Denis Vyalikh
Donostia International Physics Center, 20080 Donostia/San Sebastián, Basque Country, Spain
IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain

“Photoemission studies of strongly-correlated electrons and exotic magnetic phe-nomena in quasi-two-dimensional 4f systems”

The photoelectric effect, discovered by H. Hertz in 1887 and later on, in 1906 explained by A. Einstein, underlies a variety of spectroscopic techniques called photoelectron spectroscopy (PES). Among them ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), angle-resolved photoelectron spectroscopy (ARPES), spin- and time-resolved photoelectron spectros-copies [1]. Nowadays, PES is one of the most extensively used and continuously developing methods allowing to comprehensively study the electronic structure of molecules, solids, surfaces and interfac-es, to gain insight into the dynamic and magnetic properties of the studied objects. Furthermore, PES has wide practical implications in such fields like surface chemistry or material science, and has sig-nificantly contributed to the understanding of fundamental principles in solid state physics [1].
Brief introduction and description of photoelectron and in particular of angle-resolved photoe-lectron spectroscopy will be given. Further, several own examples of ARPES implication for investiga-tions of strongly-correlated electrons and magnetic phenomena in rare-earth (RE) intermetallic mate-rials will be shown in great details. For a long time, such materials have attracted considerable inter-est because of their exotic properties at low temperatures, which include complex magnetic phases, valence fluctuations, heavy-fermion properties, Kondo behavior and many others. All of these proper-ties stem from the interplay between almost localized 4f electrons and itinerant states.
In that regard, the class of RE compounds RET2Si2 (T is transition metal atoms) of the ThCr2Si2 type structure attracts considerable attention. Besides their unique bulk properties evolving from a delicate interplay of 4f and spd electrons, these materials serve as nice models for studying exotic physics within the non-centrosymmetric Si-T-Si-RE four layers of the Si-terminated surface. There, the spin-orbit coupling (SOC) can be tuned by choice of suitable transition metal atoms. It gradually increases by exchanging Co (3d) for Rh (4d) and further for Ir (5d). The SOC-based phenomena will be rather weak for Co 3d electrons, while they will be greatly enhanced for Ir 5d orbitals.
As a competing ingredient, exchange magnetic interaction may be exploited by inserting ele-mentary 4f magnets like Gd as the RE component. Because the orbital moment of the Gd 4f shell van-ishes (L = 0), the pure and large spin moment of Gd will be a strong and robust source of magnetic phenomena. A rotation of the 4f moments to a certain angle relative to the surface normal may be achieved by coupling to crystal-electric-field (CEF). To make use of notable CEF effects, a non-vanishing orbital moment L is needed, like for instance in Ho or Dy. Then, this option allows to im-plement a magnetic exchange field with different strength and orientation at the surface, which com-petes with the Rashba SOC field and creates additional possibilities to manipulate the properties of the 2D electrons within the considered Si-T-Si-RE system. Moreover, it will be shown that so essential property of 4f moments as their orientation in the system can be derived from classical 4f-photoelectron spectroscopy measurements.

As the next ingredient, the Kondo effect can be introduced by inserting elements with unstable 4f shell as Yb or Ce. This gives the opportunity to explore the interplay of the 2D electrons with 4f moments within a 2D Kondo lattice in the presence of spin-orbit coupling and a non-centrosymmetric environment.
It will be shown that in general such a Si-T-Si-RE system may serve as a beautiful playground for studying the fundamental properties of 2D electrons. These systems can be nicely used as a veri-table construction kit with spin-orbit, Kondo, crystal-electric field, and exchange magnetic interac-tions as building blocks. Combining them with one another gives the opportunity to design systems for different scenarios and to study the physics of 2D electrons in the presence of these competing interactions.

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