On the non-superconducting state in the phase diagram of NaxCoO2.yH2O

For the understanding of the superconducting state of NaxCoO2.yH2O (x ∼ 0.3), we have carried out various kinds of experimental studies. Among results of these studies, we show here data of the specific heats, frequency-dependent ac-magnetic susceptibility and 59Co-NQR and 2D NMR and argue what electronic state is realized in the nonsuperconducting phase, which appears in the T-3νQ phase diagram of this system, νQ being the Co-NQR quadrupole frequency. The transition observed in this phase at ∼7 K is not due to a magnetic ordering but possibly due to a charge ordering or the CDW formation. Slightly below this transition temperature, the system exhibit glassy behavior. The results indicate that the superconducting phase cannot necessarily be considered to be located near the magnetic phase.