Theory of x-ray absorption spectroscopy for ferrites

The theoretical calculation of the interaction of electromagnetic radiation with matter remains a challenging problem for contemporary ab initio electronic structure methods, in particular, for x-ray spectroscopies. This is not only due to the strong interaction between the core hole and the photoexcited electron, but also due to the elusive multiplet effects that arise from the Coulomb interaction among the valence electrons. In this work we report a method based on density functional theory in conjunction with multiplet ligand-field theory to investigate various core-level spectroscopies, in particular, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The developed computational scheme is applied to the L 2 , 3 XAS and XMCD edges of magnetite (Fe 3 O 4 ) as well as cobalt ferrite (CoFe 2 O 4 ) and nickel ferrite (NiFe 2 O 4 ). The results are in overall good agreement with experimental observations, both regarding the XAS L 2 / L 3 branching ratio, the peak positions, as well as the relative intensities. The agreement between theory and experiment is equally good for XAS and the XMCD spectra, for all studied systems. The results are analyzed in terms of e g and t 2 g orbital contribution, and the robustness of the spectra with regard to the uncertainties of the Slater parameters is investigated. The analysis also highlights the strong effect of the 2 p -3 d interaction in x-ray spectroscopy

Re‐Dichalcogenides: Resolving Conflicts of Their Structure–Property Relationship

Abstract ReX2 (X = S, Se) remains a copious source of controversies and unanswered questions due to its widely contrasting experimental and theoretical results. With the help of comparative first‐principles electronic structure and phonon calculations, the correct structures for both systems are established, which minimize the apparent divergence of different experimental results. It is demonstrated that ReS2 and ReSe2 are neither iso‐structural nor iso‐electronic. The contributions of the in‐plane and out‐of‐plane orbitals at the band‐edges of the bulk and monolayers are coordinated with their anisotropic optical response. Under moderately high pressure, both of these systems are observed to undergo a semiconductor to metal transition. With the help of a combined full‐potential density functional theory and multiplet ligand field theory (DFT+MLFT) approach, the X‐ray spectral properties of these two systems are analyzed in the light of their intricate differences of optimized structures and electronic correlations

Annual Report 2023 and Phase-I Closeout

This report summarises the activities of the CERN strategic R&D programme on technologies for future experiments during the year 2023, and highlights the achievements of the programme during its first phase 2020-2023