Microstructure and microwave surface resistance of typical YBaCuO thin films on sapphire and LaAlO₃

Abstract

Epitaxial c-oriented YBCO films laser deposited onto 3 in diameter CeO 2-buffered sapphire wafers and LaAlO 3 cylinders as well as sputter deposited onto 2 in diameter LaAlO 3 wafers were characterized by integral and spatially resolved measurements of the critical current density j c and the microwave surface resistance R s , by microstructure investigations using optical and electron microscopy and by x-ray diffraction. Epitaxial misorientations of in-plane-rotated as well as of a-axis-oriented grains were found in amounts up to 10%. The in-plane rotation seriously degraded R s while the a orientation mainly lowered j c. Moreover, a degradation of R s and of the microwave power handling could be clearly correlated with the density of microcracks occasionally found in YBCO films on sapphire. Inhomogeneities like a-axis-oriented grains were observed to 'disperse' microcracks, probably in favour of the electrical properties. The impact of further microstructure imperfections on R s , in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.

Imaging the Anisotropic Nonlinear Meissner Effect in NodalYBa2Cu3O7−δThin-Film Superconductors

“…However, we observe the same anisotropy on YBCO/MgO and YBCO/sapphire films that do not have twinned substrates. The apparent rotation of the PR by 45 o in a narrow temperature range in all of the YBCO samples also argues against a substrate-induced anisotropy. Next is the concern that the shape of the substrate dictates the anisotropy of the PR in the spiral.…”

“…3 show the YBCO/sapphire sample below (4.405 K) and above (6.641 K) the crossover temperature for this sample. We find that the PR maxima are aligned along the nodal directions of YBCO at higher temperatures, and then show a 'rotation' by 45 o at lower temperatures, [43] although the crossover temperature is sample dependent (approximately 4 -5 K for YBCO/LAO and 5 -7 K for YBCO/MgO and YBCO/sapphire). This sample dependence may be associated with different abundances of exposed {110} facets, and twinning in each sample.…”

“…This sample dependence may be associated with different abundances of exposed {110} facets, and twinning in each sample. YBCO films typically show twin separations less than 100 nm [44] and have complex bi-directional twinning, [45] meaning that our focused laser spot averages over a great many twin domains and any anisotropic effects of twinning will not be manifested on the scale of the spiral images. Note that the global nodal and anti-nodal directions are preserved in twinned structures.…”

Recent developments in the characterization of superconducting films by microwaves

“…1 shows R eff (T) of a 400nm-thick YBCO film on a 340nm-thick CeO 2 /YSZ/CeO 2 trilayer on textured Ni [41]. While the low normal-state resistance is related to the low impedance of the thin buffer and the metallic substrate, the high R s -level in the superconducting state is attributed to low-and large-angle grain boundaries or microcracks [5,[42][43][44]. These effects add not only to the total surface resistance but lead also to a larger thickness correction due to an enhanced penetration length.…”

Post Deposition Heat Treatment Effects on Ceramic Superconducting Films Produced by Infrared Nd:YAG Pulsed Laser Deposition

“…4d), the surface of the oxygen annealed Y-123 film show that the grains interconnect laterally forming larger particulates with extended grain boundaries (indicated by squares). This is a common feature of Y-123 films especially when solid state sintered targets are used as the material source for the deposition [15,21]. While the heat treatment helps in densifying the grains, it also aids in coalescence of grains and forming an alignment of the material relative to the substrate orientation [10].…”