Probing single-unit-cell resolved electronic structure modulations in oxide superlattices with standing-wave photoemission

W. Yang, R. U. Chandrasena, M. Gu, R. M. S. dos Reis, E. J. Moon, Arian Arab, M.-A. Husanu, S. Nemšák, E. M. Gullikson, J. Ciston, V. N. Strocov, J. M. Rondinelli, S. J. May, and A. X. Gray

Phys. Rev. B 100, 125119 – Published 9 September 2019

Abstract

Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice $[E u_{0.7} S r_{0.3} Mn O_{3} / L a_{0.7} S r_{0.3} Mn O_{3}] \times 15$ wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.

Received 20 February 2019
Revised 20 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.125119

Physics Subject Headings (PhySH)

Surface & interfacial phenomena

Multilayer thin films Transition metal oxides

X-ray standing waves

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

W. Yang^1,2, R. U. Chandrasena^1,2, M. Gu³, R. M. S. dos Reis⁴, E. J. Moon⁵, Arian Arab^1,2, M.-A. Husanu^6,7, S. Nemšák⁸, E. M. Gullikson⁸, J. Ciston⁴, V. N. Strocov⁶, J. M. Rondinelli³, S. J. May⁵, and A. X. Gray^1,2,*

¹Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA
²Temple Materials Institute, Temple University, Philadelphia, Pennsylvania 19122, USA
³Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
⁴National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA
⁶Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland
⁷National Institute of Materials Physics, Atomistilor 405A, Magurele 077125, Romania
⁸Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*axgray@temple.edu

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Issue

Vol. 100, Iss. 12 — 15 September 2019

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Images

Figure 1
Atomic structural modulations: experiment and theory. (a) HRSTEM-HAADF image of the superlattice along the ${[100]}_{pc}$ projection. The top-left inset shows a magnified image, highlighting local A-site projected displacements. (b) A-site cation positions and displacements ( $Δ X_{c}$ ) determined using the HAADF signal. Atomic sites are color-coded according to the amplitude and direction of the cation displacement with the uncertainties of 0.04 Å and 12.94°, respectively. (c) Atomic-plane-averaged A-site displacement amplitudes calculated via DFT+U, with the error-bars accounting for the variations within the individual A-site monolayers. (d) High-resolution ABF image along the ${[100]}_{pc}$ projection overlaid by the simulated ABF images (yellow dotted boxes). Magnified simulations for ESMO and LSMO are shown in the outsets. (e) Oxygen octahedral rotation and tilt angles, as defined in the diagram on the left side, calculated via DFT+U. The rotation angle is displayed for the equatorial oxygens, while the tilt is shown for the apical oxygens.
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Figure 2
SW-XPS experiment and x-ray optical simulations. (a) Schematic diagram of the sample and the experimental geometry, showing the soft x-ray beam, incident at the grazing angle corresponding to the first-order Bragg condition, and the resultant x-ray SW within the superlattice. (b) The best fits between the experimental (circular markers) and calculated (solid curves) SW RCs for the Eu $4 d$ , La $4 d$ , Mn $3 p$ , and Sr $3 d$ core levels. Calculated curve for Sr $3 d$ appears behind the overlayed experimental markers. (c) The resultant model of the superlattice, which self-consistently describes the shapes and amplitudes of the RCs for every constituent element in the structure (O is shown separately, in Fig. 3). The white-to-blue color scale represents the simulated intensity of the x-ray SW E-field ( $E^{2}$ ) inside the superlattice as a function of depth and grazing incidence angle. The line cuts and the corresponding E-field intensity plots on the right side show that at the grazing incidence angles of 18.7°, 19.2°, and 19.8° the SW preferentially highlights the top, middle and bottom unit-cells of ESMO, respectively.
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Figure 3
Oxygen-derived unit-cell-resolved electronic structure. (a) Two-dimensional intensity plot of the depth-dependent evolution of the $O 1 s$ core-level, with the three key line-cuts, corresponding to the depths of the bottom (green), middle (blue), and top (red) ESMO unit cells. (b) Depth-specific $O 1 s$ spectra extracted from the line cuts in (a). (c)–(f) Spectral components originating from the bottom (c), middle (d) and top (e) ESMO unit cells, as well as the oxygen-containing surface-adsorbed atmospheric contaminant (f). (g) Typical fit and spectral decomposition of the $O 1 s$ spectrum (at 19.2°) using five Voigt peaks, with the four most prominent components labeled 1–4. (h) SW RCs of the unit-cell-specific $O 1 s$ spectral components (solid symbols) and the x-ray optical RC simulations for each individual unit cell comprising the topmost ESMO layer in the superlattice. (i) Plot of the correlation between the experimental binding energies of the unit-cell-specific $O 1 s$ spectral components and the DFT+U-calculated integrated charge on the O atoms in the top three unit cells of ESMO (shown for the Eu(Sr)O and $Mn O_{2}$ planes separately).
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Figure 4
Unit-cell-resolved valence-band electronic structure. (a) DFT+U calculations of the atomic structure of the top three unit cells of ESMO. The layers exhibit structural modulations (in agreement with the HRSTEM measurements), as well as a surface-layer reconstruction characterized by the emergence of the tilted oxygen tetrahedra (see left outset). (b) Atomic-plane-resolved $O 2 p$ -projected pDOS for the atomic planes containing the equatorial and apical oxygen atoms in the topmost ESMO layer. (c) Unit-cell-resolved SW valence-band photoemission spectra, probing the corresponding depth-resolved changes in the matrix-element-weighted DOS. Spectral differences between the top and bottom experimental spectra are shown in the lower panel to help visualize most prominent excursions [features A and B (B′)].
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