Topological states in superlattices of HgTe-class materials for engineering three-dimensional flat bands

TL;DR

This study uses ab-initio calculations to explore topological phases in HgTe-based superlattices, revealing potential for three-dimensional flat bands and diverse topological states under strain and pressure.

Contribution

It uncovers how hydrostatic pressure and uniaxial strain induce various topological phases in HgTe superlattices, including flat bands, Weyl semimetals, and topological insulators.

Findings

Isoenergetic nodal lines in HgTe/CdTe superlattices can host flat bands.

HgTe/HgSe superlattices exhibit a rich phase diagram with Weyl and Dirac semimetals.

Strain tuning enables transitions between topological phases.

Abstract

In search of materials with three-dimensional flat band dispersions, using {\em ab-initio} computations, we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion.