Nonlinear dynamics of flux creep in underdoped cuprates

Abstract

We give a brief overview of transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y 1−χPr χBa 2Cu 3O 7−δ crystals. Two phenomena are especially interesting from the point of view of both fundamental physics of “vortex matter” and power applications of high- T c superconductors. First is the transition from thermally activated flux creep to temperature independent quantum flux creep which is observed in both transport and magnetic relaxation measurements at temperatures T ≤ 5 K. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Second, in the mixed state below the coupling transition, the dissipation is non-ohmic, and we speculate that, as a result, the current distribution may be unstable with respect to self-channeling, resulting in the formation of thin current-carrying layers.