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.
@article{arxiv.cond-mat/0008353,
title = {Direct Observation of Superheating and Supercooling of Vortex Matter using Neutron Diffraction},
author = {X. S. Ling and S. R. Park and B. A. McClain and S. M. Choi and D. C. Dender and J. W. Lynn},
journal= {arXiv preprint arXiv:cond-mat/0008353},
year = {2007}
}