Gap anisotropy and Fermi velocity in oriented YBa₂Cu₃O_7-δ from point contact spectroscopy

Point contact spectroscopy measurements on YBa₂Cu₃O_7-δ textured ceramics are presented. The I(V) curves measured along the Copper-Oxygen plane direction at 4.2K are characteristic of the Andreev reflection phenomenon, with a sharp break at the gap edge. This allows a rather precise determination of the in plane gap, Δ=19±1mev. Subgap structure suggests the existence of a smaller gap for some orientations in the CuO plane, of the order of 12mev. Preliminary results of measurements performed at higher temperature suggest the possibility of a non BCS s-wave temperature dependence of the gap, Δ(T). The data enabled us to set a lower bound to the in plane Fermi velocity in YBa₂Cu₃O_7-δ: v_F≥7×10⁷cm/sec, considerably higher than band structure calculations. We discuss the importance of v_F with respect to theoretical explanations for High Temperature Superconductivity.