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Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo2P2, which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo2P2, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo2P2, we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited.

The discovery of a square magnetic-skyrmion lattice in GdRu2Si2, with the smallest so far found skyrmion size and without a geometrically frustrated lattice, has attracted significant attention. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu2Si2 by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4f magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along kz at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman–Kittel–Kasuya–Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4f moments responsible for the skyrmion physics in GdRu2Si2. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based systems.

Using a reactive molecular beam with high kinetic energy (Ekin), it is possible to speed gas–surface reactions involving high activation barriers (Eact), which would require elevated pressures (P0) if a random gas with a Maxwell–Boltzmann distribution is used. By simply computing the number of molecules that overcome the activation barrier in a random gas at P0 and in a molecular beam at Ekin = Eact, we establish an Ekin–P0 equivalence curve, through which we postulate that molecular beams are ideal tools to investigate gas–surface reactions that involve high activation energies. In particular, we foresee the use of molecular beams to simulate gas surface reactions within the industrial-range (>10 bar) using surface-sensitive ultra-high vacuum (UHV) techniques, such as X-ray photoemission spectroscopy (XPS). To test this idea, we revisit the oxidation of the Cu(111) surface combining O2 molecular beams and XPS experiments. By tuning the kinetic energy of the O2 beam in the range of 0.24–1 eV, we achieve the same sequence of surface oxides obtained in ambient pressure photoemission (AP-XPS) experiments, in which the Cu(111) surface was exposed to a random O2 gas up to 1 mbar. We observe the same surface oxidation kinetics as in the random gas, but with a much lower dose, close to the expected value derived from the equivalence curve.

The search for novel technologies like spin-based electronics and suitable materials for respective devices requires a profound understanding of fundamental interactions regarding electron spin and related properties. In the same context, with ongoing device miniaturisation, surface-related phenomena become increasingly important. Here, we study the electronic and magnetic properties of quasi-2D electron states at a metallic surface under the influence of the Rashba effect and exchange coupling to localised 4f moments that order magnetically at low temperatures. Particularly, in the considered systems, both interactions are of similar strengths, a case which is rather unexplored in the literature. Our model system is the (001) surface of intermetallic LnIr2Si2 compounds with ThCr2Si2 structure, where Ln = lanthanide. With this work, we continue our long-term systematic study of the LnT2Si2 compounds with T = Rh, where the Rashba-like spin-orbit coupling is about a hundred times weaker than the exchange interaction. Using ARPES and DFT we explore with GdIr2Si2 and EuIr2Si2 two representatives of the LnIr2Si2 family, which are both characterised by the insensitivity of the 4f shell to the crystal electric field. On the other hand, they have fundamentally different bulk properties. GdIr2Si2 is a robust bulk antiferromagnet with a high ordering temperature of 87 K, whereas EuIr2Si2 is a mixed-valent material with a non-magnetic ground state in the bulk. The mean Eu valency is strongly temperature dependent, changing continuously from a nearly divalent magnetic configuration at room temperature to a nearly trivalent non-magnetic Eu state below 50K. Studying the surface states in both compounds we find that the magnitude of the Rashba-like spin-orbit interaction increases tremendously in comparison to the isoelectronic Rh compounds. This is reflected in a huge splitting of the surface state bands and emphasizes the importance of atomic spin-orbit coupling in high Z elemen, Die Suche nach neuartigen Technologien wie spinbasierte Elektronik sowie nach geeigneten Materialien für entsprechende Bauteile erfordert ein tiefgreifendes Verständnis der Wechselwirkungen des Elektronenspins und damit verbundener Materialeigenschaften. Mit der zunehmenden Miniaturisierung von Bauteilen gewinnen in diesem Zusammenhang auch Oberflächenphänomene zunehmend an Bedeutung. In dieser Arbeit untersuchen wir die elektronischen und magnetischen Eigenschaften quasizweidimensionaler elektronischer Zustände an metallischen Oberflächen unter dem Einfluss des Rashba-Effekts und der Austauschwechselwirkung mit lokalisierten 4f Momenten, die bei tiefen Temperaturen magnetisch ordnen. Dabei liegt die Besonderheit der untersuchten Systeme darin, dass beide Wechselwirkungen von vergleichbarer Stärke sind. Dieser Fall ist in der Fachliteratur bislang unterrepräsentiert. Unser Modellsystem ist die (001)-Oberfläche intermetallischer LnIr2Si2 Verbindungen mit ThCr2Si2 Struktur, wobei Ln ein Lanthanoidenelement darstellt. Dabei führen wir die langjährige und systematische Untersuchung von LnT2Si2 Verbindungen mit T = Rh fort, in denen die Rashba-artige Spin-Bahn-Kopplung ungefähr 100-mal schwächer als die Austauschwechselwirkung ist. Mit Hilfe von winkelaufgelöster Photoelektronenspektroskopie (ARPES) und Dichtefunktionaltheorie (DFT) erkunden wir mit GdIr2Si2 und EuIr2Si2 zwei Vertreter der LnT2Si2 Familie, die beide durch die Insensibilität der 4f Schale gegenüber dem Kristallfeld ausgezeichnet sind. Zugleich haben sie grundsätzlich verschiedene Volumeneigenschaften. GdIr2Si2 ist ein robuster Volumenantiferromagnet mit einer hohen Ordnungstemperatur von 87K, wohingegen EuIr2Si2 eine gemischtvalente Verbindung mit einem nicht-magnetischen Volumengrundzustand ist. Die mittlere Eu Valenz ist stark temperaturabhängig, sie ändert sich kontinuierlich von einer nahezu zweiwertigen Konfiguration bei Raumtemperatur zu einem beinahe dreiwertigen, nicht-magnetischen Eu Zustand unte

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Spin–orbit coupling induces a unique form of Zeeman interaction in momentum space in materials that lack inversion symmetry: the electron’s spin is locked on an effective magnetic field that is odd in momentum. The resulting interconnection between the electron’s momentum and its spin leads to various effects such as electric dipole spin resonance, anisotropic spin relaxation and the Aharonov–Casher effect, but also to electrically driven and optically driven spin galvanic effects. Over the past 15 years, the emergence of topological materials has widened this research field by introducing complex forms of spin textures and orbital hybridization. The vast field of Rashba-like physics is now blooming, with great attention paid to non-equilibrium mechanisms such as spin-to-charge conversion, but also to nonlinear transport effects. This Review aims to offer an overview of recent progress in the development of condensed matter research that exploits the unique properties of spin–orbit coupling in non-centrosymmetric heterostructures.

New temperature scales and remarkable differences from bulk properties have increasingly placed the surfaces of strongly correlated f materials into the focus of research activities. Applying first-principles calculations and angle-resolved photoelectron spectroscopy measurements, a strong Rashba effect and spin-split surface states at the CeIn surface of the heavy-fermion superconductor CeIrIn5 are revealed. The unveiled 4f-derived electron landscape is remarkably distinct for surface and bulk Ce implying the existence of novel temperature scales near the surface region in this material. These results show that ab initio calculations can reliably predict the unusual electronic and spin structure of surfaces of strongly correlated 4f systems where Rashba spin-orbit-coupling phenomena emerge. It is suggested that the structural blocks of such materials can be combined with magnetically active layers for engineering of novel nanostructural objects with appropriate substrates where the diversity of f-driven properties can be applied for the development of novel functionalities.

In the new stoichiometric high-temperature ironbased superconductor RbEuFe4As4, superconductivity coexists with a peculiar long-range magnetic order of the Eu 4f states; their coexistence is an enigma and a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy (ARPES), resonant photoemission spectroscopy (ResPES), Andreev reflection spectroscopy, scanning tunneling spectroscopy, and DFT band structure calculations, we have made significant progress in solving this puzzle. Our results unambiguously indicate a separation between the electronic states of Fe (superconductivity) and Eu (magnetism) and demonstrate the existence of superconducting and long-range magnetic orders almost independently of each other.