  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.
