Symposium on Neutron Scattering Studies of Vortex Structures in Superconductors J. Lynn, Chairman Neutron scattering studies of the vortex lattice in niobium and ‘8123 superconductors (invited)

The magnetic flux lattice undergoes a melting transition not only in high-T, oxide superconductors, but also in conventional superconductors, as recently observed in superconducting niobium films. Small-angle neutron scattering was used to investigate the properties of the magnetic flux lattice in a large, high-quality single crystal of niobium. The small London penetration depth of niobium gives a large magnetic scattering signal, and the use of a high-quality single crystal eliminates other unwanted scattering (from twin boundaries, voids, etc.). The signal-to-noise ratio is therefore improved by several orders of magnitude over the best available measurements of high-T, oxide superconductors. A sixfold hexagonal pattern of peaks is observed in the mixed state (H,t<HCH,,) at all temperatures. These peaks are resolution limited below the irreversibility line; above it, the width in the transverse direction increases with temperature due to the vortex dynamics. Close to Hc2, the radial widths of the peaks also broaden. The increase in broadening is a direct observation of a transition to a disordered phase. Nevertheless, the basic hexagonal pattern of peaks is maintained throughout the mixed state, indicating that a correlated flux fluid exists in the reversible regime. Some results on the vortex lattice in superconducting DyBa,Cu,O, are presented and some of the possible exotic states resulting from the coexistence of antiferromagnetic order and superconductivity are described.