Hard X-ray photoelectron diffraction (hXPD) is a well-established method for effective retrieval of structural information. We perform such measurements with momentum microscopy and hard X-ray excitations at synchrotron facilities. The strengths of XPD are its ability to analyse adsorbate geometries and its close relation to photoelectron holography. Elemental selectivity, even sensitivity to the chemical state of an atom, and high site-specificity can be exploited as unique fingerprints of atomic sites in compounds. By combining angle-resolved photoelectron spectroscopy and XPD in a single experiment, electronic and structural information can be obtained simultaneously and under identical conditions (kinetic energy of the photoelectrons, size and position of the probe spot and the probe depth). As an example, hXPD-patterns that have been recorded for a graphite single crystal at various photon energies are presented in Fig. 1.
The measurement yielded detailed Kikuchi-type diffraction patterns that showed an excellent one to-one agreement with simulations using the Bloch-wave approach, see Fig. 1(d-f). Those experimental results were nominated for DESY Highlights in 2019 and even selected for the cover of the book. Furthermore, hXPD can be used to study inversion symmetry breaking under external stimuli (temperature, strain) [3].
An additional advantage of ToF-MM microscopy is the possibility to measure X-ray photoemission spectroscopy (XPS). For example, we have studied the valence transition in EuPd2Si2 with hard XPS [4,5]. The large inelastic mean free path of the photoelectrons ensures that the results are not affected by effects of the surface (such as different stoichiometry or valence states). Thus, hXPD and XPS measurements on high-quality single crystals were performed to study valence transition in compound EuPd2Si2. We have measured the spectra for Eu 3d core-levels for temperatures 25K ≤ T ≤ 300 K (see Fig. 2, a). It was also found that the valence transition significantly changes the band structure (see Fig. 2, b-e).
[1] O. Fedchenko et al., New J. of Phys. 21, 113031 (2019).
[2] A. Winkelmann et al., New J. Phys. 10 (2008) 113002.
[3] O. Fedchenko et al. J. Phys. Soc. Jpn. 91, 091006 (2022)
[4] O. Fedchenko et al., Phys. Rev. B 109, 085130 (2024). DOI: 10.1103/PhysRevB.109.085130.
[5] S. Kölsch et al., Phys. Rew. Mat. 6, 115003 (2022). DOI: 10.1103/PhysRevMaterials.6.115003.