Andreev reflection spectroscopy of niobium point contacts in a magnetic field

The Blonder-Tinkham-Klapwijk model of the point-contact Andreev reflection (PCAR) spectroscopy does not provide an adequate description of experiments in the presence of a magnetic field. We demonstrate this using a junction between a niobium tip and a copper film. We modify the theory to explicitly take into account the contribution to conductance that stems from the normal vortex cores in the superconductor. These results have important implications for the interpretation of transport spin polarization measurements using the PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin polarization on the strength of the interface barrier and potential misassignment of the inferred spin polarization.