Optimal superconductivity inLa2−xSrxCuO4

The temperature dependence of the resistivity $\rho \mathrm{}\left(T\right)\mathrm{}$ and the thermoelectric power $\alpha \mathrm{}\left(T\right)\mathrm{}$ have been measured under hydrostatic pressure on a single-crystal film of ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$ deposited on a ${\mathrm{LaSrAlO}}_4$ substrate. The compressive biaxial stress built into the film raises $T_c$ at ambient pressure. $T_c$ increased with pressure $P<\mathrm{}3.3\mathrm{}$ kbar, but saturated at a pressure-independent $T_c=43.7\mathrm{}$ at pressures $P>\mathrm{}3.3\mathrm{}$ kbar. A low-temperature enhancement of $\alpha \mathrm{}\left(T\right)\mathrm{}$ having a $T_{\max }\approx 140$ K increased with $T_c.$ The data are discussed within the framework of a vibronic model of the superconductive phase.