Pressure dependence of the superconducting transition temperature of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>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 (P0.6 GPa) on specimens of compositions ${\mathrm{YBa}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (0\ensuremath{\le}x\ensuremath{\le}0.14) and ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ca}}_{\mathit{y}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (0\ensuremath{\le}y\ensuremath{\le}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\ensuremath{\sim}4 GPa [Klotz, Reith, and Schilling, Physica C 172, 423 (1991)].

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