Abstract:
For a long time\, rare-earth (RE) i ntermetallic materials have attracted considerable interest because of the ir rich and exotic properties including complex magnetic phases\, valence fluctuations\, heavy-fermion and Kondo behavior and non-Fermi-liquid prope rties. At the heart of the involved physics is the delicate interplay betw een itinerant electrons and the lattice of localized 4f states. Our synchr otron-based experiments aim to explore the peculiarities of f-d interactio n\, to reveal the fine electronic structure at a meV scale near the Fermi level\, and to investigate the magnetic properties of the 4f-systems. To t his end\, we combine different spectroscopic techniques that allow to cons truct a comprehensive picture of the studied phenomena on the basis of the obtained results. We actively use ultraviolet (UV) and X-ray photoelectro n spectroscopies\, angle-resolved photoelectron spectroscopy (ARPES)\, spi n-and time-resolved photoelectron spectroscopies as well as magnetic dichr oism experiments and photoelectron diffraction. In this talk\, I will give an overview of the most interesting experimental results which were mainl y obtained by UV-ARPES. In spite of its surface sensitivity\, UV-ARPES all ows not only to gain deep insight into the rich physics of rare-earthsyste ms driven by f-d interactions at the surface\, but also to probe bulk-deri ved states. With the help of our combined spectroscopic techniques we are able to comprehensively discriminate and investigate the electronic and ma gnetic properties at the surface and in the bulk of the considered 4f-mate rial. In the past years\, we systematically studied the family of the RET2 Si2 compounds where RE = Yb\, Ce\, Eu\, Gd\, Tb\, Dy\, Ho and T=Co\, Rh\, Ir\, and addressed essential and hotly discussed topics at the core of str ongly correlated and magnetic 4f electron systems. The most important find ings can be summarized as follows:
(i) Direct observation of cryst al-electric field (CEF) splittings of the 4f states in Yb-and Ce-based sys tems and their complex dispersion induced by f-d hybridization near the Fe rmi level\;
(ii) Insight into the temperature-dependent properties of the 4f-derived Fermi surface in Kondo lattices\; Disclosure of its top ology and spectral features reflecting f-d coupling at the surface and in the bulk from ARPES experiments\;
(iii) Clear evidence of the inte rplay between Dirac fermions and heavy quasi-particles in 4f-systems\;
(iv) Manifestation of strong ferromagnetic properties at the Si-termi nated surfaces of the antiferromagnets EuRh2Si2\, GdRh2Si2\, HoRh2Si2 and the mixed-valent compound EuIr2Si2. The unusual ferromagnetism at the Si-s urface in these materials is caused by exchange interaction between itiner ant two-dimensional (2D) electron states and the ordered 4f magnetic momen ts lying in the fourth atomic layer below the surface\;
v) Present ation of an efficient way for manipulating the spin and energy of 2D elect rons at the Si-terminated surface by controlling the competition between s pin-orbit and exchange magnetic interaction\;v) Demonstration of how stron g spin-orbit coupling\, lack of inversion symmetry and –as a novel ingre dient –Kondo-related phenomena act together within a few atomic layers n ear the surface and how their mutual interaction affects the electronic an d magnetic properties of suchnanostructural objects.
Our experimen ts nicely demonstrate that the Si-T-Si-RE surface block of the RET2Si2 mat erials serves as a versatile playground for studying the fundamental prope rties of 4f magnetism and f-d interactions at reduced dimensionality. It c an serve as a kind of construction kit with spin-orbit coupling\, Kondo in teraction\, crystal-electric fields and magnetic exchange with different s trengths as building blocks. Their mutual combination gives the opportunit y to design systems for different scenarios and to study the physics of 2D electron states in the presence of these competing interactions. The obta ined results create a solid platform for the time-resolved studies that ar e of particular importance for the physics and applications of magnetic ma terials. The most essential and interesting results were published in:
[1]. D. Yu. Usachov et al.\, Cubic Rashba Effect in the Surface Spin Structure of Rare-Earth Ternary Materials\, Phys. Rev. Lett. 124\, 237202 (2020).
[2]. S. Schulz et al.\, Emerging 2D-ferromagnetism and str ong spin-orbit coupling at the surface of valence-fluctuating EuIr2Si2\, n pj Quantum Materials 4 26 (2019).
[3]. M. Güttler et al.\, Dival ent EuRh2Si2 as a reference for the Luttinger theorem and antiferromagneti sm in trivalent heavy-fermion YbRh2Si2\, Nature Communications 10 796 (201 9).
[4]. A. Generalov et al.\, Strong spin-orbit coupling in the n oncentrosymmetric Kondo lattice\, Phys. Rev. B 98 115157 (2018) (Editors' Suggestion).
[5]. A. Generalov et al.\, Spin orientation of two-di mensional electrons driven by temperature-tunable competition of spin-orbi t and exchange magnetic interactions\, Nano Letters 17 811 (2017).
[6]. S. Patil et al.\, ARPES view on surface and bulk hybridization pheno mena in the antiferromagnetic Kondo lattice CeRh2Si2\, Nature Communicatio ns 7 11029 (2016).
[7]. M. Güttler\, Robust and tunable itineran t ferromagnetism at the silicon surface of the antiferromagnet GdRh2Si2\, Scientific Reports 6 24254 (2016).
[8]. K. Kummer\, et al.\, Tempe rature invariant Fermi surface in the Kondo lattice YbRh2Si2\, Phys. Rev. X 5 011028 (2015).
[9]. A. Chikina et. al.\, Strong ferromagnetism at the surface of an antiferromagnet caused by buried magnetic moments\, Nature Communications 5 3171 (2014).
[10]. M. Höppner et. al.\, Interplay of Dirac fermions and heavy quasiparticles in solids\, Nature Co mmunications 4 1646 (2013).
[11]. D.V. Vyalikh et. al.\, k-depende nce of the crystal-field splittings of 4f states in rare-earth systems\, P hys. Rev. Lett. 105\, 237601 (2010).
DTSTAMP:20260930T055604Z CREATED:20200701T220835Z LAST-MODIFIED:20200705T220530Z END:VEVENT END:VCALENDAR