Nonlinear electrodynamic response of superconducting materials near transition

One of the important problems of applied superconductivity is to understand, well describe and control the electromagnetic response due to flux motion. We show that the transport properties of type-II superconductors can be well described by a nonlinear response function. In connection with the superconducting- normal state (S-N) resistive transition we find further a model R(T, H) equation which fairly agrees with the experimental data of MgB/sub 2/ as well as the untwined high critical temperature YBCO crystals. Instead of the constant R in the Ohm's law of normal metals, combining with Maxwell equations, this nonlinear resistance function over wide range of temperature, field and current provides a useful basis for engineering design and analysis.