Fragmentation of magnetic flux in anisotropic superconductors

It is shown that in anisotropic superconductors uniform current flow can be unstable for current densities ${\mathit{j}}_{\mathit{f}1}$<j<${\mathit{j}}_{\mathit{f}2}$, where ${\mathit{j}}_{\mathit{f}1}$ and ${\mathit{J}}_{\mathit{f}2}$ essentially depend on the orientation of j. The instability leads to a stratification of the magnetic flux into macroscopic domains (macrovortices). Densities of closed shielding currents circulating inside the macrovortces can be well above the mean density of the transport current. The relation of such a macrovortex structure to the magnetic granularity of high-${\mathit{T}}_{\mathit{c}}$ oxides is discussed. The anisotropy is shown to result in the instability of both the Bean critical-state model and the uniform vortex glass with current-voltage characteristic of the form V∝exp(-const/${\mathit{j}}^{\mathrm{\mu }}$).