Electron tunnelling spectroscopy of YBa2Cu3O7-xceramics

Abstract

Electron tunnelling measurements on YBa₂Cu₃O_7-x superconducting ceramics are reported. A surface layer on the grains prevents observation of superconducting tunnelling characteristics with simple evaporated electrodes. However, point contacts between a variety of metal electrodes and the ceramic surface show a conductance characteristic with a broadened gap-like feature whose magnitude diminishes smoothly with temperature and disappears near the measure T_c of 91.7 K. The separation between conductance peaks varied between 28 and 65 mV over a sample of some 30 contacts, with a marked peak in the frequency distribution at 40 mV. The overall results are interpreted in a model where the spectra arise from proximity-induced superconductivity in a surface layer on underlying superconducting YBa₂Cu₃O_7-x with a gap of 2 Delta approximately=40 mV, corresponding to 2 Delta /kT_c approximately=5. Peak separations larger than 40 meV are ascribed to the effects of finite inter-grain resistance.