VORTEX DYNAMICS AND MELTING IN NIOBIUM

Small angle neutron scattering has been used to investigate the vortex scattering in single crystal niobium. In the mixed state (H(c1) < H < H(c2)) a sixfold hexagonal pattern of peaks is observed at all temper atures. Below the irreversibility line these peaks are resolution limi ted, while above it the width in the transverse direction increases wi th temperature due to the vortex dynamics. Close to H(c2) the radial w idths of the peaks also broaden. However, the basic hexagonal pattern of peaks is maintained throughout, indicating that a correlated flux f luid exists in the reversible regime.