Evidence for vortex tunnel dissipation in deoxygenated YBa2Cu3O6.4 thin films - art. no. 054509

We report on transport and magnetic relaxation measurements of deoxygenated YBa2Cu3O7-delta thin films. Strongly oxygen depleted samples with delta = 0.6 are produced to ensure the pure two-dimensional nature of the vortex sy stem. Linear resistivity shows a temperature dependence according to rho (l in)proportional to exp[-(T-o/T)(p)]. T-o takes a value of 230 +/- 10 K over the whole field range, and p changes from 1 +/- 0.03 at 2 T to 0.70 +/- 0. 03 at 8 T. For fields higher than 4 T, dissipation in the linear regime (lo w current densities) is dominated by quantum variable range hopping (VRH) o f vortices. At high current densities and low temperatures, nonlinear dissi pation takes place by quantum creep, characterized by a temperature-indepen dent resistivity and by a saturation of the magnetic relaxation rate.