Electron-Phonon Coupling in Two-Dimensional Silicene and Germanene

TL;DR

This study investigates electron-phonon interactions in silicene and germanene, revealing significant EPC effects in silicene similar to graphene, but much weaker in germanene, with implications for their electronic and vibrational properties.

Contribution

First-principles analysis of EPC in silicene and germanene, highlighting the differences and similarities with graphene and predicting observable phonon anomalies.

Findings

EPC matrix elements in silicene are about 50% of those in graphene.

Significant Kohn anomalies are predicted in silicene.

EPC effects in germanene are substantially weaker.

Abstract

Following the work in graphene, we report a first-principles study of electron-phonon coupling (EPC) in low-buckled (LB) monolayer silicene and germanene. Despite of the similar honeycomb atomic arrangement and linear band dispersion, the EPC matrix-element squares of the $\Gamma$-$E_g$ and K-$A_1$ modes in silicene are only about 50% of those in graphene. However, the smaller Fermi velocity in silicene compensates this reduction by providing a larger joint electronic density of states near the Dirac point. We predict that Kohn anomalies associated with these two optical modes are significant in silicene. In addition, the EPC-induced frequency shift and linewidth of the Raman-active $\Gamma$-$E_g$ mode in silicene are calculated as a function of doping. The results are comparable to those in graphene, indicating a similar non-adiabatic dynamical origin. In contrast, the EPC in germanene is found to be much reduced.