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Register a new userOscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
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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.
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In this paper, we describe the vortex dynamics under high-amplitude microwave drive and its effect on the surface resistance of superconductors. The vortex surface resistance is calculated with a Montecarlo approach, where the vortex motion equation is solved for a collection of vortex flux lines each oscillating within a random pinning landscape. This approach is capable of providing a detailed description of the microscopic vortex dynamics and in turn important insights into the microwave field amplitude dependence of the vortex surface resistance. The numerical simulations are compared against experimental data of vortex surface resistance at high microwave amplitude measured by means of bulk niobium superconducting-radio frequency cavities operating at 1.3 GHz. The good qualitative agreement of simulations and experiments suggests that the non-linear dependence of the trapped flux surface resistance with the microwave field amplitude is generated by progressive microwave depinning and vortex jumps.
By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.
The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first order vortex lattice melting transition is detected in measurements of the heat capacity or the small angle neutron scattering (SANS) intensity. Direct observation of the vortices via SANS always finds a well ordered vortex lattice. The fluctuation broadening is considered in terms of the Lowest Landau Level theory of critical fluctuations and scaling is found to occur over a large H_{c2}(T) range.
Fundamental and higher harmonics of the AC magnetic susceptibility have been measured on a LaO_0.92F_0.08FeAs sample as a function of the temperature, at various amplitudes and frequencies of the AC magnetic field, with a small superimposed DC field parallel to the AC field. The granularity of the sample has been investigated and the inter-grain and intra-grain contributions have been clearly individuated looking at both the first and third harmonics. The vortex dynamics has been also analyzed, and a comparison with the magnetic behavior of both the MgB_2 and the cuprate superconductors has been performed. Some vortex dissipative phenomena, i.e. the thermally activated flux flow and the flux creep, have been detected in the presented measurements, similar to what obtained on YBCO. Nevertheless, although the general behavior is similar, several differences have been also evidenced between these different classes of superconductors, mainly in the third harmonics. We infer that different vortex dynamics have to be included into the analysis of the magnetic response in this iron-based new material.
Hysteretic ac losses in a thin, current-carrying superconductor strip located between two flat magnetic shields of infinite permeability are calculated using Beans model of the critical state. For the shields oriented parallel to the plane of the strip, penetration of the self-induced magnetic field is enhanced, and the current dependence of the ac loss resembles that in an isolated superconductor slab, whereas for the shields oriented perpendicular to the plane of the strip, penetration of the self-induced magnetic field is impaired, and the current dependence of the ac loss is similar to that in a superconductor strip flanked by two parallel superconducting shields. Thus, hysteretic ac losses can strongly augment or, respectively, wane when the shields approach the strip.
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