References of "Johansen, T. H"
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See detailFlux penetration in a superconducting film partially capped with a conducting layer
Brisbois, Jérémy ULg; Gladilin, V. N.; Tempere, J. et al

in Physical Review B (2017), 95

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides ... [more ▼]

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps protecting the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/µ0σt, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We demonstrate this effect by investigating Nb samples with a thickness step. Some of the observed features, namely the deflection and the branching of the flux trajectories at the border of the thick centre, as well as the favoured flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations. [less ▲]

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See detailVisualizing the ac magnetic susceptibility of superconducting films via magneto-optical imaging
Motta, M.; Colauto, F.; Zadorosny, R. et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(21), 214529

We have established a link between the global ac response and the local flux distribution of superconducting films by combining magnetic ac susceptibility, dc magnetization, and magneto-optical ... [more ▼]

We have established a link between the global ac response and the local flux distribution of superconducting films by combining magnetic ac susceptibility, dc magnetization, and magneto-optical measurements. The investigated samples are three Nb films: a plain specimen, used as a reference sample, and other two films patterned with square arrays of antidots. At low temperatures and small ac amplitudes of the excitation field, the Meissner screening prevents penetration of flux into the sample. Above a certain ac drive threshold, flux avalanches are triggered during the first cycle of the ac excitation. The subsequent periodic removal, inversion, and rise of flux occurs essentially through the already-created dendrites, giving rise to an ac susceptibility signal weakly dependent on the applied field. The intradendrite flux oscillation is followed, at higher values of the excitation field, by a more drastic process consisting of creation of new dendrites and antidendrites. In this more invasive regime, the ac susceptibility shows a clear field dependence. At higher temperatures a smooth penetration occurs, and the flux profile is characteristic of a critical state. We have also shown that the regime dominated by vortex avalanches can be reliably identified by ac susceptibility measurements. [less ▲]

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