Heterostructures of Topological Insulators and Superconductors

Topological insulators (TI), such as Bi2Se 3, are a new class of quantum materials discovered recently. They are insulating in the bulk but can conduct on the surfaces. The robust surface states of three-dimensional strong TIs form a unique two-dimensional system of massless electrons, known as a helical metal, with a linear energy-momentum dispersion and spin-momentum locking. While these surface modes alone have spurred great interest, their interaction with superconductors (S) in close proximity has opened up opportunities to engineer topological superconductivity using TI-S heterostructures. This thesis is a microscopic, self-consistent theoretical investigation of the interplay between TI and superconductors. Three types of TI-based heterostructures with increasing complexity are studied in detail.;We first present a detailed study of the coupling between a metal and a topological insulator. We compute the spin-active scattering matrix for electrons coming from the metal incident on the metal-TI interface. We find that there exists a critical incident angle, where perfect spin-flip occurs as the incoming electron is reflected. We discuss the origin of this phenomena and its potential implications in spintronics. We then compute the local spectrum at the metal-TI interface, and examine its evolution from the tunneling limit (bad contact) to the strong coupling limit (good contact). The calculations are done using two complementary approaches; in a continuum model based on a k·p Hamiltonian a wave function matching approach is taken and the lattice model requires the use of lattice Green's functions. The study of metal-TI interface lays the foundation for our subsequent theory of S-TI interface.;Next we carry out microscopic, self-consistent calculations of the superconducting order parameter and pairing correlations near a S-TI interface, where S is an s-wave superconductor. We discuss the suppression of the order parameter by the topological insulator and show that triplet pairing correlations are induced by spin-flip scattering at the interface. We verify that the interface spectrum at sub-gap energies is well described by the Fu-Kane model even for strongly coupled S and TI. These sub-gap modes are interface states with spectral weight penetrating well into the superconductor. We extract the phenomenological parameters of the phenomenological Fu-Kane model from our microscopic calculations, and find they are strongly renormalized from the bulk material parameters.;Building upon such understanding of single TI-S interface, we move on to examine a TI surface in contact with two superconductors with a phase bias, namely a Josephson junction patterned on the TI surface and mediated by the helical metal. A short Josephson junction of this kind at a phase bias of pi is known to give rise to exotic quasiparticle excitations known as Majorana fermions with a linear dispersion, E ∼ k. Our self-consistent calculation of the Andreev bound states spectrum reveals, for the first time, a new regime with very different physics in these devices. We show that the subgap spectrum becomes nearly flat at zero energy when the chemical potential is sufficiently away from the Dirac point. The flat dispersion is well approximated by E ∼ k N, where N scales with the chemical potential. We find a similar linear-to-flat dispersion evolution also occurs for the subgap spectrum of a periodic superconducting proximity structure, such as a TI surface in contact with a striped superconductor.;The systematic microscopic study of TI-S heterostructures helps interpret the data from ongoing experiments on these structures. The formalism developed also forms the basis for subsequent investigation of more complicated layered materials such as the periodic array of magnetically doped TI and S which is argued to give rise to an exotic topological superconductor known as Weyl superconductor.