Disorder, metastability, and history dependence in transformations of a vortex lattice

Magnetic screening response of the superconductor $2{\mathrm{H}\ensuremath{-}\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_{2}$ with varying pinning reveals a rich evolution of the peak effect from a history-independent sharp anomaly to a broad and strongly history dependent effect with internal structure. The results display a stepwise disordering of the vortex lattice through transformations affected by both thermal fluctuations and quenched disorder.