Nonlinear dynamics of flux creep in underdoped cuprates

We give a brief overview of transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y1-xPrxBa2Cu3O7-delta crystals. T wo phenomena are especially interesting from the point of view of both fund amental physics of "vortex matter" and power applications of high-T-c super conductors. First is the transition From thermally activated flux creep to temperature independent quantum flux creep which is observed in both transp ort and magnetic relaxation measurements at temperatures T less than or equ al to 5 K. Flux transformer measurements indicate that the crossover to qua ntum creep is preceded by a coupling transition. Second, in the mixed state below the coupling transition, the dissipation is non-ohmic, and we specul ate that, as a result, the current distribution may be unstable with respec t to self-channeling, resulting in the formation of thin current-carrying l ayers.