Surface effects in x-ray absorption spectra of lanthanides: Focus on strongly correlated cerium materials

Lanthanide (Ln) materials are widely studied using x-ray absorption spectroscopy (XAS), primarily at the Ln $3d$ edge, with less attention given to the $4d$ edge. Spectral features are usually analyzed in relation to bulk properties with surface effects rarely considered. In this study, we explore surface phenomena and their impact on XAS spectra, using as examples the antiferromagnetic (AFM) Kondo lattice ${\mathrm{CeRh}}_{2}{\mathrm{Si}}_{2}$, where we thoroughly examine both the $3d$ and $4d$ edges, and AFM ${\mathrm{LnRh}}_{2}{\mathrm{Si}}_{2}$ (Ln = Tm, Tb, and Pr), where our focus is primarily on the $3d$ edge. We observe significant contributions from surface effects in the $3d$ XAS spectra, even when measured in the total electron yield regime. We find evidence of Ce $4f$ moment reorientation at the Ce-terminated surface of ${\mathrm{CeRh}}_{2}{\mathrm{Si}}_{2}$, linked to changes in the crystal electric field at the surface. By modeling the Ce $3d$ and $4d$ XAS spectra, we not only distinguish between surface and bulk properties but also reveal the fine multiplet structure and the characteristic shape of the giant resonance associated with $|{M}_{J}\ensuremath{\rangle}$ states. These findings provide insights into $4f$-derived surface phenomena and their distinction from bulk electronic properties, while also emphasizing the general importance of surface sensitivity in XAS measurements of Ln materials.