Dmitry Yu. Usachov

The interplay of strongly localized 4f electrons with itinerant spd valence states gives rise to a wide range of correlated phenomena and properties that place lanthanide materials at the focus of considerable research efforts. Beyond the bulk, their surfaces are of particular interest, where the reduced coordination, a modified crystal electric field, broken inversion symmetry in combination with strong spin–orbit coupling, and the emergence of surface states and resonances considerably reshape 4f-driven electronic and magnetic properties. This, in turn, enables novel functionalities of particular relevance for low-dimensional systems and their applications. This review summarizes how advances in photoelectron spectroscopies, together with improved crystal growth, have enabled detailed insights into bulk and surface phenomena of lanthanide-based crystals. After a brief overview of key developments from the 1970s to the 1990s, we discuss recent progress, focusing on systematic studies by the authors and collaborators that form a coherent line of research. These include the unveiling of k-resolved f–spd hybridization, the formation and evolution with temperature of f-derived Fermi surface in Kondo lattices, layer-dependent 4f magnetic anisotropy, the emergence of ferromagnetically ordered surfaces in systems with non-magnetic bulk ground state. This coherent line of research addresses core questions in the physics of 4f systems and opens opportunities for engineering novel lanthanide-based architectures, including heterostructures and supramolecular complexes with novel physical properties and functionalities.

The topological insulator GeBi ₄ Te ₇ belongs to a family of compounds consisting of alternating quintuple and septuple layers (QLs and SLs), and its surface electronic structure is determined by whether the crystal terminates with a QL or SL layer block. Using spatially and momentum-resolved photoelectron spectroscopy and STM/STS in the field emission resonance spectroscopy regime as well, we clearly revealed the charge redistribution between the QL and SL terminations, as evidenced by band bending. We attribute the charge redistribution to cation antisite defects in the SLs, which are electrically active and lead to an uncompensated surface charge. Our findings and the ability to manipulate the electronic properties of the surface have profound implications for the entire family of compounds, including the magnetic topological insulator Mn(Bi,Sb) ₄ Te ₇ .

Magnetic kagome metals host complex electronic states and real-space magnetic textures, but their small and temperature-dependent magnetic domains make experimental access difficult. Here we show that micro-focused circular-dichroic photoemission spectroscopy enables spectroscopic access to individual magnetic domains in the kagome metal DyMn ₆ Sn ₆ at low temperature. By tuning to element-specific electronic states, we image domain contrast associated with Dy 4 f levels and detect corresponding signatures from Mn core states. The energy dependence of the dichroic response is consistent with modeling and indicates ferrimagnetic alignment between Dy and Mn local moments. Measurements of Mn 3 d -derived valence bands, supported by first-principles calculations, reveal features related to orbital magnetization. These results establish element- and orbital-resolved spectroscopy of single magnetic domains and enable studies of magnetic textures and electronic structure in complex magnetic quantum materials.

By means of ab initio density functional theory (DFT) calculations, we examined the surface electronic structure of the TbIr₂Si₂ antiferromagnet, which is distinguished by the out-of-plane alignment of Tb 4f moments and a high Neel temperature. We analyzed the interplay between the spin-orbit and exchange in- ´ teractions and their effect on the dispersion of surface states resided in the projected band gap around the M¯ point of the surface Brillouin zone, and compared our theoretical findings with low-temperature angle-resolved photoemission spectroscopy (ARPES) measurements.

We reveal Dirac fermions in graphene and an underlying Au monolayer with two-dimensional ferrimagnetism. The observed features of the electronic structure are intimately linked to the network of triangular loop dislocations at the Au/Co interface.