Experimental study of noise and Josephson oscillation linewidths in bicrystal YBCO junctions

Abstract

The intensities of the noise in a bicrystal high-TC (HTS) Josephson junction have been precision-measured at 1–2 GHz frequency band at bias voltages up to 50 mV at T = 4.2 K. At large bias voltages, V > 30 mV, the dependence of current noise density was found exactly coinciding with the Schottky shot noise asymptote 2eI. At relatively low voltages, V < 4 mV, a noticeable noise rise has been registered. The broadening of Josephson oscillation linewidths ΔfJ over the values ΔfRSJ predicted by the RSJ model has been experimentally studied at different frequencies in the mm and submm wave range up to voltages V = 2 mV in connection with low-voltage noise rise. Both the features observed, the linewidth broadening and the excess noise over the noise level of thermal fluctuations, are discussed in terms of multiple Andreev reflection, giving rise to a nonequilibrium shot noise—the case which may take place in the d-wave superconducting junctions. Experimental results on noise performance are also compared with the qualitatively similar dependences of the current noise, known for the s-superconducting ballistic point-like or diffusive-type SNS junctions, where the excess low-voltage noise is manifested due to multiple Andreev reflections. Increasing the operating temperature, the thermal (equilibrium) fluctuations were found to predominate, resulting in a decrease of ratio ΔfJ/ΔfRSJ. The characteristics of the ac Josephson effect in HTS junctions measured at submm wave frequencies at temperatures close to the transition temperature TC are also discussed.