Insight into the electronic structure of the centrosymmetric skyrmion magnet GdRu2_2Si2_2

The discovery of a square magnetic-skyrmion lattice in GdRu$_2$Si$_2$, with the smallest so far found skyrmion diameter and without a geometrically frustrated lattice, has attracted significant attention, particularly for potential applications in memory devices and quantum computing. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu$_2$Si$_2$ 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 4$f$ 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 the $k_z$ 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 4$f$ moments responsible for the skyrmion physics in GdRu$_2$Si$_2$. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based devices.Comment: 13 pages, 8 figure

Colossal magnetoresistance in EuZn2_2P2_2 and its electronic and magnetic structure

We investigate single crystals of the trigonal antiferromagnet EuZn$_2$P$_2$ ($P\overline{3}m1$) by means of electrical transport, magnetization measurements, X-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES) and ab-initio band structure calculations (DFT+U). We find that the electrical resistivity of EuZn$_2$P$_2$ increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (CMR effect). Resonant magnetic scattering reveals a magnetic ordering vector of $q = (0\, 0\, \frac{1}{2})$, corresponding to an $A$-type antiferromagnetic (AFM) order, below $T_{\rm N} = 23.7\,\rm K$. We find that the moments are canted out of the $a-a$ plane by an angle of about $40^{\circ}\pm 10^{\circ}$ degrees and tilted away from the [100] - direction by $30^{\circ}\pm 5^{\circ}$. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below $\approx 150\,\rm K$, the temperature below which also the field dependence of the material's resistivity starts to increase. An analysis of the infrared reflectivity spectrum at $T=295\,\rm K$ allows us to resolve the main phonon bands and intra-/interband transitions, and estimate indirect and direct band gaps of $E_i^{\mathrm{opt}}=0.09\,\rm{eV}$ and $E_d^{\mathrm{opt}}=0.33\,\rm{eV}$, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly $T$-independent above and below $T_{\rm N}$. The comparison of the theoretical and experimental data shows a weak intermixing of the Eu 4$f$ states close to the $\Gamma$ point with the bands formed by the phosphorous 3$p$ orbitals leading to an induction of a small magnetic moment at the P sites

Classical and cubic Rashba effect in the presence of in-plane 4f magnetism at the iridium silicide surface of the antiferromagnet GdIr2Si2

We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet GdIr2Si2 above and below the Ned temperature. Using angle-resolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the (M) over bar point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Ned temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered 4f moments of the near-surface Gd layer

Electronic Structure and Coexistence of Superconductivity with Magnetism in RbEuFe4As4

In the novel stoichiometric iron-based material RbEuFe4As4, superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three-dimensional electronic structure and compare it with density functional theory calculations. Multiple super-conducting gaps were measured on various sheets of the Fermi surface. High-resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe-and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long-range magnetic orders exist independently from each other. The established electronic structure of RbEuFe4As4 opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.We thank Matthew Watson for his critical reading of the manuscript. We thank Diamond Light Source for access to beamline I05 (Proposal No. SI15074 and No. SI19041) that contributed to the results presented here. Work was done using equipment from the LPI Shared Facility Center. K.S.P. and V.M.P. acknowledge support by the Russian Scientific Foundation (RSF Project No. 21-12-00394). A.V.S. and A.S.U. acknowledge support by the Russian Foundation for Basic Research (Project No. 21-52-12043). E.V.C. acknowledges funding by Saint Petersburg State University project for scientific investigations (ID No. 73028629). S.V.E. acknowledges support from the government research assignment for ISPMS SB RAS (Project FWRW-2019-0032). R.V. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG) TRR 288 (Project A05). V.B. thanks the Goethe University Frankfurt for computational resources and Daniel Guterding for providing the FS plotting software. K.K. thanks M. Valvidares, J. Herrero, H. B. Vasili, S. Agrestini, and N. Brookes for their support during the XMCD experiment at ALBA via IHR Proposal 2019063615. D.V.V. also acknowledges support from the Spanish Ministry of Economy (MAT-2017-88374-P

Adenosine A1 receptor: Functional receptor-receptor interactions in the brain

Over the past decade, many lines of investigation have shown that receptor-mediated signaling exhibits greater diversity than previously appreciated. Signal diversity arises from numerous factors, which include the formation of receptor dimers and interplay between different receptors. Using adenosine A1 receptors as a paradigm of G protein-coupled receptors, this review focuses on how receptor-receptor interactions may contribute to regulation of the synaptic transmission within the central nervous system. The interactions with metabotropic dopamine, adenosine A2A, A3, neuropeptide Y, and purinergic P2Y1 receptors will be described in the first part. The second part deals with interactions between A1Rs and ionotropic receptors, especially GABAA, NMDA, and P2X receptors as well as ATP-sensitive K+ channels. Finally, the review will discuss new approaches towards treating neurological disorders

Kondo et sous-réseaux magnétiques dans les composés du Ce - une étude spectroscopique aux rayons X

In this work, the two different Ce-based intermetallic compounds CeRh2Si2 and CeCo2P2 have been studied by means of synchrotron radiation spectroscopic (X-ray absorption and photoelectron emission spectroscopy) and resonant inelastic X-ray scattering methods in combination with ab-initio band structure calculations. The measurements allowed for an improved understanding and interpretation of surface sensitive Ce 4f physics by studying distinct surface terminations and their effects, while being combined in the bulk with Co 3d magnetism.In the first presented study, angle-resolved photoemission spectroscopy (ARPES) was used to explore the temperature dependence of the Ce 4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Distinct spectra from the Ce- and Si-terminated surfaces were studied in a wide temperature range revealing characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The terminations exhibit different temperature dependences of the Fermi level peak suggesting that the effective Kondo temperatures at the surface and in the bulk differ significantly. The origin of this difference might lie in a greatly reduced crystal–electric-field (CEF) splitting at the surface resulting in a larger effective Kondo temperature due to a higher local-moment effective degeneracy. Additionally, different hybridization strengths could further influence the 4f peak intensities at the Fermi level. The possibility of measuring distinct surfaces lays the foundation for directly testing quantitatively many-body theories that link spectroscopy and transport properties for both the bulk and the surface, separately.In contrast to CeRh2Si2, the CeCo2P2 compound shows no signs of heavy-fermion physics in the bulk. But our surface sensitive angle-resolved photoemission measurements suggest that the 4f physics significantly change near the surface. We propose that this is a direct result of the symmetry breaking of the antiferromagnetically ordered cobalt sublattice resulting in an effective magnetic field which in turn leads to partially occupied and spin-polarized Ce 4f states near the surface. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario.At the surface, the particular interplay between the magnetic Co 3d and the Ce 4f sublattice leads to heavy-fermion physics, while the complete absence of local-moment 4f physics away from the surface region results in a reduced density of states at the Fermi level in the bulk. Usually, in metallic compounds the lifetime of high-frequency magnons is strongly reduced due to interactions with the Stoner continuum, however, due to the reduced density in the bulk, long-living magnons on the Co sublattice of CeCo2P2 can exist up to the terahertz regime. In detail, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which we experimentally verified by resonant inelastic X-ray scattering measurements. In comparison, the isostructural LaCo2P2 compound exhibits the typical strongly damped magnon behavior usually observed in metallic systems. Based on these observations, we propose the La/CeCo2P2 system as a suitable candidate for further studies on the way to metallic magnonic quantum devices.Ce travail de thèse présente des études menées sur CeRh2Si2 et CeCo2P2, deux composés intermétalliques à base de Cérium, par spectroscopie d’absorption, d’émission photoélectrique et de diffusion inélastique résonante de rayons X issus du rayonnement synchrotron, en combinaison avec des calculs ab-initio de structure de bande. En étudiant les effets de différentes terminaisons de surface, ces mesures nous permettent une meilleure compréhension et interprétation de la physique du Cérium 4f, sensible aux effets de surface et combinée au magnétisme du cobalt en volume.Dans la première étude présentée, la dépendance en température des réponses spectrales du Ce 4f en surface et en volume a été explorée par spectroscopie de photoémission résolue en angle (ARPES) dans le réseau Kondo antiferromagnétique CeRh2Si2. Des schémas 4f caractéristiques d’une hybridation faible (à la surface) et forte (en volume) ont été mis en évidence par l’étude des spectres des surfaces terminées par du Ce et du Si dans une large gamme de température. La dépendance en température du pic associé au niveau de Fermi diffère fortement pour ces deux terminaisons, ce qui suggère une différence significative de la température de Kondo en surface et en volume. Cette différence pourrait s’expliquer par une levée de dégénérescence du champ cristallin fortement réduite à la surface, entraînant une température de Kondo effective plus élevée résultant d’une dégénérescence effective accrue du moment local. De plus, des forces d’hybridation différentes pourraient jouer sur les intensités du pic 4f au niveau de Fermi. La possibilité de mesurer des surfaces bien distinctes ouvre ainsi la voie pour directement tester de manière quantitative les théories à N-corps qui lient la spectroscopie et les propriétés de transport, en volume et en surface séparément.Contrairement au CeRh2Si2, le composé CeCo2P2 ne montre aucun signe dans son volume de la physique des fermions lourds. Mais nos mesures de photoémission résolue en angle, qui sont sensibles à la surface, suggèrent que la physique des 4f change considérablement proche de la surface. Nous proposons que cela soit directement lié à la brisure de symétrie du sous-réseau du cobalt ordonné antiferromagnétiquement, entraînant l’apparition d’un champ magnétique effectif créant des états Ce 4f partiellement occupés polarisés en spin proches de la surface. Les mesures en température indiquent des variations importantes de l’intensité des 4f au niveau de Fermi, compatibles avec un scénario de type Kondo.A la surface, l’interaction particulière entre les sous-réseaux du cobalt 3d magnétique et du Ce 4f mène à la physique des fermions lourds, alors que l’absence totale de physique des moments localisés 4f loin de la surface se traduit par une densité d’états réduite au niveau de Fermi dans le volume. Habituellement dans les composés métalliques, la durée de vie des magnons de haute fréquence est fortement réduite par les interactions avec le continuum de Stoner. Cependant, de par cette densité réduite dans le volume, les magnons du sous-réseau de cobalt du composé CeCo2P2 subsistent jusque dans le régime du térahertz. Plus précisément, nos calculs ab-initio prédisent une suppression des excitations Stoner de basse énergie, ce que nous avons vérifié expérimentalement par diffusion inélastique résonante de rayons X. Par comparaison, le comportement typique de magnon fortement amorti observé habituellement dans les systèmes métalliques se retrouve dans le composé à structure identique LaCo2P2. Sur la base de ces observations, nous proposons le système La/CeCo2P2 comme un candidat potentiel pour des études plus poussées en vue d’aboutir à des dispositifs métalliques quantiques magnoniques

Kondo and magnetic sublattices in Ce compounds - a spectroscopic X-ray study

Ce travail de thèse présente des études menées sur CeRh2Si2 et CeCo2P2, deux composés intermétalliques à base de Cérium, par spectroscopie d’absorption, d’émission photoélectrique et de diffusion inélastique résonante de rayons X issus du rayonnement synchrotron, en combinaison avec des calculs ab-initio de structure de bande. En étudiant les effets de différentes terminaisons de surface, ces mesures nous permettent une meilleure compréhension et interprétation de la physique du Cérium 4f, sensible aux effets de surface et combinée au magnétisme du cobalt en volume.Dans la première étude présentée, la dépendance en température des réponses spectrales du Ce 4f en surface et en volume a été explorée par spectroscopie de photoémission résolue en angle (ARPES) dans le réseau Kondo antiferromagnétique CeRh2Si2. Des schémas 4f caractéristiques d’une hybridation faible (à la surface) et forte (en volume) ont été mis en évidence par l’étude des spectres des surfaces terminées par du Ce et du Si dans une large gamme de température. La dépendance en température du pic associé au niveau de Fermi diffère fortement pour ces deux terminaisons, ce qui suggère une différence significative de la température de Kondo en surface et en volume. Cette différence pourrait s’expliquer par une levée de dégénérescence du champ cristallin fortement réduite à la surface, entraînant une température de Kondo effective plus élevée résultant d’une dégénérescence effective accrue du moment local. De plus, des forces d’hybridation différentes pourraient jouer sur les intensités du pic 4f au niveau de Fermi. La possibilité de mesurer des surfaces bien distinctes ouvre ainsi la voie pour directement tester de manière quantitative les théories à N-corps qui lient la spectroscopie et les propriétés de transport, en volume et en surface séparément.Contrairement au CeRh2Si2, le composé CeCo2P2 ne montre aucun signe dans son volume de la physique des fermions lourds. Mais nos mesures de photoémission résolue en angle, qui sont sensibles à la surface, suggèrent que la physique des 4f change considérablement proche de la surface. Nous proposons que cela soit directement lié à la brisure de symétrie du sous-réseau du cobalt ordonné antiferromagnétiquement, entraînant l’apparition d’un champ magnétique effectif créant des états Ce 4f partiellement occupés polarisés en spin proches de la surface. Les mesures en température indiquent des variations importantes de l’intensité des 4f au niveau de Fermi, compatibles avec un scénario de type Kondo.A la surface, l’interaction particulière entre les sous-réseaux du cobalt 3d magnétique et du Ce 4f mène à la physique des fermions lourds, alors que l’absence totale de physique des moments localisés 4f loin de la surface se traduit par une densité d’états réduite au niveau de Fermi dans le volume. Habituellement dans les composés métalliques, la durée de vie des magnons de haute fréquence est fortement réduite par les interactions avec le continuum de Stoner. Cependant, de par cette densité réduite dans le volume, les magnons du sous-réseau de cobalt du composé CeCo2P2 subsistent jusque dans le régime du térahertz. Plus précisément, nos calculs ab-initio prédisent une suppression des excitations Stoner de basse énergie, ce que nous avons vérifié expérimentalement par diffusion inélastique résonante de rayons X. Par comparaison, le comportement typique de magnon fortement amorti observé habituellement dans les systèmes métalliques se retrouve dans le composé à structure identique LaCo2P2. Sur la base de ces observations, nous proposons le système La/CeCo2P2 comme un candidat potentiel pour des études plus poussées en vue d’aboutir à des dispositifs métalliques quantiques magnoniques.In this work, the two different Ce-based intermetallic compounds CeRh2Si2 and CeCo2P2 have been studied by means of synchrotron radiation spectroscopic (X-ray absorption and photoelectron emission spectroscopy) and resonant inelastic X-ray scattering methods in combination with ab-initio band structure calculations. The measurements allowed for an improved understanding and interpretation of surface sensitive Ce 4f physics by studying distinct surface terminations and their effects, while being combined in the bulk with Co 3d magnetism.In the first presented study, angle-resolved photoemission spectroscopy (ARPES) was used to explore the temperature dependence of the Ce 4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Distinct spectra from the Ce- and Si-terminated surfaces were studied in a wide temperature range revealing characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The terminations exhibit different temperature dependences of the Fermi level peak suggesting that the effective Kondo temperatures at the surface and in the bulk differ significantly. The origin of this difference might lie in a greatly reduced crystal–electric-field (CEF) splitting at the surface resulting in a larger effective Kondo temperature due to a higher local-moment effective degeneracy. Additionally, different hybridization strengths could further influence the 4f peak intensities at the Fermi level. The possibility of measuring distinct surfaces lays the foundation for directly testing quantitatively many-body theories that link spectroscopy and transport properties for both the bulk and the surface, separately.In contrast to CeRh2Si2, the CeCo2P2 compound shows no signs of heavy-fermion physics in the bulk. But our surface sensitive angle-resolved photoemission measurements suggest that the 4f physics significantly change near the surface. We propose that this is a direct result of the symmetry breaking of the antiferromagnetically ordered cobalt sublattice resulting in an effective magnetic field which in turn leads to partially occupied and spin-polarized Ce 4f states near the surface. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario.At the surface, the particular interplay between the magnetic Co 3d and the Ce 4f sublattice leads to heavy-fermion physics, while the complete absence of local-moment 4f physics away from the surface region results in a reduced density of states at the Fermi level in the bulk. Usually, in metallic compounds the lifetime of high-frequency magnons is strongly reduced due to interactions with the Stoner continuum, however, due to the reduced density in the bulk, long-living magnons on the Co sublattice of CeCo2P2 can exist up to the terahertz regime. In detail, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which we experimentally verified by resonant inelastic X-ray scattering measurements. In comparison, the isostructural LaCo2P2 compound exhibits the typical strongly damped magnon behavior usually observed in metallic systems. Based on these observations, we propose the La/CeCo2P2 system as a suitable candidate for further studies on the way to metallic magnonic quantum devices