Physics of strongly correlated electron systems

May 06, 2023

Optical spectroscopy

Neutron Scattering

Raman Scattering

TRISP spectroscopy

Theory

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.

NEWS

ERC Advanced Grant

Bernhard Keimer receives his second Advanced Grant from the European Research Council for the development of spectroscopic instrumentation for terahertz magnonics. more

Spin-Orbit Excitons in a Correlated Metal

Multi-band metals are currently of major interest in the quantum materials community, as epitomized by unconventional superconductors such as FeSe and Sr₂RuO₄. Since the valence electrons are spread out over several energy bands and Fermi surfaces, capturing their mutual interactions and their influence on the macroscopic phase behavior is a major theoretical challenge. We have carried out a combined Raman scattering and theoretical study of spin-orbit excitations in the disorder-free multi-band metal Sr₂RhO₄, where prior experiments had revealed sharp Fermi surfaces. We observe unusual but well-defined excitations around 230 meV, and identify them as excitonic transitions between the spin-orbit multiplets of the Rh ions, analogous to those recently observed in the Mott insulators Sr₂IrO₄ and RuCl₃. Our discovery of atomic-like electronic excitations in a clean d-electron metal opens up a rich source of information on electronic correlations in multi-band metals.

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Distinct spin and orbital dynamics in Sr2RuO4

Distinct spin and orbital dynamics in Sr₂RuO₄

Conduction electrons in quantum materials form delocalized states that can be quantum-coherent over macroscopic length scales, while being subject to local interactions akin to those in atomic physics. This dichotomy spawns a large variety of collective quantum phenomena and remains one of the major challenges of modern condensed matter physics. The square-lattice compound Sr₂RuO₄ has long served as a model system for the influence of atomic-scale correlations and macroscopic electronic properties – including particularly the unconventional superconducting state that forms at low temperatures. Momentum-space maps acquired with a high-resolution x-ray spectrometer have now revealed a separation of energy scales for spin and orbital correlations, which can be attributed to interactions akin to Hund’s rules in atomic physics. The results serve as a testbed for state-of-the-art many-body theories and yield fresh insight into the origin of superconductivity in Sr₂RuO₄.

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Generation of Terahertz Radiation via the Transverse Thermoelectric Effect

Terahertz (THz) radiation is a powerful tool with widespread applications ranging from imaging, sensing, and broadband communications to spectroscopy and nonlinear control of materials. Future progress in THz technology depends on the development of efficient, structurally simple THz emitters that can be implemented in advanced miniaturized devices. Researchers at the Max Planck Institute for Solid State Research and their collaborators at the Universities of Stuttgart and Dresden have demonstrated the generation of terahertz radiation via the transverse thermoelectric effect in thin films of layered conducting transition metal oxides grown on offcut substrates, such that the highly conducting layers subtend an angle with the substrate plane. Ultrafast out-of-plane temperature gradients induced by femtosecond lasers launch in-plane thermoelectric currents leading to intense THz emission. The film geometry allows efficient emission of the resulting THz field out of the film structure. The experimental scheme does not require elaborate fabrication methods and complementary microstructure elements, and is not limited to a particular material or specific operational conditions. It thus offers a simple and promising avenue for versatile THz sources and integrable emitter elements.

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Photo-induced ferromagnetism

Following up on previous evidence of an extended regime of spin-orbital fluctuations in ferromagnetic YTiO₃, a collaboration with the group of Ankit Disa and Andrea Cavalleri at the MPI for Structural Dynamics in Hamburg has shown that intense terahertz (THz) light pulses stabilize ferromagnetic order and enhance the Curie temperature by more than a factor of three. The photo-induced ferromagnetism persists for many many nanoseconds after the light exposure. more