POINT CONTACT SPECTROSCOPY IN ORIENTED YBa₂Cu₃O_7−δ

Abstract

We report point contact spectroscopy measurements on YBa 2 Cu 3 O 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 data also enabled us to set a lower bound to the inplane Fermi velocity in YBa 2 Cu 3 O 7−δ : v F ≥ 7 × 10 7 cm/sec . The value of v 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 5 mev.