Comparison of Three Methods to Determine the Energy Gap of YBa2Cu3O7-δ

We have determined the gap in the excitation spectrum of the high T_c superconductor YBa₂Cu₃O_7-δ by three independent techniques: quansiparticle tunneling with a low temperature scanning tunneling microscope, far infrared reflection spectroscopy, and Andreev reflection at a silver-YBa₂Cu₃O_7-δ interface. The results are mutually consistent and in agreement with a BCS density of states. Both the far infrared and the tunneling measurements give 2Δ/k_BT_c approx 3.6 ± 0.6 (Δ = 14 ± 2 meV), which is not imcompatible with a weak-coupling BCS state. The Andreev reflection experiment gives a lower limit for the gap value: Δ >= 12.5 ± 2 meV, and indicates a zero momentum paired (k, - k) ground state.