Superheating and supercooling of vortex matter in a Nb single crystal: Direct evidence for a phase transition at the peak effect from neutron diffraction

We report the first observation of a striking history dependence of the str ucture function of vortex matter in the peak effect regime in a Nb single c rystal by using small angle neutron scattering combined with in situ magnet ic susceptibility measurements. Metastable phases of vortex matter, superco oled vortex liquid and superheated vortex solid, have been identified. We i nterpret our results as direct structural evidence for a first-order vortex solid-liquid transition at the peak effect. DOI: 10.1103/PhysRevLett.86.71 2.