QUANTUM CREEP AND PINNING PROPERTIES OF OXYGEN-DEFICIENT YBA2CU3OXN FILMS

A high sensitivity capacitance torquemeter has been used for a compreh ensive investigation of the induced current densities and dynamic rela xation rates in a YBa2Cu3Oxn film With nominal oxygen content varying between x(n) = 6.55 and x(n) = 7.0. The dynamic relaxation rate Q does not extrapolate to zero at T = 0 K, indicating the presence of quantu m creep. By changing the oxygen content of the film it is possible to investigate the relation between the quantum creep rate Q(0) and the n ormal-state resistivity rho(n)(0) at low temperature. Although Q(0) in creases monotonically with rho(n), it is found that Q(0) is not propor tional to rho(n)(0), in contrast to the predictions of a theory based on dissipative tunneling of collectively pinned single vortices [Blatt er et al., Rev. Mod. Phys. 66, 1125 (1994)]. The experimental results imply that in YBa2Cu3O7 quantum creep takes place in a transition regi me between Hall tunneling and dissipative tunneling. For lower oxygen contents the quantum creep regime moves towards the dissipative limit. For each oxygen content the characteristic pinning energy U-c(0) at T = 0 is obtained by a linear extrapolation to T = 0 K of the T/Q versu s T curves. The critical current density j(c) at T = 0 is determined i ndependently by a linear extrapolation of the measured Inj(s) versus T curves. A power-law relation U-c(0)proportional to[j(c)(0)](p) with p approximate to 0.5 is found, indicating single vortex pinning at high er temperatures. This is confirmed by a detailed analysis of the measu red current densities and relaxation rates by means of the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48, 13 178 (1993)]. For x(n) greater than or equal to 6.6 at B-e = 0.6 T and for x(n) greater than or equal to 6.7 at B-e = 2.0 T the calculated tempe rature dependence of j(c) and U-c agrees remarkably well with a model based on three-dimensional single vortex pinning caused by spatial flu ctuations in the charge carrier mean free path. At lower oxygen conten ts and higher magnetic-fields the agreement gradually breaks down due to the increasing importance of thermal fluctuations.