Faculty Profiles - TEI Kokukei

Papers - TEI Kokukei

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Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2 center dot yH(2)O from nuclear quadrupole resonance

T Fujimoto, GQ Zheng, Y Kitaoka, RL Meng, J Cmaidalka, CW Chu

PHYSICAL REVIEW LETTERS 92 ( 4 ) 2004.1

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

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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Coexistence of Unconventional Superconductivity and Antiferromagnetic order in the heavy Fermion Compounds Ce(Rh1-xIrx)In5:NQR Studies.

G.-q. Zheng, N. Yamaguchi, K. Tanabe, Y. Kitaoka, J.L. Sarrao, J.D Thompson

Phys. Rev. B 2004

Coexistence of superconductivity and antiferromagnetism in multilayered high-Tc superconducto HgBa2Ca4Cu5Oy: Cu-NMR study

H. Kotegawa, Y. Tokunaga, Y. Araki, G. -q. Zheng, Y. Kitaoka, K. Tokiwa, K. Ito, T. Watanabe, A. Iyo, Y. Tanaka, H. Ihara

Phys Rev. B 2004

d-wave superconductivity develops out of the Fermi liquid ground state in electron-doped superconductor Pr0.91Ce0.09LaCuO4-y Reviewed

G. -q. Zheng, T. Sato, Y. Kitaoka, M. Fujita, K. Yamada

Physica C 408-410 319 - 320 2004

Anisotropic spin fluctuations in heavy-fermion superconductor CeCoIn5: In-NQR and Co-NMR studies

Y Kawasaki, S Kawasaki, M Yashima, T Mito, GQ Zheng, Y Kitaoka, H Shishido, R Settai, Y Haga, Y Onuki

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 72 ( 9 ) 2308 - 2311 2003.9

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

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 < T < 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 < 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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Fermi-liquid ground state in the n-type Pr0.91LaCe0.09CuO4-y copper-oxide superconductor (vol 90, art no 197005, 2003)

GQ Zheng, T Sato, Y Kitaoka, M Fujita, K Yamada

PHYSICAL REVIEW LETTERS 91 ( 10 ) 2003.9

Extended quasiparticle states outside the vortex cores in a d-wave superconductor

GQ Zheng

INTERNATIONAL JOURNAL OF MODERN PHYSICS B 17 ( 18-20 ) 3509 - 3512 2003.8

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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Unconventional superconductivity in ferromagnetic UGe2: a Ge-73 nuclear magnetic resonance/nuclear quadrupole resonance study

H Kotegawa, S Kawasaki, A Harada, Y Kawasaki, K Okamoto, GQ Zheng, Y Kitaoka, E Yamamoto, Y Haga, Y Onuki, KM Itoh, EE Haller

JOURNAL OF PHYSICS-CONDENSED MATTER 15 ( 28 ) S2043 - S2046 2003.7

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

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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Effect of chemical pressure on the magnetic order in heavy electron system CeRhIn5: In-115 NQR study of CeRh1-xIrxIn5

N Yamaguchi, H Kan, GQ Zheng, Y Kitaoka, JL Sarrao, JD Thompson

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 388 543 - 544 2003.5

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

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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Sb-NQR probe for superconducting properties in the Pr-based filled-skutterudite compound PrRu4Sb12

M Yogi, H Kotegawa, Y Imamura, GQ Zheng, Y Kitaoka, H Sugawara, H Sato

PHYSICAL REVIEW B 67 ( 18 ) 2003.5

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

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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Cu-13-NMR study of single-layer high-T-c cuprate Bi2.1Sr1.9CuO6

A Sakai, GQ Zheng, Y Kitaoka, B Liang, CT Lin

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 388 251 - 252 2003.5

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

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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Magnetism and superconductivity in CeRhIn5 under chemical and hydrostatic pressures

GQ Zheng, S Kawasaki, N Yamaguchi, K Tanabe, T Mito, H Kan, Y Kitaoka, Y Haga, D Aoki, Y Onuki, JL Sarrao, JD Thompson

PHYSICA B-CONDENSED MATTER 329 450 - 451 2003.5

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

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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Microscopic coexistence of antiferromagnetism and superconductivity in HgBa2Ca4Cu5Oy : Cu-NMR study

H Kotegawa, Y Tokunaga, Y Araki, GQ Zheng, Y Kitaoka, K Tokiwa, T Watanabe, A Iyo, H Kito, Y Tanaka, H Ihara

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 388 237 - 238 2003.5

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

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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Unconventional superconductivity in CeIn3: In-115-NQR study under pressure

S Kawasaki, Y Kawasaki, T Mito, GQ Zheng, Y Kitaoka, S Araki, R Settai, Y Onuki

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 388 541 - 542 2003.5

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

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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Fermi-liquid ground state in the n-type Pr0.91LaCe0.09CuO4-y copper-oxide superconductor Reviewed

GQ Zheng, T Sato, Y Kitaoka, M Fujita, K Yamada

PHYSICAL REVIEW LETTERS 90 ( 19 ) 2003.5

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

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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Unconventional superconductivity and quasi-2D magnetic fluctuations in Ce(Ir,Rh)In-5

GQ Zheng, K Tanabe, S Kawasaki, H Kan, Y Kitaoka, D Aoki, Y Haga, Y Onuki

ACTA PHYSICA POLONICA B 34 ( 2 ) 435 - 438 2003.2

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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Coexistence of antiferromagnetism and superconductivity near the quantum criticality of the heavy-fermion compound CeRhIn5

T Mito, S Kawasaki, Y Kawasaki, GQ Zheng, Y Kitaoka, D Aoki, Y Haga, Y Onuki

PHYSICAL REVIEW LETTERS 90 ( 7 ) 2003.2

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

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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Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: An Sb nuclear quadrupole resonance study

H Kotegawa, M Yogi, Y Imamura, Y Kawasaki, GQ Zheng, Y Kitaoka, S Ohsaki, H Sugawara, Y Aoki, H Sato

PHYSICAL REVIEW LETTERS 90 ( 2 ) 2003.1

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

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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Magnetic Properties of CeRhIn5 under Pressure Probed by 115In-NQR

S. Kawasaki, T. Mito, G.-q. Zheng, Y. Kawasaki, Y. Kitaoka, D. Aoki, Y. Haga, R. Settai, Y. Onuki

Acta Phys. Polonica B 2003

Gapless Magnetic and Quasi-particles Excitations due to the Coexistence of Antiferromagnetism and Superconductivity in CeRhIn5 : 115In-NQR Study under Pressure

S. Kawasaki, T. Mito, Y. Kawasaki, G.-q. Zheng, Y. Kitaoka, D Aoki, Y Haga, Y. Onuki

Phys. Rev. Lett. 2003

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