## Interaction of Phonons and Dirac Fermions on the (001) Surface of the Topological Crystalline Insulator Pb_{1-x}Sn_{x}Se

**Speaker:**Samuel Kalish

**When:**November 7, 2018 (Wed), 02:30PM to 03:30PM (add to my calendar)

**Location:**SCI 352

*This event is part of the Departmental Seminars. *

Interaction of Phonons and Dirac Fermions on the (001) Surface of the Topological Crystalline Insulator Pb*{1-x}Sn*{x}Se

Dissertation Committee:

Michael El-Batanouny, Claudio Chamon, Karl Ludwig, John Butler, Wanzheng Hu

Abstract:

Nontrivial topological crystalline insulator phases are protected by a crystal symmetry and not by time-reversal symmetry as is the case of topological insulators. In this talk, I will begin by discussing the theory of topological crystalline insulators. I will then argue the merits of helium atom surface-scattering as a probe of surface dynamics, and describe the techniques used. Next, I will present inelastic scattering measurements to determine the surface phonon dispersion of the (001) surface of the topological crystalline insulator Pb*{0.7}Sn*{0.3}Se. This material exhibits a temperature-dependent topological transition, so I have made measurements of the surface phonon dispersion curves in both the trivial and topological phases. I carry out calculations of the surface lattice dynamics to interpret these measurements. I find that a vertical shear surface resonance branch around 1.9 THz dramatically changes on going from the trivial to the topological phase. This remarkable change is associated with the emergence of surface Dirac fermions. I use the measured dispersion of this resonance branch to determine the corresponding momentum-dependent mode-specific electron-phonon coupling λ_ν (q).