Observation of Dirac Node Formation and Mass Acquisition in a Topological Crystalline Insulator

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Science  29 Aug 2013:
DOI: 10.1126/science.1239451

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In topological crystalline insulators (TCIs), topology and crystal symmetry intertwine to create surface states with unique characteristics. The breaking of crystal symmetry in TCIs is predicted to impart mass to the massless Dirac fermions. Here, we report high-resolution scanning tunneling microscopy studies of a TCI, Pb1-xSnxSe, which reveal the coexistence of zero mass Dirac fermions protected by crystal symmetry with massive Dirac fermions consistent with crystal symmetry breaking. In addition, we show two distinct regimes of the Fermi surface topology 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.

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