STRUCTURE AND CORRELATIONS OF THE FLUX-LINE-LATTICE IN CRYSTALLINE NBTHROUGH THE PEAK EFFECT

We have measured the structure of the field cooled flux line lattice ( FLL) in single crystal Nb using small angle neutron scattering. Augmen ted by transport and thermodynamic data, a scenario for the dramatic d isordering of the FLL near the peak effect emerges. A precursor to the peak effect is an hexatic FLL as positional order is lost. As the cri tical current rises, the orientational and longitudinal correlation le ngths also fall, leading to an amorphous array of lines at the critica l current maximum.