X-ray spectroscopy

The research in our group focuses on the investigation of transition-metal oxide heterostructures by means of X-ray spectroscopy with the goal to design new materials with functional properties.
Transition-metal oxides with strong electron correlations host a large variety of technologically interesting phases, ranging from insulators, metals, high-temperature superconductors, ferro-, ferri- and antiferromagnets, ferroelectrics, etc. In an epitaxial heterostructure where two different oxides with possibly competing phases are brought together and form an atomically sharp interface very interesting physics can emerge. One phase can dominate the other, possibly with some modifications or, a new phase is stabilized, which has no bulk-analogue in either constituent. Such new phases may be realized only in some atomic layers near the interface or comprise the complete heterostructure depending on the particular reconstruction. Several individual contributions may be relevant: (i) structural modifications such as changes in bond distances and angles resulting from epitaxial constraints, (ii) charge transfer, (iii) electronic confinement, and (iv) the alteration of magnetic exchange interactions.
We aim to understand the spin, orbital, charge and lattice reconstructions in oxide heterostructures with atomic layer precision. To this end, we mainly use synchrotron-based resonant X-ray scattering and reflectometry. These techniques are very powerful to investigate the local electronic and magnetic structure in multilayers, since they combine the sensitivity to spatial variations of diffraction with the spectroscopic information of X-ray absorption. The successful application of these methods requires the presence of high-quality epitaxial multilayers with atomically sharp interfaces. In collaboration with the Thin Film Technology group, we grow such samples by either pulsed layer deposition or ozone-assisted molecular beam epitaxy. To obtain information about structural quality and changes induced by the heteroepitaxy, we usually rely on hard X-ray diffraction data measured with synchrotron and in-house sources and combine it with local information from electron microscopy (provided by collaborations).
Another important aspect is the close collaboration with theory groups that perform ab initio calculations. These calculations are important for our work in several ways. On one hand they help to guide the design of new heterostructures with interesting phases and to facilitate the experimental search for particular order patterns, on the other hand they are important to understand the complex resonant X-ray scattering form factors.
In terms of material composition, we focus on heterostructures made up of rare-earth nickelates (RNiO₃ with R = rare-earth ion), vanadates (RVO₃), and high-temperature cuprates. These materials have different spin, charge, and orbital ordered phases in a relatively narrow range of parameters and are therefore very interesting both as model systems and from a technological point of view.
Our research is funded by the Max Planck Minerva Program, the Deutsche Forschungsmeinschaft through TRR80 – project G1, and by the Integrated Quantum Science and Technology collaborative (IQST).

Selected Publications

“Complex magnetic order in nickelate slabs“ Nature Physics 14, 1097-1102 (2018) M. Hepting, R. J. Green, Z. Zhong, M. Bluschke, Y. E. Suyolcu, S. Macke, A. Frano, S. Catalano, M. Gibert, R. Sutarto, F. He, G. Cristiani, G. Logvenov, Y. Wang, P. A. van Aken, P. Hansmann, M. Le Tacon, J.-M. Triscone, G. A. Sawatzky, B. Keimer, and E. Benckiser

“Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure“ Phys. Rev. Materials 2, 035001 (2018) F. Wrobel, B. Geisler, Y. Wang, G. Christiani, G. Logvenov, M. Bluschke, E. Schierle, P. A. van Aken, B. Keimer, R. Pentcheva, and E. Benckiser

“Transfer of Magnetic Order and Anisotropy through Epitaxial Integration of 3d and 4f Spin Systems“ Phys. Rev. Lett. 118, 207203 (2017) M. Bluschke, A. Frano, E. Schierle, M. Minola, M. Hepting, G. Christiani, G. Logvenov, E. Weschke, E. Benckiser, and B. Keimer

“Comparative study of LaNiO₃/LaAlO₃ heterostructures grown by pulsed laser deposition and oxide molecular beam epitaxy“ Appl. Phys. Lett. 110, 4 (2017) F. Wrobel, A. F. Mark, G. Christiani, W. Sigle, H.-U. Habermeier, P. A. van Aken, G. Logvenov, B. Keimer, and E. Benckiser

“Quantitative determination of bond order and lattice distortions in nickel oxide heterostructures by resonant x-ray scattering“ Phys. Rev. B 93, 165121 (2016) Y. Lu, A. Frano, M. Bluschke, M. Hepting, S. Macke, J. Strempfer, P. Wochner, G. Cristiani, G. Logvenov, H.-U. Habermeier,   M. W. Haverkort, B. Keimer, and E. Benckiser

“Lattice distortions and octahedral rotations in epitaxially strained LaNiO₃/LaAlO₃ superlattices“ Appl. Phys. Lett. 104, 221909 (2014) M. K. Kinyanjui, Y. Lu, N. Gauquelin, M. Wu, A. Frano, P. Wochner, M. Reehuis, G. Christiani, G. Logvenov, H.-U. Habermeier, G. A. Botton, U. Kaiser, B. Keimer, and E. Benckiser

“Strain and composition dependence of the orbital polarization in nickelate superlattices“ Phys. Rev. B 88, 125124 (2013) M. Wu, E. Benckiser, M. W. Haverkort, A. Frano, Y. Lu, N. Nwankwo, S. Brück, P. Audehm, E. Goering, S. Macke, V. Hinkov, P. Wochner, G. Christiani, S. Heinze, G. Logvenov, H.-U. Habermeier, and B. Keimer

“Orbital Reflectometry of Oxide Heterostructures“ Nature Materials 10, 189 (2011) E. Benckiser, M.W. Haverkort, S. Brück, E. Goering, S. Macke, A. Franó, X. Yang, O. K. Andersen, G. Cristiani, H.-U. Habermeier, A.V. Boris, O. Zegkinoglou, P.Wochner, H.-J. Kim, V. Hinkov, and B. Keimer