Two coexisting vortex phases in the peak effect regime in a superconductor

The critical current in the vortex phase of a type-II superconductor such a s NbSe2 displays a striking anomaly in the vicinity of the superconductor-t o-normal-metal transition. Instead of going to zero smoothly, it rebounds t o a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect(1-3), has remai ned an unsolved problem for 40 years. Here we use a scanning a.c. Hall micr oscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter ph ases with intrinsically different pinning strengths coexist on a macroscopi c scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects(4-6) and an omalous voltage response(4,5) commonly seen when external parameters such a s temperature, magnetic field or transport current are varied. We argue tha t the observed phase coexistence is, in fact, the hallmark of a disorder-dr iven non-thermal phase transition.