Electron and hole transmission through superconductor — Normal metal interfaces

Abstract

We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (T c = 1.2 K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1 K. We analysed the point contacts with the modified BTK theory that includes Dynes' lifetime as a fitting parameter Γ in addition to superconducting energy gap 2∆ and normal reflection described by Z. For contact areas from 1 nm 2 to 10000 nm 2 the BTK Z parameter was 0.5, corresponding to transmission coefficients of about 80%, independent of the normal metal. The very small variation of Z indicates that the interfaces have a negligible dielectric tunneling barrier. Fermi surface mismatch does not account for the observed transmission coefficient.

Andreev-reflection spectroscopy with superconducting indium—A case study

“…The BTK parameters of our contacts with superconducting In correspond well with those of superconducting Al [39]. Unlike 2∆ 0 (R) of Al, that increases with R, the In contacts have a rather constant 2∆ 0 ≈ 1.2 meV.…”

“…For contacts with the other normal-conducting metals Γ ranges from 10 µeV to 100 µeV. However, most notable is Z ≈ 0.5 over up to five orders of magnitude in normal state resistance R like for superconducting Al [39]. Point contacts with superconducting Nb, measured at T = 4.2 K, have slightly larger average Z but with a wider variation [42].…”

“…Our contacts were fabricated using the shear (crossed wire) method by gently touching one sample wire with the other one as described by J. I. Pankove [37] and more recently by Chubov et al [38]. Point contacts with In were more difficult to fabricate than those with aluminium (Al) [39]. Very often, when we tried to set the resistance, the contact either opened with a vacuum gap between the electrodes or closed with an extremely small resistance of order 1 mΩ which is unsuitable for spectroscopy.…”

“…However, we can not notice any change of contact properties that would indicate a transition between the two transport regimes. Therefore, in our earlier paper on contacts with superconducting Al [39] we have implicitly assumed that large resistance contacts would be ballistic, and, without noticing a changing behaviour, that they would stay ballistic when the contact resistance is reduced. Maybe the converse argument is more appropriate.…”

Mechanisms of normal reflection at metal interfaces studied by Andreev-reflection spectroscopy

“…During the last couple of years we have found similar Z % 0:5 values for many S-N combinations over a wide range of contact resistances or lateral contact sizes that can not be explained by a dielectric barrier. [8][9][10] Since point contacts between conventional metals and heavy-fermion compounds with up to two orders of magnitude smaller v F also have Z % 0:4 À 0:5, 7 Fermi-velocity mismatch does not seem to be a valid approach. That is why our initial interpretation of point-contact experiments with superconducting niobium and conventional metals, 8 which we will revise here, was based on the mismatch of Fermi momentum.…”