Dynamic measurement of flux flow resistivity in YBa/sub 2/Cu/sub 3/O/sub 7/ wires

Polycrystalline samples of YBa/sub 2/Cu/sub 3/O/sub 7/ have been pushed deep into the flux flow regime using submillisecond feedback-controlled current ramps. Thermal stability has been maintained in the 65-93 K range in spite of current densities that exceed J/sub c/ by a factor of four or more. Linear I-V relations are observed, and the measured flux flow resistivity shows a simple dependence on the microstructure of the samples. The resistivity of unoriented ceramic samples shows essentially no dependence on magnetic field between 0.05 T and 0.5 T, and shows a slight dependence on temperature that mirrors the linear behavior of the normal state resistivity down to about 72 K. Below 72 K the flux flow resistivity begins to rise with dropping temperature. In contrast, directionally solidified samples show a much lower flux flow resistivity with almost no temperature dependence. A strong field dependence appears in this latter case, one that is reminiscent of the Bardeen-Stephen flux flow resistivity of low T/sub c/ materials.<
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