Emergent non-Hermitian boundary contributions to charge pumping and electric polarization
The phenomenon of charge pumping and the modern theory of electric polarization are reconsidered by analytically taking into account emergent non-Hermitian contributions. These are accounted for through the use of an extended definition of the velocity operator and are determined by means of a dynamic Hellmann-Feynman theorem (DHFT) that we derive here for the first time. The DHFT introduces generalized Berry curvatures and it is valid for calculating observables nonperturbatively, hence with results valid to all orders of the external fields. By using the extended velocity operator we rigorously show how the charge pumping is linked up with the boundaries of the material (with the non-Hermiticity being essential for this connection), and by means of the DHFT we show that the well-known topological quantization of the pumped charge breaks down due to a nonintegrable Aharonov-Anandan phase in driven non-equilibrium processes whenever the periodic gauge cannot be applied to the Floquet-Bloch states. Likewise, we show that the electronic polarization change has an additional non-Hermitian contribution, which is overlooked in the modern theory of electric polarization. The non-Hermitian contribution is by definition a bulk quantity that may equally be evaluated as a boundary quantity due to a symmetric structure that allows the bulk integration to be transformed into a boundary one. This non-Hermitian contribution is very sensitive to the realistic boundary conditions imposed on the wavefunctions and it is therefore expected to be significant in biased insulators where charge accumulation over their boundaries is present during the process that causes the polarization change. Finally, we show how a well-defined surface-charge theorem can be formulated in terms of the boundary non-Hermitian contribution.