Faculty Profiles - TEI Kokukei

Papers - TEI Kokukei

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Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog

Z. Li, S. Kawasaki, T. Oka, T. Tabuchi, Y. Ooe, M. Ichioka, Z. A. Ren, Z. X. Zhao, J. L. Luo, N. L. Wang, X. C. Wang, Q. Q. Liu, C. Q. Jin, C. T. Lin, Guo-qing Zheng

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 492 - 496 2011.5

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

As-NMR/NQR study of pressure-induced superconductivity in CaFe2As2

S. Kawasaki, T. Oka, T. Tabuchi, X. F. Wang, X. H. Chen, Guo-qing Zheng

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 501 - 504 2011.5

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

As-NQR study of superconductivity in LaRAsO1-xFx (R=Fe and Ni)

T. Oka, T. Tabuchi, Z. Li, S. Kawasaki, G. F. Chen, J. L. Luo, N. L. Wang, Guo-qing Zheng

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 497 - 500 2011.5

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 <= T <= 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

Nodeless energy gaps of single-crystalline Ba0.68K0.32Fe2As2 as seen via As-75 NMR

Z. Li, D. L. Sun, C. T. Lin, Y. H. Su, J. P. Hu, Guo-qing Zheng

PHYSICAL REVIEW B 83 ( 14 ) 140506(R)-1-140506(R)-4 2011.4

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 -> 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

Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi(2)Sr2-xLaxCuO6+delta Revealed by Cu-63,Cu-65-Nuclear Magnetic Resonance in Very High Magnetic Fields

Shinji Kawasaki, Chengtian Lin, Philip L. Kuhns, Arneil P. Reyes, Guo-qing Zheng

PHYSICAL REVIEW LETTERS 105 ( 13 ) 2010.9

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 >= 0.8, which suggests that the density of states plunges to zero upon approaching the Mott insulating phase.

DOI： 10.1103/PhysRevLett.105.137002

As-75 NQR and NMR Studies of Superconductivity and Electron Correlations in Iron Arsenide LiFeAs

Zheng Li, Yosuke Ooe, Xian-Cheng Wang, Qing-Qing Liu, Chang-Qing Jin, Masanori Ichioka, Guo-qing Zheng

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 79 ( 8 ) 2010.8

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 <= 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

Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

Guo-qing Zheng, K. Matano, S. Kawasaki, Z. Li, G. L. Sun, D. L. Sun, C. T. Lin, G. F. Chen, J. L. Luo, N. L. Wang, Z. A. Ren, Z. X. Zhao

JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 23 ( 5 ) 609 - 612 2010.7

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

Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2

S. Kawasaki, T. Tabuchi, X. F. Wang, X. H. Chen, Guo-qing Zheng

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 23 ( 5 ) 054004-1-054004-6 2010.5

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 <= P <= 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

Evidence for a full energy gap in the nickel pnictide superconductor LaNiAsO1-xFx from As-75 nuclear quadrupole resonance

T. Tabuchi, Z. Li, T. Oka, G. F. Chen, S. Kawasaki, J. L. Luo, N. L. Wang, Guo-qing Zheng

PHYSICAL REVIEW B 81 ( 14 ) 2010.4

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 < T < 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

59Co NMR at pulsed high magnetic fields Reviewed

Guo-Qing Zheng, K. Katayama, M. Kandatsu, N. Nishihagi, S. Kimura, M. Hagiwara, K. Kindo

J. Low. Temp. Phys. 159 280 2010.1

As-75 NMR studies of superconducting Ba0.72K0.28Fe2As2 Reviewed

Z. Li, K. Matano, D. L. Sun, G. L. Sun, C. T. Lin, M. Ichioka, G. -Q. Zheng

Physica C: Superconductivity and its Applications 470 S495 - S496 2009.12

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

Spin-Echo NMR in Pulsed High Magnetic Fields up to 48 T

Guo-qing Zheng, Kouta Katayama, Masahide Nishiyama, Shinji Kawasaki, Naoki Nishihagi, Shojiro Kimura, Masayuki Hagiwara, Koichi Kindo

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 9 ) 095001-1-095001-2 2009.9

Superconducting state in the noncentrosymmetric Mg9.3Ir19B16.7 and Mg10.5Ir19B17.1 revealed by NMR

K. Tahara, Z. Li, H. X. Yang, J. L. Luo, S. Kawasaki, Guo-qing Zheng

PHYSICAL REVIEW B 80 ( 6 ) 060503-1-060503-4 2009.8

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

Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba0.72K0.28Fe2As2: NMR in a single crystal

K. Matano, Z. Li, G. L. Sun, D. L. Sun, C. T. Lin, M. Ichioka, Guo-Qing Zheng

EPL 87 ( 2 ) 27012-1-27012-5 2009.7

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

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

Measurement of electron correlations in LixCoO2 (x=0.0-0.35) using Co-59 nuclear magnetic resonance and nuclear quadrupole resonance techniques

S. Kawasaki, T. Motohashi, K. Shimada, T. Ono, R. Kanno, M. Karppinen, H. Yamauchi, Guo-qing Zheng

PHYSICAL REVIEW B 79 ( 22 ) 220514-1-220514-4 2009.6

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

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 -> 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

Superconducting Gap and Valence Band of Mg10Ir19B16 Studied by Laser and Synchrotron Photoemission Spectroscopy Reviewed

Yoshida Rikiya, Okazaki Hiroyuki, Iwai Keisuke, Noami Kengo, Muro Takayuki, Okawa Mario, Ishizaka Kyoko, Shin Shik, Li Zheng, Luo Jianlin, Zheng Guo-qing, Oguchi Tamio, Hirai Masaaki, Muraoka Yuji, Yokoya Takayoshi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 3 ) 34705 - 034705-4 2009.3

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

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

Bulk-sensitive spectroscopic studies on noncentrosymmetric superconducting system of Mg–Ir–B Reviewed

R. Yoshida, H. Okazaki, K. Iwai, K. Noami, T. Muro, M. Okawa, K. Ishizaka, S. Shin, Z. Li, J.L. Luo, G.-Q. Zheng, T. Oguchi, M. Hirai, Y. Muraoka, T. Yokoya

Physica C-Superconductivity and Its Applications 469 1034 - 1036 2009

Two superconducting gaps in LaFeAsO0.92F0.08 revealed by As-75 nuclear quadrupole resonance

S. Kawasaki, K. Shimada, G. F. Chen, J. L. Luo, N. L. Wang, Guo-qing Zheng

PHYSICAL REVIEW B 78 ( 22 ) 220506-1-220506-4 2008.12

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

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

Hydration-induced anisotropic spin fluctuations in NaxCoO2 center dot 1.3H(2)O superconductor

K. Matano, C. T. Lin, Guo-qing Zheng

EPL 84 ( 5 ) 57010-1-57010-5 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY

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

(123)Sb-NQR study of unconventional superconductivity in the filled skutterudite heavy-fermion compound PrOs(4)Sb(12) under high pressure up to 3.82 GPa

S. Kawasaki, K. Katayama, H. Sugawara, D. Kikuch, H. Sato, Guo-qing Zheng

PHYSICAL REVIEW B 78 ( 6 ) 064510-1-064510-7 2008.8

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

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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