The electrical properties of YBa2Cu3O7-delta thin films of various thicknesses grown upon MgO substrates

We have studied the vortex-related phase-transition-like behaviour in high quality YBa2Cu3O7-delta (YBCO) films of thickness ranging from 10 to 200 nm grown upon MgO substrates using a DC inverted cylindrical magnetron sputte ring (ICMS) technique. Good scaling collapse of the current-voltage isother ms from the region near the transition has been observed under an algorithm based on the hypothesis that there is a continuous thermodynamic phase tra nsition such as the one proposed for the transition between a vortex-glass and a vortex-liquid. The scaling works for all film thicknesses, and from z ero magnetic field through the fields of the order of I T. Even though we h ave not been able to explore the lower scaling branch of the 10 nm film at higher fields, it is noteworthy that the same algorithm, rather than the on e proposed for 2D behaviour. collapses the data. For the 30-200 nm thick fi lms, we observe systematic variation of the scaling exponents z and nu with film thickness and magnetic field; in particular, z increases substantiall y with decreasing film thickness. For the 10 nm film, the exponents found a re similar to those of very thick films. We point out that the magnitudes o f z nu and the range of temperatures over which good scaling collapse is ob served are not consistent with an interpretation based on a continuous ther modynamic phase transition, which clearly fails to capture some important e lements of the physics. (C) 2000 Elsevier Science B.V. All rights reserved.