Vortex avalanches and magnetic flux fragmentation in superconductors

We report results of numerical simulations of non isothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hotspot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hotspot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. Then the hotspot undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multi-filamentary pattern of magnetization currents.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let

Flux penetration into an artificially granular high-Tc superconductor

A YBa2Cu3O7- thin film is patterned into a hexagonal close-packed lattice of disks (2r=50 m) which are touching each other at the circumferences in order to enable the flow of an intergranular current. Such a sample was suggested by Koblischka et al. [Appl. Phys. Lett. 70, 514 (1997)] as a model for a layered granular structure like in a (Pb,Bi)2Sr2Ca2Cu3O10+ (Bi-2223) tape. The magnetization measurements reveal an anomalous position of the low-field peak (central peak) similar to Bi-2223 tapes. Magneto-optic imaging is employed to visualize the local-field distributions. At low magnification, the flux patterns in the intergranular area between the disks are observed. The observations at high magnification reveal the flux penetration and pinning within the disks. It is shown that such samples may serve as model samples for granular high-Tc superconductor