Bu. Runge et al., Magneto-optic characterization of defects and study of flux avalanches in high-T-c superconductors down to nanosecond time resolution, LASER PHYS, 10(1), 2000, pp. 53-59
Optical methods offer an intrinsic high potential for experiments with exce
llent spatial and in particular also temporal resolution. Using the Faraday
effect we carried out magneto-optical investigations of high-T-c supercond
uctor thin films in a polarization microscope. Small defects in the superco
nducting material which possess a lower critical current density disturb th
e homogeneous penetration of magnetic flux into a sample when an external m
agnetic field is applied after zero field cooling. This is true even if the
defects are below the sample surface or when the superconducting sample is
covered by a thin layer of another material, e.g., gold, and can be used t
o characterize samples with diameters up to 3 ". For studies of the dynamic
s of magnetic flux in a superconducting sample, a pump-probe setup has been
used. An instability which causes magnetic flux to enter the sample in den
dritic form [1] is triggered by local heating with a focused ns or fs laser
pulse. Part of the beam is separated by a beam splitter, passed through a
variable delay line of suitable length and used for illumination of the sam
ple. For YBa2Cu3O7-delta thin films a spreading velocity of (5 +/- 2) x 10(
4) m/s is found which is an order of magnitude higher than the velocity of
sound. The total area of the dendritic structure formed is found to depend
linearly on the change Delta B-ext of the external magnetic flux density ap
plied before the trigger pulse. No dependence on the pulse duration has bee
n observed, suggesting a purely thermal nature of the trigger process.