Article

# Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na x CoO 2 · y H 2 O

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Division of Chemistry, Kyoto University, Kioto, Kyōto, Japan
(Impact Factor: 1.59). 12/2003; 72(12):3041-3044. DOI: 10.1143/JPSJ.72.3041
Source: arXiv

ABSTRACT

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor NaxCoO2\cdotyH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, a SC bilayer-hydrate sample with Tc ˜ 4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitative agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T_1T of the SC sample shows a gradual increase below 100 K down to Tc, whereas 1/T_1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T_1T and chibulk in the SC sample, the increase in 1/T_1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related to the unconventional superconductivity in this compound. The implication of this finding is discussed.

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##### Article: Possible unconventional superconductivity in Na_ {x} CoO_ {2}· yH_ {2} O probed by muon spin rotation and relaxation
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ABSTRACT: The superconducting property of sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O (Tc=4.5 K), has been studied by means of muon spin rotation/relaxation down to 2 K. It was found that the zero-field muon spin relaxation rate is independent of the temperature, indicating that no static magnetism appears in this compound, at least above 2 K. The result also provides evidence of no breaking of time-reversal symmetry for the superconductivity. Meanwhile, the muon Knight shift at 60 kOe, which is proportional to the local spin susceptibility, shows no obvious reduction below Tc. Considering these observations, possible unconventional superconductivity of Na0.35CoO2∙1.3H2O is discussed.
Physical review. B, Condensed matter 10/2003; 70(13). DOI:10.1103/PhysRevB.70.134508 · 3.66 Impact Factor
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##### Article: Spin resonance peak in Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O superconductors: A probe of the pairing symmetry
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ABSTRACT: Motivated by the recent discovery of superconductivity in layered Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compound, we investigate theoretically the spin dynamics in the superconducting state with three possible $p_{x}+ip_{y}$-, $d+id'$-, and $f$-wave pairing symmetries in two-dimensional triangular lattices. We find that a spin resonance peak, which is elaborated to have a close relevance to the relative phase of the gap function and the geometry of the Fermi surface, appears in both in-plane and out-of-plane components of the spin susceptibility $\chi$ for the spin-singlet $d+id'$-wave pairing, while only in the out-of-plane(in-plane) component of $\chi$ for the spin-triplet $p_{x}+ip_{y}$-wave ($f$-wave) pairing. Because the pairing symmetry in this compound is still hotly debated at present, this distinct feature may be used to probe or determine it unambiguously with future neutron scattering experiments.
• ##### Article: Superconducting Transition Temperatures of Cu(Co1-xMx)2S4
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ABSTRACT: Superconducting transition temperatures Tc have been measured for s-wave superconductors Cu(Co1-xMx)2S4 with varying x. The system is often considered to be a three dimensional version of the triangular lattice, because it has a three dimensional network of corner-sharing (Co1-xMx)4 tetrahedra. As the M atom, Cr, Ir and Rh, which can substitute for Co, are used. Although Tc decreases linearly in x, the suppression rates |dTc/dx| are much smaller than the values expected from the pair breaking due to magnetic impurities in s-wave superconductors. They are, of course, much smaller than those expected for the pair breaking due to non-magnetic impurities in superconducting systems with anisotropic order parameter. The behavior is very similar to that observed in the newly found superconductor NazCoO2\cdotyH2O, which has two-dimensional layers of CoO2 triangular lattice. It suggests that a same mechanism of the Tc suppression is working in the two systems and presents useful information on the symmetry of the superconducting order parameter of NazCoO2\cdotyH2O. Brief arguments of a possible mechanism of the Tc suppression of the present system are given.
Journal of the Physical Society of Japan 01/2004; 73(7):1938-1943. DOI:10.1143/JPSJ.73.1938 · 1.59 Impact Factor