EQUILIBRIUM VORTEX-LINE CONFIGURATIONS AND CRITICAL CURRENTS IN THIN-FILMS UNDER A PARALLEL FIELD

Vortex-line equilibrium configurations and critical currents for type- II superconducting films at zero temperature are studied theoretically . The films are assumed to be of thickness less than or equal to the p enetration depth, free of pinning by imperfections, and to be subjecte d to a magnetic field parallel to the film surfaces and to a transport current perpendicular to the field. By numerical minimization of the exact London-theory energy expression, using-simulated annealing techn iques, the equilibrium configurations are determined with great accura cy over a wide range of fields and currents. These consist of chains o f straight vortex lines whose number depends both on the field and on the current. Transitions involving a change by one in the chain number are found to take place either if the field is increased at zero tran sport current or if the transport current is increased at constant fie ld. At the transitions, there is a considerable rearrangement in the v ortex-line positions and a small, discontinuous, change in their numbe r. The equilibrium configurations with chain numbers that are not too small form a nearly perfect triangular lattice, centered with respect to the film surfaces. However, small deviations from this arrangement are found to be important in determining the behavior near the transit ion and when the transport current approaches the critical current. It is found that the critical current has a a nonmonotonic dependence on the field. The zero-temperature equilibrium phase diagram in the fiel d-transport current plane is reported.