Spin-charge Separated Solitons in a Topological Band Insulator

In this paper we construct a simple, controllable, two dimensional model based on a topological band insulator. It has many attractive properties. (1) We obtain spin-charge separated solitons that are associated with $\pi$ fluxes. (2) It suggests an alternative way to classify $Z_2$ topological band insulator without resorting to the sample boundary. (3) When the $\pi$ fluxes are dynamical variables, as in a correlated insulator with emergent gauge fluxes, these solitons are propagating bosonic excitations and their condensation triggers a phase transition into a planar ferromagnet.