Peter Krüger

Peter Krüger

Affiliation: Chiba University

Research Interests: surface physics, theoretical spectroscopy

Title: Dichroism in X-ray absorption and photoemission spectroscopy for the geometrical and magnetic characterization of novel materials

Abstract: I present some of my recent work (1-4) on the theory and computation of polarization-dependent X-ray absorption (XAS) and photoemission spectroscopy of functional materials. (1) Transition metal phthalocyanines (Pc) present interesting properties for spintronic applications such as a large magnetic anisotropy and intermediate spin states. The electronic and magnetic state of Mn-Pc adsorbed on graphite can be manipulated by nitrogen adsorption [J. L. Zhang et al. Nanoletters 15, 3181 (2015)]. Here, X-ray absorption spectra at the nitrogen K-edge and Mn L-edges are computed using DFT and ligand field theory and good agreement with experiment is obtained. (2) The crystal phase transformations observed in titanium dioxide nanoribbons, going from NaHTi3O7 titanate over the TiO2–B to the anatase phase are analyzed using polarization-dependent XAS in scanning transmission X-ray microscopy [1]. Strong linear dichroism is observed in single nanoribbons, reflecting preferential O-2p to Ti-3d bond orientation in the low symmetry crystal structures. It is shown how the crystal orientation in the nanoparticles can be inferred from the polarization-dependent absorption spectra. (3) In angle resolved photoemission of graphite a strong and complex circular dichroism (CD) signal is observed and well reproduced using multiple scattering calculations [2]. It is shown that photoelectron scattering plays a substantial role in the formation of the CD. (4) Altermagnets are a new class of materials with great prospects for technology. Through DFT calculations, many materials have been predicted to have an altermagnetic phase, but the experimental evidence is scarce. Here we show that a the CD signal in responant photoelectron diffraction (RPED) [3] allows to clearly identify the altermagnet state. The theory is successfully applied to the altermagnet MnTe [4]. [1] Chemical Bond Modification upon Phase Transformation of TiO2 Nanoribbons Revealed by Nanoscale X-ray Linear Dichroism, P. Krüger, M. Sluban, P. Umek, P. Guttmann and C. Bittencourt, J. Phys. Chem. C 121, 17038 (2017). [2] Observation and theory of of strong circular dichroism in angle- resolved photoemission from graphite, P. Krüger and F. Matsui, J. Electron Spectrosc. Related Phenom. 258, 147219 (2022). [3] Theory of circular dichroism in angle-resolved resonant photoemission from magnetic surfaces, R. Sagehashi, G. Park and P. Krüger, Physical Review B 107, 075407 (2023). [4] Circular dichroism in resonant photoelectron diffraction as a direct probe of sublattice magnetization in altermagnets, P. Krüger, arXiv:2504.08380 (2025)