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

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, the electron-phonon (e-ph) coupling matrix elements were computed using the frozen-phonon approach. They found that in silicene the average e-ph coupling matrix element square over the Fermi-surface < g(qv)(2)>(F) is about 50% of those in graphene, but in germanene is weaker and nearly negligible. However, Yan et al. [Phys. Rev. B 88, 121403 (2013)] argue that the smaller Fermi velocity in silicene compensates the reduced < g(qv)(2)>(F) , leading to phonon linewidths (gamma(qv)) slightly larger than those in graphene. In this Comment, we show that the DFPT and the frozen-phonon results of Yan et al. [Phys. Rev. B 88, 121403 (2013)] 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 < g(qv)(2)>(F) and gamma(qv). in silicene are one order of magnitude smaller than in graphene, in contrast to the conclusions of Yan et al.