INFLUENCE OF MAGNETIC-FIELD AND PINCUS STATES IN NORMAL-METAL-SUPERCONDUCTOR TUNNEL-JUNCTIONS

Normal-metal-superconductor tunneling, when applied to metallic superc onductors with well-known bulk properties, yields useful information o n barrier properties. In particular, it is quite common to observe dev iations from the predictions of a homogeneous barrier model, Nb-oxide barriers being the most-investigated example. In discussing these shor tcomings, high local nearly metallic transparency instead of a tunneli ng probability of order 10(-8) is an often applied concept. We here pr esent experimental results on Ag/Nb and Ag/Ta tunneling junctions whic h, in addition to a commonly observed finite zero-bias conductance, sh ow a pronounced, characteristic magnetic field dependence. A theory is developed on the basis of the Bogoliubov-de Gennes equations which ta kes into account the influence of the shielding current as well as tun neling of quasiparticles with nonperpendicular incidence, typical for inhomogeneous tunneling barriers with high local transparency. The fie ld effect due to quasiparticles with non-normal incidence is nearly an tisymmetric with respect to the gap voltage. Below Delta/e the effect is positive on account of resonant transmission to the Pincus-type bou nd states appearing for quasiparticle momenta antiparallel to the shie lding current. Above Delta/e, a compensating negative contribution is obtained. The results are quantitatively discussed with respect to the ir dependence on barrier transparency and angular spreading of quasipa rticle injection. We find good agreement with the experiments for a br oad angular distribution of injected quasiparticles, typical for inhom ogeneous barriers with high local transparency. Furthermore difference s by an order of magnitude between Nb- and Ta-based junctions can be f ully accounted for. On the other hand, strong confinement of quasipart icle injection in forward direction which is typical for an ideal, hom ogeneous barrier, yields marked deviations from experiment. Thus, we c an conclude a broad angular distribution of quasiparticle transmission , characteristic for high local transparency.