  The dynamics of transient current distributions in superconducting YBCO thin
films were investigated during and immediately following an external field
ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of
dynamic current profiles. A number of qualitatively unique and previously
unobserved features are seen in this novel analysis of the evolution of
supercurrent during penetration. As magnetic field ramps up from zero, the
dynamic current profile is characterized by strong peaks, the magnitude of
which exceed the conventional critical current density (as determined from
static current profiles). These peaks develop close to the sample edges,
initially resembling screening currents but quickly growing in intensity as the
external field increases. A discontinuity in field and current behaviour is
newly observed, indicating a novel transition from increasing peak current
toward relaxation behaviour. After this transition, the current peaks move
toward the centre of the sample while reducing in intensity as magnetic
vortices penetrate inward. This motion slows exponentially with time, with the
current distribution in the long-time limit reducing to the expected Kim-model
profile.
