3,423 research outputs found
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
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
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