Correlation of structural quality with superconducting behavior in epitaxial thin films of Ba2YCu3O7−δ on LaAlO3(100)

Abstract

The correlations between structural quality and superconducting behavior in 1000-Å-thick Ba2YCu3O7−δ (BYCO) films grown on LaAlO3(100) from the coevaporation of BaF2, Y, and Cu, followed by an optimized ex situ annealing process are reported. Epitaxial films with smooth, laminar morphology and excellent crystallinity can be grown to have critical current density Jc values nearly identical to single crystals. This finding contrasts with the typical observation that Jc values in thin films of BYCO are very high compared to those of single crystals. This is attributed to a greater density of flux pinning sites due to structural defects within the films. The most crystalline films presented here have penetration length λ∼2000 Å with temperature dependencies described well by the Bardeen–Cooper–Schrieffer (BCS) theory. Material disorder of two types can be controlled by the high-temperature stage Ta of the annealing process. The first type is point defects and dislocations the same size or smaller than the coherence length ξab, which Rutherford backscattering/channeling suggests decrease in number with increasing Ta. The second is crevices, pinholes, and microcracks, which are at least one to two orders of magnitude larger than ξab. At Ta < 850 °C, crevices, which create areas of nonuniform thickness, occur due to incomplete epitaxial growth and correlate with the presence of weak links. Hence film resistivity is high, Tc is low, and λ is large. As Ta is increased, the film morphology becomes smoother and all electrical properties improve, except for Jc in nonzero applied magnetic fields, since the improved epitaxy correlates with reduced flux pinning. By Ta= 900 °C, the BYCO films are similar to single crystals in both cation alignment and Jc behavior. Above this annealing temperature, pinholes and microcracks develop and increase in both size and density with increasing Ta. Although these relatively large defects do not act as weak links, they do affect magnetic screening (and hence λ), to result in an anomalous temperature dependence that masks the intrinsic BCS behavior.