Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface

We investigate the applicability of spin-polarization measurements using Andreev reflection in a point-contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As. Although we observe conventional Andreev reflection in nonmagnetic (Ga,Be)As epilayers, our measurements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentration the conductance spectra can only be adequately described by a broadened density of states and a reduced superconducting gap. We suggest that these pair-breaking effects stem from inelastic scattering in the metallic impurity band of (Ga,Mn)As and can be explained by introducing a finite quasiparticle lifetime or a higher effective temperature. For (Ga,Mn)As with 8% Mn concentration and 140 K Curie temperature we evaluate the spin polarization to be 83±17%.