Charla Invitada: Hysteretic effective pinning and vortex lattice reordering near the order-disorder transition: An interplay between plastic and elastic energy barriers?

V. BEKERIS; G. PASQUINI; M. MARZIALI BERMÚDEZ; E. R. LOUDEN; M. ESKILDSEN

Natal

Workshop; 16th international workshop of vortex matter in superconductors.; 2017

History effects related to plasticity, glassy behavior and metastable configurations in the vicinity of an order-disorder transition (ODT) are common features in a variety of systems, among which vortex matter becomes a prototype model. In superconducting materials with randomly distributed weak pinningcenters, as clean NbSe2, the stable vortex phase at low temperature and low magnetic fields is an ordered Bragg Glass (BG) that, with increasing field and/or temperature, undergoes an ODT to a strongly pinned disordered phase, whose fingerprint is the Peak Effect (PE) anomaly. It is well known that a field-cooled vortex lattice (VL) remains trapped below the PE in a more strongly pinned disordered configuration; with the help of high dc currents or large shaking magnetic fields the system can reach the stable ordered BG, free of dislocations, which has lower effective pinning. In the last years, direct evidence confirmed the existence of a narrow in-between transitional region, where a dynamic assistance gives rise to bulk VL configurations with intermediate degree of dislocation density. These dynamically-originated configurations correlate with the observed intermediate linear ac response, associated with the effective pinning. On the other hand, hysteretic effective pinning observed after cooling and/or warming the system without anydynamic assistance are poorly understood up to now. InthistalkIwillbrieflyreviewthekeyresultssupportingtheexistenceofthetransitionalregionandthe VL dynamicre ordering[1,2,3]. I will then present new results[4] that reveal qualitative differences between thermal and dynamic history effects. By combining small angle neutron scattering (SANS) with ac susceptibility measurements in clean NbSe2 single crystals, we carried out a systematic study of the effective pinning and structural characterization of the VL after various thermal histories. We observed metastablesuperheated and supercooled VL configurations that coexist with a hysteretic effective pinning responsedue to thermal cycling of the system. A novel scenario, governed by the interplay between (lower) elastic and (higher) plastic energy barriers, is proposed as an explanation for our observations: Plastic barriers, which prevent the annihilation or creation of topological defects, require dynamic assistance to be overcome. Conversely, thermal hysteresis in the pining response is ascribed to low energy barriers, which inhibit rearrangement within a single VL correlation volume and are easily overcome as the relative strength of competing interactions changes with temperature. Our findings suggest a novel scenario to describe the order-disorder transition and the PE anomaly.