研究者詳細 - 鄭　国慶

論文 - 鄭　国慶

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P-31-NMR study in single crystal SmRu4P12 査読

Satoru Masaki, Takeshi Mito, Shinji Wada, Hitoshi Sugawara, Daisuke Kikuchi, Hideyuki Sato, Masashi Takigawa, Naoya Takeda, Guo-qing Zheng

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 206 - 208 2008年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

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

Spin-singlet superconductivity with multiple gaps in PrFeAsO0.89F0.11

K. Matano, Z. A. Ren, X. L. Dong, L. L. Sun, Z. X. Zhao, Guo-Qing Zheng

EPL 83 ( 5 ) 57001-1-57001-4 2008年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY

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

NQR investigation on multipolar ordering systems 査読

T. Mito, S. Tomisawa, S. Masaki, M. Takemura, S.Wada, H. Harima, Y. Kato, M. Kosaka, D. Kikuchi, H. Sato, H. Sugawara, N. Takeda, G.-q. Zheng

Physica 403 ( 5-9 ) 1633 - 1635 2008年

Ru-NQR studies on filled skutterudite compounds RRu4P12 (R= La, Nd, Sm) 査読

S. Masaki, T. Mito, M.Takemura, S. Wada, H. Harima, D. Kikuchi, H. Sato, H. Sugawara, N. Takeda, G.-q. Zheng

Physica 403 ( 5-9 ) 1630 - 1632 2008年

Evidence for d-wave pairing in the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O 査読

K. Matano, Guo-Qing Zheng, D. P. Chen, C. T. Lin

Physica B: Condensed Matter 403 1107 - 1108 2007年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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

Pressure dependence of the superconducting transition and electron correlations in NaxCoO2 center dot 1.3H(2)O

E. Kusano, S. Kawasaki, K. Matano, Guo-Qing Zheng, R. L. Meng, J. Cmaidalka, C. W. Chu

PHYSICAL REVIEW B 76 ( 10 ) 100506(R)(1)-100506(R)(4) 2007年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

J. Laverock, S. B. Dugdale, J. A. Duffy, J. Wooldridge, G. Balakrishnan, M. R. Lees, G.-Q. Zheng, D. Chen, C. T. Lin, A. Andrejczuk, M. Itou, Y. Sakurai

PHYSICAL REVIEW B 76 ( 5 ) 052509(1)-052509(4) 2007年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Observation of symmetry lowering associated with the metal-insulator transition in SmRu4P12 by Ru-101-NQR

Satoru Masaki, Takeshi Mito, Masayuki Takemura, Shinji Wada, Hisatomo Harima, Daisuke Kikuchi, Hideyuki Sato, Hitoshi Sugawara, Naoya Takeda, Guo-qing Zheng

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 76 ( 4 ) 043714 (1)-043714 (4) 2007年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

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

NMR study of the successive two phase transitions in SmRu4P12

Masaki Satoru, Mito Takeshi, Shinji Wada, Naoya Takeda, Daisuke Kikuchi, Hideyuki Sato, Hitoshi Sugawara, Guo-qing Zheng

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 ( 2 ) E175 - E177 2007年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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

Magnetic-field induced ferromagnetism as the origin for the colossal magneto-resistance in La1.2Sr1.8Mn2O7

Y. Shiotani, J. S. Gardner, J. L. Sarrao, Guo-qing Zheng

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 ( 2 ) E661 - E663 2007年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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

Novel superconductivity at the magnetic critical point in heavy-fermion systems: a systematic study of NQR under pressure 招待査読

Y. Kitaoka, S. Kawasaki, Y. Kawasaki, T. Mito, G-q Zheng

JOURNAL OF PHYSICS-CONDENSED MATTER 19 ( 12 ) 125202-1-125202-18 2007年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

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

Evidence for point nodes in the superconducting gap function in the filled skutterudite heavy-fermion compound PrOs4Sb12 : Sb-123-NQR study under pressure

Kouta Katayama, Shinji Kawasaki, Masahide Nishiyama, Hitoshi Sugawara, Daisuke Kikuchi, Hideyuki Sato, Guo-qing Zheng

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 76 ( 2 ) 023701 (1)-023701(4) 2007年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

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

Spin triplet superconducting state due to broken inversion symmetry in Li2Pt3B

M. Nishiyama, Y. Inada, Guo-qing Zheng

PHYSICAL REVIEW LETTERS 98 ( 4 ) 047002(1) - 047002(4) 2007年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

NMR and NQR studies of the pairing symmetry in the superconducting state of NaxCoO2 center dot 1.3H(2)O 査読

K. Matano, G.-Q. Zheng, D. P. Chen, C. T. Lin, J. Cmaidalka, R. L. Meng, W. Chu

Journal of Magnetism and Magnetic Materials 310 ( 2 ) 687 - 689 2006年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER

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

Spin singlet pairing in the superconducting state of NaxCoO2 center dot 1.3H(2)O: Evidence from a Co-59 Knight shift in a single crystal

Guo-qing Zheng, Kazuaki Matano, D. P. Chen, C. T. Lin

PHYSICAL REVIEW B 73 ( 18 ) 2006年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Evidence for nodes in the superconducting energy gap of the skutterudite compound PrOS4Sb12

M. Nishiyama, T. Kato, H. Sugawara, D. Kikuchi, H. Sato, H. Harima, G. -Q. Zheng

PHYSICA B-CONDENSED MATTER 378-80 192 - 193 2006年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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

Evolution of the physical properties with Na content in the cobalt oxide superconductors NaxCoO2 center dot 1.3H(2)O

K Matano, GQ Zheng, J Cmaidalka, Y Sun, RL Meng, CW Chu

PHYSICA B-CONDENSED MATTER 378-80 871 - 872 2006年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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

NMR study of unique properties in SmRu4P12

T Mito, S Masaki, N Oki, S Noguchi, S Wada, N Takeda, D Kikuchi, H Sato, H Sugawara, GQ Zheng

PHYSICA B-CONDENSED MATTER 378-80 224 - 225 2006年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

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) < T < 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

Field-induced ferromagnetic order and colossal magnetoresistance in La1.2Sr1.8Mn2O7: A La-139 NMR study

Y Shiotani, JL Sarrao, GQ Zheng

PHYSICAL REVIEW LETTERS 96 ( 5 ) 2006年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Na content dependence of superconductivity and the spin correlations in NaxCoO2 center dot 1 center dot 3H(2)O

GQ Zheng, K Matano, RL Meng, J Cmaidalka, CW Chu

JOURNAL OF PHYSICS-CONDENSED MATTER 18 ( 5 ) L63 - L70 2006年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

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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