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Severing Group: x-ray spectroscopies on narrow band materials

Heavy Fermion compounds are conducting materials in which the charge carriers move as if they were 1000 times heavier than free electrons. Strong interactions among the electrons are responsible for this mass enhancement. The heavy fermion phenomenon can be observed in many solids containing cerium (Ce), ytterbium (Yb), and uranium (U), where the 4f or 5f electrons hybridize with the conduction electrons, forming ultra-narrow bands. Heavy fermion compounds serve as important model materials for discovering novel phases, such as unconventional or multiphase superconductivity, the coexistence or competition of magnetism and superconductivity, among others. The small (meV) characteristic energy scale in this material class allows for the high tunability of electronic properties.

Our recent focus is on U intermetallic compounds. Also, in these compounds the hybridization of f and conduction electrons plays a crucial role in driving the physics. However, unlike in the case of 4f electrons, the 5f electrons are much more strongly hybridized, leading to a lack of clear hierarchy in interactions, such as Coulomb repulsion, spin-orbit coupling, hopping, and crystal field. This poses a significant challenge in modelling, as it raises the question of whether the electronic structure should be described with an itinerant band approach or with an impurity model that takes local degrees of freedom into account. The complexity is exacerbated by the fact that in uranium intermetallics crucial pieces of information—such as the formal valence, the filling of the 5f shell, or the relevant symmetries of the 5f—are often missing due to experimental difficulties. Traditional strategies fall short, necessitating the development of new methods.

We employ various x-ray spectroscopic methods to investigate the electronic structure of the afore-mentioned materials. While some of these methods are well established, others are at the forefront of what is technically possible.

Using the linear dichroism in x-ray absorption (XAS) at the Ce M4,5-edges, we demonstrated, for example, that the 4f orbital anisotropy correlates with the CeRh1-xIrxIn5 phase diagram (PNAS 2015). Most recently, we provided evidence for a Kondo induced quasi-quartet ground state in the newly discovered multiphase superconductor CeRh2As2 (PRL 2024).

Non-resonant inelastic x-ray scattering (NIXS), also known as x-ray Raman Scattering (XRS), utilizing hard x-rays, probes multipolar excitations. In the case of Ce and U intermetallics, these multipolar excitations are much more excitonic than their dipole counterparts, so that, for the first time, a large dichroism in U intermetallic systems was observed. This provided the means to determine their ground state symmetry, akin to the linear dichroism in XAS. With NIXS, we have determined ground state wave functions in URu2Si2 and related compounds (PNAS 2016 and PNAS 2020). Furthermore, we have shown that singlet magnetism in U intermetallics is not so uncommon (PRB 2023).

Resonant inelastic X-ray scattering (RIXS) in the soft X-ray regime proves effective for Ce compounds. Utilizing Ce M-edge RIXS, we successfully addressed the long-standing problem of CeRh3B2, a material combining a high Kondo temperature with an exceptionally high Curie temperature. The solution came through determining its crystal-field scheme as function of temperature (see PRB 2022). Unfortunately, RIXS in the soft X-ray regime does not work for U intermetallics due to a lack of intensity. However, we have demonstrated that RIXS works very well at the U M-edge in the tender X-ray regime (3.5 keV), utilizing the unique RIXS end station at beamline P01 at DESY, upgraded by the Keimer group from the Max-Planck Institute for Solid State Research in Stuttgart. These RIXS data reveal ff excitations that serve as a fingerprint of the relevant U configuration. Our initial findings have been published (PRB 2023), and ongoing work is expected to be published in 2024.

Last but not least, we use photoelectron spectroscopy (PES) - both core level and valence band - with soft and hard x-rays (HAXPES) with the satellite structures providing information about the covalence. In Ce compounds, the spectra are well understood and well described within a configuration interaction model (see, for example, PRL 2021 and PRB 2021). However, this approach does not work for U intermetallics due to the much stronger hybridization. Only trends can be given (PRB 2023). To obtain a more quantitative insight into the covalence of U intermetallic compounds, we utilize the energy dependence of the cross-sections in PES to disentangle the different orbital contributions to the valence band. Our collaborators (Hariki Atsushi from Osaka Prefecture University, Osaka, Japan) use this information to perform a material specific DFT+DMFT calculation. By tuning parameters such as double counting and Coulomb repulsion to match the spectra, they are able to quantitatively determine the contributions of the U 5fn configurations in the intermediate valent ground and they are able to simulate the U 4f core-level data.



All work is performed in collaboration with the Max-Planck Institute for Chemical Physics of Solids in Dresden.



DFG project (2012-2015): Period I) Crystal-field investigations in rare earth compounds using linear polarized soft x-ray absorption spectroscopy.

DFG project (2016-2018): Period II) Crystal-field investigations in rare earth compounds using linear polarized soft x-ray absorption spectroscopy.

DFG project (2013-2017): Valence and orbital states of rare earth Heavy Fermion compounds close to the quantum critical point: Resonant and non-resonant inelastic X-ray scattering investigations.

DFG project (2016-2019): Correlated topological insulators: spectroscopic investigations combined with band structure calculations.

DFG project (2018-2021): Period I) From hidden to large-moment-antiferromagnetic order in URu2Si2: an x-ray study of 5f occupation and wave function symmetry.

DFG project (2022-2025 April): Period II) From hidden to large-moment-antiferromagnetic order in URu2Si2: an x-ray study of 5f occupation and wave function symmetry.


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