Electrostatic effects and band-bending in doped topological insulators

We investigate the electrostatic effects in doped topological insulators by developing a self consistent scheme for an interacting tight binding model. The presence of bulk carriers, in addition to surface electrons, generates an intrinsic inhomogeneous charge density in the vicinity of the surface and, as a result, band bending effects are present. We find that electron doping and hole doping produce band bending effects of similar magnitude and opposite signs. The presence of additional surface dopants breaks this approximate electron-hole symmetry and dramatically affects the magnitude of the band bending. Applying a gate potential can generate a depletion zone characterized by a vanishing carrier density. We find that the density profile in the transition zone between the depleted region and the bulk is independent of the applied potential. In thin films the electrostatic effects are strongly dependent on the carrier charge density. In addition, we find that substrate induced potentials can generate a Rashba type spin-orbit coupling in ultra thin topological insulator films. We calculate the profiles of bulk and surface states in topological insulator films and identify the conditions corresponding to both types of states being localized within the same region in space.Comment: 9 pages, 10 figure

Non-locality of zero-bias anomalies in the topologically-trivial phase of Majorana wires

We show that the topologically trivial zero bias peak (ZBP) emerging in semiconductor Majorana wires due to soft confinement exhibits correlated splitting oscillations as a function of the applied Zeeman field, similar to the correlated splitting of the Majorana ZBP. Also, we find that the presence of a strong impurity can effectively cut the wire in two and destroy the correlated splitting in both the trivial and the Majorana regimes. We identify a strong nonlocal effect that operates only in the topologically trivial regime and demonstrate that the dependence of the ZBP on the confining gate potential at the opposite end in Majorana wires with two normal metal end-contacts represents a powerful tool for discriminating between topologically trivial and nontrivial ZBPs.Comment: published version, 4+ pages, 4 figure

Tunneling conductance in semiconductor-superconductor hybrid structures

We study the differential conductance for charge tunneling into a semiconductor wire--superconductor hybrid structure, which is actively investigated as a possible scheme for realizing topological superconductivity and Majorana zero modes. The calculations are done based on a tight-binding model of the heterostructure using both a Blonder-Tinkham-Klapwijk approach and a Keldysh non-equilibrium Green's function method. The dependence of various tunneling conductance features on the coupling strength between the semiconductor and the superconductor, the tunnel barrier height, and temperature is systematically investigated. We find that treating the parent superconductor as an active component of the system, rather than a passive source of Cooper pairs, has qualitative consequences regarding the low-energy behavior of the differential conductance. In particular, the presence of sub-gap states in the parent superconductor, due to disorder and finite magnetic fields, leads to characteristic particle-hole asymmetric features and to the breakdown of the quantization of the zero-bias peak associated with the presence of Majorana zero modes localized at the ends of the wire. The implications of these findings for the effort toward the realization of Majorana bound states with true non-Abelian properties are discussed.Comment: published version, 15+ pages, 12 figure

Robustness of Topological Superconductivity in Proximity-Coupled Topological Insulator Nanoribbons

We study the low-energy physics of topological insulator (TI) nanoribbons proximity-coupled to s-wave superconductors (SCs) by explicitly incorporating the proximity effects that emerge at the TI-SC interface. We construct a low-energy effective theory that incorporates the proximity effect through an interface contribution containing both normal and anomalous terms and an energy-renormalization matrix. We show that the strength of the proximity-induced gap is determined by the transparency of the interface and the amplitude of the low-energy TI states at the interface. Consequently, the induced gap is strongly band-dependent and collapses for bands containing states with low amplitude at the interface. We find that states with energies within the bulk TI gap have surface-type character and, in the presence of proximity-induced or applied bias potentials, have most of their weight near either the top or the bottom surface of the nanoribbon. As a result, single interface TI-SC structures are susceptible to experiencing a collapse of the induced gap whenever the chemical potential is far enough from the value corresponding to the bulk TI Dirac point and crosses weakly coupled bands. We also find that changing the chemical potential in single-interface structures using gate potentials may be ineffective, as it does not result in a significant increase of the induced gap. On the other hand, we find that symmetric structures, such as a TI nanowire sandwiched between two superconductors, are capable of realizing the full potential of TI-based structures to harbor robust topological superconducting phases.Comment: published version, 15+ pages, 16 figure

Induced spin texture in semiconductor/topological insulator heterostructures

We show that a semiconductor thin film can acquire a non-trivial spin texture due to the proximity effect induced by a topological insulator. The effect stems from coupling to the topological surface states and is present even when the insulator is doped. We propose a semiconductor/topological insulator heterostructure as a device that allows measuring interface properties and probing surface states in uncompensated samples. We also find that the topological insulator surface modes can be significantly broadened and shifted by the presence of metallic contacts.Comment: 6 pages, 2 figures, published versio