Temperature dependence of clusters with attracting vortices in superconducting Niobium studied by neutron scattering

We have investigated the intermediate mixed state of a superconducting niobium sample by Very Small Angle Neutron Scattering.We show that this state is stabilized through a sequence where a regular vortex lattice appears, which then coexists with vortex clusters before vanishing at low temperature. Vortices in clusters have a constant periodicity regardless of the applied field, exhibit a temperature dependence close to the one of the penetration depth. The clusters disappear in the high temperature limit. All the results agree with an explanation in terms of vortex attraction due non local effects, and indicate a negligible role of pinning. Phase coexistence between Abrikosov vortex lattice and vortex clusters is reported showing the first order nature of the boundary line.