Simulations of Dynamical Ordering in Pinned Vortex Systems

We model a vortex system in a sample with bulk pinning and superficial pinning generated by a magnetic decoration. We perform a sequence of finite temperature numerical experiments in which external forces are applied to obtain a dynamically ordered vortex lattice. We analyze the final structures and the behavior of the total energy of the system. The phase diagram of vortices in high temperature superconductors in the presence of pinning potentials has a rich variety of phases and transitions between them 1,2. Several techniques have been implemented to artificially create pinning centers 3. In particular, the structure of the vortex system in the presence of a superficial pinning potential has been studied by means of Bitter decorations 4. Experiments show that there is no observable change in the critical current (the minimal current needed to depin the vortex lattice) after the decoration is performed. This indicates that the bulk pinning dominates the vortex dynamics. In a previous work we show that, for a set of parameters that are consistent with the experimentally observed structures, the depinning force is dominated by the bulk pinning 5. Here we present additional results in which a sequence of numerical experiments have been performed to show that: a) When the external force exceeds the depinning force, the vortex lattice becomes dynamically ordered, even with a quasi-periodical Bitter pinning. This order is similar to the one predicted for a vortex system displaced by high forces over a random potential 6. b) For a wide range of bulk pinning, polycrystalline structures are obtained after a gradual warming up from the dynamically ordered vortex lattice or by cooling down the system from high temperatures. c)These final structures are not reproducible due to the dense bulk pinning that randomly nucleates