Co NQR measurements under hydrostatic pressure on superconducting cobaltate [formula omitted

Abstract

Co nuclear quadrupole resonance (NQR) measurements were performed on the bilayer hydrate cobaltate compound Na x ( H 3 O ) z CoO 2 · y H 2 O under hydrostatic pressures to study the pressure effect on the superconducting (SC) and magnetic properties. We found that the hydrostatic pressure increases the NQR frequency ν Q and suppresses the SC transition temperature T c . The relationship between ν Q and T c observed in the present pressure-dependent measurements is different from that observed in sample-dependent NQR measurements. Nuclear spin–lattice relaxation rate 1 / T 1 was also measured under several pressures. The results indicate that the magnetic fluctuations are suppressed with applying pressures. The pressure effect on T c can be understood as the suppression of magnetic fluctuations, which act as the glue to form the cooper pairs. From the pressure effect on asymmetric parameter η of the electric-field gradient, we suggest a possible explanation for the difference between sample dependence and pressure dependence.