  Transport as well as magnetic relaxation properties of the mixed state were
studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d 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 in the mixed
state below the coupling transition, where dissipation is nonohmic, the current
distribution may be unstable with respect to self-channeling resulting in the
formation of very thin current-carrying layers.
