Current density in YBCO-based Tapes Studied over 8 Decades of Dissipation*

Many applications of superconductors require conduction of high density electric currents in a magnetic field, with minimal dissipation. We investigated the dependence of current density $J$ on electric field $E$ due to motion of depinned vortices, over a range of $\sim $10$^{8}$ in $E$. The materials are pre-commercial YBa$_{2}$Cu$_{3}$O$_{\sim 7}$ coated conductors (3.5$\mu $m) on buffered Hastelloy substrates prepared by SuperPower, Inc. Experimental methods include conventional 4-probe electrical transport at the highest $E$ fields; inductive measurements of magnetic moment $m\sim J$ using a swept magnetic field d$H$/d$t\sim E$ at lower $E$ fields; and time dependent ``flux creep'' measurement where d$m$/d$t\sim E$. At $T$ = 77 K, a power law variation $E\sim J^{n}$ is found. The resulting $E(J)$ dependencies become steeper, i.e., the characteristic $n$-value increases, as $J$ is reduced, reflecting a diverging activation energy for vortex movement. The inductive studies are easily extended to lower temperatures and a wide range of magnetic fields. Implications for applications will be discussed.