Some temperaturehyphen;dependent properties of niobium tunnel junctions

摘要

Some measurements of the electrical properties of Nb tunnel junctions in the temperature range 2ndash;9thinsp;deg;K are described. These include (i) the quasiparticle tunneling current, (ii) the gap voltage 2Dgr;/e, (iii) the dc Josephson currenti1, and (iv) the London penetration depth lgr;. 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 ofi1, are consistent with the proximityhyphen;effect model proposed by Schwidtal to describe Nbhyphen;NbOxhyphen;Pb junctions. The temperature and frequency dependences of the penetration depth show fair agreement with the BCS theory in the local rsquo;rsquo;dirtyrsquo;rsquo; limit. Some results are also given on the variation of junction capacitance with Josephson current density.