A second-order effect causing the layer structure of arsenic

The role of second-order perturbations in the interlayer interactions of gray arsenic is discussed. A hypothetical structural change from the germanium (or diamond) to the arsenic structures is considered using a two-layer model in terms of second-order Jahn-Teller distortions. We show from the concept of transition density or transition force that the second-order term originating from mixing of the pseudodegenerate bands near the Fermi level leads to the well-known layer structure of arsenic. The interesting structure of arsenic is a consequence of orbital interactions near the Fermi level.