Topological states in multi-orbital HgTe honeycomb lattices.

W Beugeling,E Kalesaki,Y.-M Niquet,C Morais Smith,C Delerue,D Vanmaekelbergh

doi:10.1038/ncomms7316

Abstract

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin–orbit coupling. Here we show theoretically that 2D honeycomb lattices of HgTe can combine the effects of the honeycomb geometry and strong spin–orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin–orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase.

Highlights

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice
The absence of a bandgap in its spectrum prevents its use as a field-effect transistor, and its weak spin–orbit coupling (SOC) hampers the possibility to realize the quantum spin Hall effect (QSHE)[7] and use it for quantum spintronics
Theoretical investigations have shown that CdSe superlattices formed in such a way exhibit Dirac cones at two energies and nearly dispersionless bands. These flat bands are connected to the nearby higher energy bands, and the SOC gaps in the conduction band are very small[16]

Summary

Introduction

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin–orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. We consider three different types of HgTe layers with superimposed honeycomb geometry and present atomistic tight-binding (TB) calculations of their conduction band structure that we accurately describe by a 16-band effective model Such lattices take advantage of the multi-orbital degrees of freedom in the honeycomb setup, allied to the strong SOC17. We conclude that, depending on the position of the Fermi level, QSHE could be observed in these structures, and fractional QSHE or fractional Chern insulator phases, as on-site and NN Coulomb-interaction parameters are found in the energy range required for their realization

Methods

Results

Conclusion