Crystal Field Effect Induced Topological Crystalline Insulators In Monolayer IV-VI Semiconductors

TL;DR

This paper predicts that monolayer IV-VI semiconductors can be 2D topological crystalline insulators with large band gaps, due to a crystal field effect causing band inversion, offering new avenues for 2D topological material design.

Contribution

It reveals a novel topological phase in monolayer IV-VI semiconductors driven by crystal field effects, expanding the understanding of 2D topological insulators.

Findings

Monolayer PbTe has a band gap up to 260 meV.

The topological phase arises from crystal field-induced band inversion.

These materials can host tunable Dirac fermion systems.

Abstract

Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X=Ge, Sn, Pb and Y= S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in monolayer PbTe, providing a materials platform for realizing two-dimensional Dirac fermion systems with tunable band gap. This unexpected nontrivial topological phase stems from the strong {\it crystal field effect} in the monolayer, which lifts the degeneracy between $p_{x,y}$ and $p_z$ orbitals and leads to band inversion between cation $p_z$ and anion $p_{x,y}$ orbitals. This crystal field effect induced topological phase offers a new strategy to find and design other atomically thin 2D topological materials.