Signatures of interaction-induced helical gaps in nanowire quantum point contacts

Abstract

Spin-momentum locking in a semiconductor device with strong spin-orbit coupling (SOC) is thought to be an important prerequisite for the formation of Majorana bound states 1-3 . Such a helical state is predicted in one-dimensional (1D) nanowires subject to strong Rashba SOC and spin-mixing 4 -its hallmark being a characteristic re-entrant behaviour in the conductance. Here, we report direct experimental observations of the re-entrant conductance feature, which reveals the formation of a helical liquid, in the lowest 1D subband of an InAs nanowire. Surprisingly, the feature is very prominent also in the absence of magnetic fields. This behaviour suggests that exchange interactions have a substantial impact on transport in our device. We attribute the opening of the pseudogap to spin-flipping two-particle backscattering 5-7 . The all-electric origin of the ideal helical transport could have important implications for topological quantum computing.A 1D conductor with strong SOC is predicted 1,2,8 to represent a viable host for Majorana bound states. These zero-energy states feature characteristic non-Abelian exchange statistics 8 and can be created by mimicking spinless p-wave Cooper pairing using a semiconductor nanowire with a helical state and inducing s-wave superconductivity. InAs and InSb nanowires are promising host materials to explore the existence and nature of Majorana bound states 9,10 . To this end, it is essential to both establish transport in 1D subbands and induce a helical state in the nanowire. The usual mechanism that is considered to open a helical gap involves an external Zeeman field oriented perpendicular to the uniaxial spinorbit field 4 . The magnitude of the spin-orbit energy relative to the Zeeman energy is partly responsible for the size of the topological energy gap that will protect the zero-energy Majorana modes 11 . However, Oreg et al. 2,12 and Stoudenmire et al. 13 have pointed out that such an energy gap can also result from strong electronic correlations. Several mechanisms have been proposed along these lines: for example, spin-flipping two-particle backscattering 7 and hyperfine interaction between nuclear spins and a Luttinger liquid 14 , both of which can open a gap. The latter mechanism has been invoked to explain a conductance reduction by a factor of two at low temperatures in a GaAs quantum wire 15 , but no re-entrant behaviour is predicted within this framework.Other than Quay et al. 3 , we report on a re-entrant conductance feature in the lowest subbands of InAs nanowire quantum point contacts (QPCs), which offer the desired strong SOC (see Supplementary Section 1). Moreover, our proposed spin-mixing mechanism does not necessarily rely on external time-reversal symmetry-breaking terms: while the effect is pronounced in the presence of an external magnetic field, it persists also in its absence. Guided by the observation 16 of the Landé g factor enhancement for the lowest subband 17 and by signatures of the 0.7 anomaly 18 , we identify the important role of exchange int...

Missing Shapiro steps and the 8π -periodic Josephson effect in interacting helical electron systems

“…Without induced superconductivity, the system only has a partial gap, and so does not contain localized bound states. Recent data 43 shows a dip in the conductance of InAs wires with an applied magnetic field, consistent with the appearance of a helical gap. The same authors also find a similar signature without applied field, possibly arising from a partial spin-umklapp gap.…”

“…In a wire with RSOC and electron-electron interactions this gap will lead to a reduction of the normal-state conductance of from 2e 2 /h to e 2 /h as the chemical potential approaches the Dirac point, which may already have been seen in InAs nanowires. 43 We would like to point out that the required strong interactions have already been seen in these wires. 44 Umklapp scattering has also recently been invoked to explain the reduction in conductance observed in InAs/GaSb topological insulator edge states.…”

“…However, the level of disorder in current state-of-the-art nanowires and nanowire junctions is so low as to demonstrate ballistic electron transport. 43,57,58 In such clean wires, and without explicit TRS breaking, it may even be possible to see the existence of degenerate, localized zero-energy bound states. To investigate the form of these bound states, we use the unfolding transformation described in Refs.…”

Dynamic Transport in a Quantum Wire Driven by Spin–Orbit Interaction

“…The velocity of the excitations is strongly renormalized by SOI, which opens a way to fine-tune the charge and spin response of 1D electrons by changing the gate potential. One of the modes softens upon increasing the gate potential to enhance the current response as well as the power dissipated in the system.Introduction.-Today we are witnessing the burst of interest in the ballistic electron transport in quantum wires [1][2][3][4][5]. For the last three decades the quantum wires formed by electrostatic gating of a high-mobility two-dimensional (2D) electron gas have been the favorite playground to study quantum many-body effects in one-dimensional (1D) electron systems [6], where a strongly correlated state known as the Tomonaga-Luttinger liquid emerges as a result of the electron-electron (e-e) interaction [7].…”

Nonmonotonic response and light-cone freezing in fermionic systems under quantum quenches from gapless to gapped or partially gapped states

“…Namely, a spin-orbit coupled (SOC) quantum wire in the presence of an applied magnetic field [63][64][65][66] and a chain of spin-less 1D fermions. For the latter, the gapping quench mechanism is either induced by a staggered potential (SP) or by the sudden switch-on of fermion-fermion interactions 67 .…”