Nonlinear phononics in Bi$_2$Te$_3$ nanoscale thin films: A theoretical approach

TL;DR

This paper uses first-principles calculations and group theory to analyze nonlinear phonon interactions in Bi$_2$Te$_3$ thin films, explaining experimental observations of coherent phonon generation via THz pulses.

Contribution

It introduces a theoretical model that identifies cubic phonon-phonon couplings as key to understanding nonlinear phononics in Bi$_2$Te$_3$ nanostructures, supported by first-principles calculations.

Findings

Cubic phonon-phonon interactions are significant in Bi$_2$Te$_3$.

Theoretical results match experimental transmittance data.

Group theory helps identify dominant phonon couplings.

Abstract

Density Functional Theory (DFT) calculations not only allow to predict the vibrational and optical properties of solids but also to understand and disentangle the mechanisms playing a key role in the generation of coherent optical phonons. Recent experiments performed on a Bi$_2$Te$_3$ nanofilm have shown that a THz pulse launches at least a coherent $A_{1g}^1$ phonon as the transient transmittance measured using an isotropic detection scheme displays oscillations with a frequency matching the frequency of the $A_{1g}^1$ mode measured in Raman experiments. Such an observation can be explained by invoking either a sum frequency process or cubic/quartic phonon-phonon couplings as considered for Bi$_2$Se$_3$, a parent compound of Bi$_2$Te$_3$. By resorting to group theory and calculating energy surfaces from first-principles, the main phonon-phonon couplings can be identified. Furthermore, a minimal model can be built to compute the dynamics of the Raman active modes coupled to the infrared active mode driven by the experimental THz pulse. Our model firmly establishes that cubic phonon-phonon interactions are relevant as the agreement between the computed and experimental transmittance is noteworthy.