  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.

Superconductivity;Mesoscale and Nanoscale Physics