Narrow Results

The magnetic field B dependence of the critical current I_c for the vortex phase of a disordered superconductor is studied numerically at zero temperature. The I_c(B) increases rapidly near the upper critical field B_c2 similar to the peak effect (PE) phenomenon observed in many superconductors. The real space configuration across the PE changes continuously from a partially ordered domain (polycrystalline) state into an amorphous state. For B≥0.4B_c2, the topological defect density n_d(b) increases as e^αBk with k>1. There is no evidence of a phase transition in the vicinity of the PE suggesting that an order-disorder transition is not essential for the occurrence of the PE phenomenon. An alternative view is presented wherein the vortex system with high dislocation density undergoes jamming at the onset of the PE.

We report on the dynamic as well as static properties of vortices in the low-temperature (T) solid phase of a thick amorphous Mo_xGe_1-x film with weak pinning. The vortex-solid state is considered to be a weakly disordered vortex lattice, which is inferred from the mode-locking resonance over the broad field region. The peak effect of the critical current I_c with magnetic field is clearly visible down to the lowest T measured (0.2 K), which corresponds to 3.3% of the superconducting transition temperature. We find that largest voltage noise originating from current-driven vortices appears in the field region just prior to the peak field at which I_c takes a peak. This is in accord with the result reported in NbSe₂ single crystals.

We have investigated the role of pinning density on the properties of the peak effect in the critical current density in superconducting systems. This study was conducted using a series of molecular dynamic simulations on two systems of nanostructures of periodic square arrays of pinning sites with different pining densities. We have found that the peak occurred in both systems only at zero temperature and for specific values of pinning strength. The most interesting result was that in both systems, the peak was found to occur only at nearly 0.5 fraction of the first matching field for all values of pinning strength. The properties of the peak were found to depend mainly on the initial positions of the vortices with respect to the positions of the pinning sites. The critical dipping force at the peak was found to increase linearly with the pinning strength and to have larger values for the system with the smaller density of pinning sites. The dependence of the relative height of the peak on the pinning strength was found be nearly the same in both systems. One-dimensional linear channels of moving vortices along the direction of the driving force were observed at the dip just before the peak.