Investigation of the electrical dissipation properties of in-plane aligned a-axis YBCO films grown on (100) LaSrGaO 4 substrates

Abstract

a-axis YBa 2Cu 3O 7− δ (YBCO) films have been grown on tetragonal (100) LaSrGaO 4 (LSGO) substrates over a PrBa 2Cu 3O 7− δ (PBCO) template layer, using inverted cylindrical magnetron (ICM) sputtering. Detailed X-ray diffraction studies showed that more than 98% of the grains are in-plane aligned with the substrate axes. We have studied the vortex-related phase-transition-like behaviour in these films from dc electrical measurements made with the transport current along the b- and c-axes, and in magnetic fields up to 1 T. Good scaling collapse of the current–voltage isotherms for a wide range of temperatures about the transition is observed when using an algorithm based upon a continuous thermodynamic phase transition such as that between a vortex–liquid and a vortex–glass. We note that the critical scaling exponents are by no means “universal” as expected by some authors and that the exponent z is far higher than expected from analogies with spin–glass systems. The wide range of temperature over which collapse occurs and the very high values of z make the usual association of z and zν with a length and a time scale implausible. Importantly, the observed “transition” temperature, T t, differs appreciably between the two transport current directions for the same magnetic field and field angle. These observations cast doubt on an interpretation for the vortex-related behaviour based on a continuous thermodynamic phase transition such as that between a vortex–liquid and a vortex–glass and alternative possibilities must be explored.