Surface phonons in two-layer thin films of GeSe
  This paper presents ab inition calculations of the surface phonon spectra of GeSe layered semiconductor compound, based on the Density Functional Perturbation Theory (DFPT). The surface has been imitated by a structure of periodically arranged slabs of two layers of GeSe crystal separated from other identical layers by the layers of vacuum sufficiently wide to ignore the effect of the upper boundary of the double-layer upon its lower boundary. We discuss the character of the surface modes located in the gaps, in the pockets, and in the area of allowed phonon states for the bulk GeSe crystals, as well as outside (above and below) the boundaries of the bulk phonon states. 
F.M. Hashimzade, D.A. Huseinova, Z.A.Jahangirli, B.H. Mehdiyev
cond-mat.mtrl-sci