Low voltage excess noise and shot noise in YBCO bicrystal junctions

The spectral density of background noise emitted by symmetric bicrystal YBaCuO Josephson junctions on sapphire substrates have been measured by a low noise cooled HEMT amplifier for bias voltages up to V approximate to 50 mV. At relatively low voltages V < 4 mV a noticeable noise rise has been registered. At large bias voltages V > 30 mV a clear dependence of noise power. exactly coinciding to the asymptote of the Schottky shot noise function, has been observed for the first time. Experimental results are discussed in terms of multiple Andreev reflections which may take place in d-wave superconducting junctions with low transparency D much less than 1. (C) 2002 Elsevier Science B.V. All rights reserved.