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Vesna Srot1 Yi Wang1 Matteo Minola1 Marco Salluzzo2 3 Gabriella Maria De Luca3 2 Bernhard Keimer1 Peter van Aken1

1, Max Planck Institute for Solid State Research, Stuttgart, , Germany
2, CNR-SPIN Napoli Complesso Monte Sant Angelo via Cinthia, Napoli, , Italy
3, E. Pancini, Complesso Monte Sant Angelo via Cinthia, Napoli, , Italy

Intriguing phenomena arising at the atomic scale have been found in functional complex oxide materials in recent years due to significant progress in technical and methodological development in scanning transmission electron microscopy (STEM). New possibilities for direct visualization of light elements have become feasible with introduction of the annular bright-field (ABF)-STEM technique [1]. Simultaneous acquisition of high-angle annular dark-field (HAADF)- and ABF-STEM images has enabled combined imaging of materials systems consisting of light and heavy elements.
In this work, high quality NdBa2Cu3O7 (NBCO) thin films have been deposited on SrTiO3 (STO) substrate by sputtering. The stacking sequence of NBCO layers in direction of the crystallographic c-axis is as follows: CuO-BaO-CuO2-Nd-CuO2-BaO [2]. The perovskite-type structure layers of NBCO are separated by CuO2 planes with Nd atoms present in-between the copper-oxygen planes. Chains of CuO run parallel to the copper-oxygen planes with barium atoms placed between the planes and chains. Variation of the oxygen content in NBCO results in major changes of its physical properties [2]. In the case of non-stoichiometric NdBa2Cu3O7-x, x denotes the amount of O vacancies present in the CuO chain. A previous report shows high dependency of Tc on the charge balance between the copper-oxygen chains and copper-oxygen planes [3]. The chain serves as a charge reservoir from which electrons are transferred to the copper-oxygen planes due to a decrease in oxygen content.
We have employed atomically resolved quantitative STEM imaging to investigate the NBCO film lattice with special emphasis on Cu-O distortion in NBCO by using an aberration-corrected JEOL JEM-ARM200F microscope equipped with a DCOR probe corrector operated at 200 kV. To improve the signal-to-noise ratio and to effectively reduce the image distortion, HAADF- and ABF-STEM images were acquired as series of 10 frames using a short acquisition time (1µs/pixel). Afterwards the images were aligned and merged. The simultaneously acquired HAADF- and ABF-STEM images enabled us to quantitatively analyze the local lattice and copper-apical-oxygen distances.
In addition, by employing extensive STEM image simulations, the lowest detectable oxygen concentrations present in the CuO chains, as visible in ABF images, were related to the sample thickness.

Reference:
[1] S.D. Findlay et al., Appl Phys Lett 95, 191913-1 (2009)
[2] H. Shaked et al., Phys Rev B 41, 4173 (1990)
[3] R. J. Cava et al., Physica C 165, 419 (1990)

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