Large a-b anisotropy of the expansivity anomaly at Tc in untwinned YBa2Cu3O7- delta.

The response of the YBa 2 Cu 3 O 7-δ 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 T c , which suggests that superconductivity favors a symmetric (b=a) CuO 2 plane. Little effect is seen along the c axis