Direct Observation of Superheating and Supercooling of Vortex Matter   using Neutron Diffraction
  We report the first observation of a striking history dependence of the structure function of the vortex matter in the peak effect regime in a Nb single crystal by using small angle neutron scattering combined with {\it in situ} magnetic susceptibility measurements. Metastable phases of vortex matter, supercooled vortex liquid and superheated vortex solid, have been identified. We interpret our results as direct structural evidence for a first-order solid-liquid transition at the peak effect. 
X. S. Ling, S. R. Park, B. A. McClain, S. M. Choi, D. C. Dender, and   J. W. Lynn