Pressure dependence of the superconducting transition temperature of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ as a function of carrier concentration: A test for a simple charge-transfer model

Measurements of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ vs purely hydrostatic pressure (P<0.6 GPa) on specimens of compositions ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (0≤x≤0.14) and ${\mathrm{Y}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Ca}}_{\mathit{y}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (0≤y≤0.06) yield an approximately linear relation between ${\mathit{dT}}_{\mathit{c}}$/dP and the carrier concentration induced in the ${\mathrm{CuO}}_2$ planes by chemical substitution with ${\mathit{dT}}_{\mathit{c}}$/dP ranging from -0.6 K/GPa (y=0.06) to +2.2 K/GPa (x=0.14). A simple phenomenological model clarifies the ${\mathit{dT}}_{\mathit{c}}$/dP behavior and provides a physical origin for the observed anomalous peak in ${\mathit{T}}_{\mathit{c}}$(P) at P∼4 GPa [Klotz, Reith, and Schilling, Physica C 172, 423 (1991)].