Large a-b anisotropy of the expansivity anomaly at ${\mathit{T}}_{\mathit{c}}$ in untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The response of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ lattice to superconducting order is studied using an ultrahigh-resolution capacitance dilatometer. The onset of superconductivity is accompanied by highly anisotropic jumps of the expansivities in the a-b plane. This leads to a reduction of the orthorhombic splitting below ${\mathit{T}}_{\mathit{c}}$, which suggests that superconductivity favors a symmetric (b=a) ${\mathrm{CuO}}_2$ plane. Little effect is seen along the c axis. Superconducting fluctuations are for the first time clearly observed in the lattice expansion.