Physics of strongly correlated electron systems

Optical spectroscopy

A metallic object with attached panels displaying rows of spheres and arrows, suggesting directional flow on a checkered red grid.

Neutron Scattering

Raman spectroscopy setup with several lenses and mirrors, and one green laser beam.

Raman Scattering

TRISP spectroscopy

Two complex 3D structures with smooth surfaces, one labeled "isospin up" and the other "isospin down," featuring red and blue color gradients.

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

Denise Christovam wins the 2026 Elisabeth Bauser Fellowship

On January 14th, 2026, the Max Planck Institute for Solid State Research in Stuttgart awarded the Dr. Elisabeth Bauser Fellowship to Denise S. Christovam and Jiyeon Kim. Denise obtained her doctoral degree at TU Dresden in 2025, with a thesis on x-ray spectroscopy of cerium- and uranium-based materials. She will continue this line of research as a postdoctoral researcher in the Solid State Spectroscopy Department. Denise commented: “I hope to learn a great deal and to deepen my knowledge of the techniques I have been working with. I cannot think of a better place to gain a deeper understanding of how RIXS and optical Raman spectroscopy work, and to study cuprates and high-temperature superconductivity, than Professor Keimer’s department." more

Bernhard Keimer Named Honorary Senator of the University of Stuttgart

At its 2025 Annual Celebration, the University of Stuttgart awarded its Honorary Senatorship to Bernhard Keimer. The distinction recognizes not only his scientific achievements but also his long-standing commitment to strengthening the university and supporting its strategic development.
In his laudation, Rector Peter Middendorf emphasized Keimer’s significant impact on the university — as a researcher, as Chair of the University Council, and as a reliable partner in research and education. His international scientific recognition, including a Leibniz Prize and two ERC Advanced Grants, further reflects the breadth of his contributions. more

Uniaxial-Pressure Boosts Excitonic Fluctuations

Studying the properties of Ta₂NiSe₅ to test the hypothesis that its ground state is an excitonic insulator has been a decade-long research project combining the efforts of our research groups from two departments at the Max Planck Institute for Solid State Research: Solid State Spectroscopy (Bernhard Keimer) and Quantum Materials (Hidenori Takagi), together with the Scientific Facility Crystal Growth. Our experiments on Raman scattering under uniaxial compressive strain reveal enhanced excitonic fluctuations accompanied by reduced monoclinic distortions, supporting the excitonic mechanism of the phase transition in Ta₂NiSe₅. more

Unveiling the relationship between charge order and the pseudogap in a homogeneous high-temperature superconductor

Our team at the Max Planck Institute for Solid State Research, in collaboration with the European Synchrotron Radiation Facility (ESRF) and the Karlsruhe Institute of Technology, has used resonant x-ray scattering to observe a sharp onset of charge order in the stoichiometric cuprate superconductor YBa₂Cu₄O₈ (Y124). Charge order, characterized by spatially periodic modulations of electronic charge density, is a universal feature of the cuprates’ normal state, but it is strongly affected by materials-specific disorder that broadens its onset and complicates research into its origin. The sharp onset we discovered in the minimally disordered Y124 compound coincides with the onset of the "pseudogap", suggesting a common origin of both phenomena. more

Electrical transport and simultaneous x-ray (neutron) scattering. Schematic of the experimental setup for in situ scattering and resistance measurements. The samples are placed in an atmosphere-controlled chamber connected to a vacuum pump and gas mixtures.

Understanding the Role of Hydrogen and Oxygen in Electronic Phase Changes of Nickelates

Hydrogen is the most abundant element in the universe. This makes it attractive for use in sustainable technologies, such as energy storage and fuel cells, but also in novel electronic components. In contact with transition metal-oxygen compounds, it can reversibly change electrical resistance or magnetism, thereby creating functionality. However, three fundamental questions always arise: how much hydrogen is in the material, in what form is it incorporated and how stable is the compound? more

Giniyat Khaliullin wins the James C. McGroddy Prize for New Materials

The American Physical Society awarded the James McGroddy Prize to Giniyat, together with George Jackeli and Hide Takagi, for their “seminal theoretical and experimental research, materials design and discoveries that pioneered the exploration of novel forms of topological quantum matter in spin-orbit assisted Mott insulators realized in transition metal oxides”.
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Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇

Recently, superconductivity was discovered under high pressure in the layered nickelate La₃Ni₂O₇, with a remarkably high transition temperature of 80K. However, the microscopic mechanism driving the superconductivity in the material remains a subject of ongoing debate. In a joint effort, researchers from the Solid State Spectroscopy Department, Crystal Growth Scientific Facility, Quantum Materials Department, and the Stuttgart Center for Electron Microscopy, alongside collaborators from several partner institutions, uncovered a surprisingly distinct crystal structure of La₃Ni₂O₇, which will have to be the basis of future efforts to elucidate the electronic structure and Cooper pairing mechanisms in this material.
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Detailed depiction of a quantum sensor with electronic connections (I+, I-, V+, V-) and a laser system examining a layered material, symbolizing advanced research instrumentation.

Imaging the Current-Driven Insulator-to-Metal Transition in Calcium Ruthenate with Transport-ARPES

Mott insulators, such as Ca₂RuO₄, are materials that would be metallic in the absence of interactions but behave as insulators due to strong electron-electron correlations. When current is applied, a directional dependence is imposed on the electrons, creating a new pathway towards a metallic state. Using photoemission spectroscopy in the presence of transport currents, we and our collaborators from the Max Planck-UBC-Tokyo Center for Quantum Materials and the Lawrence Berkeley National Laboratory have found that the current-driven transition leads to a unique electron population of the orbitals. These results yield new insight into the mechanisms driving this unusual non-equilibrium phase transition.
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Imprinted atomic displacements drive spin-orbital order in a vanadate

The atomic structure of some crystals, such as quartz, can be recognized with the naked eye by looking at their facets. In others, these are difficult to distinguish, even by x-ray diffraction. What happens when a second material is grown, atom by atom, on only slightly different facets of a single crystal? Are the physical properties of the deposited material changed? We pursued these questions in a spectroscopic study which has now been published in Nature Physics.
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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