Sr14-xCaxCu24O41 is a fascinating member in the family of cuprates, not only because of its peculiar crystal structure where two distinct units, corner-shared CuO2 chains and edge-shared Cu2O3 ladders, coexist within the unit cell but also because it is the only known superconductor with a non two-dimensional CuO2 plane structure. Indeed, the superconducting state theoretically predicted by E. Dagotto and T.M. Rice  was first observed experimentally in Sr0.4Ca13.6Cu24O41.84 below Tc = 12 K and for pressures starting from 3 GPa .
Independently of the Ca composition, Sr14-xCaxCu24O41 is an intrinsically hole-doped compound with 6 holes per formula unit, leading to an average Cu valence of +2.25. A central issue for understanding the mechanisms leading to superconductivity in this compound is therefore to measure accurately the carrier distribution among CuO2 chains and Cu2O3 ladders. This task has been undertaken shortly after the discovery of superconductivity in this system  but is still a matter of intense debate due to the very scattered nature of the results. For instance, depending on the technique, reported hole concentrations in the ladder layers of Sr3Ca11Cu24O41 vary from ~1 to ~4.5 holes/formula unit [4,5].
All these results have been obtained with techniques that have a relatively poor spatial resolution ranging from several hundred nanometers to a few micrometers. In this work, we exploit the unmatched spatial-resolution of STEM-EELS to measure local hole concentration in superconducting Sr3Ca11Cu24O41 at the atomic scale and provide, for the first time, a real-space measurement where spatial separation between chains and ladders is achieved . As shown by F.C. Zhang and T.M. Rice , in doped cuprates, the hole strongly binds to the four O atoms surrounding the central Cu through Cu3d-O2p in-plane sigma hybridization within the CuO4 plaquettes. As such, the local hole concentration can be monitored very efficiently through the O-K pre-edge structures as shown in Figure 1. These experimental results, combined with inelastic scattering calculations, demonstrate unambiguously that holes lie preferentially within the CuO2 chains of the structure.
In summary, this work illustrates how the combination of near-edge fine-structure analyses with atomic resolution in the aberration corrected STEM can improve the understanding of the electronic properties of complex oxides, such as Cu-based superconductors .
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 The experimental work has been performed at the Canadian Centre for Electron Microscopy, a national facility supported by NSERC, the Canada Foundation for Innovation and McMaster University. Financial support by the Austrian Science Fund (FWF) under grant nr. J3732-N27 is gratefully acknowledged.
To cite this abstract:Matthieu Bugnet, Guillaume Radtke, Stefan Löffler, Peter Schattschneider, David Hawthorn, Hanna A. Dabkowska, Graeme M. Luke, George A. Sawatzky, Gianluigi A. Botton; Quantifying the hole distribution in cuprates: Atomic-resolution near-edge fine-structures of the superconductor Sr3Ca11Cu24O41. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/quantifying-the-hole-distribution-in-cuprates-atomic-resolution-near-edge-fine-structures-of-the-superconductor-sr3ca11cu24o41/. Accessed: January 21, 2022
EMC Abstracts - https://emc-proceedings.com/abstract/quantifying-the-hole-distribution-in-cuprates-atomic-resolution-near-edge-fine-structures-of-the-superconductor-sr3ca11cu24o41/