Current-induced organization of vortex motion in type-II superconductors

When a magnetic field is applied to a type-II superconductor, it penetrates the sample in localized tubes of magnetic flux associated with quantized current vortices; under appropriate conditions, these vortices form an ordered lattice, In a material free of crystal defects, transport currents force this lattice to move and dissipate energy, giving the material a non-zero resistance, The presence of defects, however, can inhibit vortex motion, or even I pin vortices to specific locations. Thus, to engineer materials with improved properties it is important to understand the motion of a driven vortex lattice in the presence of different kinds of pinning defects(1,2). Recent research has investigated vortex-lattice dynamics in the cases of weak, uniform pinning and strong but non-uniform pinning. Here we consider a different regime, in which the barriers to vortex motion at sample surfaces' also play a crucial role. Our experiments on dean, detwinned YBa2Cu3O7-delta crystals at temperatures around 80-90K reveal an interplay between surface pinning and weak bulk pinning that leads to the formation of a defect superstructure in the vortex lattice, This current-induced organization is similar to phenomena observed in the dynamics of sliding charge-density waves, and represents a fundamentally new kind of vortex dynamics.