The investigation of the atomic structure of individual defects is critical to the understanding and
precise controlling of the physical properties of materials. And although defects are sometimes
detrimental to functionality, in high temperature superconductors (HTS) are necessary for
providing pinning of magnetic flux and allowing high currents to be carried. Moreover, a strong
enhancement on the vortex pinning in HTS YBa₂Cu₃O₇ (YBCO) films is also found to be
controlled by nanostrain [1], which is attributed to elastic distortions of the crystal lattice at the
nanoscale level. Using aberration-corrected Scanning Transmission Electron Microscopy
(STEM) we explore the complex defect landscape of YBCO nanocomposite thin films.
Combining High Angle Annular Dark Field (HADDF) with Low Angle Annular Dark Field
(LAADF) and local strain analyses we are able to map and quantify the lattice deformations
associated to the defects, which will ultimately determine their self-assembling behavior as well
as their mutual interaction. Our atomic scale investigation shows that the presence of mainly
randomly oriented nanoparticles generates incoherent interfaces within the epitaxial YBCO
matrix, which drastically increases the density of defects, yielding a ramified network of
inhomogeneously distributed nanostrained regions where the crystalline perfection of the
superconductor is perturbed.
Finally, we will compare the microstructure of conventional high-quality solution-derived
trifluoroacetate-YBCO nanocomposites with new fluorine-free films based on a novel transientliquid
assisted growth method (TLAG), which provides ultra-high growth rates with a consequent
influence on the defects landscape. Accordingly, TLAG envisages an enormous potential for lowcost
and high-performance coated conductors.

We acknowledge funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES, ERC-AdG-
2014-669504 ULTRASUPERTAPE and MINECO MAT2014-51778-C2-1-R.

References
[1] Llordés at al., Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in
high-temperature superconductors. Nature Materials, 11, 329 (2012).

Figures:

FIGURE. Top. HAADF-LAADF images showing embedded Ba2YTaO6 nanoparticles within the YBCO matrix as well as Y248 intergrowths. The inset shows the contrast enhancement seen in LAADF imaging due to the strain field associated to the Y248 partial dislocation. Bottom. Local strain analysis showing the deformation of the YBCO lattice due to the presence of two Y248 intergrowths. The deformation is found to be highly localized around the partial dislocations (arrows). Local strains ensuing from the partial dislocations induce a breaking in the twin boundary vertical coherence and a reduction of the twin spacing, as shown in the xx map.

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