Motion of a Klein–Gordon kink in an external field

The drift velocity of a kink of the one-dimensional nonlinear Klein–Gordon equation is calculated in the framework of the linear response approximation. The kink drifts by virtue of an external field that lifts the degeneracy of the ground state of the unperturbed system. It is shown that in the leading low-temperature approximation the drift velocity can be expressed in the usual manner in terms of a diffusion coefficient.