<i>T</i>–3ν_Q Phase Diagram of Hydrated Cobalt Oxides Na_<i>x</i>CoO₂·<i>y</i>H₂O

Abstract

The superconducting transition temperatures of Na x CoO 2 · y H 2 O prepared by various methods have been found to exhibit the systematic dependence on the nuclear quadrupole frequency ν Q . In the T –3ν Q phase diagram, the superconducting region of 3ν Q , spreading over the region between ∼12.0 and ∼12.9 MHz, is divided into two regions by the nonsuperconducting phase between ∼12.5 and ∼12.6 MHz. We have investigated the origin of the nonsuperconducting phase by detailed measurements of 59 Co-NQR and 2 D-NMR, and the results indicate that the nonsuperconducting phase is possibly in the charge-density-wave or the charge disproportionate state, in which the magnetic moments induced inhomogeneously by this transition become frozen at low temperatures.