Non-Helical Spin Textures and Proximity-Induced Superconductivity in Topological Insulators

We study how interface potentials present at the boundary between a topological insulator (TI) and a superconductor (SC) affect the proximity-induced superconductivity of the TI surface state. We construct an effective Hamiltonian for the TI-SC interface states and analyze how the spin-momentum locking of these states, which reflects the reduced symmetries of the interface, influences the induced topological superconductivity. We consider TIs coupled to both spin-singlet and spin-triplet SC's, and show that the type of superconducting order induced in the TI's surface state sensitively depends on the symmetries which are broken at the TI-SC boundary. We identify the conditions necessary for the proximity coupling to different parent SCs. To consider the experimental consequences of such interface potentials we calculate the dispersion and the density of states for the Andreev edge states, the spin currents carried by these states, and the tunneling conductance spectra for parent SCs with several qualitatively different order parameters.