We investigated pressure dependence of the electrical resistivity of the only copper-free layered-perovskite superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$ The characteristic temperature $T_M,$ where the out-of-plane resistivity ${\rho }_c$ changes from metallic behavior to nonmetallic behavior with increasing temperature, increases with hydrostatic pressure. In contrast, the absolute value of ${\rho }_c$ at $T_M$ is almost independent of pressure. Such a change over in ${\rho }_c$ is ascribable to the crossover from the diffusion process of quasiparticles between the adjacent layers to the thermally assisted hopping process. The pressure effects on ${\rho }_c$ are possibly characterized by the enhanced transfer integral along the c axis by pressure. On the other hand, in the temperature dependence of the in-plane resistivity, we observed an intriguing hump that appears at low T less than $T_M$ in the pressure range from 1.5 to 5 GPa, and vanishes on applying higher pressure. This behavior may be comprehensible from the view of the two-dimensional ferromagnetic spin fluctuations enhanced by pressure.

• Received 15 June 1998

DOI:https://doi.org/10.1103/PhysRevB.58.15062