An experiment is performed in which a topological line defect (S=-(1/2)) is forced to move with constant speed c under the action of an applied voltage V. We argue that the line speed is determined by a competition between the viscous damping and the free energy that the system gains by displacing the line, so that c=βV, with β=1/2γb(${\mathrm{\epsilon }}_{\mathrm{a}}$K/π${)}^{1/2}$ and b a number ∼1 determined only by viscous effects close to the core of the defect.

• Received 16 October 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.222