FLUXON DYNAMICS IN LONG JOSEPHSON-JUNCTIONS IN THE PRESENCE OF A TEMPERATURE-GRADIENT OR SPATIAL NONUNIFORMITY

Fluxon dynamics in nonuniform Josephson junctions was studied both exp erimentally and theoretically. Two types of nonuniform junctions were considered: the first type had a nonuniform spatial distribution of cr itical and bias currents and the second had a temperature gradient app lied along the junction. An analytical expression for the I-V curve in the presence of a temperature gradient or spatial nonuniformity was d erived. It was shown that there is no static thermomagnetic Nernst eff ect due to Josephson fluxon motion despite the existence of a force pu shing fluxons in the direction of smaller self-energy (from the cold t o the hot end of the junction). A phenomenon, the ''zero crossing flux flow step'' (ZCFFS) with a nonzero voltage at a zero applied current, was observed in nonuniform long Josephson junctions. The phenomenon i s due to the existence of a preferential direction for the Josephson v ortex motion. ZCFFS's were observed at certain magnetic fields when th e critical current in one direction but not the other becomes zero. Po ssible applications of nonuniform Josephson junctions in flux flow osc illators and as a superconducting diode are discussed.