Observation of Dirac node formation and mass acquisition in a topological crystalline insulator

TL;DR

This study uses high-resolution STM to observe how crystal symmetry breaking induces mass in Dirac fermions in a topological crystalline insulator, revealing Lifshitz transition and potential for quantum engineering.

Contribution

It provides direct experimental evidence of mass generation in Dirac fermions through symmetry breaking in a TCI, and characterizes the Lifshitz transition in Pb1-xSnxSe.

Findings

Observation of massless Dirac fermions protected by crystal symmetry

Identification of mass acquisition mechanism via symmetry breaking

Detection of Lifshitz transition with Fermi surface topology change

Abstract

In the recently discovered topological crystalline insulators (TCIs), topology and crystal symmetry intertwine to create surface states with a unique set of characteristics. Among the theoretical predictions for TCIs is the possibility of imparting mass to the massless Dirac fermions by breaking crystal symmetry, as well as a Lifshitz transition with a change of Fermi surface topology. Here we report high resolution scanning tunneling microscopy studies of a TCI, Pb1-xSnxSe. We demonstrate the formation of zero mass Dirac fermions protected by crystal symmetry and the mechanism of mass generation via symmetry breaking, which constitute the defining characteristics of TCIs. In addition, we show two distinct regimes of fermiology separated by a Van-Hove singularity at the Lifshitz transition point. Our work paves the way for engineering the Dirac band gap and realizing interaction-driven topological quantum phenomena in TCIs.