Thermally activated flux motion and quantum creep in YBa₂Cu₃O₇ and Y₂Ba₄Cu₈O₁₆ films

Abstract

The dynamical relaxation rate Q dln js/dln(dB/dt) where js is the current induced in a superconductor when the external field is swept at a rate dB/dt, has been measured in YBa2Cu3O7 and Y2Ba4Cu8O16 films of 100 nm and 180 nm thickness as a function of magnetic field at temperatures between 2 K and Tc. The data show i) a non zero relaxation at low T due to quantum creep, ii) a plateau between 15 and ∼ 50 K and iii) a divergence when T approaches the irreversibility line. The data are analyzed within the collective pinning model. At low temperatures it is shown that both js and Q are influenced by quantum creep.