37 research outputs found
Depinning and dynamics of AC driven vortex lattices in random media
We study the different dynamical regimes of a vortex lattice driven by AC
forces in the presence of random pinning via numerical simulations. The
behaviour of the different observables is charaterized as a function of the
applied force amplitude for different frequencies. We discuss the
inconveniences of using the mean velocity to identify the depinnig 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 new hysteretic
effects which are absent in the DC driven systems.Comment: 6 pages, 4 figure
Dynamic reorganization of vortex matter into partially disordered lattices
We report structural evidence of dynamic reorganization in vortex matter in
clean NbSe by joint small angle neutron scattering and ac-susceptibility
measurements. The application of oscillatory forces in a transitional region
near the order-disorder transition results in robust bulk vortex lattice
configurations with an intermediate degree of disorder. These
dynamically-originated configurations correlate with intermediate pinning
responses previously observed, resolving a long standing debate regarding the
origin of such responses.Comment: 9 pages, 7 figures. To be published in Physical Review Letter
Thermal patterning of superconducting films
Includes bibliographical references (page 600).The transient shape control of a superconducting high Tc film by selective optical heating has been addressed in this work, using a non-conventional technique that combines the application of a pulsed magnetic field and the optical heating of the superconducting film by a synchronized pulsed laser. A zero field cooled (ZFC) film was partially protected by a mask before exposing it to the laser radiation. The temperature at the illuminated area rises causing very fast local lowering of the vortex pinning force or may even rise above Tc. As a result, flux penetrates completely the heated area and an effective smaller sample remains in a superconducting critical state. Flux penetration is measured with a pick-up coil for different positions of the mask. Results are described within the critical state model for a strip, where the time integrated voltage signal is approximated to the flux difference between the initial ZFC sample and the final critical state of the "new" (cold) ZFC sample
Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
In this work we study by ac susceptibility measurements the evolution of the
solid vortex lattice mobility under oscillating forces. Previous work had
already shown that in YBCO single crystals, below the melting transition, a
temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular)
can heal the vortex lattice (VL) and increase its mobility, but a temporarily
asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into
a more pinned disordered state. In this work we present evidence that the
mobility of the VL is reduced for large vortex displacements, in agreement with
predictions of recent simulations. We show that with large symmetric
oscillating fields both an initially ordered or an initially disordered VL
configuration evolve towards a less mobile lattice, supporting the scenario of
plastic flow.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Peak effect in YBCO crystals: Statics and dynamics of the vortex lattice
Oscillatory dynamics and quasi-static Campbell regime of the vortex lattice
(VL) in twinned YBa2Cu3O7 single crystals has 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 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.Comment: 8 pages, 5 figure
Nonconventional short-time dc magnetometer for superconducting films
Includes bibliographical references.A nonconventional technique for the measurement of magnetic relaxation in superconducting films at short times (~ 10-5 s) is described. This technique combines the application of a pulsed magnetic field and a synchronized high-energy pulsed laser. Remanent magnetic relaxation in (Y;Gd)Ba2Cu3O7-δ thin films is reported over five decades time at reduced temperatures above 0.8.This work was partially supported by Fundació Sauberán, EX018-UBA and EX285-UBA grants
Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets
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 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 Xac(H) in a
broad temperature range. Due to the coherence length divergence at Tc, a
crossover to wire network behavior is experimentally found. While pinning
occurs in a wide temperature range up to Tc, wire network behavior 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
Plasticity and memory effects in the vortex solid phase of twinned YBa2Cu3O7 single crystals
We report on marked memory effects in the vortex system of twinned YBa2Cu3O7
single crystals observed in ac susceptibility measurements. We show that the
vortex system can be trapped in different metastable states with variable
degree of order arising in response to different system histories. The pressure
exerted by the oscillating ac field assists the vortex system in ordering,
locally reducing the critical current density in the penetrated outer zone of
the sample. The robustness of the ordered and disordered states together with
the spatial profile of the critical current density lead to the observed memory
effects