Fractional Charges in the Su-Schrieffer-Heeger Model

Abstract

The Su-Schrieffer-Heeger(SSH) model has been widely used to study the topological property of 1D systems. It is claimed that there is fractional charge at the boundary of the nontrivial phase while none at that of trivial phase. However, this conclusion is in direct contradiction to the modern theory of polarization(MTP). We solve this paradox by showing that the polarization of SSH model depends only on the distribution of the positive charges and is irrelevant to the Zak phase defined in previous works. Thus the distribution of positive charges alone determines whether the SSH chain is topological or not. Similarly, we show the polarization defined by Berry connection can not be used to characterize topological property of a 2D generalization of SSH model.