STRUCTURE AND CORRELATIONS OF THE FLUX LINE LATTICE IN CRYSTALLINE NB THROUGH 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. Augmented by transport and thermodynamic data, a scenario for the dramatic disordering 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 critical current rises, the orientational and longitudinal correlation lengths also fall, leading to an amorphous array of lines at the critical current maximum.