Current-Voltage Characteristics in a Superconducting Bi-2223
Tape in the Range of Very Low Electric Field

E. S. Otabe, T. Matsushita, T. Kodama, M. Fukuda
Kyushu Institute of Technology

K. Itoh
National Research Institute for Metals

Current-voltage characteristics in the range of very low
electric field were analyzed from a relaxation of the
magnetization in a superconducting Bi-2223 tape.  The
magnetization was measured using a SQUID magnetometer.  It
was found that the observed current-voltage curves in this
range scaled well as predicted by the vortex glass-liquid
transition theory similarly to those in the usual range of
the electric field.  However, the obtained transition field
was much lower than that obtained in the higher field range.
At the same time, the dynamic critical index, z, was much
larger than usual values.  The former result suggests that
the E-J curve varies from concave upword in the higher E
range to concave downword in the lower E range.  This is
consistent with the prediction of TAFF(thermally assisted
flux flow) model but contradictory with the prediction of
the vortex glass-liquid transition theory.  The experimental
result was compared with the theoretical analysis using the
flux creep-flow theory in which the distribution of the
effective flux pinning strength was taken into account.  It
was found that the theoretical result explains approximately
the observed result.  This shows that the essential
mechanism which governs the transport property in the
superconducting tape is the flux pinning.