Magneto-optical imaging of transient vortex states in superconductors

A high temporal resolution magneto-optical system is employed to follow the crystallization process of the quasiordered vortex state in Bi₂Sr₂CaCu₂O_8+δ crystals following a sudden change in the applied magnetic field. Two types of experiments are performed. In the first one the sample is suddenly exposed to a steady magnetic field smaller than the vortex order-disorder transition field, B_od. In the second type of experiment the sample is initially exposed to an external field larger than B_od, and then the field is suddenly reduced. The two types of experiments reveal growth of the quasiordered state proceeding in opposite directions: from the sample center toward its edge in the first experiment, and from the sample edge toward the center in the second experiment. This motion enables tracing of the time evolution of the thermodynamic quasiordered vortex phase in the early stages of its formation.