Electron-phonon (de)coupling in 2D

Abstract

Electron phonon coupling in (quasi)-two-dimensional structures within the first order theory is strongly constrained by symmetry, and the resulting deficiencies are manifested in a number of striking phenomena: electronic system dynamically decoupled from lattice, absence of totally symmetric phonons, Jahn-Teller theorem violation (with spontaneous symmetry breaking), insufficiency of the Kohn singularity conditions, nonlinear acoustic branches. Here presented analysis of layered structures enlightens that this phenomena are enforced in highly symmetric structures (including here superconducting CuO 2 sheet, atomically thin hexagonal boron nitride, graphene. e.g.). General group theoretical results covering all possible quasi-two-dimensional crystal structures are illustrated on real materials, as well as on a few hypothetical compounds. • Unified insight to a number of exceptional characteristics of layer. • Significant deficiencies of electron-phonon coupling in highly symmetric layered structures. • Number of totally symmetric phonons reduced or annulled by symmetry. • Breakdown of Jahn-Teller theorem in highly symmetric layered structures: possible degenerate ground states. • Existence of the electronic states decoupled from the ionic subsystem. • Standard Kohn's requirement in some layers is not sufficient to cause anomaly (because of decoupled phonons). • Jahn-Teller breaking excludes Dirac cones in electronic dispersions.