Anisotropic pressure dependence ofTcin single-crystalYBa2Cu3O7via thermal expansion

Abstract

High-resolution anisotropic-thermal-expansion measurements of single-crystalline and oriented-grained ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ at the superconducting transition are presented for the first time. Discontinuities in the thermal-expansion coefficient ${\mathrm{\ensuremath{\alpha}}}_{\mathit{a}\mathit{b}}$[\ensuremath{\Delta}${\mathrm{\ensuremath{\alpha}}}_{\mathit{a}\mathit{b}}$=(15--23)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$], measured with a capacitance dilatometer, are found to occur in both samples. No discontinuity in ${\mathrm{\ensuremath{\alpha}}}_{\mathit{c}}$(\ensuremath{\Vert}\ensuremath{\Delta}${\mathrm{\ensuremath{\alpha}}}_{\mathit{c}}$\ensuremath{\Vert}1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$) is observed in either sample, although ${\mathrm{\ensuremath{\alpha}}}_{\mathit{c}}$(T) shows a distinct change of slope at ${\mathit{T}}_{\mathit{c}}$. The specific-heat discontinuity \ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$ of both samples was also measured and is used, along with the \ensuremath{\Delta}\ensuremath{\alpha}'s, to calculate the dependence of ${\mathit{T}}_{\mathit{c}}$ on uniaxial pressure and uniaxial strain to first order. ${\mathit{T}}_{\mathit{c}}$ is predicted to increase with pressure applied perpendicular to the c axis (${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{a}\mathit{b}}$=0.04--0.09 K/kbar) and to be insensitive to pressure parallel to the c axis. Uniaxial strain, on the other hand, is found to increase ${\mathit{T}}_{\mathit{c}}$ about equally in both directions.