Inequivalent Berry phases for the bulk polarization

TL;DR

This paper clarifies the subtleties in defining Berry phases for bulk polarization in insulators, showing that while the Berry phase's value depends on its definition, the total charge transported in topological processes remains invariant.

Contribution

It elucidates the different definitions of Berry phases for polarization, their dependence on the chosen framework, and confirms the topological invariance of charge transport during Thouless pumping.

Findings

Berry phase definitions depend on the chosen formulation.

The total charge transported in Thouless pumping is topologically invariant.

Different Berry phase definitions correspond to different real-space current measurements.

Abstract

We discuss characterization of the polarization for insulators under the periodic boundary condition in terms of the Berry phase, clarifying confusing subtleties. For band insulators, the Berry phase can be formulated in terms of the Bloch function in the momentum space. More generally, in the presence of interactions or disorders, one can instead use the many-body ground state as a function of the flux piercing the ring. However, the definition of the Bloch function and the way describing the flux are not unique. As a result, the value of the Berry phase and its behavior depend on how precisely it is defined. In particular, identifying the Berry phase as a polarization, its change represents a polarization current which also depends on the definition. We demonstrate this by elucidating mutual relations among different definitions of the Berry phase, and that they correspond to the current measured differently in the real space. Despite the non-uniqueness of the polarization current, the total charge transported during a Thouless pumping process is independent of the definition, reflecting its topological nature.