Growth mechanism and electrical transport properties of superconducting YBa₂Cu₃O₇₋ₓ thin films grown by pulsed-laser ablation

The growth mechanism and electrical transport properties of YBa₂Cu_{3,/sub>O7-x, (YBCO) superconducting epitaxial thin films grown by pulsed-laser ablation (PLA) have been investigated. Using x-ray diffraction (XRD), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM), we found that the YBCO thin films grow unit-cell by unit-cell, and films with the c-axis perpendicular to the substrate grow essentially in a Stranski-Krastanov (layer-growth transforming to island-growth) mode. The change from layer-growth to island-growth strongly depends on the lattice constant match between film and substrate. On a MgO substrate, layer-growth only exists for the first one or two YBCO unit-cells, while on SrTiO3 the transition from layer-growth to island-growth occurs much later. We also observed the existence of terraced islands that have a spiral growth feature in the center of the island. However, the spiral growth changes sensitively with the growth conditions (such as temperature) and the choice of substrate. In ultrathin films (< 10 unit-cells), the crystallography and critical temperature are related to the state of strain release in the films. Furthermore, we found that the number density and size of particles, generated in the films by laser ablation can be minimized by changing the growth conditions, especially by decreasing the deposition rate. The transport properties of these as-grown (in situ oxygen annealed) epitaxial YBCO thin films have been described in terms of thermally activated flux motion. The analysis employs a pinning potential that depends on the current density J, the temperature T, and the applied magnetic field B. These dependencies are determined from a range of measurements: the activated resistivity for fields B > Birr; the pinning force density Fp = JcB for B < Birr; the I-V curves for B > and B < Birr; and from the resistive transitions in a magnetic field, where Birr is the irreversibility field. The results are consistent with thermally activated processes and a modified Anderson-Kim model with a pinning potential barrier given by U ∝ exp(-J/Jco(1-t)^1.8B^-1. The exponential J-dependence duplicates the behavior U ∝ J^-mu; (μ ≥ 0.8) of the collective flux creep and vortex glass models in the regime J ≤ Jco, while properly describing the finite pinning potential observed in the flux flow regime as J → 0, Within the context of the flux creep model, results of the set of measurements are self-consistent, and reveal that the vortex glass phase may only exist in the region of low temperatures or of low magnetic fields. After studying heavy-ion irradiated YBCO films, we observed that the irreversibility line shifts close to the flux-melting line, i.e., the thermally activated flux flow (TAFF) region is reduced, and the glassy region is also extended because of strong pinning. Using Brandt's flux depinning model, the pinning potential U and depinning current density J0 were calculated. The results indicate that there are three flux motion regions: glassy phase, giant flux creep, and thermally activated flux flow in the mixed state of high-Tc. superconductors.}