Anisotropic Pressure Dependence of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>: Evidence for the Tunneling-Unit Model

The superconducting transition temperature ${T}_{c}$ of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ shows a decrease with uniaxial pressure in the a direction and an approximately equal increase with pressure in the b direction near ${T}_{c}\ensuremath{\approx}91.5\mathrm{K}$. The jump in the thermal expansivity near ${T}_{c}$ is also approximately equal and of opposite sign in the a and b directions. We show that the recently derived tunneling-unit model for high ${T}_{c}$ superconductivity explains the anisotropic pressure dependence of ${T}_{c}$ and the resulting jump in the thermal expansivity near ${T}_{c}$ in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. For $\ensuremath{\delta}\ensuremath{\approx}0$ our results are in close quantitative agreement with measurements. We suggest further experiments to clarify the nature of high ${T}_{c}$ superconductivity in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ for $0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.5$.

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