Large a - b anisotropy of the expansivity anomaly at T sub c in untwinned YBa sub 2 Cu sub 3 O sub 7 minus. delta

The response of the YBa{sub 2}Cu{sub 3}O{sub 7{minus}{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 {ital a}-{ital b} plane. This leads to a reduction of the orthorhombic splitting below {ital T}{sub {ital c}}, which suggests that superconductivity favors a symmetric ({ital b}={ital a}) CuO{sub 2} plane. Little effect is seen along the {ital c} axis. Superconducting fluctuations are for the first time clearly observed in the lattice expansion.