Robust Level Coincidences in the Subband Structure of Quasi 2D Systems

TL;DR

This paper investigates the complex structure of level coincidences in the energy bands of quasi-2D systems, revealing robust crossings that signal topological phase transitions and the existence of gapless phases.

Contribution

It uncovers the rich parameter space of level coincidences and identifies robust crossings that are protected by symmetry, advancing understanding of topological phase transitions in quantum wells.

Findings

Robust level coincidences cannot be removed by small symmetry-preserving perturbations.

Different topological phases are separated by a gapless metallic phase.

The study uses realistic models of HgTe/CdTe quantum wells.

Abstract

Recently, level crossings in the energy bands of crystals have been identified as a key signature for topological phase transitions. Using realistic models we show that the parameter space controlling the occurrence of level coincidences in energy bands has a much richer structure than anticipated previously. In particular, we identify robust level coincidences that cannot be removed by a small perturbation of the Hamiltonian compatible with the crystal symmetry. Different topological phases that are insulating in the bulk are then separated by a gapless (metallic) phase. We consider HgTe/CdTe quantum wells as a specific example.