Reentrant topological phase transitions in a disordered spinless superconducting wire

In a one-dimensional spinless p-wave superconductor with coherence length \xi, disorder induces a phase transition between a topologically nontrivial phase and a trivial insulating phase at the critical mean free path l=\xi/2. Here, we show that a multichannel spinless p-wave superconductor goes through an alternation of topologically trivial and nontrivial phases upon increasing the disorder strength, the number of phase transitions being equal to the channel number N. The last phase transition, from a nontrivial phase into the trivial phase, takes place at a mean free path l = \xi/(N+1), parametrically smaller than the critical mean free path in one dimension. Our result is valid in the limit that the wire width W is much smaller than the superconducting coherence length \xi

Supersymmetry in the Majorana Cooper-Pair Box

Over the years, supersymmetric quantum mechanics has evolved from a toy model of high energy physics to a field of its own. Although various examples of supersymmetric quantum mechanics have been found, systems that have a natural realization are scarce. Here, we show that the extension of the conventional Cooper-pair box by a 4pi-periodic Majorana-Josephson coupling realizes supersymmetry for certain values of the ratio between the conventional Josephson and the Majorana- Josephson coupling strength. The supersymmetry we find is a "hidden" minimally bosonized supersymmetry that provides a non-trivial generalization of the supersymmetry of the free particle and relies crucially on the presence of an anomalous Josephson junction in the system. We show that the resulting degeneracy of the energy levels can be probed directly in a tunneling experiment and discuss the various transport signatures. An observation of the predicted level degeneracy would provide clear evidence for the presence of the anomalous Josephson coupling.Comment: 10 pages, 5 figure

Measurement of spin-dependent conductivities in a two-dimensional electron gas

Spin accumulation is generated by injecting an unpolarized charge current into a channel of GaAs two-dimensional electron gas subject to an in-plane magnetic field, then measured in a non-local geometry. Unlike previous measurements that have used spin-polarized nanostructures, here the spin accumulation arises simply from the difference in bulk conductivities for spin-up and spin-down carriers. Comparison to a diffusive model that includes spin subband splitting in magnetic field suggests a significantly enhanced electron spin susceptibility in the 2D electron gas

Chirality blockade of Andreev reflection in a magnetic Weyl semimetal

A Weyl semimetal with broken time-reversal symmetry has a minimum of two species of Weyl fermions, distinguished by their opposite chirality, in a pair of Weyl cones at opposite momenta $\pm K$ that are displaced in the direction of the magnetization. Andreev reflection at the interface between a Weyl semimetal in the normal state (N) and a superconductor (S) that pairs $\pm K$ must involve a switch of chirality, otherwise it is blocked. We show that this "chirality blockade" suppresses the superconducting proximity effect when the magnetization lies in the plane of the NS interface. A Zeeman field at the interface can provide the necessary chirality switch and activate Andreev reflection.Comment: 15 pages, 9 figures. V2: added investigation of the dependence of the chirality blockade on the direction of the magnetization and (Appendix C) calculations of the Fermi-arc mediated Josephson effec

Universal spatial correlations in random spinor fields

We identify universal spatial fluctuations in systems with non trivial spin dynamics. To this end we calculate by exact numerical diagonalization a variety of experimentally relevant correlations between spinor amplitudes, spin polarizations and spin currents both in the bulk and near the boundary of a confined two-dimensional clean electron gas in the presence of spin-orbit interaction and a single magnetic impurity. We support or claim of universality with the excellent agreement between the numerical results and system-independent spatial correlations of a random field defined on both the spatial and spin degrees of freedom. A rigorous identity relating our universal predictions with response functions provides a direct physical interpretation of our results in the framework of linear response theory.Comment: 4 pages, 4 figure

Current-induced nuclear spin polarization in (Bi 1-x Sb x)2 Te 3

Three-dimensional topological insulators (3DTIs) host conducting surface states, while the bulk remains insulating. These surface states are spin-momentum locked, which opens the road to spintronic applications. We investigate the hyperfine interaction between spin-momentum locked electrons and nuclear spins. A DC source-drain bias implies a nonzero electron spin polarization, which is transfered to a nonzero nuclear spin polarization via spin-flip interactions. Inversely, thermal relaxation of a nonzero nuclear spin polarization drives a charge current. This shows as an inductive current, observable at timescales comparable to the nuclear spin-flip rate. At DC timescales, the nonzero nuclear polarization is expected to affect resistance as a function of applied bias. Moreover, the additional in-plane magnetic field stemming from nuclear polarization is expected to affect phase coherence lengths. We search for signatures of current-induced nuclear polarization in the 3DTI (Bi 1-x Sb x)2 Te 3 (BST). The high nuclear spin abundancy makes this an excellent candidate material. BST thin-films are deposited by molecular beam epitaxy, which allows for tuning the position of Fermi level and Dirac point within the bulk band gap. The experiments focus on probing effects of non-zero nuclear polarization using DC signals. However, careful consideration is required to distinguish signatures of other effects, such as electron-electron interactions and Joule heating

Resonant states and order-parameter suppression near pointlike impurities in d-wave superconductors

We examine the role of order-parameter suppression in the development of low-energy peaks ͑i.e., resonances͒ in the tunneling density of states near a nonmagnetic impurity in a d-wave superconductor. Without order-parameter suppression, the zero-energy resonance appears only in the unitary ͑i.e., strong impurity͒ limit. However, suppression makes the resonance appear even when the impurity is much weaker. To model this situation, we make the physical hypothesis that the order parameter is reduced whenever one electron of a Cooper pair encounters the impurity, a hypothesis that retains the exact solvability of the problem. In this way, we determine that suppression of the order parameter drives the effective strength of the impurity towards the unitary limit. We determine the order-parameter reduction variationally, and show that the ratios between the main energy scales-the bandwidth and superconducting gap-strongly affect this reduction and, in consequence, the position and width of the resonance

Multiple Andreev reflections in two-dimensional Josephson junctions with broken time-reversal symmetry

Andreev bound states (ABS) occur in Josephson junctions when the total phase of the Andreev and normal reflections is a multiple of $2\pi$. In ballistic junctions with an applied voltage bias, a quasi-particle undergoes multiple Andreev reflections before entering the leads, resulting in peaks in the current-voltage $I(V)$ curve. Here we present a general model for Josephson junctions with spin-active interlayers i.e., magnetic or topological materials with broken time-reversal symmetry. We investigate how ABS change the peak positions and shape of $I(V)$, which becomes asymmetric for a single incident angle. We show how the angle-resolved $I(V)$ curve becomes a spectroscopic tool for the chirality and degeneracy of ABS.Comment: 5 pages, 3 figure

Breakdown of universality in three-dimensional Dirac semimetals with random impurities

Dirac-Weyl semimetals are unique three-dimensional (3D) phases of matter with gapless electrons and novel electrodynamic properties believed to be robust against weak perturbations. Here, we unveil the crucial influence of the disorder statistics and impurity diversity in the stability of incompressible electrons in 3D semimetals. Focusing on the critical role played by rare impurity configurations, we show that the abundance of low-energy resonances in the presence of diluted random potential wells endows rare localized zero-energy modes with statistical significance, thus lifting the nodal density of states. The strong nonperturbative effect here reported converts the 3D Dirac-Weyl semimetal into a compressible metal even at the lowest impurity densities. Our analytical results are validated by high-resolution real-space simulations in record-large 3D lattices with up to 536 000 000 orbitals

Ehrenfest-time-dependent excitation gap in a chaotic Andreev billiard

A semiclassical theory is developed for the appearance of an excitation gap in a ballistic chaotic cavity connected by a point contact to a superconductor. Diffraction at the point contact is a singular perturbation in the limit $\\hbar\\to 0$, which opens up a gap $E_{\\rm gap}$ in the excitation spectrum. The time scale $\\hbar/E_{\\rm gap}\\propto\\alpha^{-1}\\ln\\hbar$ (with $\\alpha$ the Lyapunov exponent) is the Ehrenfest time, the characteristic time scale of quantum chaos