Direct observation of apical oxygen vacancies in the high-temperature superconductor $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$

Direct observation of apical oxygen vacancies in the high-temperature superconductor $YB a_{2} C u_{3} O_{7 - x}$

Steven T. Hartman, Bernat Mundet, Juan-Carlos Idrobo, Xavier Obradors, Teresa Puig, Jaume Gázquez, and Rohan Mishra

Phys. Rev. Materials 3, 114806 – Published 22 November 2019

Abstract

The properties of the high-temperature superconductor $YB a_{2} C u_{3} O_{7 - x}$ (YBCO) depend on the concentration of oxygen vacancies ( $V_{O}$ ). It is generally agreed upon that $V_{O}$ form in the CuO chains, even at low concentrations where the critical temperature for superconductivity peaks ( $x \approx 0.07$ ), with only a handful of reports suggesting the presence of $V_{O}$ at the apical sites. In this paper, we show direct evidence of apical $V_{O}$ in optimally doped YBCO samples. Using density-functional-theory calculations, we predict that isolated $V_{O}$ are equally favorable to form in either the CuO chains or the apical sites, which we confirm using atomic-resolution scanning transmission electron microscope imaging and spectroscopy. We further show that apical $V_{O}$ lead to significant lattice distortions and changes in the electronic structure of YBCO, indicating they should be considered on an equal footing with chain $V_{O}$ to understand the superconducting properties of YBCO in the optimal doping region.

Received 3 April 2019
Revised 27 September 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.114806

Physics Subject Headings (PhySH)

Defects Electronic structure Impurities in superconductors Point defects Structural properties Superconducting phase transition Vacancies

Cuprates High-temperature superconductors

Density functional calculations

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Steven T. Hartman ^1,*, Bernat Mundet^2,*, Juan-Carlos Idrobo³, Xavier Obradors², Teresa Puig², Jaume Gázquez ^2,†, and Rohan Mishra ^4,1,‡

¹Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA
²Institut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, 08193 Barcelona, Spain
³Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁴Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, USA

^*These authors contributed equally to this work.
^†Corresponding authors: jgazqueza@gmail.com
^‡Corresponding authors: rmishra@wustl.edu

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Vol. 3, Iss. 11 — November 2019

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Figure 1
(a) The atomic structure of fully oxygenated $YB a_{2} C u_{3} O_{7}$ ( $YBC O_{7}$ ). The locations of the four possible oxygen vacancies are marked. (b) Oxygen vacancy formation energy as a function of oxygen stoichiometry, using the same colors and symbols as (a). The solid blue line shows $T_{c}$ , based on experimental data (Ref. [17], while the solid brown line shows $J_{C}$ (Ref. [2]. O(2) and O(3) are combined because they have nearly identical formation energies.
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Figure 2
(a) Raw ABF image of a YBCO thin film with a YBCO-124 intergrowth viewed along the [100] zone axis. (b) The same ABF image with its contrast inverted to show the atomic columns as bright spots. The image has been filtered to reduce the noise. Scale bar: 1 nm. (c) Panel showing two horizontal intensity profiles measured along the BaO planes near to the intergrowth. (d) A simulated contrast-inverted ABF image of a YBCO supercell relaxed with 25% oxygen vacancies in one of the apical planes, marked with a red box, which corresponds to the red intensity trace.
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Figure 3
(a) The change in the interlayer separations for the experimental STEM images and the DFT-calculated YBCO structure with apical vacancies. Each separation is labeled in the schematic. (b) Structural relaxation around an apical $V_{O}$ as calculated using DFT. The left panel shows the location of the vacancy in the pristine $YBC O_{7}$ , while the right panel is a wireframe of the relaxed structure with vectors showing the atomic displacements. The vectors follow the same color scheme as the atoms; red vectors show O movement, blue Cu, magenta Ba, and yellow Y. The vector length is magnified fivefold for clarity.
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Figure 4
(a) EELS O $K$ -edge spectra obtained from the chain O(1) (blue) and plane O(2), O(3) (red) of optimally doped YBCO thin films. The prepeak is marked with an arrow. (b) Comparison of Cu $L$ -edge spectra from $Cu O_{2}$ planes closer to the film surface (black) with more apical $V_{O}$ and farther from it with fewer apical $V_{O}$ (green). (c) Comparison of O $K$ -edge spectra from the same $Cu O_{2}$ planes as in (b). (d) The orbital-projected DOS of the superconducting $Cu O_{2}$ plane atoms directly adjacent to an apical vacancy. The planar $O p_{x}$ , Cu $3 d_{z^{2}}$ and Cu $3 d_{x^{2} - y^{2}}$ states are in magenta, gold, and blue, respectively. The states near 3 eV, highlighted by an arrow correspond to the axial orbital as proposed by Pavarini et al. [21]. The black line in the DOS plots corresponds to pristine YBCO, shown for comparison. (e) Schematic of the plane-chain system before and after the apical oxygen is removed.
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Physical Review Materials

Direct observation of apical oxygen vacancies in the high-temperature superconductor YBa2Cu3O7−x

Steven T. Hartman, Bernat Mundet, Juan-Carlos Idrobo, Xavier Obradors, Teresa Puig, Jaume Gázquez, and Rohan Mishra

Phys. Rev. Materials 3, 114806 – Published 22 November 2019

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Direct observation of apical oxygen vacancies in the high-temperature superconductor $YB a_{2} C u_{3} O_{7 - x}$