Pressure dependence of the superconducting transition and electron correlations in Nax Co O2·1.3 H2 O

We report Tc and Co59 nuclear quadrupole resonance measurements on the cobalt oxide superconductor Nax Co O2·1.3 H2 O (Tc =4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. Tc decreases with increasing pressure at an average rate of -0.49±0.09 K/GPa. At low pressures P≤0.49 GPa, the decrease of Tc is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of Tc is mainly due to a reduction of the DOS. These results indicate that the electronic and magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T1 at P=0.49 GPa shows a T3 variation below Tc down to T∼0.12 Tc, which provides compelling evidence for the presence of line nodes in the superconducting gap function.