ELECTRON TUNNELLING SPECTROSCOPY OF YBA2CU3O7-X CERAMICS

Electron tunnelling measurements on YBa//2Cu//3O//7// minus //x superconducting ceramics are reported. A surface layer on the grains prevents observation of superconducting tunneling 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 measured 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 YBa//2Cu//3O//7// minus //x with a gap of 2 DELTA approximately equals 40 mv, corresponding to 2 DELTA /kT//c approximately equals 5. Peak separations larger than 40 mev are ascribed to the effects of finite inter-grain resistance.