Local enhancement of the upper critical field in niobium point contacts

Abstract

We have measured the values of the upper critical fieldHc2 in point contacts between a niobium tip and a copper film, and alsobetween a copper tip and a niobium film. In both experiments, theHc2 of a point contact (up to 2 T at 4.2 K) is significantly enhanced over the independently measuredbulk critical fields of the Nb tip material and the Nb film. Due to the local enhancement ofHc2 in the niobium point contacts, it is possible to obtain robust and well resolved point-contactspectra in fields in excess of 1 T, which is of high significance for studying the spin polarizationin ferromagnetic materials and magnetic semiconductors. We ascribe the increase ofHc2 to strong plastic deformations in Nb at the point contact.