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Orbital reflectometry of oxide heterostructures

Abstract

The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties1. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties2,3,4,5,6,7,8, but could not thus far be probed in a quantitative manner9,10,11. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of 3% in the occupation of Ni eg orbitals in adjacent atomic layers of a LaNiO3–LaAlO3 superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.

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Figure 1: Sketch of the LNO–LAO superlattice with layer stacks of four pseudo-cubic unit cells (u.c., see the red box) investigated in this work.
Figure 2: Momentum-dependent X-ray reflectivity of the (4 u.c.//4 u.c.) × 8 LNO–LAO superlattice.
Figure 3: Polarization-dependent XAS spectrum (FY) across the Ni L2,3 edges.
Figure 4: Energy scans of the reflectivity data with constant momentum transfer qz.

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Acknowledgements

We acknowledge financial support from the Deutsche Forschungsgemeinschaft within the framework of the TRR80, project C1. The authors thank G. Khaliullin, V. Kiryukhin, and G. A. Sawatzky for discussions. We acknowledge the provision of synchrotron radiation and the assistance from W. Mahler and B. Zada at the UE562-PGM1 beamline at Helmholtz-Zentrum Berlin—BESSY II. We thank M. Dudek for making the hard-X-ray reflectivity measurements, and S. Heinze for taking the atomic force microscopy image.

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Contributions

E.B. carried out the experiments and analysed the data. M.W.H. made substantial contributions to the data analysis and carried out the cluster calculations. S.B. and E.G. designed and set the experiment up. S.B., E.G. and S.M. developed the analysis tool ReMagX. G.C. and H-U.H. grew the superlattices by pulsed laser deposition. A.F., E.B., A.V.B. and P.W. characterized the samples by high-resolution X-ray diffraction. X.Y. and O.K.A. carried out the LDA+U calculations. I.Z. and H.J.K. assisted in the experiments. V.H. worked on the data collection and analysis. E.B., M.W.H., V.H. and B.K. wrote the paper. V.H. and B.K. coordinated the project.

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Correspondence to Vladimir Hinkov or Bernhard Keimer.

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The authors declare no competing financial interests.

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Benckiser, E., Haverkort, M., Brück, S. et al. Orbital reflectometry of oxide heterostructures. Nature Mater 10, 189–193 (2011). https://doi.org/10.1038/nmat2958

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