Asymmetric versus symmetric $\rm{HgTe/Cd_{x}Hg_{1-x}Te}$ double quantum wells: Band gap tuning without electric field

TL;DR

This study explores how asymmetry in HgTe/Cd_xHg_{1-x}Te double quantum wells enables band gap tuning without external electric fields, revealing potential for topological phase transitions.

Contribution

It demonstrates that asymmetry-induced potential differences can control band gaps and hybridization in quantum wells, a novel approach compared to electric field tuning.

Findings

Asymmetry opens a band gap absent in symmetric wells.

The band gap and hybridization are tunable via well widths and composition.

Inverted band ordering suggests possible topological phases.

Abstract

We investigate the electron states in double asymmetric $\rm{HgTe/Cd_{x}Hg_{1-x}Te}$ quantum wells grown along the $[001]$ direction. The subbands are computed by means of the envelope function approximation applied to the 8-band Kane $\bf{k}\cdot\bf{p}$ model. The asymmetry of the confining potential of the double quantum wells results in a gap opening which is absent in the symmetric system where it can only be induced by an applied electric field. The band gap and the subbands are affected by spin-orbit coupling which is a consequence of the asymmetry of the confining potential. The electron-like and hole-like states are mainly confined in different quantum wells, and the enhanced hybridization between them opens a spin-dependent hybridization gap at a finite in-plane wavevector. We show that both the ratio of the widths of the two quantum wells and the mole fraction of the $\rm{Cd_{x}Hg_{1-x}Te}$ barrier control both the energy gap between the hole-like states and the hybridization gap. The energy subbands are shown to exhibit inverted ordering, and therefore a nontrivial topological phase could emerge in the system.