Dynamical-charge neutrality at a crystal surface

TL;DR

This paper extends the concept of dynamical charge neutrality to crystal surfaces, demonstrating that surface ions in polar insulators have different dynamical charges than in the bulk, supported by calculations on SiC surfaces.

Contribution

It introduces a new sum rule for dynamical charges at crystal surfaces, revealing differences from bulk charges in polar insulators.

Findings

Surface ions in polar insulators do not share bulk dynamical charges.

The sum rule for dynamical charges applies to surfaces, not just bulk or molecules.

Calculations on SiC surfaces validate the extended sum rule.

Abstract

For both molecules and periodic solids, the ionic dynamical charge tensors which govern the infrared activity are known to obey a dynamical neutrality condition. This condition enforces their sum to vanish (over the whole finite system, or over the crystal cell, respectively). We extend this sum rule to the non trivial case of the surface of a semiinfinite solid and show that, in the case of a polar surface of an insulator, the surface ions cannot have the same dynamical charges as in the bulk. The sum rule is demonstrated through calculations for the Si-terminated SiC(001) surface.