] superconductor [Formula presented] ([Formula presented]). At low temperatures well below [Formula presented], both [Formula presented] and [Formula presented] measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a [Formula presented]-wave superconductor. The density of states (DOS) around the saddle point is found to be [Formula presented], with [Formula presented] and [Formula presented] being the normal-state DOS. © 2002 The American Physical Society.

DOI： 10.1103/PhysRevLett.88.077003

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We find contrastive response of the pseudogap (PG) to high magnetic fields up to 28.5 T based on 63Cu NMR measurements. In the slightly overdoped TlSr2CaCu2O6.8, the PG is strongly field dependent and shown to be due to the superconducting fluctuations. By contrast, the PG in the underdoped YBa2Cu4O8 does not depend on magnetic fields up to 28.5 T. These results imply that there exists a field-insensitive PG up to a certain doping level beyond which it is taken over by the superconducting fluctuations-induced one. © 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(01)00820-6

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DOI： 10.1103/PhysRevLett.87.209701

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We find contrastive response of the pseudogap (PG) to high magnetic fields up to 28.5 T based on Cu-63 NMR measurements. In the slightly overdoped TlSr2CaCu2O6.8, the PG is strongly field dependent and shown to be due to the superconducting fluctuations. By contrast, the PG in the underdoped YBa2CU4O8 does not depend on magnetic fields up to 28.5 T. These results imply that there exists a field-insensitive PG up to a certain doping level beyond which it is taken over by the superconducting fluctuations-induced one. (C) 2001 Elsevier Science B.V. All rights reserved.

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We report unusual magnetic and superconducting (SC) characteristics in multilayered CuO2 planes in Hg- and Cu-based high-Tc cuprates through the 63Cu-NMR measurements. These compounds, in which the number of CuO2 planes (n) ranges from 3 to 5 in a unit cell, include crystallographically inequivalent outer (OP) and inner (IP) CuO2 plane that are surrounded by pyramidal and square oxygen, respectively. The Knight shift (63K) at the OP and IP exhibits respective characteristic temperature dependence, consistent with its own doping level. Using an experimental relation between the spin part in 63K at room temperature and the doping level in a CuO2 layer Nh, we show that Nh(OP) at the OP is larger than Nh(IP) at the IP for all the systems and its difference ΔNh=Nh(OP)-Nh(IP) increases as either a total carrier content σ or n increases. At ΔNh's exceeding a critical value, the pseudogap behavior in the normal state is seen alone at the IP, and a bulk SC transition does not set in simultaneously at the IP and OP. A SC nature at the OP becomes consistent with a mean-field behavior only below Tc2 that is significantly lower than Tc. Reduction in Tc with increasing n is associated with an increase in ΔNh. It is a rather remarkable aspect that a Tc is not always reduced even though these multilayered high-Tc compounds are heavily overdoped. This arises, we show, because the IP remains underdoped and keeps a high value of Tc, while the OP is predominantly overdoped. This may be a microscopic origin for the lowest anisotropic SC characteristics reported to date in Cu-based multilayered high-Tc compounds.

DOI： 10.1103/PhysRevB.64.064515

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We report unusual magnetic and superconducting (SC) characteristics in multilayered CuO2 planes in Hg- and Cu-based high-T-c cuprates through the Cu-63-NMR measurements. These compounds, in which the number of CuO2 planes (n) ranges from 3 to 5 in a unit cell, include crystallographically inequivalent outer (OP) and inner (IP) CuO2 plane that are surrounded by pyramidal and square oxygen, respectively. The Knight shift (K-63) at the OP and IP exhibits respective characteristic temperature dependence, consistent with its own doping level. Using an experimental relation between the spin part in K-63 at room temperature, and the doping level in a CuO2 layer N-h, we show that N-h,(OP) at the OP is larger than Nh(IP) at the IP for all the systems and its difference DeltaN(h)=N-h(OP) -N-h(IP) increases as either a total carrier content delta or n increases. At DeltaN(h)'s exceeding a critical value, the pseudogap behavior in the normal state is seen alone at the IP. and a bulk SC transition does not set in simultaneously at the IP and OP. A SC nature at the OP becomes consistent with a mean-field behavior only below T-c2 that is significantly lower than T-c. Reduction in T-c with increasing n is associated with an increase in DeltaN(h). It is a rather remarkable aspect that a T-c is not always reduced even though these multilayered high-T-c compounds are heavily overdoped. This arises, we show, because the IP remains underdoped and keeps a high value of T-c, while the OP is predominantly overdoped. This may be a microscopic origin for the lowest anisotropic SC characteristics reported to date in Cu-based multilayered high-T-c compounds.

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We report In-115 nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field H-int at the In site is substantially reduced from H-int= 1.75 kOe at P=0 to 0.39 kOe at P=1.23 GPa, while the Neel temperature slightly changes with increasing P. This suggests that either the size in the ordered moment M-Q(P) or the angle theta(P) between the direction of M-Q(P) and the tetragonal c axis is extrapolated to zero at P*=1.6+/-0.1 GPa at which a bulk SC transition is no longer emergent. In the SC state at P=2.1 GPa, the nuclear spin-lattice relaxation rate (115)(1/T-1) has revealed a T-3 dependence without the coherence peak just below T-c, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic fluctuations in the normal state is also discussed.

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The perovskites of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO) were synthesized by the sol-gel method. The two perovskites were mixed into polycrystalline materials, (1 - m)LCMO + mLSMO, where m is the relative weight ratio of LSMO. The compound with m similar to 0.3-0.5 has the lowest semiconductor-metal transition temperature T-p and the largest low-T resistivity. Moving away from this ratio, T-p increases while the low-T resistivity decreases with increasing or decreasing m. Measurements of the magnetoresistance (MR) show that the mixed polycrystals exhibit a sizable MR effect in low magnetic fields which increases upon cooling and reaches its largest value for T --&gt; 0 K. It is also interesting to observe the almost T-independent MR effect over a wide temperature window below 300 K for the mixed materials with smaller m.

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We report Cu-NQR results under pressure on heavy-fermion (HF) superconductor CeCu2Si2 close to a magnetic phase, Ce0.99Cu2.02Si2 and CeCu2(Si0.98Ge0.02)(2). It is found that an application of a minute pressure exceeding 0.3 GPa markedly suppresses low-energy magnetic fluctuations observed at ambient pressure (P = 0) in Ce0.99CU2.02Si2, Correspondingly, anomalous superconducting (SC) characteristics evolve into conventional HF-SC ones with increasing pressure. In the latter. the Ge substitution works as a negative chemical pressure. CeCu2(Si0.98Ge0.02)(2) reveals a magnetic phase transition at T-N = 0.75 K followed by the SC phase transition at T-c similar to 0.4 K at P = 0. It is suggested experimentally that the magnetic fluctuations seen in Ce0.99Cu2.02Si2 at P = 0 possesses a quantum critical character of both SC and magnetically ordered states. (C) 2001 Elsevier Science B.V. All rights reserved.

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We report measurements of the In-115 nuclear spin-lattice relaxation rate (1/T-1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. Ar 0.4 less than or equal to T less than or equal to 100 K, 1/T-1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that I/T1T, subtracting that for LaIrIn5, follows a (1 / tau + theta)(3/4) variation with theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The hulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T-1 follows a T-3 variation, which suggests unconventional superconductivity with Line-node gap.

DOI： 10.1103/PhysRevLett.86.4664

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We report Cu nuclear-quadrupole-resonance results under pressure (P) on a homogeneous CeCu2Si2 (T-c = 0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic (AF) phase. This exotic superconducting (SC) phase evolves into the typical heavy-fermion SC phase at minute pressures exceeding P(c)similar to0.2 GPa. The nuclear spin-lattice relaxation data in a P range 0.85 - 2.58 GPa are shown to be accountable by the spin-fluctuation theory based on the nearly AF Ferni-liquid model. In 0 less than or equal toP&lt;P-c&lt;similar to&gt;0.2 GPa, by contrast, we conclude that the exotic SC phase manifests itself under the unconventional normal state where AF waves propagate over a long range without any trace of AF order and thereby the heavy-fermion state breaks up.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We report Cu-63-Knight shift measurement on multilayered high-T-c cuprate oxides that include inequivalent outer (OP) and inner (IP) CuO2 planes in a unit cell with number of planes n = 3-5. Using an experimental relation between the spin part of Knight shift (K-s) and the carrier concentration (N-h) reported in n = 1 and 2 cuprates, the local carrier concentrations Nh(OP) in the OP and Nh(IP) in the IP have been deduced. We have found that Nh(OP) is larger than Nh(IP) in all the systems. The difference in the doping level increases as total-carrier content delta and n increase. Imbalance between Nh(OP) and Nh(IP) is suggested to be caused by a mechanism that the electrostatic potential associated with the apical oxygen has more attraction for holes in the OP than in the IP. It is also suggested that T-c of Hg 1223 (n = 3) is the highest (T-c = 133 K) to date, due to N-h(IP) optimized to N-h.optimum similar to 0.2 From the fact that N-h(OP)&gt; N-h.optimum, we propose that if Nh(OP) could also be optimized in addition to optimized N-h(IP). T-c might be raised higher than 133 K. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00122-0

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From the Cu-63 Knight shift and the nuclear spin-lattice relaxation rate under magnetic fields from zero up to 28 T in the underdoped (U) YBa2Cu4O8 and the slightly overdoped (O) TlSr2CaCu2O6.8 We find that the spin-gap (pseudogap) behaviors show contrasting response to magnetic fields in the U and O compounds. In the O compound. the spin-gap is strongly field dependent and can be accounted for by the superconducting fluctuation effects, but that in the U compound shows no field dependence. The implication is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00095-0

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We report 115In nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field Hint at the In site is substantially reduced from Hint = 1.75 kOe at P = 0 to 0.39 kOe at P = 1.23 GPa, while the Néel temperature slightly changes with increasing P. This suggests that either the size in the ordered moment MQ(P) or the angle θ(P) between the direction of MQ(P) and the tetragonal c axis is extrapolated to zero at P* = 1.6±0.1 GPa at which a bulk SC transition is no longer- emergent. In the SC state at P =2.1 GPa, the nuclear spin-lattice relaxation rate 115(1/T1) has revealed a T3 dependence without the coherence peak just below Tc, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic fluctuations in the normal state is also discussed.

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We report (formula presented) nuclear-quadrupole-resonance (NQR) measurements of the pressure (formula presented)-induced superconductor (formula presented) in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field (formula presented) at the In site is substantially reduced from (formula presented) at (formula presented) to 0.39 kOe at (formula presented) while the Néel temperature slightly changes with increasing P. This suggests that either the size in the ordered moment (formula presented) or the angle (formula presented) between the direction of (formula presented) and the tetragonal c axis is extrapolated to zero at (formula presented) at which a bulk SC transition is no longer emergent. In the SC state at (formula presented) the nuclear spin-lattice relaxation rate (formula presented) has revealed a (formula presented) dependence without the coherence peak just below (formula presented) giving evidence for the unconventional superconductivity. The dimensionality of the magnetic fluctuations in the normal state is also discussed. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevB.63.220507

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We report Cu nuclear-quadrupole-resonance results under pressure (P) on a homogeneous CeCu2Si2 (Tc=0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic (AF) phase. This exotic superconducting (SC) phase evolves into the typical heavy-fermion SC phase at minute pressures exceeding Pc∼0.2 GPa. The nuclear spin-lattice relaxation data in a P range 0.85 - 2.58 GPa are shown to be accountable by the spin-fluctuation theory based on the nearly AF Fermi-liquid model. In 0≤P< Pc∼0.2 GPa, by contrast, we conclude that the exotic SC phase manifests itself under the unconventional normal state where AF waves propagate over a long range without any trace of AF order and thereby the heavy-fermion state breaks up.

DOI： 10.1103/physrevb.63.140501

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Based on measurements of the (CU)-C-63 Knight shift and the nuclear spin-lattice relaxation rate under magnetic fields up to 28 T, we find that the pseudogap in the slightly overdoped TISr2CaCu2O6.88 (T-c=68 K) is due to Cooper pair density fluctuations. Such fluctuations are also found in the underdoped YBa2Cu4O8 (T-c=74 K), but are small and only survive up to T-sf similar or equal to T-c+15K=90 K, which is much lower than the held-independent pseudogap temperature T* similar or equal to 160 K. The implication on the phase diagram is discussed. At low temperatures in the superconducting state, a field-induced finite density of states was found in both compounds and is interpreted as due to the quasiparticles outside a vortex core in a d-wave superconductor.

DOI： 10.1016/S0921-4534(00)00703-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Inst of Physics  

Measurements in the slightly overdoped superconductor TlSr2CaCu2O6.8 at both zero magnetic field and high fields up to 28 T were made. It was found that the reduction of the Knight shift and 1/T1T above Tc is strongly field dependent and follows a simple scaling relation. Overall, the results indicate that an electron correlation-driven pseudogap would terminate at some doping level before entering the overdoped regime.

DOI： 10.1103/PhysRevLett.85.405

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

From measurements of the Cu-63 Knight shift (K) and the nuclear spin-lattice relaxation rate (1/T-l) under magnetic fields from zero up to 28 T in the slightly overdoped high-T-c. superconductor TlSr2CaCu2O6.8 (T-c = 68 K), we find that the pseudogap behavior, i.e., the reductions of 1/TlT and K above T-c from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.

DOI： 10.1103/PhysRevLett.85.405

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The responses to the magnetic field (H) of the superconductivity and the pseudogap in the underdoped high- T-c cuprate YBa2Cu4O8 (T-c = 74 K) are reported up to 23.2 T based on NMR measurements. For temperatures T less than or equal to 15 K, the Hand T-variation of the Cu-63 nuclear spin-lattice relaxation rate (1/T-1) is 1/T-1 proportional to TH and also the Knight shift increases with increasing H. We interpret our results as evidence for H-induced quasiparticle states that extend from the d-wave vortex centers with an ungapped spectrum. Even though H = 23.2 T reduces T-c by 19 K, no effect is seen on the normal-state pseudogap which persists in the temperature region where the superconductivity is destroyed by the field, suggesting that it is a consequence of strong electron correlations. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)01161-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BALTZER SCI PUBL BV  

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2 polycrystals: Ce0.99Cu2.02Si2 (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce0.975Cu2Si2 (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although T-c stays nearly constant in the measured pressure range, P less than or equal to 14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T --&gt;0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER WIEN  

This paper introduces nuclear magnetic resonance works in the strongly correlated superconductors: heavy-Fermion, high-T-C superconductors and Sr2RuO4. The analyses strongly support the spin-fluctuation-mediated superconductivity model in a high-T-C superconductor.

DOI： 10.1007/BF03162372

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DOI： 10.1016/S0921-4526(99)01170-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BALTZER SCI PUBL BV  

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2 polycrystals: Ce0.99Cu2.02Si2 (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce0.975Cu2Si2 (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although T-c stays nearly constant in the measured pressure range, P less than or equal to 14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T --&gt;0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality.

DOI： 10.1023/A:1012635716291

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BALTZER SCI PUBL BV  

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2 polycrystals: Ce0.99Cu2.02Si2 (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce0.975Cu2Si2 (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although T-c stays nearly constant in the measured pressure range, P less than or equal to 14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T --&gt;0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality.

DOI： 10.1023/A:1012635716291

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER WIEN  

This paper introduces nuclear magnetic resonance works in the strongly correlated superconductors: heavy-Fermion, high-T-C superconductors and Sr2RuO4. The analyses strongly support the spin-fluctuation-mediated superconductivity model in a high-T-C superconductor.

DOI： 10.1007/BF03162372

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Based on measurements of the 63Cu Knight shift and the nuclear spin-lattice relaxation rate under magnetic fields up to 28 T, we find that the pseudogap in the slightly overdoped TlSr2CaCu2O6.88 (Tc=68 K) is due to Cooper pair density fluctuations. Such fluctuations are also found in the underdoped YBa2Cu4O8 (Tc=74 K), but are small and only survive up to Tsf ≃ Tc+15K=90 K, which is much lower than the field-independent pseudogap temperature T* ≃ 160 K. The implication on the phase diagram is discussed. At low temperatures in the superconducting state, a field-induced finite density of states was found in both compounds and is interpreted as due to the quasiparticles outside a vortex core in a d-wave superconductor. © 2000 Elsevier Science B.V. All righls reserved.

DOI： 10.1016/S0921-4534(00)00703-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

The responses to the magnetic field (H) of the superconductivity and the pseudogap in the underdoped high-T-c cuprate YBa2Cu4O8 (T-c=74 K) are reported up, to 23.2 T based on NMR measurements. Even though H=23.2 T reduces T-c by 19 K, no effect is seen on the normal-state pseudogap, which continues in the temperature region where the superconductivity is destroyed by the field. This suggests that the pseudogap, is a consequence of strong electron correlation effects. I;br temperatures T less than or equal to 15 It, the H- and T-variation of the Cu-63 nuclear spin-lattice relaxation rate (1/T-1) is 1/T-1 proportional to TH and also the Knight shift increases with increasing H. We interpret our results as evidence for H-induced quasiparticle states that extend from the d-wave vortex centers with an ungapped spectrum.

DOI： 10.1142/S021797929900388X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KLUWER ACADEMIC/PLENUM PUBL  

The carrier distribution among the inequivalent CuO2 planes in multilayer cuprates is investigated by Cu-63 Knight shift measurement with varying the carrier concentration, delta and the number of CuO2 plane in a unit cell, n. We Sound that 70 (60)% of total carriers are doped selectively into two outer CuO2 planes in n = 3 (4). This indicates resultantly that the inhomogeneity of carrier distribution increases with increasing delta and n in multilayer cuprates.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The responses to the magnetic field (H) of the pseudogap and the superconductivity in the underdoped high T-c cuprate YBa2Cu4O8 (T-c=74 K) are reported up to 23.2 T based on NMR measurements. Even though 23.2 T reduces T, by 26%, no effect is seen on the normal-state pseudogap, which persists in the temperature range where the superconductivity is destroyed by the field. For temperatures T less than or equal to 15 K, the spin Knight shift increases with increasing H and also the H and T variation of the Cu-63 nuclear spin-lattice relaxation rate (1/T-1) is 1/T-1 proportional to TH. A possible explanation for this finding is offered as due to the H-induced quasiparticle states that extend from the d-wave vortex centers with ungapped spectrum. [S0163-1829(99)50438-4].

DOI： 10.1103/PhysRevB.60.R9947

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We report systematic Cu-NMR study on HgBa2Ca3Cu4O10+y with T-c = 123 K comprising respective 4-fold and 5-fold two CuO2 planes surrounded by square and pyramidal oxygens. Possible structure and temperature dependence of the normal-state pseudogap have been extracted from the steep decrease of the spin contribution in the Knight shift, K-s below T-K*similar to 220K. By assuming the linear increase of a size of pseudogap between T-K* and T-c with the d(x2-y2) symmetry, the T-dependence of K-s/K-n in both 4- and 5-fold is reproduced below T-K*. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(98)01066-7

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High-pressure NMR measurements on a single crystal of hole-doped two-leg ladder compound, Sr(2)Ca(12)Cu(24)O(41), have been carried out up to similar to 1.7 GPa. The activated T-dependences of T(1) and Knight shift at 1.7 GPa assured the persistence of the spin gap in this system. (C) 1998 Elsevier Science B.V, AII rights reserved.

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Recent NMR measurements on high-ir, and related material Sr2RuO4 by the Osaka group are reviewed. The following results are presented: (1) Study of YBa2Cu3O7, + Ni has revealed that the attractive force is spin in origin. (2) Analysis of T-1, T-2 and Knight shift (K) in various high-T-c, materials supports the antiferromagnetic spin-fluctuation mediated superconducting model. (3) Temperatures at which a pseudogap starts to open obtained by 1/T1T, dK/dT, photoemission and resistivity agree with each other in the Bi2Sr2CaCu2O8 + delta system. (4) O-17 Knight shift measurements in the related material Sr2RuO4 suggests that a triplet pairing is realized in this compound as in the heavy-fermion superconductor UPt3. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(98)01015-1

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We report magnetic properties of the antiferromagnet YMn2 as functions of pressure and temperature (T) obtained from the 55Mn spin-lattice relaxation rate and the Knight shift. An energy-band narrowing due to electron correlation is indicated by the Knight shift measurement and the application of pressure is found to enhance the conduction electron bandwidth. In the pressure-induced paramagnetic state, the system undergoes a crossover from an itinerant spin fluctuation (SF) regime at low T to a local SF regime at high T. It is pointed out that this magnetic crossover of SF resembles that of high-temperature superconductors. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevB.59.13973

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We report magnetic properties of the antiferromagnet YMn2 as functions of pressure and temperature (T) obtained from the 55Mn spin-lattice relaxation rate and the Knight shift. An energy-band narrowing due to electron correlation is indicated by the Knight shift measurement and the application of pressure is found to enhance the conduction electron bandwidth. In the pressure-induced paramagnetic state, the system undergoes a crossover from an itinerant spin fluctuation (SF) regime at low T to a local SF regime at high T. It is pointed out that this magnetic crossover of SF resembles that of high-temperature superconductors. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevB.59.13973

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Recent NMR measurements on high-Tc and related material Sr2RuO4 by the Osaka group are reviewed. The following results are presented: (1) Study of YBa2Cu3O7 + Ni has revealed that the attractive force is spin in origin. (2) Analysis of T1, T2 and Knight shift (K) in various high-Tc materials supports the antiferromagnetic spin-fluctuation mediated superconducting model. (3) Temperatures at which a pseudogap starts to open obtained by 1/T1T, dK/dT, photoemission and resistivity agree with each other in the Bi2Sr2CaCu2O8+δ system. (4) 17O Knight shift measurements in the related material Sr2RuO4 suggests that a triplet pairing is realized in this compound as in the heavy-fermion superconductor UPt3. © 1999 Published by Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(98)01015-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

High-pressure NMR measurements on a single crystal of hole-doped two-leg ladder compound, Sr2Ca12Cu24O41, have been carried out up to ∼1.7 GPa. The activated T-dependences of T1 and Knight shift at 1.7 GPa assured the persistence of the spin gap in this system. © 1998 Elsevier Science B. V. All rights reserved.

DOI： 10.1016/S0921-4526(98)00807-2

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We report the NQR study on the pressure-induced paramagnetic state in the antiferroinagrietic (AF) intermetal-lic compound YMn2 with the Néel temperature TN=100 K at ambient pressure. Froin the T variation of the nuclear spin-lattice relaxation rate, 56(lJT1) of 55Mn above the critical pressure of 4 kbar, the spin fluctuation feature is found to change below a temperature Taf. At higher temperature than Taf, the nuclear relaxation behavior is well described in terms of the self-consistent renomalized (SCR) spin fluctuation theory for nearly AF metals, whereas below Taf a deviation is significant and 1JT1T exhibits a weak T variation. It is pointed out that Taf coincides with the temperature below which a T2 law in electrical resistivity is valid as expected for a Feimi liquid ground state. We proposed that these features are understood from the standpoint that the development of AF spin fluctuation remains in short-range associated with the singlet formation among Mn spins in each tetrahedron as suggested by the inelastic neutron experiments on Y1−xScxMn2. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

DOI： 10.4131/jshpreview.7.626

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

We have measured the spin-lattice relaxation rate 1/T1 in an as-grown single crystal of YNi2B2C (Tc = 13.8 K) in the superconducting state down to 0.9 K. 1/T1 shows no Hebel-Slichter peak just below Tc, decreasing sharply as T3, and followed by a slow dependence of 1/T1 ∝ Tat low temperatures. The pairing state is discussed on the basis of T1 result. © 1998 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(98)00205-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

63Cu spin-lattice relaxation rate has been measured in annealed and as-grown samples of Tl2Ba2Ca2CU3O10. We found for the first time that Tc increases as a result of decrease of the residual density of states at the Fermi level due to the improvement of crystal order by annealing. A 2-dimensional d-wave superconductivity model accounts for the increase of Tc of 8K in terms of the change in potential scattering due to crystal imperfection which is likely caused by Tl-substitution of Ca site.

DOI： 10.1016/S0921-4534(97)00785-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

The 63Cu spin-lattice relaxation rate has been measured in an annealed sample of Tl2Ba2Ca2Cu3O10 with Tc = 123 K, and compared with that in the previously reported asgrown sample with Tc = 115 K. We found, for the first time, that Tc increases as a result of a decrease in the residual density of states (DOS), Nres, at the Fermi level due to the improvement of crystal ordering by annealing. Based on the T1 result, Nres normalized by the value at Tc, Nres/N0, was found to be reduced from 0.30 to 0.18 after annealing. A 2-dimensional d-wave superconductivity model accounts for the increase of Tc in terms of the change in potential scattering due to crystal imperfection, which is probably caused by Ca/Tl inter-substitution.

DOI： 10.1143/JPSJ.66.1880

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Measurements of the Knight shift of Cu-63 and the nuclear spin-lattice relaxation rate of Tl-205, (205)(1/T-1), have revealed that TlSr2(Lu0.7Ca0.3)Cu2Oy with T-c = 40 K consisting of the bi-CuO2 layer without the CuO chain is In the underdoped regime with the spin pseudogap, which is evidenced by the significant decrease of (205)(1/T1T) below 120 It far above T-c and the decrease of the Cu-63 spin Knight shift upon cooling. Although the disorder is introduced into the adjacent Ca layers to the CuO2 plane by the substitution of Lu for Ca sites, the spin-gap behavior is presented not to be masked. From this result, it is suggested that the absence of the spin-gap behavior in La2-xSrxCuO4 is neither due to the disorder introduced into the adjacent LaO layers to the CuO2 plane by the substitution of Sr for La sites nor the absence of the CuO chain.

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63Cu, 17O and 205Tl NMR have been performed in the high-Tc superconductor Tl2Ba2Ca2Cu3O10 whose Tc(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO2 plane have been extracted from the nuclear quadrupole frequency vQ. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T1 and T2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa2Cu3O7, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher Tc is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, Tc would go up to 132 K which is quite close to the record Tc realized in the Hg based compound.

DOI： 10.1016/0921-4534(96)00092-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Superconducting properties in HgBa2Ca2Cu3O8+delta, with a transition temperature of T-c=133 K, have been investigated by measurements of the nuclear spin-lattice relaxation rate, (63)(1/T-1). It has been found that the T1T=const behavior, observed well below T-c, arises from combined relaxation channels to vortex cores, as well as from a residual density of states (DOS), N-res, at the Fermi level associated with the gapless superconductivity. From the value of T1T=const in the latter channel, the DOS fraction normalized by its value at T-c, N-res/N-Tc, has been deduced to be as small as similar to 0.05. Based on the two-dimensional gapless d-wave model, the T-c reduction rate, Delta T-c/T-c0, is estimated to be as small as 0.02 for N-res/N-0 similar to 0.05. It is pointed out that these measures of the quality of HaBa(2)Ca(2)Cu(3)O(8+delta) explain why T-c reaches such a high value.

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The Gaussian spin-echo decay rate, 1/T2G, of 63Cu, which is related to the antiferromagnetic spin correlation length, ξ, has been measured in overdoped TlSr2CaCu2O7-δ with Tc = 70 K, 52 K, and 10 K. It has been found that the decrease of ξ under a critical value makes the pairing interaction weaken, resulting in the marked suppression in Tc with increasing holes.

DOI： 10.1007/BF02583853

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Results of our NMR measurements of high Tc superconductors are reviewed. From systematic measurements of NQR frequency, Knight shift, nuclear spin lattice relaxation rate and nuclear transverse relaxation rate over a wide hole concentration range from light to heavy doped compounds together with the impurity and pressure effect, following results are concluded. The numbers of the holes at the respective sites of Cu and O site in the CuO2 plane govern the several physical properties of the system. There are optimum values of these hole numbers to produce highest Tc. The superconducting properties are well explained consistently by the d-wave pairing model. Tc is found to be correlated intimately with the antiferromagnetic spin fluctuation parameters that governs the nuclear relaxation. The mechanism for the superconductivity is magnetic in origin.

DOI： 10.1007/BF02548129

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.1143/JPSJ.65.2341

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

The existence of local moments in both Zn- and Ni-doped YBa2Cu4O8 (Y1248) is confirmed by a Curie-law T-dependence of NMR linewidth and the Cu relaxation at low T. The reduction of Cu-NQR signal intensity in Ni-doped Y1248 suggests that, in contrast to the case of Ni-doped YBa2Cu3O7 (Y1237), the Cu spin correlation around Ni is destroyed. This effect may be responsible for the larger reduction of Tc by Ni-doping than in Y1237. For Cu sites which are away from an impurity by two lattice constants, 1/T1T continues to increase below T*, at which it has a maximum in the non-doped material. The effect in reducing T*, which is sometimes called the spin gap temperature, is much more pronounced in Zn-doped samples, suggesting that the reduction of T* is mainly due to the induction of magnetic moments in the dx 2-y2 orbital, which may modify Cu spin fluctuation.

DOI： 10.1016/0921-4534(96)00083-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

63Cu NMR measurements have been carried out in HgBa2Ca2Cu3O8+δ (Hg1223) with the highest transition temperature, Tc = 133 K, to date. The narrow NMR spectrum and the small residual density of states (DOS), Nres, at the Fermi level in the superconducting state indicate that the Hg1223 possesses good quality.

DOI： 10.1016/0921-4534(95)00830-6

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Superconducting properties in (Formula presented), with a transition temperature of (Formula presented) K, have been investigated by measurements of the nuclear spin-lattice relaxation rate, (Formula presented). It has been found that the (Formula presented) behavior, observed well below (Formula presented), arises from combined relaxation channels to vortex cores, as well as from a residual density of states (DOS), (Formula presented), at the Fermi level associated with the gapless superconductivity. From the value of (Formula presented) in the latter channel, the DOS fraction normalized by its value at (Formula presented), (Formula presented), has been deduced to be as small as ∼0.05. Based on the two-dimensional gapless (Formula presented)-wave model, the (Formula presented) reduction rate, (Formula presented), is estimated to be as small as 0.02 for (Formula presented). It is pointed out that these measures of the quality of (Formula presented) explain why (Formula presented) reaches such a high value. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.R8906

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We report extensive measurements of the Knight shift K, the nuclear spin-lattice relaxation rate 1/(Formula presented), and the Gaussian spin-echo decay rate 1/(Formula presented) of (Formula presented) in overdoped (Formula presented)(Formula presented)(Formula presented) (Tl1212) with (Formula presented) = 70 K, 52 K, and 10 K, in order to elucidate the origin of the reduction in (Formula presented) with increasing holes and to identify the symmetry of the order parameter. In the normal state, it is shown that 1/(Formula presented)T obeys the Curie-Weiss law, pointing to the presence of the antiferromagnetic (AF) spin correlation. From the analyses of 1/(Formula presented) and 1/(Formula presented), it is found that the increase of the hole content in Tl1212 compounds makes the characteristic energy of the AF spin fluctuation around a zone boundary, Q=(π/a,π/a), (Formula presented), transfer to a higher-energy region and concomitantly reduces the magnetic correlation length (Formula presented) significantly. The AF spin correlation is concluded to become less distinct in going from the optimum-doped to the overdoped regime. In the superconducting state, the T dependences of K and 1/(Formula presented) have revealed that the superconductivity is in the gapless regime with a finite density of states at the Fermi level. The NMR results are consistently interpreted in the d-wave model in which the impurity scattering is incorporated in terms of the unitarity limit as demonstrated in most of the high-(Formula presented) cuprates so far. Eventually, the reduction in (Formula presented) from 70 K to 52 K in Tl1212 is concluded to be not due to the impurity effect associated with the oxygen content. In the previous works, the enhancement of (Formula presented) from 93 K in (Formula presented)(Formula presented)(Formula presented) with double (Formula presented) layers to 115-135 K in (Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented) and (Formula presented)(Formula presented)(Formula presented)(Formula presented) with triple (Formula presented) layers was shown to be due to the increase in (Formula presented) with (Formula presented) unchanged appreciably. This finding was compatible with the relationship of (Formula presented)∝(Formula presented)(Formula presented)exp(-1/λ) based on the spin-fluctuation-induced mechanism for the d-wave superconductivity. Within the same scheme, the origin of the marked decrease in (Formula presented) irrespective of increasing (Formula presented) in Tl1212 is proposed to be due to the significant reduction in (Formula presented) which makes the pairing interaction weaken and λ in the above formula reduced. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.54.10131

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The spin-lattice relaxation rate 1/T1 and the Knight shift were measured for La1.85Sr0.15CuO4 and YBa2Cu4O8 under pressure to 1.4 GPa. The results together with the previously reported pressure dependence of the nuclear quadrupole frequency, νQ, give evidence that the local hole distribution changes under pressure. In La1.85Sr0.15CuO4, the hole density in the Cu 3d orbit decreases, accompanying a suppression of the antiferromagnetic spin fluctuation of the low-energy component. In YBa2Cu4O8, by contrast, the spin fluctuation changes little while the total hole number in the CuO2 plane increases. We argue that such changes in the hole distribution are responsible for the Tc increase in the respective compounds. We also found that the so-called "spin-gap" behavior in YBa2Cu4O8 becomes more pronounced under pressure. © 1995.

DOI： 10.1016/0921-4534(95)00029-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Various types of ground states in strongly correlated electron systems have been systematically investigated by means of NMR/NQR at low temperatures under high magnetic field and pressure. We focus on two well-known heavy-electron families, CeCu2X2 (X = Si and Ge) (Ce(122)) and UM2Al3 (M = Ni and Pd) (U(123)). The Cu NQR experiments on CeCu2X2 under high pressure indicate that the physical property of CeCu2Ge2 at high pressure, i.e. above the transition at 7.6GPa from antiferromagnetic (AF) to superconductivity, are clearly related to that of CeCu2Si2 at ambient pressure. In addition to the H-T phase diagram established below 7 T, NMR and specific heat experiments on polycrystal CeCu2.05Si2 have revealed the presence of a new phase above 7 T. In a high-quality polycrystal of UPd2Al3 with a record high-T-c of 2 K at ambient pressure and the narrowest AI NQR line width, the nuclear-spin lattice relaxation rate, (27)(1/T-1) measured in zero field has been found to obey the T-3 law down to 0.13 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. Thus it seems that all heavy-electron superconductors exhibit lines of zero gap, regardless of their different magnetic properties.

DOI： 10.1016/0921-4526(94)00365-3

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Other Link： http://orcid.org/0000-0001-5081-2777

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We report a 63Cu and 17O NMR study in the high-Tc superconductor Tl2Ba2Ca2Cu3O10 with Tc (max) = 127 K. The results of T1 and T2G reveal that the characteristic energy of the antiferromagnetic spin fluctuation ΓQ is higher than that in YBa2Cu3O7, while the magnetic correlation lengths ξ do not differ much from each other. The high Tc in this compound is thus related to large ΓQ, suggesting that the superconductivity is spin fluctuation mediated. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.64.3184

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The Knight shift, the nuclear spin-lattice relaxation rate, 1/T1, and the transverse relaxation rate, 1/T2G, of 63Cu have been separately measured for the square and the pyramidal CuO2 planes in HgBa2Ca2Cu3O8+δ (Hg1223) with the highest transition temperature, Tc= 133 K to date. From the quantitative analysis of 1/T1 and 1/T2G, it is shown that the product of the staggered susceptibility and the characteristic energy of spin fluctuations around a zone boundary, χQΓQ, for the square CuO2 plane site is larger than that for YBa2Cu3O7 (YBCO7), although that for the pyramidal CuO2 plane site is almost the same. This enhancement of χQΓQ for the square site is considered to be responsible for the higher Tc in Hg1223 than in YBCO7. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.64.4561

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From the NMR and NQR studies, we show the unconventional superconducting properties in the antiferromagnetic (AF) heavy-fermion (HF), UPd2Al3 and high-Tc cuprate systems. In UPd2Al3 with TN = 14.5 K and Tc = 2 K, the superconductivity is shown to be of d-wave pairing type with lines of vanishing gap on the Fermi surface. The potential scattering associated with some defects at the Al site is the cause for the Tc suppression and the appearance of the density of states (DOS) at the Fermi level. This unusual impurity effect is consistent with a d-wave scenario, because the potential scattering leads to a gap opening for an anisotropic s-wave scenario. In high-Tc cuprate which are argued to be either a d-wave or an anisotropic s-wave, the potential scatterer like Zn in the plane induces the gapless superconductivity as well, which can not be interpreted in terms of an anisotropic s-wave scenario if the order parameter does not change sign largely. It is shown from the NMR experiment at the Al site for Al-doped La1.85Sr0.15CuO4 that local moments at near neighbor Cu sites induced around Al impurity do not play a role to suppress the Tc and to induce the DOS at all. The NMR studies in novel superconductors have provided important clues to identify their symmetry of order parameter to be d-wave. © 1995.

DOI： 10.1016/0022-3697(95)00098-4

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The hole numbers at Cu and oxygen sites in the CuO2 plane in a wide variety of high-Tc materials have been extracted from the measured nuclear quadrupole frequency, vQ. The hole number in the [formula omitted] orbit, [formula omitted] is found to be more than 0.85 in La2-xSrxCuO4 (x=0.075-0.24), while decreases to 0.74 in Tl2Ba2Ca2Cu3O10. Thus estimated [formula omitted] is supported by the result that the orbital Knight shift along the c-axis, [formula omitted] increases in proportion to [formula omitted] In contrast to the decrease of [formula omitted] the hole number in the O-2pσ orbit, [formula omitted] increases in going from the La- to the Tl-systems. It has been found that Tc is determined by two parameters about holes, i.e., [formula omitted] and [formula omitted] The former tells the microscopic “doping level” of the CuO2 plane, while a smaller [formula omitted] results in a weaker antifer-romagnetic spin fluctuations of low energy. It has been found that not only a proper [formula omitted] but also a sufficiently small [formula omitted] are needed to obtain a high value of Tc. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.64.2524

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

It is demonstrated that the NMR results on the nuclear relaxation behavior and the impurity/imperfection effects are consistent with the d-wave model with the node line at the Fermi surface.

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The 63Cu and 17O nuclear spin-lattice relaxation rates in YBa2Cu4O8 have been measured in the temperature and magnetic field ranges of 1.4-270 K and 4.0-11 T, respectively. In the mixed state, 1/T1 for both 63Cu and 17O are enhanced by the magnetic field. Most strikingly, in a high field of 11 T, 63(1/T1) and 17(1/T1) decrease in proportion to the temperature, T, below T=15 K and T=10 K, respectively. The magnetic-field dependence of 1/T1 is shown to be dominated by spin diffusion to the vortex cores, implying that the electronic state in the cores is characterized by the T1T=const. law. This result indicates that the electronic state in the vortex cores is described in terms of the Fermi-liquid picture, in sharp contrast to the unusual properties above Tc. © 1994.

DOI： 10.1016/0921-4534(94)90370-0

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Result of extensive Cu NMR and NQR studies in the superconducting La2-x Srx,CuO4 with x=0.075∼0.24 have been presented. The rapid decrease of the nuclear spin-lattice relaxation rate, 1/T1, and the spin Knight shift below Tc confirm microscopically the existence of the superconducting phase with lower Tc than 38 K for 0.10≤x0.24. For x=0.10, magnetic anomalies emerge at low-T in some small parts of the sample, e.g. T-independent T1 and broadening of spectrum, having some relevance to the magnetic order in La2-xBaxCuO4 with x=0.125. For x&gt
0.15 the T1, T-const, behavior and the existence of the residual spin Knight shift far below Tc have revealed that the superconductivity is of a gapless type with a finite density of states at the Fermi level caused by the mixture of different structures, i.e. tetragonal and orthorhombic. La2-xSrxCuO4, high-Tc superconductivity, Cu nmr and nqr, d-wave gapless superconductivity, antiferromagnetic spin correlation, structural instability. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.63.700

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Both 63Cu NQR frequency νQ and 1/T1T were found to decrease under high pressure up to 14kbar, indicating the decrease of hole density at Cu site. The enhancement of Tc under pressure is explained as due to the transfer of hole density from Cu to oxygen, which increases the effective carrier density. © 1994.

DOI： 10.1016/0921-4534(94)92014-1

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DOI： 10.1016/0921-4534(94)92041-9

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The Cu-63 Knight shifts have been measured for the heavily doped high-T(c) superconductors TlSr2CaCu2O7-y, with T(c) = 64 and 45 K. The spin part of the shift, K(s), is almost T-independent in the normal state and increases with decreasing T(c). From the analysis of the anisotropy in K(s) in both compounds, the transferred hyperfine field from the nearest-neighbor Cu spins has been deduced to be larger than that in YBa2Cu3O7, similar to the results for Tl2Ba2CuO6+y. A systematic correlation has been found in the heavily doped superconductors that the spin susceptibility and the nuclear quadrupole frequency increase with decreasing T(c), while 1/T1T is suppressed, as reported previously.

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A 17O NMR study has been carried out for La2-xSrxCuO4 in the lightly- (x=0.075) and heavily-doped (x=0.24) regions in the normal state. Together with the data previously reported for x=0.15, it was found that the nuclear quadrupole frequency νQ in the apical oxygen (O(1)) does not change with x, suggesting that the doped hole does not go into the LaO layer at all. On the contrary, νQ in the planar oxygen (O(2)) increases appreciably with Sr doping. By analyzing νQ of Cu and O(2), it was shown that the increase of holes on the planar O 2pσ orbit is more appreciable than that in the Cu 3d orbit. From the ratio of the hole density in Cu to that in oxygen, and the spin fluctuation probed by 63(T1T), it is suggested that the depression of Tc in the "heavily-doped" region is neither due to the over-doping nor to the collapse of the two-dimensionality, but to other causes. The temperature dependences of the Knight shift 17K in two samples are in sharp contrast with each other. In the lightly-doped region, 17K decreases with decreasing temperature. In the heavily-doped region, however, 17K increases slightly with decreasing temperature, becoming almost T-independent below 100 K. An increase of the anisotropic part of spin Knight shift 17Ks,ani with x also indicates that the hole density at the O 2pσ orbit increases with Sr doping. © 1993.

DOI： 10.1016/0921-4534(93)90206-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Kluwer Academic Publishers  

Results of our systematic NMR study in high Tc cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La1.85Sr0.15CuO4. YBa2Cu3O7 and Tl2Ba2CuO6+y. 1/T1 of63Cu in the CuO2 plane in the normal state follows essentially a Curie-Weiss law at high temperature and T1T = const. law at low temperature. The temperature dependence of 1/T1 and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation between Tc and vQ, and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum in Tc. © 1993 J.C. Baltzer AG, Science Publishers.

DOI： 10.1007/BF00567615

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The magnetic and superconducting properties in high-Tc cuprates have been investigated over a wide doping range by NMR and NQR of 63Cu and 17O in the lightly-doped La2⇔xSrxCuO4] (LSCO), YBa2Cu4O8 (Y124) and the heavily-doped Bi2Sr2CaCu2O8 (BSCO), Tl2Ba2CuO6+y (TBCO) and the Zn- and Ni-doped YBa2Cu3O7 (YBCO7). From extensive studies in the normal state, it is extracted that the uniform, χs(T) and the staggered, χQ(T) susceptibilities originating from the antiferromagnetic (AF) spin correlation are suppressed with increasing hole content and furthermore, the T-dependence of χs(T) and χQ(T) is changed from the T-increasing behavior with temperature and the Curie-Weiss one into the T-independent behavior, respectively. The superconducting properties cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of NMR results, if the finite density of states at the Fermi level is taken into consideration associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-Tc cuprates. © 1993.

DOI： 10.1016/0022-3697(93)90198-Z

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The nuclear-spin-lattice relaxation rate, 1/Tu and the Knight shift, Kab, of 63Cu in the plane have been measured for YBa2(Cu1-xZnJC)408 (x=0.01 and 0.02) in order to unravel the underlying problem of spin dynamics in YBa2Cu408. It was found that the substitution of Zn into the Cu02 plane causes a continuous increase of 1/TXT down to Tc in contrast to the marked reduction of l/T{Tat far above Tc for the undoped sample, while the Knight shift is invariant. These findings show that the suppression of the spin fluctuation spectral weight around Q=(n, n) at low energies is removed by substituting Zn into the Cu02 plane, while that around q-{0, 0) is not affected appreciably. The ^-dependent change of spin-fluctuations with Zn-doping may be crucial in elucidating the cause of the decrease of 63(l/7,1r) above Tc. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.62.2591

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BALTZER SCI PUBL BV  

Results of our systematic NMR study in high T, cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La1.85Sr0.15CuO4. YBa2Cu3O7 and Tl2Ba2CuO6+y. 1/T-1 of Cu-63 in the CuO2 plane in the normal state follows essentially a Curie Weiss law at high temperature and T1T = const. law at low temperature. The temperature dependence of 1/T-1 and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation between T-c and v(Q), and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum in T-c.

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17O NMR has been carried out in YBa2Cu4O8 with Tc = 74 K. The nuclear quadrupole tensors in the oxygen sites were determined and compared with those in other high-Tc cuprates. The hole densities in the oxygen and copper sites have been extracted from νz(O) and νQ(Cu) by assuming that the electric field gradient is dominated by the on-site holes. We propose that the hole distribution (the extent of covalency) between the planar oxygens and copper, which is changed by doping and pressure, is a hidden parameter determining the spin dynamics and the superconducting transition temperature Tc in high-Tc cuprates. It was found that 1/T1T and the Knight shift of the planar oxygens decrease with lowering temperature in the normal state. The nuclear relaxation rates of 17O and 63Cu in the CuO2 plane exhibit different behaviors at temperatures higher than ∼ 120 K, reflecting the different q-window of spin fluctuations, while both of them share the same T-dependence below ∼ 120 K down to 55 K. This scaling of 1/T1 between 17O and 63Cu, which is also seen in YBa2Cu3O7 (Tc = 92 K) and YBa2Cu3O6.63 (Tc = 60 K), seems to be essential for high-Tc superconductivity. © 1992.

DOI： 10.1016/0921-4534(92)90882-D

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：VERLAG Z NATURFORSCH  

Recent NQR and NMR experiments on La2-xSrxCuO4, YBa2Cu3O7, Tl2Ba2CuO6+y and other compounds are reviewed. The antiferromagnetic spin fluctuations decrease in the order of La, Y, Tl compounds. In the normal state the T-dependence of (T1T)-1 is considered to follow the Curie-Weiss law at high temperature and the (T1T) = const law at low temperature. The T dependence of 1/T1 below T(c) suggests strongly d-wave pairing. Correlations of the NQR frequency of Cu-63 and O-17 with the antiferromagnetic spin fluctuations and the superconducting transition temperature are discussed.

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Dependence of NQR frequency, nu-Q(Cu) on pressure up to 1.3GPa is reported for La1.85Sr0.15CuO4 and YBa2Cu4O8. Correlations are found between some physical quantities and the ratio of the hole density in Cu2 to that in O2(3), n(Cu2)/n(O2, 3) which is extracted from nu-Q's. It is proposed tha there exists an optimum of n(Cu2)/n(O2, 3) for T(c) and that the increase of T(c) under pressure is due to the approach of n(Cu2)/n(O2, 3) to the optimum through the local hole transfer.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN KIKAI-SHINKO BUILDING  

O-17 and Cu-63 NMR measurements have been made in L1.85Sr0.15CuO4 with T(c) = 38 K. Both O-17 Knight shift and quadrupole tensor were determined from the analyses of the O-17 spectrum of oriented powder. The quadrupole frequencies of the planar oxygen and Cu nuclei are consistently interpreted by the analyses of Hanzawa et al. made in YBa2Cu3O7 with the electric field gradients due to the on-site holes at Cu and O sites. Then the weight factor of the Cu-3d(x2 - y2) and 2p-sigma wave functions are deduced, the latter having the larger weight. From the Knight shifts vs the bulk susceptibility plots, the hyperfine coupling constants for O-17 and Cu-63 were almost the same as those in YBa2Cu3O7. Both shifts decrease significantly below T(c) having residual parts of 63K(ab) = 0.38% and 17K(c) = 0.02% at 4.2 K, which were larger than those in YBa2Cu3O7.

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The pressure dependence of the 63Cu nuclear quadrupole resonance (NQR) frequency, νQ, and Tc have been measured for La1.85Sr0.15CuO4 and YBa2Cu4O8 under high pressure up to 1.3GPa. νQ was found to decrease with increasing pressure in La1.85Sr0.15CuO4, while νQ in the CuO2 plane increases with pressure in YBa2Cu4O8. It is evidenced that charge transfer takes place under pressure. Based on the analysis that νQ arises mainly from the on-site holes, a schematic model relating Tc and the ratio of the in-plane hole concentration in the Cu3d and O2p orbitals, n(Cu)/n(O), is constructed. It is proposed that n(Cu)/n(O) is a primarily important factor governing Tc and that the enhancement of Tc under pressure is due to the change of the ratio approaching the most favorable value (∼1). © 1991.

DOI： 10.1016/0038-1098(91)90478-E

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DOI： 10.1016/0921-4534(91)91606-5

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The nuclear spin lattice relaxation rate, T1-l, for 63Cu in zero field from 8 K down to 0.3 K, has been measured in Nb-Cu multilayered thin films where both the Nb- and Cu-layer thicknesses are much smaller compared to the coherence lengths. A BCS-like temperature dependence of T1-1 replaced gradually by a Korringa-law-like one at low temperatures has been found. A dependence of the relaxation on the Cu-layer thickness at low temperatures has also been found. It has been proposed and confirmed by a series of experiments that superconductivity in the Cu-layer induced by the proximity effect has an anisotropic energy gap, reflecting the geometrical feature of a thin film. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.60.599

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The nuclear spin-lattice relaxation rate, T1-1, for 63Cu in zero field from 4.2K down to 0.3K, has been measured in a series of NbCu multilayers. A BCS-like temperature dependence of T1-1 followed gradually by A Korringa-law-like one at low temperatures, has been found. It has been confirmed that such behavior of T1-1 comes from the anisotropic energy gap in the Cu-layers. © 1990.

DOI： 10.1016/S0921-4526(90)81090-B

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Language：English   Publisher：The Physical Society of Japan (JPS)  

A series of measurements on the nuclear spin-lattice relaxation rate, $T_{1}^{-1}$, of 63Cu in superconducting multilayer Nb–Cu has been performed over a temperature range from 4.2 K down to 0.3 K, using the field cycling method. $T_{1}^{-1}$ decreases rapidly after an initial increment just below $T_{\text{c } }$. At low temperatures, however, the rate of the decrease slows down, and $T_{1}^{-1}$ decreases in proportion to temperature. It has been concluded that such behavior of $T_{1}^{-1}$ originates from a distribution of the energy gap of the Cooper pairs in Cu. The electrons which are propagated nearly normal to the S–N interface have a finite energy gap predicted by McMillan, while the electrons propagated nearly parallel to the interface are gapless and behave like normal electrons.

DOI： 10.1143/JPSJ.58.39

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An anomalous Cu NMR signal with extremely small e2qQ has been observed in the oxide superconductor Nd1.85Ce0.15 CuO4-y The Knight shift decreased significantly as temperature decreased below Tc The temperature-independent T1 of 63Cu suggests that the relaxation is dominated by the fluctuation of the Nd-moment, and no magnetic order is achieved for this content of Ce. © 1989, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.58.1910

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