Abstract
We have discovered two novel types of planar defects that appear in heteroepitaxial YBa2Cu3O7−δ (YBCO123) thin films, grown by pulsed-laser deposition (PLD) either with or without a La2/3Ca1/3MnO3 (LCMO) overlayer, using the combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and electron energy loss spectroscopy (EELS) mapping for unambiguous identification. These planar lattice defects are based on the intergrowth of either a BaO plane between two CuO chains or multiple Y-O layers between two CuO2 planes, resulting in non-stoichiometric layer sequences that could directly impact the high-Tc superconductivity.
Highlights
In an earlier study of YBCO123 thin films heterostructured with La2/3Ca1/3MnO3 (LCMO) overlayers, we observed nanoscale domains of YBCO124 and YBCO247 containing CuO-chain intergrowths, which were attributed to the heteroepitaxial strain and shown to cause attenuation of T c
We focus on defect phases associated with Ba-O and Y-O intergrowths in heteroepitaxial YBCO123 thin films both with and without LCMO overlayers
As we observed in a prior study,[7] whereas an unilayer YBCO film experiences epitaxial strain from only the substrate, the YBCO layer in a bilayer LCMO/YBCO film is subjected to epitaxial strain from both the underlying substrate and the LCMO overlayer
Summary
A great deal of interest has been focused on thin-film heterostructures comprising YBa2Cu3O7−δ (YBCO123) and other complex oxides, because of novel interfacial interactions[1,2,3] that could affect the high-critical temperature (T c) superconductivity.[4,5] YBCO123 in heteroepitaxial form is known to contain a variety of crystalline defects, as its layered perovskite structure is sensitive to lattice strains induced by the epitaxial mismatch.[6,7,8,9,10,11] Since such defects may affect the microscopic pairing mechanism, it is important to determine their lattice structures at the atomic scale. As shown in 2(a), the D1 defect type we observed can be systematically identified in terms of BaO-CuO1−δ intergrowth using atomic-scale EELS mapping whereby an extra Ba layer is detected in the Ba M4,5 map (pointed out with a green arrow in 2(a)).
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