IIT Kanpur / TIFR

Preamble: Vortex matter in the mixed state of superconductors is governed by energy scales associated with vortex-vortex interaction, thermal fluctuations and vortex pinning. Consequently, the static and dynamic phases of the vortex state are quite rich. In my thesis, I presented results of my experimental studies on the static and driven phases of vortex matter in high quality single crystals of high-T_c and low-T_c superconductors with weak intrinsic pinning as well as artificially introduced pinning arrays.

Static phases of vortex matter in a sample with intrinsic pinning centers: Using high sensitivity magneto-optical imaging (MOI) on a Bi₂Sr₂CaCu₂O₈ (BSCCO) single crystal we have identified signatures of a novel interaction-driven freezing transition from a dilute vortex liquid to vortex solid phase along with a solid-liquid phase coexistence regime. We have constructed a phase boundary for the low field liquid - solid transition line in the H - T vortex phase diagram. We have studied the effect of pinning strength on this phase boundary. We have also evaluated the entropy change associated with the low field phase transformation [1].

Driven phases of vortex matter: Upon applying a transport current to a superconductor; moving vortices scatter electrons and voltage is generated. By recording voltage/time series we have uncovered a new non-equilibrium driven jammed vortex state. The entry from a free flow - into a jammed vortex state is observed either after waiting for long time at constant drive or by steadily accelerating the vortices. Depinning of the jammed vortex state is also highly unusual and is associated with giant vortex-velocity fluctuations with life-times exhibiting critical divergence on approaching the threshold depinning force value. Our results have been compared with recent studies on similar effects found in driven colloidal systems [2,3].

Effects of artificial pinning on static phases of vortex matter:

(a) We have used Focused Ion Beam (FIB) to generate arrays of nanopins (blind holes) on surfaces of single crystals of 2H-NbSe₂ and BSCCO. SQUID magnetization measurements in these samples provide evidence for a crossover from a weak to strong pinning state, the crossover being driven by changing magnetic field. The observation of the crossover at fields much larger than the saturation field of the blind holes is proposed to be due to the jamming of mobile interstitial vortices driven through the blind hole lattice [4,5].

(b) We have shown how the local static configuration of vortices can be significantly altered with nanopatterning. Our studies reveal a distinct glassy nature and enhanced local irreversibility of the vortex state inside a nanopatterned region in a crystal. We find evidence for a barrier towards entry (exit) of vortices into (out of) the patterned region. Increasing and decreasing applied magnetic field leads to strong exclusion and confinement of vortices and consequently leads to buildup of large gradients in vortex density in the vicinity of the patterned region. This barrier is found to originate from large shielding currents circulating around the periphery of the patterned region. The variation of these shielding currents with applied magnetic field accounts for the observed changes in vortex distribution in and around the patterned region. Our studies reveal the existence of multiple critical current density states inside the patterned region. We have shown that the metastable dilute vortex configuration inside the patterned region can be reconfigured by repeated application of a field step [6,7].