Dynamics of flux creep in underdoped single crystals of Y1-xPrxBa2Cu3O7-delta

Transport as well as magnetic relaxation properties of the mixed state were studied on strongly underdoped Y1-xPrxBa2Cu3O7-delta crystals. We observed two correlated phenomena-a coupling transition and a transition to quantum creep. The distribution of transport current below the coupling transition is highly nonuniform, which facilitates quantum creep. We speculate that i n the mixed state below the coupling transition, where dissipation is non-O hmic, the current distribution may be unstable with respect to self-channel ing, resulting in the formation of very thin current-carrying layers.