Physical phenomena at interfaces of complex oxide hetero-structures have stimulated intense research activities due to the occurrence of a broad range of electric and magnetic functionalities that do not pertain to the constituting single phases alone. Interface effects have been proven to be a powerful tool for improving or even inducing novel functionalities [1, 2]. In the case of interface superconductivity, the interatomic structure relaxation and charge transfer play a key role. In this work, we combine atomic-resolved quantitative STEM imaging with analytical STEM-EELS/EDX analysis to investigate interface effects in La2CuO4 (LCO)-based hetero-structures, i.e. superlattices of two-dimensionally (2D) Sr-doped LCO and LCO/La2-xSrxNiO4 (LSNO) hetero-structures, both exhibiting Tc up to 40 K, despite its non-superconducting constituents. We lay emphasis on detailed and quantitative STEM analysis to understand the interplay between interface effects (cation and electron hole redistribution, as well as local lattice and oxygen octahedral distortion) and high-temperature interfacial superconductivity.
With atomically resolved EDXS and EELS analyses, we investigate the cation intermixing, i.e. La/Sr and Ni/Cu, at the interface of superlattices. Oxygen K-edge fine structure analysis were used to quantify the electron hole population. The electron hole profile reveals that charge redistribution occurs at the interface. Atomic-resolved high-angle annular dark-field (HAADF) and annular bright-field (ABF) images, which were simultaneously acquired, were used to evaluate the local lattice and copper-apical-oxygen distortions at these interfaces. The interatomic structure analysis shows that the copper-apical-oxygen distance has a remarkable variation near the interface . We observe an anomalous expansion of the apical oxygen–oxygen distance at the downward side of the interface, and a substantial shrinkage of the apical oxygen–oxygen distance on the upward side in the growth direction of the thin film. Finally, the correlation of these results with the observed Tc will be discussed.
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