Quantum Creep in $Y_{1-\mathit{x}}{\mathrm{Pr}}_{\mathit{x}}{{\mathrm{Ba}}_2{\mathrm{Cu}}_{3}O}_{7-\mathit{\delta }}$ Crystals: Magnetic Relaxation and Transport

We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ crystals. A transition from thermally activated flux creep to temperature-independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures $T\le 5\mathrm{K}$. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Based on these observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts.