Electron-phonon coupling in two-dimensional silicene and germanene

Following the work in graphene, we report a first-principles study of electron-phonon coupling (EPC) in low-buckled monolayer silicene and germanene. Despite the similar honeycomb atomic arrangement and linear-band dispersion, the EPC matrix-element squares of the Gamma-Eg and K-A(1) modes in silicene are only about 50% of those in graphene. However, the smaller Fermi velocity in silicene compensates for this reduction by providing a larger joint electronic density of states near the Dirac point, giving rise to comparable phonon linewidths. 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-Eg mode in silicene are calculated as a function of doping. The results are comparable to those in graphene, indicating a similar nonadiabaticdynamical origin. In contrast, the EPC in germanene is found to be much reduced.