Interplay of exciton condensation and quantum spin Hall effect in InAs/GaSb bilayers

We study the phase diagram of the inverted InAs/GaSb bilayer quantum wells. For small tunneling amplitude between the layers, we find that the system is prone to formation of an s-wave exciton condensate phase, where the spin-structure of the order parameter is uniquely determined by the small spin-orbit coupling arising from the bulk inversion asymmetry. The phase is topologically trivial and does not support edge transport. On the contrary, for large tunneling amplitude, we obtain a topologically non-trivial quantum spin Hall insulator phase with a p-wave exciton order parameter, which enhances the hybridization gap. These topologically distinct insulators are separated by an insulating phase with spontaneously broken time-reversal symmetry. Close to the phase transition between the quantum spin Hall and time-reversal broken phases, the edge transport shows quantized conductance in small samples, whereas in long samples the mean free path associated with the backscattering at the edge is temperature independent, in agreement with recent experiments.Comment: v. 2, 9 pages, 5 figure

Minimal circuit for a flux-controlled Majorana qubit in a quantum spin-Hall insulator

We construct a minimal circuit, based on the top-transmon design, to rotate a qubit formed out of four Majorana zero-modes at the edge of a two-dimensional topological insulator. Unlike braiding operations, generic rotations have no topological protection, but they do allow for a full characterization of the coherence times of the Majorana qubit. The rotation is controlled by variation of the flux through a pair of split Josephson junctions in a Cooper pair box, without any need to adjust gate voltages. The Rabi oscillations of the Majorana qubit can be monitored via oscillations in the resonance frequency of the microwave cavity that encloses the Cooper pair box.Comment: Contribution for the proceedings of the Nobel Symposium on topological insulators. 8 pages, 6 figure

Andreev-Bragg reflection from an Amperian superconductor

We show how an electrical measurement can detect the pairing of electrons on the same side of the Fermi surface (Amperian pairing), recently proposed by Patrick Lee for the pseudogap phase of high-$T_c$ cuprate superconductors. Bragg scattering from the pair-density wave introduces odd multiples of $2k_{\rm F}$ momentum shifts when an electron incident from a normal metal is Andreev-reflected as a hole. These Andreev-Bragg reflections can be detected in a three-terminal device, containing a ballistic Y-junction between normal leads $(1,2)$ and the superconductor. The cross-conductance $dI_1/dV_2$ has the opposite sign for Amperian pairing than it has either in the normal state or for the usual BCS pairing.Comment: 5 pages, 6 figures; v2 includes study of disorder and interface barrie

Effects of disorder on Coulomb-assisted braiding of Majorana zero modes

Majorana zero modes in one-dimensional topological superconductors obey non-Abelian braiding statistics. Braiding manipulations can be realized by controlling Coulomb couplings in hybrid Majorana-transmon devices. However, strong disorder may induce accidental Majorana modes, which are expected to have detrimental effects on braiding statistics. Nevertheless, we show that the Coulomb-assisted braiding protocol is efficiently realized also in the presence of accidental modes. The errors occurring during the braiding cycle are small if the couplings of the computational Majorana modes to the accidental ones are much weaker than the maximum Coulomb coupling.Comment: 7 pages, 4 figures, this is the final, published versio

Physical Principles of the Amplification of Electromagnetic Radiation Due to Negative Electron Masses in a Semiconductor Superlattice

In a superlattice placed in crossed electric and magnetic fields, under certain conditions, the inversion of electron population can appear at which the average energy of electrons is above the middle of the miniband and the effective mass of the electron is negative. This is the implementation of the negative effective mass amplifier and generator (NEMAG) in the superlattice. It can result in the amplification and generation of terahertz radiation even in the absence of negative differential conductivity.Comment: 5 pages, 3 figure

Suppressed absolute negative conductance and generation of high-frequency radiation in semiconductor superlattices

We show that space-charge instabilities (electric field domains) in semiconductor superlattices are the attribute of absolute negative conductance induced by small constant and large alternating electric fields. We propose the efficient method for suppression of this destructive phenomenon in order to obtain a generation at microwave and THz frequencies in devices operating at room temperature. We theoretically proved that an unbiased superlattice with a moderate doping subjected to a microwave pump field provides a strong gain at third, fifth, seventh, etc. harmonics of the pump frequency in the conditions of suppressed domains.Comment: 8 pages. Development of cond-mat/0503216 . Version 2: Final version, erratum is include