Ballistic quantum spin Hall state and enhanced edge backscattering in strong magnetic fields

TL;DR

This paper demonstrates that the quantum spin Hall state persists under strong magnetic fields, revealing a transition to quantum Hall states and showing how edge backscattering affects conductance, providing a practical measurement approach.

Contribution

It shows the persistence of the QSH state in strong magnetic fields and introduces a method to probe edge backscattering via magnetoresistance measurements.

Findings

QSH state persists in strong magnetic fields

Transition from QSH to QH regimes near Landau level crossing

Edge backscattering causes power-law decay in conductance with magnetic field

Abstract

The quantum spin Hall (QSH) state, observed in a zero magnetic field in HgTe quantum wells, respects the time-reversal symmetry and is distinct from quantum Hall (QH) states. We show that the QSH state persists in strong quantizing fields and is identified by counter-propagating (helical) edge channels with nonlinear dispersion inside the band gap. If the Fermi level is shifted into the Landau-quantized conduction or valence band, we find a transition between the QSH and QH regimes. Near the transition the longitudinal conductance of the helical channels is strongly suppressed due to the combined effect of the spectrum nonlinearity and enhanced backscattering. It shows a power-law decay 1/B^2N with magnetic field B, determined by the number of backscatterers on the edge, N. This suggests a rather simple and practical way to probe the quality of recently realized quasiballistic QSH devices using magnetoresistance measurements.