Faculty Profiles - TEI Kokukei

Papers - TEI Kokukei

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Charge fluctuations and nodeless superconductivity in quasi-one-dimensional Ta4Pd3Te16 revealed by Te-125-NMR and (181)nTa-NQR

Z. Li, W. H. Jiao, G. H. Cao, Guo-qing Zheng

PHYSICAL REVIEW B 94 ( 17 ) 2016.11

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

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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New superconductivity dome in LaFeAsO1-xFx far away from magnetism and accompanied by structural transition

J. Yang, Guo-qing Zheng

HYPERFINE INTERACTIONS 237 ( 1 ) 2016.10

Language：English Publishing type：Research paper (international conference proceedings) Publisher：SPRINGER

We report on the discovery and novel physics of a new superconductivity dome in LaFeAsO1-xFx with high-doping rate (0.25 <= x <= 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 <= 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 > 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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Spin-rotation symmetry breaking in the superconducting state of CuxBi2Se3

K. Matano, M. Kriener, K. Segawa, Y. Ando, Guo-qing Zheng

NATURE PHYSICS 12 ( 9 ) 852 - 854 2016.9

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

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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Orbital order and spin nematicity in the tetragonal phase of the electron-doped iron pnictides NaFe1-xCoxAs

R. Zhou, L. Y. Xing, X. C. Wang, C. Q. Jin, Guo-qing Zheng

PHYSICAL REVIEW B 93 ( 6 ) 060502(1)-060502(6) 2016.2

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

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 <= x <= 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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19aBN-9 NMR/NQR studies of the non-centrosymmetric superconductor

Yatagai R., Mori H., Maeda S., Matano K., Zheng G.-q.

Meeting Abstracts of the Physical Society of Japan 71 1985 - 1985 2016

20pBM-2 Magnetic field induced CDW in the pseudogap state of Bi2201 superconductor revealed by Cu-NMR

Kitahashi M., Kawasaki S., Lin Chengtian T., Zheng Guo-qing

Meeting Abstracts of the Physical Society of Japan 71 2071 - 2071 2016

Doping-enhanced antiferromagnetism in Ca1-xLaxFeAs2 Reviewed

Shinji Kawasaki, Tomosuke Mabuchi, Satoki Maeda, Tomoki Adachi, Tasuku Mizukami, Kazutaka Kudo, Minoru Nohara, Guo-Qing Zheng

Physical Review B - Condensed Matter and Materials Physics 92 ( 18 ) 2015.11

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

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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Doping-enhanced antiferromagnetism in Ca1-xLaxFeAs2

Shinji Kawasaki, Tomosuke Mabuchi, Satoki Maeda, Tomoki Adachi, Tasuku Mizukami, Kazutaka Kudo, Minoru Nohara, Guo-qing Zheng

PHYSICAL REVIEW B 92 ( 18 ) 180508(1)-180508(4) 2015.11

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

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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New Superconductivity Dome in LaFeAsO1-xFx Accompanied by Structural Transition

Yang Jie, Zhou Rui, Wei Lin-Lin, Yang Huai-Xin, Li Jian-Qi, Zhao Zhong-Xian, Zheng Guo-Qing

CHINESE PHYSICS LETTERS 32 ( 10 ) 107401(1)-107401(7) 2015.10

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

High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1-xFx (x <= 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 <= x <= 0.75, where the maximal critical temperature T-c at x(opt) = 0.5-0.55 is even higher than that at x <= 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 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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Ferromagnetic Spin Fluctuation and Unconventional Superconductivity in Rb2Cr3As3 Revealed by As-75 NMR and NQR

J. Yang, Z. T. Tang, G. H. Cao, Guo-qing Zheng

PHYSICAL REVIEW LETTERS 115 ( 14 ) 147002(1)-147002(5) 2015.9

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

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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Superconductivity and the electronic phase diagram of LaPt2-xGe2+x Reviewed

S. Maeda, K. Matano, R. Yatagai, T. Oguchi, Guo-Qing Zheng

Physical Review B - Condensed Matter and Materials Physics 91 ( 17 ) 2015.5

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

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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Superconductivity and the electronic phase diagram of LaPt2-xGe2+x

S. Maeda, K. Matano, R. Yatagai, T. Oguchi, Guo-qing Zheng

PHYSICAL REVIEW B 91 ( 17 ) 174516(1)-174516(7) 2015.5

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

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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Coexistence of multiple charge-density waves and superconductivity in SrPt2As2 revealed by As-75-NMR/NQR and Pt-195-NMR

Shinji Kawasaki, Yoshihiko Tani, Tomosuke Mabuchi, Kazutaka Kudo, Yoshihiro Nishikubo, Daisuke Mitsuoka, Minoru Nohara, Guo-Qing Zheng

PHYSICAL REVIEW B 91 ( 6 ) 060510(1)-060510(5) 2015.2

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

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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16pCD-3 Coexistence of multiple charge-density waves and superconductivity in SrPt_2As_2 revealed by ^<75>As-NMR/NQR and ^<195>Pt-NMR

Kawasaki S., Tani Y., Mabuchi T., Kudo K., Nishikubo Y., Mitsuoka D., Nohara M., Zheng Guo-qing

Meeting Abstracts of the Physical Society of Japan 70 1704 - 1704 2015

21pBA-3 NMR study of Sr_2VFeAsO_3

Ueshima K., Han F., Zhu X., Wen Hai-Hu, Kawasaki S., Zheng Guo-qing

Meeting Abstracts of the Physical Society of Japan 70 1989 - 1989 2015

22aBA-5 NMR/NQR study of Ca_<1-x>La_xFeAs_2

Mabuchi T., Kawasaki S., Mizukami T., Kudo K., Nohara M., Zheng G. -q.

Meeting Abstracts of the Physical Society of Japan 70 2059 - 2059 2015

22pBC-10 NMR/NQR study of the structural phase transition in the superconductors LaPt_<2-x>Ge_<2+x>

Maeda S., Matano K., Yatagai R., Oguchi T., Zheng Guo-qing

Meeting Abstracts of the Physical Society of Japan 70 2244 - 2244 2015

24pDC-5 Cu-NMR study of pseudogap state of copper oxide superconductors Bi_2Sr_<2-x>La_xCuO_<6+δ>

Kitahashi M., Kawasaki S., Lin C.T., Zheng Guo-qing

Meeting Abstracts of the Physical Society of Japan 70 2485 - 2485 2015

Magnetism and superconductivity in Sr2VFeAsO3 revealed by As-75- and V-51-NMR under elevated pressures

Keiji Ueshima, Fei Han, Xiyu Zhu, Hai-Hu Wen, Shinji Kawasaki, Guo-qing Zheng

PHYSICAL REVIEW B 89 ( 18 ) 184506-1-184506-6 2014.5

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

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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Spin-singlet superconductivity with a full gap in locally noncentrosymmetric SrPtAs

K. Matano, K. Arima, S. Maeda, Y. Nishikubo, K. Kudo, M. Nohara, Guo-qing Zheng

PHYSICAL REVIEW B 89 ( 14 ) 140504(R)-1-140504(R)-4 2014.4

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

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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