Physics of strongly correlated electron systems

The department uses neutron and X-ray diffraction and spectroscopy as well as optical spectroscopy and Raman scattering to explore the structure and dynamics of materials with strong electron correlations. We also have a strong effort in the development of new spectroscopic methods. As the close collaboration between experimentalists and theorists is essential for the progress in this field, a small theory group operates within the department.


Martin Bluschke wins Springer Thesis Prize
Collective quantum phenomena of correlated electrons are central topics of research in condensed matter physics. Martin Bluschke has exploited chemical and epitaxial degrees of freedom to manipulate charge and spin ordering phenomena in two families of transition metal oxides, while taking advantage of state-of-the-art resonant x-ray scattering (RXS) methods to characterize their microscopic origin in a comprehensive manner. In copper-oxide thin films, he discovered an unusual three-dimensionally long-range-ordered charge density wave state that persists well above room temperature, much higher than charge-ordered states in other high-temperature superconductors (Nature Comm. 9, 2978 (2018)). By combining crystallographic and spectroscopic measurements, he was able to trace the origin of this phenomenon to the epitaxial relationship with the underlying substrate. His discovery opens new perspectives for the investigation of charge order and its influence on the electronic properties of the cuprates. In a second set of RXS experiments on superlattices with alternating nickel and dysprosium oxides, he discovered several temperature- and magnetic-field-induced magnetic phase transitions. He was able to explain these observations in a model based on transfer of magnetic order and magneto-crystalline anisotropy between the Ni and Dy subsystems, thus establishing a novel model system for the interplay between transition-metal and rare-earth magnetism (Phys. Rev. Lett. 118, 207203 (2017)). In December 2019, Martin graduated with distinction from the Technical University of Berlin. The prize he received implies that this doctoral thesis will be published as a book in the Springer Theses series. more
Dr. Yi Lu wins the Otto-Hahn-Medal of the Max Planck Society and the Karl-Freudenberg-Prize of the Heidelberg Academy of Sciences

Dr. Yi Lu wins the Otto-Hahn-Medal of the Max Planck Society and the Karl-Freudenberg-Prize of the Heidelberg Academy of Sciences

Dr. Yi Lu has won both the Otto-Hahn-Medal of the Max Planck Society and the Karl-Freudenberg-Prize of the Heidelberg Academy of Sciences for his outstanding Ph.D. thesis which combines experimental, analytical, and numerical research on the collective dynamics of quantum materials.
<strong>Bond order and spin excitations in nickelate thin-film structures</strong>
Over the past several years, rare-earth nickelates (RNiO3) have been the subject of intense investigation because of their rich phase diagram that comprises a sharp temperature-driven metal-to-insulator transition and an unusual antiferromagnetic ground state. Recently, we used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni L3 edge to study simultaneously the bond and magnetic orders that develop in these materials and that can be controlled via heterostructuring. Following our initial measurement of magnetic excitations in bulk-like films [1,2] we have now observed striking variations of the spin dynamics in different nickelate heterostructures and were able to explain these data in terms of a microscopic model [3]. The results show that RIXS gives valuable insights into the interplay of different collective ordering phenomena in this prototypical transition-metal oxide.

[1] Y. Lu et al., Phys. Rev. X 8, 031014 (2018)
[3] K. Fürsich et al., Phys. Rev. B 99, 165124 (2019) - Editors' Suggestion more
<p><strong>Spin waves from a microcrystal</strong></p>

An international team of scientists from the Max Planck Institute for Solid State Research, DESY, the University of Stuttgart, Postech, and the University of Tokyo has determined the spectrum of collective magnetic excitations (“spin waves”) in a ruthenium-oxide antiferromagnet, which exhibits an unusually high magnetic ordering temperature. Such measurements are important because they yield insight into the magnetic interactions between spins inside the material, but they usually require large single crystals that are difficult to synthesize. By using the newly developed IRIXS spectrometer at PETRA III, the research team was now able to obtain a complete set of measurements on a microcrystal invisible to the naked eye. The experiment demonstrates the power of the IRIXS method as a novel probe of elementary excitations in a large class of magnetic materials.

“Spin waves and spin-state transitions in a ruthenate high-temperature antiferromagnet”
H. Suzuki, H. Gretarsson, H. Ishikawa, K. Ueda, Z. Yang, H. Liu, H. Kim, D. Kukusta, A. Yaresko, M. Minola, J. A. Sears, S. Francoual, H.-C. Wille, J. Nuss, H. Takagi, B. J. Kim, G. Khaliullin, H. Yavaş and B. Keimer, Nature Materials (2019)

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