27 research outputs found
Highly skewed current-phase relation in superconductor-topological insulator-superconductor Josephson junctions
Three-dimensional topological insulators (TI's) in proximity with
superconductors are expected to exhibit exotic phenomena such as topological
superconductivity (TSC) and Majorana bound states (MBS), which may have
applications in topological quantum computation. In
superconductor-TI-superconductor Josephson junctions, the supercurrent versus
the phase difference between the superconductors, referred to as the
current-phase relation (CPR), reveals important information including the
nature of the superconducting transport. Here, we study the induced
superconductivity in gate-tunable Josephson junctions (JJs) made from
topological insulator BiSbTeSe2 with superconducting Nb electrodes. We observe
highly skewed (non-sinusoidal) CPR in these junctions. The critical current, or
the magnitude of the CPR, increases with decreasing temperature down to the
lowest accessible temperature (T ~ 20 mK), revealing the existence of
low-energy modes in our junctions. The gate dependence shows that close to the
Dirac point the CPR becomes less skewed, indicating the transport is more
diffusive, most likely due to the presence of electron/hole puddles and charge
inhomogeneity. Our experiments provide strong evidence that superconductivity
is induced in the highly ballistic topological surface states (TSS) in our
gate-tunable TI- based JJs. Furthermore, the measured CPR is in good agreement
with the prediction of a model which calculates the phase dependent eigenstate
energies in our system, considering the finite width of the electrodes as well
as the TSS wave functions extending over the entire circumference of the TI
Effect of strain on stripe phases in the Quantum Hall regime
Spontaneous breaking of rotational symmetry and preferential orientation of
stripe phases in the quantum Hall regime has attracted considerable
experimental and theoretical effort over the last decade. We demonstrate
experimentally and theoretically that the direction of high and low resistance
of the two-dimensional (2D) hole gas in the quantum Hall regime can be
controlled by an external strain. Depending on the sign of the in-plane shear
strain, the Hartree-Fock energy of holes or electrons is minimized when the
charge density wave (CDW) is oriented along [110] or [1-10] directions. We
suggest that shear strains due to internal electric fields in the growth
direction are responsible for the observed orientation of CDW in pristine
electron and hole samples.Comment: 10 pages, 3 figure
Influence of disorder on antidot vortex Majorana states in 3D topological insulators
Topological insulator/superconductor two-dimensional heterostructures are
promising candidates for realizing topological superconductivity and Majorana
modes. In these systems, a vortex pinned by a pre-fabricated antidot in the
superconductor can host Majorana zero-energy modes (MZMs), which are exotic
quasiparticles that may enable quantum information processing. However, a major
challenge is to design devices that can manipulate the information encoded in
these MZMs. One of the key factors is to create small and clean antidots, so
that the MZMs, localized in the vortex core, have a large gap to other
excitations. If the antidot is too large or too disordered, the level spacing
for the subgap vortex states may become smaller than temperature. In this
paper, we numerically investigate the effects of disorder, chemical potential,
and antidot size on the subgap vortex spectrum, using a two-dimensional
effective model of the topological insulator surface. Our model allows us to
simulate large system sizes with vortices up to 1.8 m in diameter. We also
compare our disorder model with the transport data from existing experiments.
We find that the spectral gap can exhibit a non-monotonic behavior as a
function of disorder strength, and that it can be tuned by applying a gate
voltage.Comment: 10 pages, 6 figure