Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O

TL;DR

This study investigates how hydrostatic pressure affects the superconducting transition temperature and electron correlations in Na_xCoO_2·1.3H_2O, revealing the roles of spin correlations and density of states in superconductivity.

Contribution

It provides the first detailed analysis of pressure effects on T_c and electron correlations in this cobalt oxide superconductor, highlighting the importance of electronic and magnetic states.

Findings

T_c decreases with pressure at -0.49 K/GPa.

Spin correlations weaken at low pressures.

Density of states reduction dominates at high pressures.

Abstract

We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the decrease of T_c 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 T_c is mainly due to a reduction of the DOS. These results indicate that the electronic/magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling evidence for the presence of line nodes in the superconducting gap function.