Microscopic investigation of moving flux line structures in superconductors using neutron diffraction and µSR

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, while the flux lines are caused to move by a transport current. Using small-angle neutron scattering, we are able to demonstrate directly the alignment of the nearest-neighbour direction of flux lines while they are moving. The results are similar but not identical to those from two-dimensional simulations. We also see the destruction of the flux lattice by plastic flow and the bending of flux lines. We have also employed muSR, which measures the distribution of field values within the mixed state. to obtain information about the speed and degree of order of the motion of flux lines past the unions. Such measurements give complementary information to that obtained by neutron techniques. The general aspects of these measurements, of interest to those working in other areas, are the creation of order in non-equilibrium situations and the interplay of the ordering occurring during dissipation and the ordering caused by the crystal structure of the superconductor which is host to the flux lines.