Ferromagnetic uranium-based superconductors: formal 5f configuration and degree of covalence

• Applicant: Dr. Andrea Severing
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567326535

The formal oxidation state, here referred to as the formal configuration, and the degree of covalence in ferromagnetic uranium-based superconductors will be determined. Uranium intermetallic compounds exhibit a rich array of complex properties, including the coexistence of ferromagnetism and superconductivity. These phenomena arise from the intricate interplay between band formation and electron correlations involving the 5f electrons. However, a quantitative description of the electronic structure of these materials remains an open question—specifically, whether an itinerant approach or an embedded impurity model that explicitly accounts for local degrees of freedom provides the better starting point. Experimental information is scarce, partly due to the fact that the limited applicability of a single-Slater-determinant approach is not immediately apparent. For instance, x-ray absorption (XAS) spectra are typically broad, obscuring atomic-like multiplet structures, while core-level photoelectron spectroscopy (PES) spectra generally feature a broad main emission line with, at most, one satellite. These factors often lead to conflicting interpretations of x-ray data for uranium intermetallic compounds. Recently, high-resolution valence-band inelastic x-ray scattering (VB-RIXS) at the U M4,5 edges has become available so that 5f-5f (multiplet) excitations can be directly measured. They serve as fingerprints of the formal 5f configuration. Additionally, we have developed a method to obtain reliable, material-specific parameters for density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. By exploiting the energy dependence of photoionization cross-sections in valence-band photoelectron spectroscopy (VB-PES), we can disentangle the correlated 5f states from non-correlated, non-5f states. This enables us to fine-tune computational parameters in DFT+DMFT to reproduce spectra obtained using both soft and hard x-rays, leading to reliable values, particularly for the double-counting correction parameter (dc). This correction accounts for correlation effects that are accounted for in the DFT as well as DMFT part. These DFT+DMFT calculations, based on our VB-PES data, are carried out in collaboration with A. Hariki and J. Kuneš. Having established these necessary methods, we now aim to systematically investigate uranium intermetallic compounds, initially focusing on ferromagnetic superconductors. We believe that our research will contribute to a deeper understanding of uranium-based intermetallic compounds, particularly unconventional superconductivity.

• DFG Programme: Research Grants