where σ σ = ± ± 1 is the topological charge. For σ σ = 1 the solution is called a kink, and for σ σ = − − 1 it is an antikink. The kink width γ γ is determined by the kink velocity v , where v is measured in units of the sound velocity c and is γ γ ( v ) = 1 / 1 − − v 2 . For kink motion with v 2 ≪ ≪ c 2 , the width approximates 1. The energy of the kink in dimensionless units is