Raman Studies of Surface Phonons in the Topological Insulators Bi2Se3 and Bi2Te3

Abstract

Temperature- and wavelength- dependent Raman studies of the topological insulators Bi2Se3 and Bi2Te3 are presented. At low temperatures and in the A symmetry scattering channel, we observe additional weak modes which we ascribe to surface phonons. First-principles LDA calculations support such assignment in that they reveal surface phonons of A symmetry at frequencies which coincide with the extra lines observed experimentally. Moreover, wavelength-dependent measurements show that the surface phonon in Bi2Se3 displays an asymmetric Fano lineshape when the excitation frequency is near a resonance with a previously reported surface exciton [Kung et al., Phys. Rev. Lett. 119, 136802 (2017)]. The lineshape asymmetry, which is a result of an interference between two scattering channels, reinforces our assignment of the extra peaks as due to surface modes and provides a measure of its coupling to surface electronic excitations.