Correlation between Superconducting Transition Temperature T_c and Increase of Nuclear Spin-Lattice Relaxation Rate Devided by Temperature 1/T_1T at T_c in the Hydrate Cobaltate Na_xCoO_2・yH_2O(Condensed Matter : Electronic Structure, Electrical, Magnetic

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We have performed Co-nuclear quadrupole resonance (NQR) studies on Na_xCoO_2・yH_2O compounds with different Na (x) and hydrate (y) contents. Two samples with different Na contents but nearly the same T_c values (x = 0.348, T_c = 4.7K ; x = 0.339, T_c = 4.6K) were investigated. The spin-lattice relaxation rate 1/T_1 in the superconducting (SC) and normal states is almost the same for the two samples except just above T_c. NQR measurements were also performed on different-hydrate-content samples with different T_c values, which were prepared from the same Na-content (x = 0.348) sample. From measurements of 1/T_1 using the different-hydrate-content samples, it was found that a low-T_c sample with T_c = 3.9K has a larger residual density of states (DOS) in the SC state and a smaller increase of 1/T_1T just above T_c than a high-T_c sample with T_c = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between T_c and the increase in DOS just above T_c. This increase, which is seemingly associated with the two-dimensionality of the CoO_2 plane, is considered to be one of the most important factors for the occurrence of superconductivity.
社団法人日本物理学会の論文
2004-08-15