Confinement of fluxons by surface columnar defects in Bi1.8Pb0.33Sr1.87Ca2Cu3Oy tapes

We study the effects of surface columnar defects created along about 5% of the sample thickness by means of 0.25 GeV Au ions at different doses on Bi1.8Pb0.33Sr1.87Ca2Cu3Oy high-quality tapes on the vortex dynamics in the sample bulk. Strong phenomena of vortex localization inside the bulk are revealed by shifts of the irreversibility lines (IL's) as well as by their after-irradiation shape. The enhanced IL's exhibit specific characteristics, such as a Bose-glass-like behavior up to quite high fields, with a dose-dependent onset point. Moreover, the irreversible regime expands with decreasing defect density. Such results are consistent with the setting up of confined vortices morphologies. Experimental data concerning the IL with the field orthogonal to the columnar defect direction as well as critical current density enhancements are also reported and discussed

An improved method for quantitative magneto-optical analysis of superconductors

We report on the analysis method to extract quantitative local electrodynamics in superconductors by means of the magneto-optical technique. First of all, we discuss the calibration procedure to convert the local light intensity values into magnetic induction field distribution and start focusing on the role played by the generally disregarded magnetic induction components parallel to the indicator film plane (in-plane field effect). To account for the reliability of the whole technique, the method used to reconstruct the electrical current density distribution is reported, together with a numerical test example. The methodology is applied to measure local magnetic field and current distributions on a typical YBa2Cu3O7−x good quality film. We show how the in-plane field influences the MO measurements, after which we present an algorithm to account for the in-plane field components. The meaningful impact of the correction on the experimental results is shown. Afterwards, we discuss some aspects about the electrodynamics of the superconducting sample

On the optimized mixing of order and disorder in high-Tc ceramic superconductors

Coherent excitations of localized states, represented by a selfsimilar Cayley tree, produce a gap in the density of states, which is deeper as the number of generations increases. The model can suitably describe systems in which an optimized concentration of "defects" allows self similarity of the state-tree and coherent coupling between clusters of localized states. A renormalization of the state-tree leads to a hierarchy of the energy eigenvalues

Switching response of MgB2 thin-film microwave resonators due to local nonlinear Joule heating

We investigated the switching response of MgB2 coplanar waveguide resonators to increase of rf currents. A model based on the heat balance equation is able to account for the observed behaviors, including the presence of a thermal bistability regime. The model also reproduces the temperature dependence of the limiting currents. Results are consistent with the hypothesis that a hot spot is formed due to local Joule heating in correspondence with a weak link.Weak links are modeled as long S-N-S junctions, originating from the transition to the normal state of rows of MgB2 grains. This thermal process is proposed as the main mechanism setting the performance limit of polycrystalline MgB2 films in microwave application