Phase transitions in two tunnel-coupled HgTe quantum wells. Bilayer graphene analogy and beyond

TL;DR

This paper investigates quantum phase transitions in tunnel-coupled HgTe quantum wells, revealing a novel metal phase with coexisting gapless bulk and edge states, and draws analogies to bilayer graphene's quantum Hall effects and tunable band gaps.

Contribution

It uncovers a new 'bilayer graphene' phase in HgTe quantum wells with unique properties and introduces a novel quantum phase transition involving subband inversion.

Findings

Discovery of a metal phase with gapless bulk and helical edge states.

Identification of a quantum phase transition driven by subband inversion.

Demonstration of electrically tunable band gap leading to quantum spin Hall state.

Abstract

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the 'single-valley' analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this 'bilayer graphene' phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.