Abstract
The superconducting current density j s of YBa 2Cu 3O 7−δ(123) and YBa 2Cu 4O 8 (Y124) epitaxial thin films is studied down to 100 mK as function of the applied magnetic field B e. Torque magnetometry makes it possible to measure j s in magnetic fields up to 7 T in a wide range of magnetic field sweep-rates dB e/dt. From these data we extract the dynamic relaxation rate Q >2.trbnd; dlnj s/d ln (dB e/dt) which extrapolates to Q(0) = 2.2% for Y123 and Q(0) = 2.6% for Y124 at T = 0 K. This provides evidence for vortex motion by tunneling. We calculate the normal state resistivity ϱ n(0) using Q(0) ≈ ϱ n(0) for the case of ohmic dissipation in the vortex cores. The values of ϱ n(0) are comparable for both compounds and similar to ϱ n(0) found for Bi 2Sr 2CaCu 2O 8+°. This indicates that CuO-chains play a minor role and dissipation is mainly taking place in the CuO-planes.
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