Observation of antiphase coherent phonons in the warped Dirac cone of Bi$_2$Te$_3$

TL;DR

This study reveals how ultrafast coherent phonons are coupled with anisotropic electronic states in Bi$_2$Te$_3$, showing antiphase behavior linked to warping effects, using advanced spectroscopy and theoretical calculations.

Contribution

It demonstrates the observation of antiphase coherent phonons in Bi$_2$Te$_3$ and highlights the anisotropic electron-phonon coupling related to warping of the Dirac cone.

Findings

Antiphase coherent phonon oscillations observed in Bi$_2$Te$_3$

Anisotropic coupling between Dirac fermions and phonons confirmed

Qualitative agreement with density-functional theory calculations

Abstract

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical A$_{1g}$ coherent-phonon mode in Bi$_2$Te$_3$. We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the $\overline{\Gamma K}$ and the $\overline{\Gamma M}$ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi$_2$Te$_3$.