G. Pasquini

Ordered, disordered, and coexistent stable vortex lattices in NbSe2 single crystals.

The peak effect (PE) in the critical current density of type II superconductors has been related to an order-disorder transition in the vortex lattice (VL), but its underlying physics remains a controversial issue. Intrinsic to the PE are strong metastabilities that frequently mask the stationary VL configurations. We follow shaking and thermal protocols in NbSe2 single crystals to access these configurations and examine them by linear ac susceptibility measurements that avoid VL reorganization. We identify three different regions. For T<T1(H), stable VL configurations are maximally ordered. For T>T2(H), configurations are fully disordered and no metastability is observed. In the T1<T<T2 region, we find temperature-dependent stable configurations with an intermediate degree of disorder, possibly associated with the coexistence of ordered and disordered lattices throughout the PE.

Dynamics of superconducting vortices driven by oscillatory forces in the plastic-flow regime

We study experimentally and theoretically, the reorganization of superconducting vortices driven by oscillatory forces near the plastic depinning transition. We show that the system can be taken to configurations that are tagged by the shaking parameters but keep no trace of the initial conditions. In experiments performed in

Depinning and dynamics of ac driven vortex lattices in random media

NbSe_2

Evidence for vortex staircases in the whole angular range due to competing correlated pinning mechanisms

crystals, the periodic drive is induced by ac magnetic shaking fields and the overall order of the resulting configuration is determined by non invasive ac susceptibility measurements. With a model of interacting particles driven over random landscapes, we perform molecular dynamics simulations that reveal the nature of the shaking dynamics as fluctuating states similar to those predicted for other interacting particle systems.

Carbon Nanotubes Addition Effects on MgB2 Superconducting Properties

We study the different dynamical regimes of a vortex lattice driven by ac forces in the presence of random pinning via numerical simulations. The behavior of the different observables is characterized as a function of the applied force amplitude for different frequencies. We discuss the inconveniences of using the mean velocity to identify the depinning transition and we show that instead, the mean quadratic displacement of the lattice is the relevant magnitude to characterize different ac regimes. We discuss how the results depend on the initial configuration and we identify hysteretic effects which are absent in the dc driven systems.