Topological charge quantization on localized imperfections in crystalline insulators and the nearsightedness principle of Kohn

TL;DR

This paper demonstrates that the excess charge on localized impurities in crystalline insulators is topologically quantized, linking it to the nearsightedness principle of electronic charge distribution and boundary charge invariants.

Contribution

It introduces a topological invariant for impurity charge quantization in insulators and connects it with the nearsightedness principle of electronic density.

Findings

Excess impurity charge is a topological winding number.

Localized perturbations only cause local charge redistributions.

Adiabatic impurity variations affect impurity-bound state occupancy.

Abstract

We study the quantization of the excess charge on $N$ localized (ultra-screened) impurities in $d$-dimensional crystalline insulating systems. Solving Dyson's equation, we demonstrate that such charges are topological, by expressing them as winding numbers of appropriate functionals of bulk position space Green's functions. We discuss the ties of our topological invariant with the nearsightedness principle of W. Kohn, stating that the electronic charge density at fixed chemical potential depends on the external field only locally, meaning that localized perturbations by external fields may only result in localized charge redistributions. We arrive at the same conclusion by demonstrating that an adiabatic perturbation comprised of a variation of impurities' positions and/or strengths may only result in the change in the occupancy of impurity-localized bound states sitting, energy-wise, close to the Fermi level. Finally, we conclude by discussing the relations of the nearsightedness principle with the topological invariants characterizing the boundary charge.