Co NQR measurements under hydrostatic pressure on superconducting cobaltate Nax(H3O)zCoO2·yH2ONax(H3O)zCoO2·yH2O

Co nuclear quadrupole resonance (NQR) measurements were performed on the bilayer hydrate cobaltate compound Nax(H3O)zCoO2·yH2ONax(H3O)zCoO2·yH2O 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 νQνQ and suppresses the SC transition temperature TcTc. The relationship between νQνQ and TcTc observed in the present pressure-dependent measurements is different from that observed in sample-dependent NQR measurements. Nuclear spin–lattice relaxation rate 1/T11/T1 was also measured under several pressures. The results indicate that the magnetic fluctuations are suppressed with applying pressures. The pressure effect on TcTc 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 ηη of the electric-field gradient, we suggest a possible explanation for the difference between sample dependence and pressure dependence.