Quantized fluctuations in the Josephson oscillations of a shunted superconducting point contact

Abstract

By detecting the oscillation of a tuned circuit resonant at frequency v0 coupled to a shunted superconducting point contact we have observed oscillations not only at the Josephson voltage V=φ0v0, where φ0 is the flux quantum, but also at voltages Vn=gn(T)φ0v0, where gn(T) is a continuous monotonic function of temperature. For most temperatures gn(T)=N(T)/n, where N and n are integers, however there are ranges of temperature over which g1(T) varies rapidly through nonintegral values and the voltages Vn are not harmonically related. The level of oscillation of the tuned circuit was used to measure the power spectrum of the voltage waveform at the point contact and the linewidths were found to be oscillatory functions of temperature with minima when V1=Nφ0v0 and maxima when V1=(N+1/2)φ0v0. The Josephson oscillation of the shunted point contact consists of pulses of N flux quanta (Nφ0) crossing the point contact at a repetition frequency v0. The temperature dependencies are interpreted in terms of fluctuations in N. The relationship between these results and some temperature dependent features observed on the I-V curves of point contacts and some implications for noise thermometry are discussed.