Experimental observation of the transition from weak link to tunnel junction

Abstract

An extension to Morelands break junction technique is developed in order to obtain a clean and stable, mechanically adjustable junction. As a function of an externally applied force the coupling of two electrodes can be varied in vacuum. Experiments are described of a junction with niobium electrodes at 4.2 K which undergo a continuous change in normal resistance R N, from 1 to 10 9 Ω upon applying an increasing force. In this resistance range we discern a transition from a weak link regime to a tunnel regime. The current voltage ( I–V) curves are reproducible upon adjustment changes in the whole resistance range. In the weak link regime the two electrodes of the junction are in physical contact with each other. The product of the critical current and normal resistance is compared with predictions of Ambegaokar-Baratoff and Kulik-Omelyanchuk. The product of the excess current and normal resistance shows a logarithmic increase for low R Nvalues and decreases for the highest R N values in the weak link regime. Subharmonic gap structure, originating from multiple Andreev reflections is observed over a wide range of R N. In the transition regime the two electrodes are not in contact but there is still a large overlap of the superconducting and quasiparticle wave functions. In this regime a finite slope of the “critical current part” in the current voltage curve is observed. The I–V curves show features characteristics for both a weak link and a tunnel junction. In the tunnel regime there exists a vacuum gap between the electrodes and the Josephson coupling is suppressed. A considerable subgap current is observed, where the product of the subgap current and normal resistance is constant over almost four orders of magnitude of R N. A decreasing conductance near zero bias shows up in this regime. The normal resistane exhibits an exponential behaviour upon variations in the vacuum gap. The absolute stability of the distance between the two junction electrodes is estimated to be better than 0.5 pm over a 100 mV voltage range.