Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi2Se3

TL;DR

This study uses Raman spectroscopy to measure how plasmon-enhanced electron-phonon interactions in Dirac surface states of Bi2Se3 thin films vary with thickness, revealing a non-monotonic behavior with a maximum around 8-10 nm.

Contribution

It provides the first detailed analysis of electron-phonon coupling in Dirac surface states as a function of film thickness, highlighting plasmon-phonon interactions.

Findings

Maximum electron-phonon coupling at 8-10 nm thickness

Four-fold enhancement compared to bulk samples

Non-monotonic dependence due to plasmon-phonon coupling

Abstract

Raman measurements of a Fano-type surface phonon mode associated with Dirac surface states (SS) in Bi2Se3 topological insulator thin films allowed an unambiguous determination of the electron-phonon coupling strength in Dirac SS as a function of film thickness ranging from 2 to 40 nm. A non-monotonic enhancement of the electron-phonon coupling strength with maximum for the 8 - 10 nm thick films was observed. The non-monotonicity is suggested to originate from plasmon-phonon coupling which enhances electron-phonon coupling when free carrier density in Dirac SS increases with decreasing film thickness and becomes suppressed for thinnest films when anharmonic coupling between in-plane and out-of-plane phonon modes occurs. The observed about four-fold enhancement of electron-phonon coupling in Dirac SS of the 8 - 10 nm thick Bi2Se3 films with respect to the bulk samples may provide new insights into the origin of superconductivity in this-type materials and their applications.