Jueves 5 de julio 2012

12h30-13h30 “Static and driven phases of vortex matter in superconductors with intrinsic and nanopatterned pins”

Prof: Gorky Shaw (Senior Research Fellow, Dept.of Physics, IIT Kampur, Kampur, U.P., India)

Resumen:

The vortex state in a superconductor is strongly influenced by thermal fluctuations and quenched random disorder (pinning) in the superconductor. In this seminar, I will present results of studies on the effect of artificially introduced pinning centers on the static phases of vortex matter as well as studies on static and driven phases of vortex matter in superconductors with intrinsic pinning.
We have used Focused Ion Beam (FIB) to generate array of nanopins (blind holes) on surfaces of single crystals of BSCCO and 2H-NbSe2. High sensitivity magneto-optical imaging (MOI) investigations reveal evidence for a barrier towards entry and exit of vortices in the patterned region which leads to large gradients in vortex density in the vicinity of the patterned region. Large shielding currents, comparable to Meissner currents associated with geometrical barriers present at the sample edges, are found to circulate around the periphery of the patterned region, and result in the aforementioned barrier. Bulk magnetization measurements in these samples provide evidence of a driven weak to strong pinning crossover.
Using local MOI studies on a BSCCO single crystal we have identified signatures of a low-field vortex liquid to vortex solid phase transformation via a coexistence regime. We have constructed phase boundaries for the low field vortex liquid – solid transition in the H-T vortex phase diagram and studied the effect of pinning strength on the phase boundaries.
Using a new protocol, via transport current measurements in single crystals of 2H-NbS2, we found evidence of unusual jamming of flowing ordered vortex state which is driven either by attempting to steadily accelerate the vortex state or by waiting for long time at constant drive. Depinning of the jammed vortex state is associated with novel giant vortex-velocity fluctuations, the life-time of which is found to exhibit a critical divergence on approaching the depinning force value of the jammed state.