Determination of spin polarization of metals and gap of superconductors by spin-polarized Andreev reflection spectroscopy

Abstract

Andreev reflection spectroscopy (ARS) is a unique technique that can determine the spin polarization of a metal as well as the gap of a superconductor but d-wave superconductors are seldomly used in ARS due to the lack of quantitative model to extract parameters from experiments. Here we calculate spin-polarized ARS based on the modified Blonder-Tinkham-Klapwijk model for both s- and d-wave superconductors of a three-dimensional (3D) interface with a point angle. We show that the 3D interface and the point angle can drastically affect the spectra for both s- and d-wave ARS. For s-wave, the 3D effect is similar to that of an interfacial barrier but it can generate a variety of rich features in d-wave ARS and induce Andreev bound states (ABS). The spin polarization can linearly suppress both s- and d-wave spectra including ABS. We show that the ABS states can be controlled by the spin polarization, the point angle and the interfacial barrier but very high polarization is required to completely suppress the ABS. Furthermore, the Fermi wavevector mismatch as well as the carrier effective mass mismatch can cause effects very similar to that of an interfacial barrier for both s- and d-wave ARS.