Traditionally, ARPES and XAS have been key tools for investigating the electronic and magnetic properties of lanthanide (Ln)-based materials, with a primary focus on bulk 4f-driven phenomena. This is particularly true for numerous XAS measurements performed at the Ln M_4,5 and N_4,5 edges, where spectral lineshape analysis is predominantly discussed in the context of bulk effects. In general, the surfaces and their properties of Ln materials have received considerably less attention than the bulk. However, it is reasonable to anticipate that 4f-driven physics at the surface could be even richer and compelling than in the bulk. The lack of inversion symmetry, spin-orbit coupling, the emergence of surface states and resonances, relaxation and reconstruction, as well as modifications of the CEF near and at the surface, all serve as driving forces for novel 4f-driven phenomena, distinct phases, and characteristic energy scales that differ dramatically from those in the bulk.

In this talk, I will present an overview of key results obtained by our team on surface 4f-driven phenomena and their characteristic temperature scales in lanthanide systems such as LnRh₂Si₂ and LnIr₂Si₂, using ARPES and XAS. Also, I will focus on Ce-based materials, known for their exotic bulk properties, including CeRh₂Si₂ and CeIrIn₅. We will consider the valence-fluctuating material EuIr₂Si₂, where Eu near the surface exhibits divalent behavior in contrast to its non-magnetic valence-fluctuating bulk properties, leading to the emergence of strong 2D ferromagnetism in the Si–Ir–Si–Eu surface block with own temperature scale, accompanied by a cubic Rashba spin–orbit coupling effect.

Our findings reveal that 4f magnetic moments in such crystals reorient near the surface due to modifications in the crystal electric field compared to the bulk. By analyzing modeled spectra for each M_J state in comparison with experimental data, we can conclusively determine the orientation of magnetic moments both in the bulk and at the surface. Furthermore, we provide clear evidence of unexpectedly different Kondo-related properties at the Ce surface and in the bulk, as observed in CeRh₂Si₂. The obtained results call for detailed studies of properties emerging at the surfaces of strongly correlated Ce-, Eu-, and Yb-based materials with quasi-2D structure. They also point to a route for designing novel functional and quantum materials using thin layers of 4f-electron systems as building blocks, where different combinations of fundamental interactions can be realized.

References:

[1]. Gustav Bihlmayer, Paul Noël, Denis V. Vyalikh, Evgueni V. Chulkov and Aurélien Manchon, Rashba-like physics in condensed matter, Nature Reviews Physics 4 642 (2022)

[2]. S. Schulz et al., Emerging 2D-ferromagnetism and strong spin-orbit coupling at the surface of valence-fluctuating EuIr₂Si₂, npj Quantum Materials 4 26 (2019).

[3]. A. Generalov et al., Spin orientation of two-dimensional electrons driven by temperature-tunable competition of spin-orbit and exchange magnetic interactions, Nano Letters 17 811 (2017).

[4]. S. Patil et al., ARPES view on surface and bulk hybridization phenomena in the antiferromagnetic Kondo lattice CeRh₂Si₂, Nature Communications 7 11029 (2016).

[5]. M. Güttler, Robust and tunable itinerant ferromagnetism at the silicon surface of the antiferromagnet GdRh₂Si₂, Scientific Reports 6 24254 (2016).

[6]. A. Chikina et. al., Strong ferromagnetism at the surface of an antiferromagnet caused by buried magnetic moments, Nature Communications 5 3171 (2014).

[7]. D. Yu. Usachov et al., Surface effects in x-ray absorption spectra of lanthanides: Focus on strongly correlated cerium materials, Physical Review B 112 035137 (2025).

[8]. D. Yu. Usachov et al., Probing surface and bulk ground states of lanthanides: 4f moment orientation through 4d x-ray absorption spectroscopy, Physical Review B 110 075157 (2024).

[9]. D. Yu. Usachov et al., Estimating the Orientation of 4f Magnetic Moments by Classical Photoemission, The Journal of Physical Chemistry Letters 13 7861 (2022).

[10]. M. Mende at el., Strong Rashba Effect and Different f-d Hybridization Phenomena at the Surface of the Heavy-Fermion Superconductor CeIrIn₅, Advanced Electronic Materials 2100768 (2021).