In Dirac materials like graphene and topological insulators, electrons behave like relativistic particles with no mass. This is a direct consequence of the form of the low energy effective Hamiltonian describing these electrons and has important implications for realizing physical properties predicted for high-energy particles, now in the laboratory setting. Topological crystalline insulators are recently discovered topological materials [1,2] where topology and crystal symmetry intertwine to create relativistic massless electrons. Among the theoretical predictions for topological crystalline insulators is the possibility of imparting mass to these massless Dirac fermions by breaking crystal symmetry. In this talk I will discuss our recent experimental and theoretical investigations of a topological crystalline insulators, Pb1-xSnxSe [3,4]. We performed scanning tunneling microscopy (STM) studies at low temperatures and as a function of magnetic field. By analyzing two types of STM data: Fourier transforms of interference patterns and Landau level spectroscopy, we reveal the coexistence of zero mass Dirac fermions protected by crystal symmetry with massive Dirac fermions resulting from crystal symmetry breaking. In addition, I will discuss our recent data on the evolution of the mass as well as the Dirac surface states as we go through a quantum phase transition from the topological to trivial regime .
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 Y. Okada, et al., Observation of Dirac node formation and mass acquisition in a topological crystalline insulator, Science 341, 1496-1499 (2013)
 Ilija Zeljkovic, et al., Mapping the unconventional orbital texture in topological crystalline insulators, Nature Physics 10, 572–577 (2014)
 Ilija Zeljkovic, et al., Dirac mass generation from crystal symmetry breaking on the surfaces of topological crystalline insulators, arXiv:1403.4906