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 ${\mathrm{\alpha }}_{\mathit{a}\mathit{b}}$[Δ${\mathrm{\alpha }}_{\mathit{a}\mathit{b}}$=(15–23)×${10}^{\mathrm{-}8}$ ${\mathrm{K}}^{\mathrm{-}1}$], measured with a capacitance dilatometer, are found to occur in both samples. No discontinuity in ${\mathrm{\alpha }}_{\mathit{c}}$(‖Δ${\mathrm{\alpha }}_{\mathit{c}}$‖<1×${10}^{\mathrm{-}8}$ ${\mathrm{K}}^{\mathrm{-}1}$) is observed in either sample, although ${\mathrm{\alpha }}_{\mathit{c}}$(T) shows a distinct change of slope at ${\mathit{T}}_{\mathit{c}}$. The specific-heat discontinuity Δ${\mathit{C}}_{\mathit{p}}$ of both samples was also measured and is used, along with the Δα’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.

• Received 4 December 1989

DOI:https://doi.org/10.1103/PhysRevB.41.11299