Comment on "Electron-phonon coupling in two-dimensional silicene and germanene"

TL;DR

This paper critically examines previous calculations of electron-phonon coupling in silicene and germanene, revealing that earlier results overestimated the coupling strength in silicene by an order of magnitude.

Contribution

The authors demonstrate that previous DFPT and frozen phonon results for silicene's electron-phonon coupling are inconsistent and provide corrected, systematically lower estimates.

Findings

Silicene's electron-phonon coupling is an order of magnitude weaker than previously reported.

DFPT calculations show smaller phonon linewidths in silicene compared to earlier studies.

The corrected results suggest weaker electron-phonon interactions in silicene than previously thought.

Abstract

In their work, Yan et al. [Phys. Rev. B 88, 121403 (2013)] employing density functional perturbation theory (DFPT) calculations, demonstrate that silicene and germanene show weaker Kohn anomalies in the $\Gamma$-$E_g$ and $K$-$A_1$ phonon modes, compared to graphene. Furthermore, they compute the electron phonon (e-ph) coupling matrix elements using the frozen phonon approach and found that in silicene the average e-ph coupling matrix-element square over the Fermi surface, $\langle g_{{\bf q}\nu}^2\rangle_{F}$, is about 50 % of those in graphene, but in germanene is weaker and nearly negligible. However, Yan et al. argues that the smaller Fermi velocity in silicene compensates the reduced $\langle g_{{\bf q}\nu}^2\rangle_{F}$, leading to phonon linewidths ($\gamma_{{\bf q}\nu}$) slightly larger than those in graphene. In this Comment, we show that the DFPT and the frozen phonon results of Yan et al. for silicene are inconsistent. Additionally, we have evaluated the e-ph coupling using direct DFPT calculations, analytical relations, and frozen phonon calculations, and we found systematically that $\langle g_{{\bf q}\nu}^2\rangle_{F}$ and $\gamma_{{\bf q}\nu}$ in silicene are one order of magnitude smaller than in graphene, in contrast to the conclusions of Yan et al.