HgTe superlattices for 3D flat bands

In this study, we explored how to engineer topological phases in superlattices of HgTe-type materials. In particular, we found nodal-line semimetals, which are a promising platform for the study of 3D flat-band physics.

Quantum wells in thin heterostructures of HgTe and CdTe are examples of quantum-spin Hall insulators - a topological phase with unusual electron transport and other exotic phenomena. Thick samples of HgTe can realize a related topological insulator phase. To make a material topological, we need to somehow twist its electronic bands, which is called a band inversion. Some materials, like HgTe and HgSe, already feature such a band inversion. Other materials, like CdTe, are not topological by themselves. By combining different, topological and non-topological, materials in a thick heterostructure, we can induce and play around with band inversions to engineer new types of topological phases.

In our work, by means of computational ab-initio calculations, we looked into heterostructures of alternating layers of HgTe and HgSe, as well as combinations of HgTe and CdTe. We found that the considered HgTe/HgSe superlattices realize an ideal Weyl semimetal, which is a semimetallic topological phase. Here, the electronic bands cross only in single points. This is in contrast to insulating topological phases where the electronic bands don’t cross at all.

The HgTe/CdTe superlattice even realizes a topological nodal-line semimetal - a topological phase where the crossings of the electronic bands form loops. Previously, in a related study, we showed that such materials can be used to generate electronic bands that are flat in all directions. This gives rise to tunable correlated phases turning the material into, for instance, a magnet or even a superconductor.

Therefore, the HgTe/CdTe superlattices considered here are a promising new material platform for the study of 3D flat-band physics.

R. Islam, B. Ghosh, G. Cuono, A. Lau, W. Brzezicki, A. Bansil, A. Agarwal, B. Singh, T. Dietl, and C. Autieri,
Topological states in superlattices of HgTe class of materials for engineering three-dimensional flat bands,
Physical Review Research 4, 023114 (2022), arXiv:2112.15548