Microscopic Theory of Chern Polarization

Abstract: The modern theory of polarization does not apply in its original form to systems with non-trivial band topology. Chern insulators are one such example since they are insulating in the bulk but exhibit metallic edge states, complicating the definition of polarization. Wannier functions formed a key ingredient of the original modern theory of polarization, but it has been considered that these cannot be applied to Chern insulators since they are no longer exponentially localized and the Wannier center is no longer gauge invariant. In this Letter, we provide an unambiguous definition of absolute polarization for a Chern insulator in terms of the Zak phase. We obtain our expression by studying the non-quantized fractional charge bound to lattice dislocations and it can be computed directly from bulk quantities and makes no assumption on the edge state filling. Our result is fully consistent with previous results on the quantized charge bound to dislocations in the presence of crystalline symmetry. At the same time, our result is more general since it also applies to Chern insulators which do not have crystalline symmetries other than translations.