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**UNC-CH PHYSICS AND ASTRONOMY COLLOQUIUM**

Vidya Madhavan, Boston College

**“Symmetry protected Dirac Fermions in Topological Crystalline Insulators”**

In Dirac materials like graphene and topological insulators, electrons behave like relativistic Dirac particles with zero 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[3,4,5]. We performed scanning tunneling microscopy (STM) studies at low temperatures and as a function of magnetic field. We find that the STM images and spectra reveal the coexistence of zero mass Dirac fermions protected by crystal symmetry with massive Dirac fermions resulting from crystal symmetry breaking[3,4]. We further track the evolution of the resultant mass as well as the Dirac surface states as we go through a quantum phase transition from the topological to trivial regime [5] and find that the mass is controlled by the surface state penetration depth in these systems.