Studies on the superconducting state of NaxCoO2·yH2O – Overview

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.