Infrared absorbance of silicene and germanene

Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K' contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731626]