V. Bekeris

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

Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa 2 Cu 3 O 7 single crystals

NbSe_2

Vortex lattice dynamics in Nb films with competing intrinsic random and artificial periodic pinning

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.

Depinning and dynamics of ac driven vortex lattices in random media

We report on transport and ac susceptibility studies below the peak effect in twinned YBa 2 Cu 3 O 7 single crystals. We find that disorder generated at the peak effect can be partially inhibited by forcing vortices to movewith an ac driving current. The vortex system can be additionally ordered below a well-defined temperature where elastic interactions between vortices overcome pinning-generated stress and a plastic-to-elastic crossover seems to occur. The combined effect of these two processes results in vortex structures with different mobilities that give place to history effects.

Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets

We study vortex lattice (VL) dynamics in patterned Nb films containing dense periodic arrays of sub 50 nm magnetic nanodots or holes, by means of ac susceptibility measurements. For both types of samples, we observe matching effects within a wide temperature range, determined by the periodicity of the strong artificial pinning potential. Below a crossover reduced temperature t ∗ = 0.75, the ac vortex mobility and matching fields are different for increasing or decreasing fields. Low temperature field cooling experiments indicate that the hysteretic behavior is not related to different intensities of magnetic induction B in the ac penetrated region of the samples. The fact that hysteresis appears in both patterned samples when the intrinsic random pinning becomes stronger, together with the absence of hysteresis in a reference unpatterned Nb film, suggests that the interplay between random and periodic pinning, independent of its origin, induces VL metastability below t ∗ , which leads to the observed irreversibility. (Some figures in this article are in colour only in the electronic version)

Peak effect in YBCO crystals: statics and dynamics of the vortex lattice

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.

History effects and pinning regimes in solid vortex matter

Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as are available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e. holes) arrays, vortex pinning is the main mechanism for field induced oscillations in resistance R(H), critical current Ic(H), magnetization M(H) and ac-susceptibility χ ac(H) in a broad temperature range. Due to the coherence length divergence at Tc, a crossover to wire network behaviour is experimentally found. While pinning occurs in a wide temperature range up to Tc, wire network behaviour is only present in a very narrow temperature window close to Tc. In this temperature interval, contributions from both mechanisms are operational but can be experimentally distinguished.

Nonconventional short-time dc magnetometer for superconducting films

The oscillatory dynamics and quasi-static Campbell regime of the vortex lattice (VL) in twinned YBa2Cu3O7 single crystals have been explored at low fields near the peak effect (PE) region by linear and non-linear ac susceptibility measurements. We show evidence that the PE is a dynamic anomaly observed in the non-linear response, and is absent in the Labusch constant derived from the linear Campbell regime. Static properties play a major role however, and we identify two H(T) lines defining the onset and the end of the effect. At H1(T) a sudden increase in the curvature of the pinning potential wells with field coincides with the PE onset. At a higher field, H2(T), a sudden increase in linear ac losses, where dissipative forces overcome pinning forces, marks the end of the Campbell regime and, simultaneously, the end of the PE anomaly. Vortex dynamics was probed in frequency dependent measurements, and we find that in the PE region, vortex dynamics goes beyond the description of a power law with a finite creep exponent for the constitutive relation.

Effects of phonon dimensionality in the specific heat of multiwall carbon nanotubes at low temperatures

We propose a phenomenological model that accounts for the history effects observed in ac susceptibility measurements in