Article

# 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: Complex cobalt oxides known as cobaltites are reviewed, including LnCoO3-based perovskite-structured rare-earth cobaltites (where Ln is lanthanum or a lanthanide), quasi-two-dimensional and quasi-one-dimensional cobaltites of the types LnCo2O5+δ, La2CoO4, and Ca3Co2O8, and NaxCoO2 yH2O superconducting compounds. Key experimental and theoretical results are presented, with emphasis on the interplay between charge, spin, and orbital degrees of freedom. Two problems of specific relevance to cobaltites — the spin state instability of Co3+ ions in LnCoO3, and the nature of superconductivity in NaxCoO2 yH2O — are also given significant attention.
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##### Article: Isotope Effect on the Superconducting Transition Temperature of NaxCoO2.yH2O
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ABSTRACT: The effect of the 16O-->18O substitution on the superconducting transition temperatures Tc of NaxCoO2.yH2O has been studied for several sets of samples. Each set has two kinds of samples, one with the oxygen sites in the CoO2 layers filled with 16O atoms and another with these sites partially substituted with 18O isotope. These two kinds of samples were prepared in parallel in whole preparation processes. For all the sample sets, we have not found any significant difference of the Tc values between the non-substituted and substituted samples, which indicates that the oxygen isotope effect is not appreciable in NaxCoO2.yH2O. The result, however, does not necessarily exclude the possibility of the phonon mechanism of the superconductivity, because the strong correlation effect suppresses the isotope effect as pointed out previously.
04/2008;
• ##### Article: Magnetic Excitations of NaxCoO2\cdotyD2O Neutron Scattering
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ABSTRACT: In neutron inelastic measurements on aligned crystals of NaxCoO2\cdotyD2O, two magnetic peaks have been observed. One peak corresponds to the contribution of ferromagnetic fluctuations and another peak, that of antiferromagnetic fluctuations. Both have two-dimensional characteristics. The intensity of ferromagnetic fluctuation decreases with decreasing temperature T and becomes inappreciable below 25 K. In contrast, antiferromagnetic fluctuation remains finite at 5 K. We have also carried out specific heat measurements on several samples with different values of the 59Co-nuclear quadrupole frequency (nuQ) within the region 12.0 < nuQ3\cong 3nuQ < 12.9 MHz, where the superconducting, nonsuperconducting and superconducting phases appear in the transition temperature (Tc)-nuQ3 phase diagram with varying nuQ3, and found that the electronic specific heat coefficient gamma does not change so significantly as to indicate a topological change of the Fermi surface with nuQ. These results imply that hole pockets, the existence of which is suggested by band calculations near the K points in the reciprocal space, do not exist in the entire superconducting nuQ3 region and support that the superconducting pair symmetry does not change with nuQ3 in the phase diagram.
Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/2008; 77(7).