Influence of local symmetry on lattice dynamics coupled to topological surface states

TL;DR

This study reveals how local symmetry breaking at the surface of topological insulator Bi2Te3 influences coupled electron-lattice dynamics, showing surface states interact with a broad spectrum of vibrational modes, which can be used to identify and manipulate surface states.

Contribution

It demonstrates that surface symmetry breaking naturally leads to coupling with a wide range of vibrational modes, expanding understanding of surface phonon spectra in topological insulators.

Findings

Surface states couple to a continuum of bulk and surface vibrational modes.

Symmetry breaking at the surface explains the broad vibrational spectrum.

Coherent phonon spectra can serve as fingerprints for surface state identification.

Abstract

We investigate coupled electron-lattice dynamics in the topological insulator Bi2Te3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wavevectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman- and infrared-branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wavefunctions via ultrafast optical excitation.