Effect of a spin-active interface on proximity-induced superconductivity in topological insulators

We examine the effect of a spin-active interface on the symmetry of proximity-induced superconducting pairing amplitudes in topological insulators. We develop a model to investigate the leading order contribution to the pairing amplitude considering 3 different kinds of spin-active interfaces: 1) those that induce spin-dependent scattering phases, 2) those that flip the spin of incident electrons, and 3) interfaces that both induce spin-dependent phases and flip the spins of incident electrons. We find that in cases (1) and (3) odd-frequency triplet pairing is induced in the TI while for case (2) no odd-frequency pairing is induced to leading order. We compare our results to those for normal metals and ferromagnetic materials finding that the nontrivial spin structure of the TI leads to qualitatively different behavior.