MEASUREMENTS OF THE DYNAMIC CONDUCTANCE ON SS-POINT AND NS-POINT CONTACTS

Mechanically point contacts made of Ta, Al, Ag (anvil) and of Ta, Ag ( needle) were investigated in the temperature range between 4.2 K and 1 .3 K and in magnetic fields up to 0.8 T. For point contacts consisting of two superconducting electrodes we measured the current voltage cha racteristics and also the dynamic conductance. The characteristics wit h a negative differential conductance and the occurrence of an excess current point to the appearance of Andreev-multiple reflection at an S NS-interface in the contact region. The investigated current voltage c haracteristics and the dynamic conductance of typical NS-point contact s (Ta - Ag and Ta - Al) point to a microconstriction with a barrier of arbitrary strength at the interface between the normalconductor and t he superconductor. We performed theoretical calculations for T = 0 and also for temperatures up to the critical temperature using the model of Blonder, Tinkham and Klapwijk (BTK model). In this model the Andree v-reflection is the main scattering mechanism at the NS-interface but also elastic scattering and transmission processes are taken into cons ideration. The comparison of our theoretical results with the experime ntal results and also the determined excess current at all investigate d point contacts lead to the assumption that there exist metallic cont acts with very small barrier strengths at the NS-interface where the A ndreev-reflection is the dominant process and the other scattering mec hanisms play a subdominant role.