| The behavior of vortices in 2D and layered type-II superconductors is studied experimentally with a model system comprised of superconducting/insulating MoGe/Ge multilayers. By relating the measured transport properties to sample parameters, we probe the relative importance of vortex pinning, thermal fluctuations, and interlayer coupling in the system. At low magnetic fields, we observe that the pinning of vortices near sample edges may be substantially increased by edge pinning. However, with appropriate patterning and measurement techniques, we may avoid the influence of sample edges and study the intrinsic behavior of vortices in 2D and multilayer MoGe/Ge films. In thin 2D films, with short vortex translational correlation lengths, the motion of vortices proceeds by thermal activation, and we relate the observed activation energies to possible mobile configurations of vortices. In thicker 2D films, with longer vortex translational correlation lengths, the dynamics of vortices crosses over from thermal activation at long length scales to Kosterlitz-Thouless melting dynamics at short length scales. The characteristic length associated with this crossover may be related to the vortex translation correlation length. In MoGe/Ge multilayers, we have demonstrated directly that the magnetic forces that couple vortex discs in different layers are negligible in comparison to pinning forces and thermal fluctuations. With little or no Josephson coupling, the vortex discs in each layer move independently of discs in adjacent layers. Only when the anisotropy of the multilayer is less than a characteristic anisotropy does the hopping of vortices in different layer become correlated. It seems likely that this characteristic anisotropy is set by competition between interlayer coupling forces and the pinning forces. In multilayers of lower anisotropy, the measured critical current densities drop sharply at a field dependent temperature well below the temperature where interlayer correlated vortex hopping begins. Since the pinning potential is a smooth function of temperature, this drop in the critical current density may represent an abrupt increase in the rigidity of the system of moving vortices. This thesis highlights the profound importance of static disorder in determining the behavior of vortices in systems of reduced dimensionality. |
