Propriedades de equilÃbrio e de transporte da matÃria de vÃrtices em nanoestruturas supercondutoras

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

In the present thesis, we study theoretically the equilibrium properties and dynamics of the vortex matter in two classes of nanostructured superconductors: (i) mesoscopic samples, with dimensions comparable with the penetration depth; and (ii) films with periodic array of artificial traps. In both cases, the vortex lattice symmetry is strongly dependent on the sample spacial parameters. In this way, the vortex lattice properties may be artificially controlled by changing these parameters. Vortex penetration and further evolution inside a mesoscopic film under a parallel, externally applied magnetic field is simulated by a Langevin dynamics algorithm. The vortex structure and forces acting on it is calculated by solving the London equation with the appropriate boundary conditions. The surface barrier, resulting from the competition between the vortex self-force, which attracts vortices towards the surface, and the Meissner screening force, which pulls the vortices towards the sample center, is taken into account. The vortex lattice is shown to be composed of vortex chains parallel to the film surfaces. The field dependent magnetization curves present strong histeresis, which is due to the surface barrier acting against vortex entrance and exit. The resulting vortex states are long-lived metastable states which undergo structural transitions where one vortex chain is created or destroyed, depending on the film magnetic history. The elastic response of the vortex lattice in a square pinning array to small excitations is studied. The vortex lattice ground-state configurations are found using Monte Carlo simulated annealing minimization. A simple model was developed to compute analytically the linear response of the vortex lattice to small ac 10 currents. It is found that the frequency spectrum of the ac complex resistivity is determined by two characteristic frequencies: one is due to the coupling between vortices and the artificial traps and the other one is due to the coupling between trapped and intersticial vortices. Molecular dynamics simulations are performedto state the range of validity of the model. The transport properties of vortices interacting with a periodic pinning potential are also studied. A mean-field treatment of the Langevin equations for the whole vortex lattice, which is assumed to flow elastically, as a strong drivingcurrent is applied, was developed to compute voltageâcurrent characteristics and the dynamical crystallization process. It is shown that the broken translational symmetry of the periodic potential induces strongly anisotropic response, with vortex lattice motion directed preferentially along high symmetry axes of the pinning lattice. The transverse force necessary to depin vortices from these high symmetry directions is shown to depend on the pinning potential itself as well as on the center of mass velocity and the temperature. The crystallization temperature is shown to scale with the center of mass velocity v as 1/v2, in contrast with the 1/v low for vortex motion in disordered superconductors

ASSUNTO(S)

nanoestruturas fisica dinÃmica de vÃrtices sistemas mesoscÃpicos filmes finos

ACESSO AO ARTIGO

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