Spectroscopic Methods

Spectroscopic methods open up new windows on atomic-scale correlations in quantum materials. The development of spectroscopic instrumentation is a key element of research in the department.
 

A major recent effort has focused on commissioning and upgrading of the IRIXS spectrometer for resonant inelastic x-ray scattering with “tender” x-rays (photon energies E = 1-5 keV) at the PETRA-III synchrotron at DESY in Hamburg, which was funded in part by an Advanced Grant from the European Research Council (ERC AdG Com4Com, 2016-2022). Whereas RIXS spectrometers for soft (E ≤ 1 keV) and hard (E ≥ 5 keV) x-rays are operational at many modern synchrotrons, IRIXS provides unique access to the L-edges of transition metal compounds with 4d valence electrons, and to the M-edges of actinides with 5f electrons. In the framework of our new ERC project (ERC AdG SpecTera, 2024-2029) we are now combining IRIXS with Raman scattering and resonant elastic x-ray scattering (REXS) to explore new avenues of research on terahertz magnonics.

Magnetic ground state of the dimer-based hexagonal perovskite Ba₃⁢ZnRu_2⁢O₉
S. Hayashida, H. Gretarsson, P. Puphal, M. Isobe, E. Goering, Y. Matsumoto, J. Nuss, H. Takagi, M. Hepting, and B. Keimer
Physical Review B 111, 104418 (2025)
Nearly linear orbital molecules on a pyrochlore lattice
A. Krajewska, A.N. Yaresko, J. Nuss, S. Bette, A.S. Gibbs, M. Blankenhorn, R.E. Dinnebier, D.P. Sari, I. Watanabe, J. Bertinshaw, H. Gretarsson, K. Ishii, D. Matsumura, T. Tsuji, M. Isobe, B. Keimer, H. Takagi, and T. Takayama
Science Advances 10, 41 (2024)
Resonant inelastic x-ray scattering at the actinide M_4,5-edges
M. Sundermann, H. Hahn, D.S. Christovam, M.W. Haverkort, R. Caciuffo, B. Keimer, L.H. Tjeng, A. Severing, and H. Gretarsson
Physical Review Research 7, 043081 (2025)
 

The department operates two instruments at the FRM-II reactor: the triple-axis spin-echo (TRISP) neutron spectrometer, a unique instrument that offers μeV energy resolution and is capable of measuring the momentum-dependent lifetimes of collective excitations such as phonons and magnons; and the NREX neutron reflectometer for which we have built a powerful environment for hydrogen intercalation. Both instruments are currently undergoing extensive upgrades during the shutdown of the FRM-II.

Neutron Spin-Echo Instrumentation for Magnetic Scattering
Thomas Keller, Heiko Trepka, Klaus Habicht, Bernhard Keimer
Phys. Status Solidi B, 2100164 (2021)
Complementary Insights from Neutron and Resonant X-Ray Reflectometry for the Study of Perovskite Transition Metal Oxide Heterostructures
Eva Benckiser, Yury Khaydukov, Laura Guasco, Katrin Fürsich, Padma Radhakrishnan, Gideok Kim and Bernhard Keimer
Phys. Status Solidi B 259, 2100253 (2022)