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.
@article{arxiv.1305.2823,
title = {Observation of Dirac node formation and mass acquisition in a topological crystalline insulator},
author = {Yoshinori Okada and Maksym Serbyn and Hsin Lin and Daniel Walkup and Wenwen Zhou and Chetan Dhital and Madhab Neupane and Suyang Xu and Yung Jui Wang and R. Sankar and Fangcheng Chou and Arun Bansil and M. Zahid Hasan and Stephen D. Wilson and Liang Fu and Vidya Madhavan},
journal= {arXiv preprint arXiv:1305.2823},
year = {2014}
}