TRANSITION TO GAPLESS SUPERCONDUCTIVITY IN YBA2(CU1-XZNX)3O7

Point-contact spectra of YBa2(Cu(1-x)Zn(x))3O7/Ag are studied at various temperatures. The differential resistance dV/dI of the point contacts shows gap-related structures below T(c) which can be attributed to Andreev reflection. Evaluation of many spectra for each sample taken at 4.2 K-yields a wide distribution of voltages DELTA/e at which these structures occur. The upper limit varies roughly as expected from the depression of T(c) by Zn-doping from DELTA/e = 29 mV (x = 0) to 9 mV (x = 0.05), while the lower limit decreases much faster and disappears for x = 0.05. Hence, the Zn doped samples exhibit a tendency to gapless superconductivity as suggested earlier on the basis of specific-heat measurements.