NEUTRON-SCATTERING STUDIES OF THE VORTEX LATTICE IN NIOBIUM AND R123 SUPERCONDUCTORS

The magnetic flux lattice undergoes a melting transition not only in h igh-T-c oxide superconductors, but also in conventional superconductor s, as recently observed in superconducting niobium films. Small-angle neutron scattering was used to investigate the properties of the magne tic flux lattice in a large, high-quality single crystal of niobium. T he small London penetration depth of niobium gives a large magnetic sc attering signal, and the use of a high-quality single crystal eliminat es other unwanted scattering (from twin boundaries, voids, etc.). The signal-to-noise ratio is therefore improved by several orders of magni tude over the best available measurements of high-T-c oxide supercondu ctors. A sixfold hexagonal pattern of peaks is observed in the mixed s tate (H-c1<H<H-c2) at all temperatures. These peaks are resolution lim ited below the irreversibility line; above it, the width in the transv erse direction increases with temperature due to the vortex dynamics. Close to H-c2, the radial widths of the peaks also broaden. The increa se in broadening is a direct observation of a transition to a disorder ed phase. Nevertheless, the basic hexagonal pattern of peaks is mainta ined throughout the mixed state, indicating that a correlated flux flu id exists in the reversible regime. Some results on the vortex lattice in superconducting DyBa2Cu3O7 are presented and some of the possible exotic states resulting from the coexistence of antiferromagnetic orde r and superconductivity are described.