Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures

TL;DR

This study investigates the fragility of Dirac cone splitting in topological crystalline insulator heterostructures, revealing its extreme sensitivity to surface details and the effects of encapsulation on topological surface states.

Contribution

It provides the first detailed experimental analysis of how atomic-scale surface modifications affect Dirac cone splitting in topological heterostructures.

Findings

Dirac cone splitting is highly sensitive to surface atomic details.

Sub-monolayer coverage can eliminate Dirac cone splitting.

Quantized states appear at large overlayer thicknesses.

Abstract

The 'double Dirac cone' 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb$_{1-x}$Sn$_{x}$Se feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb$_{1-x}$Sn$_{x}$Se(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each $\bar{X}$ is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.