Some temperature-dependent properties of niobium tunnel junctions

Abstract

Some measurements of the electrical properties of Nb tunnel junctions in the temperature range 2-9 °K are described. These include (i) the quasiparticle tunneling current, (ii) the gap voltage 2Δ/e, (iii) the dc Josephson current i1, and (iv) the London penetration depth λ. With regard to (i) the tunnel current is well described by an empirical model in which the density of states within the forbidden gap of the top electrode is finite and sharply peaked at the gap edges. This, and the temperature dependence of i1, are consistent with the proximity-effect model proposed by Schwidtal to describe Nb-NbOx-Pb junctions. The temperature and frequency dependences of the penetration depth show fair agreement with the BCS theory in the local "dirty" limit. Some results are also given on the variation of junction capacitance with Josephson current density.