Emergence of quasi-long-range order below the Bragg glass transition.

We report small angle neutron scattering rocking-curve measurements of the flux line lattices in the peak effect region in a niobium single crystal. It is found that upon cooling in a magnetic field, the transverse orientational order as well as the longitudinal translational order grow rapidly with decreasing temperature, indicating diminishing population of defects in the ordering vortex matter. Surprisingly, during subsequent warming, longitudinal order increases with increasing temperature, presumably due to annealing of flux-lattice screw dislocations. The observed behavior indicates the gradual emergence of the Bragg glass phase from entangled vortex matter in the peak effect region.