Factors that control Jc in high Tc superconductors

Abstract

A study of critical currents, flux creep and vortex line energy was undertaken for a variety of materials to determine some of the factors that control the intragranular critical current. Creep measurements for Nd 1.85Ce 0.15CuO 4 and TI 2Ba 2Ca 2Cu 3O 10, plotted as a function of reduced temperature, t = T/ T c, show essentially the same behaviour. This implies that the Nd compound, which has a much longer coherence distance and may have many electron carriers, has the same effective pinning potential to kT ratio at the same reduced temperature. Hence these two factors do not seem to be dominant in controlling J c. A change in the vortex line energy, however, seems to be very important. With the addition of small amounts of Fe, the line energy and J c both fall exponentially with Fe content in Y 1Ba 2(Cu 1− x , Fe x) 3O 7−σ. Hence the free energy per unit length of vortex is a very important factor controlling the intragranular critical currents.