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 Bi2Sr2CaCu2O8 + delta crystals following a sudden change in the applied magnetic field. T wo types of experiments are performed. In the first one the sample is sudde nly exposed to a steady magnetic field smaller than the vortex order-disord er transition field, B-od. In the second type of experiment the sample is i nitially 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 quas iordered state proceeding in opposite directions: from the sample center to ward 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 ev olution of the thermodynamic quasiordered vortex phase in the early stages of its formation. (C) 2001 American Institute of Physics.