Surface vibrational modes of the topological insulator Bi$_2$Se$_3$ observed by Raman spectroscopy

TL;DR

This study uses Raman spectroscopy to identify and analyze surface vibrational modes in Bi$_2$Se$_3$, revealing surface phonons linked to lattice distortions and electron-phonon interactions, enhancing understanding of topological insulator surface properties.

Contribution

First observation and symmetry analysis of surface phonon modes in Bi$_2$Se$_3$, linking them to surface lattice distortions and electron-phonon coupling effects.

Findings

Identification of four additional surface phonon modes.

Surface modes are related to lattice distortions and symmetry reduction.

Evidence of electron-phonon coupling through Fano lineshape.

Abstract

We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi$_2$Se$_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm$^{-1}$ are associated with Raman active $A_{1g}$ bulk phonon modes, the other two at 136 and 158 cm$^{-1}$ are associated with infrared active bulk phonons with $A_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from $D_{3d}$ in the bulk to $C_{3v}$ on the crystal surface. In particular, the 158 cm$^{-1}$ surface phonon mode shows a Fano lineshape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.