Quantum Creep in Y_1-xPr_xBa_2Cu_3O_7$ Crystals: Magnetic Relaxation and Transport

W e report transport and m agnetic relaxation m easurem ents in the m ixed state of strongly underdoped Y 1 (cid:0) x P r x B a 2 C u 3 O 7 (cid:0) (cid:14) crystals. A transition from therm ally activated (cid:13)ux creep to tem perature independent quantum (cid:13)ux creep isobserved in both transportand m agnetic relaxation attem peratures T (cid:20) 5 K . Flux transform erm easurem entsindicate thatthe crossoverto quantum creep ispreceded by a coupling transition.Based on these observations we argue thatbelow the coupling transition the currentiscon(cid:12)ned within a very narrow layer beneath the currentcontacts.