?h symmetry and electron-phonon interaction in two-dimensional crystalline systems

The coupling of electrons and phonons is governed wisely by the symmetry properties of the crystal structures. In particular, for two-dimensional (2D) systems, it has been suggested that the electrons do not couple to phonons with pure out-of-plane distortion, as long as there is a ah symmetry. We show that such a statement is correct when constituents of the unit-cell layer are only located in the ah symmetric plane; a prominent example of such a system is graphene. For those 2D crystals in which atoms are vertically located away from the horizontal symmetric plane (e.g., 1H transition metal dichalcogenides), acoustic flexural modes do not couple to the electrons up to linear order, while optical flexural phonons, which preserve ah symmetry, do couple with the electrons. Our conclusions are supported by an analytic argument together with numerical calculations using density functional perturbation theory.