Defect motion and lattice pinning barriers in Josephson-junction ladders

We study the motion of domain wall defects in a fully frustrated Josephson-junction ladder system, driven by small applied currents. For small system sizes, the energy barrier E{sub B} to the defect motion is computed analytically via symmetry and topological considerations. More generally, we perform numerical simulations directly on the equations of motion, based on the resistively-shunted junction model, to study the dynamics of defects, varying the system size. Coherent motion of domain walls is observed for large system sizes. In the thermodynamical limit, we find E{sub B}=0.1827 in units of the Josephson coupling energy.