Anomalous behavior of the $E_u^1$ infrared-active phonon mode in a Bi$_{2-x}$Sr$_x$Se$_3$ crystal

TL;DR

This study investigates the temperature-dependent spectral behavior of the $E_u^1$ phonon mode in Bi$_{2-x}$Sr$_x$Se$_3$, revealing coupling to surface Dirac electrons and enhanced electron-phonon interactions at the surface.

Contribution

It provides the first observation of Fano-like phonon line shapes and surface phonon softening linked to Dirac electron coupling in a topological insulator.

Findings

$E_u^1$ mode softens upon cooling

Pronounced Fano-like spectral line shape at low temperatures

Surface phonon softening indicates enhanced electron-phonon interaction

Abstract

We have studied spectral evolution of the $E_u^1$ phonon line of a topological insulator Bi$_{2-x}$Sr$_x$Se$_3$ with temperature. Unlike the Raman-active phonons, the $E_u^1$ mode demonstrates softening upon cooling the crystal, and the corresponding spectral line acquires a pronounced Fano-like shape at temperatures $T\lesssim100$ K. We interpret the latter effect as a signature of specific coupling of the bulk infrared active phonons to surface Dirac electrons. Using coherent resonant excitation of the $E_u^1$ mode as a surface sensitive tool, we have detected softening of the surface counterpart of the bulk $E_u^1$ phonon mode upon strontium doping. This observation can be an evidence of enhanced electron-phonon interaction at the surface of the Bi$_{2-x}$Sr$_x$Se$_3$ crystal.