Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaxCoO2\cdotyH2O

Journal of the Physical Society of Japan (Impact Factor: 2.09). 01/2003; 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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    ABSTRACT: We report electronic transport measurements on high quality floating zone grown NaxCoO2 and Na0.41CoO2·0.6H2O single crystals. We find an in-plane electronic scattering minimum near 11 K and a clear charge ordering at approximately 50 K. The electronic and magnetic properties in hydrated and nonhydrated Na0.41CoO2 samples are similar at higher temperature, but evolve in markedly different ways below ∼50 K, where a strong ferromagnetic tendency is observed in the hydrated sample. Model calculations show the relationship of this tendency to the structure of the Fermi surface. The results, particularly the clear differences between the hydrated and nonhydrated material show a substantially enhanced ferromagnetic tendency upon hydration. Implications for superconductivity are discussed.
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    ABSTRACT: Results of experimental studies carried out to clarify the superconducting state of NaxCoO2·yH2O (x∼0.3; y∼1.3) are presented. Various means such as NMR, neutron magnetic scattering, and specific-heats and magnetic-susceptibility measurements have been applied to both poly- and single-crystalline samples to obtain internally consistent data. Effects of non-magnetic impurities and oxygen isotope substitution on the superconducting transition temperature Tc have also been studied. Data obtained for the mother system NaxCoO2 have also presented useful information on the superconducting state of NaxCoO2·yH2O. In the course of these studies, we have shown by measuring Knight shifts of 59Co and 23Na of aligned crytalline samples that the Cooper pairs are in the singlet state. Neutron inelastic scattering measurements on large single crystals have presented the firm evidence for the disappearance of the low energy ferromagnetic excitation with decreasing T, excluding the triplet Cooper pairing in NaxCoO2·yH2O consistently with the Knight shift. In the Tc–νQ phase diagram, it has been confirmed that there is a nonsuperconducting phase, which divides the superconducting region into two, νQ being the nuclear quadrupole frequency. The possible different symmetries between the two superconducting phases suggested by this characteristic νQ dependence shown in the phase diagram, has been carefully examined mainly by specific heat measurements and NMR. However, any difference between the pair states of these phases has not been found. The appearance of the nonsuperconducting phase should be considered due to an instability which appears within a single superconducting phase.
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Kenichi Yoshimura