TIME EVOLUTION OF POINT-CONTACT RESISTANCES OF HIGH-T(C) SUPERCONDUCTORS

The time increase of the differential resistance of YBa2Cu3Ox/In, Au a nd Bi2Sr2Ca2Cu3Oy/In, As point contacts was experimentally observed. T he results are discussed in the frame of the oxygen diffusion from the surface layer of high-T(c) superconductors. Our computer simulation o f the oxygen diffusion in YBa2Cu3Ox (YBCO) and Bi2Sr2Ca2Cu3Oy (BSCCO) surface layers gives good agreement with experimental data and yields diffusion coefficients of oxygen motion at 300 K of D(B) almost-equal- to 3.2 X 10(-19) cm2/s for BSCCO and D(Y) almost-equal-to 3.7 x 10(-18 ) cm2/s for YBCO. From the comparison of the two studied materials it clearly follows that the surface of BSCCO is more stable than YBCO; ne vertheless, the surface degradation processes due to oxygen diffusion are significant for both materials within room temperature range. The sharp drop in the time increase of the point contact resistance at tem perature T(r) almost-equal-to 220 K for YBCO and almost-equal-to 185 K for BSCCO is interpreted by oxygen reordering in the unit cells of hi gh-T(c) superconductors.