Sharp gap edge and determination of the fermi velocity in Y₁Ba₂Cu₃O_7-δ by point contact spectroscopy

We report point contact spectroscopy measurements on Y₁Ba₂Cu₃O_7-δ thin films and textured bulk ceramics. The I(V) curves measured along the copper-oxygen ab plane direction 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 = 20 meV. Subgap structure suggests the existence of a smaller gap for some orientations in the ab plane, of the order of 12 meV. The ratio of the zero bias conductance to the normal state one, in the ab direction measurements, enabled us to set a lower bound to the Fermi velocity in Y₁Ba₂Cu₃O_7-δ in this direction:υ_F>-7 × 10⁷ cm/sec. The value of υ_F has a crucial importance in the determination of the theoretical explanation for superconductivity in the high-temperature superconductors. For out-of-plane measurements, only a weak zero bias anomaly is observed at energies up to 5meV.