Induced spin textures at 3\u3ci\u3ed\u3c/i\u3e transition metal–topological insulator interfaces

While some of the most elegant applications of topological insulators, such as the quantum anomalous Hall effect, require the preservation of Dirac surface states in the presence of time-reversal symmetry breaking, other phenomena such as spin-charge conversion rather rely on the ability for these surface states to imprint their spin texture on adjacent magnetic layers. In this Rapid Communication, we investigate the spin-momentum locking of the surface states of a wide range of monolayer transition metals (3d-TM) deposited on top of Bi2Se3 topological insulators using first-principles calculations. We find an anticorrelation between the magnetic moment of the 3d-TM and the magnitude of the spin-momentum locking induced by the Dirac surface states. While the magnetic moment is large in the first half of the 3d series, following Hund’s rule, the spin-momentum locking is maximum in the second half of the series. We explain this trend as arising from a compromise between intra-atomic magnetic exchange and covalent bonding between the 3d-TM overlayer and the Dirac surface states. As a result, while Cr and Mn overlayers can be used successfully for the observation of the quantum anomalous Hall effect or the realization of axion insulators, Co and Ni are substantially more efficient for spin-charge conversion effects, e.g., spin-orbit torque and charge pumping

Induced Spin-texture at 3dd Transition Metal/Topological Insulator Interfaces

While some of the most elegant applications of topological insulators, such as quantum anomalous Hall effect, require the preservation of Dirac surface states in the presence of time-reversal symmetry breaking, other phenomena such as spin-charge conversion rather rely on the ability for these surface states to imprint their spin texture on adjacent magnetic layers. In this work, we investigate the spin-momentum locking of the surface states of a wide range of monolayer transition metals (3$d$-TM) deposited on top of Bi$_{2}$Se$_{3}$ topological insulators using first principles calculations. We find an anticorrelation between the magnetic moment of the 3$d$-TM and the magnitude of the spin-momentum locking {\em induced} by the Dirac surface states. While the magnetic moment is large in the first half of the 3$d$ series, following Hund's rule, the spin-momentum locking is maximum in the second half of the series. We explain this trend as arising from a compromise between intra-atomic magnetic exchange and covalent bonding between the 3$d$-TM overlayer and the Dirac surface states. As a result, while Cr and Mn overlayers can be used successfully for the observation of quantum anomalous Hall effect or the realization of axion insulators, Co and Ni are substantially more efficient for spin-charge conversion effects, e.g. spin-orbit torque and charge pumping.Comment: 5 pages, 7 figure

Janus Monolayers of Magnetic Transition Metal Dichalcogenides as an All-in-One Platform for Spin-Orbit Torque

We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit-torque memories. Using first principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of non-volatile magnetic random access memories.Comment: 5 pages, 4 figure

A systematic study on Pt based, subnanometer-sized alloy cluster catalysts for alkane dehydrogenation: effects of intermetallic interaction

Platinum-based bimetallic nanoparticles are analyzed by the application of density functional theory to a series of tetrahedral Pt3X cluster models, with element X taken from the P-block, preferably group 14, or from the D-block around group 10. Almost identical cluster geometries allow a systematic investigation of electronic effects induced by different elements X. Choosing the propane-to-propene conversion as the desired dehydrogenation reaction, we provide estimates for the activity and selectivity of the various catalysts based on transition state theory. No significant Brønsted-Evans-Polanyi-relation could be found for the given reaction. A new descriptor, derived from an energy decomposition analysis, captures the effect of element X on the rate-determining step of the first hydrogen abstraction. Higher activities than obtained for pure Pt4 clusters are predicted for Pt alloys containing Ir, Sn, Ge and Si, with Pt3Ir showing particularly high selectivity

Vers une description théorique des propriétés d'adsorption et de spectroscopie vibrationnelle de molécules aromatiques aux interfaces solvant/métal

L'objectif de cette thèse est de déterminer les structures d'adsorption de molécules aromatiques complexes (quinoline et un de ses dérivés chiraux, cinchonidine) sur la surface Pt(111) à partir de calculs théorie de la fonctionnelle de la densité. Celles-ci doivent permettre de mieux comprendre le rôle de la cinchonidine dans l'hydrogénation énantiosélective d'a-kéto-esters en catalyse hétérogène. En premier lieu les structures d'adsorption compétitives de ces molécules ont été calculées via un modèle classique gaz/métal. Puis les spectres vibrationnels mesurés ont été interprétés à partir de nos simulations. Ensuite, l'influence du solvant CCl4 sur l'adsorption de la quinoline a été étudiée de manière explicite par deux modèles (statique et dynamique), en corrigeant la DFT des forces de dispersion (approche semi-empirique). Les calculs montrent que la présence du solvant ne change pas la stabilité relative des formes d'adsorption. Cependant, elle affecte les vibrations de l'adsorbat en couplage fort avec les variations du potentiel électrostatique à l'interface et l'accord entre expérience et théorie est sensiblement amélioré.The main goal of the thesis is the determination of the adsorption structures of complex aromatic molecules (quinoline and one of its chiral derivatives, cinchonidine) on the Pt(111) surface from density functional theory calculations. These properties are expected to improve our understanding of the role of cinchonidine in the enantioselective hydrogenation of a-keto-esters in heterogeneous catalysis. First the competitive adsorption structures of these molecules have been calculated through a classical gas/metal interface model. Then the measured vibrational spectra have been interpreted from our simulations. Second the influence of the CCl4 solvent on quinoline adsorption has been studied explicitely from two models (static and dynamic), by correcting DFT with dispersion forces (semi-empirical approach). The calculations show that the presence of the solvent does not change the stability order of the adsorption forms. However it affects the adsorbate vibrations in strong coupling with the variations of the electrostatic potential at the interface, and the agreement between experience and theory is significantly improved.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Theoretical elucidation of the selectivity changes for the hydrogenation of unsaturated aldehydes on Pt(111)

International audienceOn the basis of density functional theory calculations and an original use of a generalized Brönsted–Evans–Polanyi relationship, the key question of the change of selectivity has been solved for hydrogenation of three unsaturated aldehydes (acrolein, crotonaldehyde and prenal) on a Pt(1 1 1) surface. This study supports the idea that the selectivity in favor of the unsaturated alcohol (UOL) is controlled by adsorption thermodynamics of this partially hydrogenated product while the selectivity in favor of the other compound in competition (saturated aldehyde, SAL) obeys a more subtle kinetic control. The present work demonstrates the efficiency and the potentiality of the exposed correlation

Elusive Dzyaloshinskii-Moriya interaction in Fe 3 GeTe 2 monolayer

International audienceUsing symmetry analysis and density functional theory calculations, we uncover the nature of Dzyaloshinskii-Moriya interaction in Fe3GeTe2 monolayer. We show that while such an interaction might result in small distortions of the magnetic texture on the short range, on the long range Dzyaloshinskii-Moriya interaction favors in-plane Néel spin-spirals along equivalent directions of the crystal structure. Whereas our results show that the observed Néel skyrmions cannot be explained by the Dzyaloshinskii-Moriya interaction at the monolayer level, they suggest that canted magnetic texture shall arise at the boundary of Fe3GeTe2 nanoflakes or nanoribbons and, most interestingly, that homochiral planar magnetic textures could be stabilized

Ab-Initio Investigations of Magnetic Properties and Induced Half-Metallicity in Ga1−xMnxP (x = 0.03, 0.25, 0.5, and 0.75) Alloys

Ab-initio calculations are performed to examine the electronic structures and magnetic properties of spin-polarized Ga1−xMnxP (x = 0.03, 0.25, 0.5, and 0.75) ternary alloys. In order to perceive viable half-metallic (HM) states and unprecedented diluted magnetic semiconductors (DMSs) such as spintronic materials, the full potential linearized augmented plane wave method is utilized within the generalized gradient approximation (GGA). In order to tackle the correlation effects on 3d states of Mn atoms, we also employ the Hubbard U (GGA + U) technique to compute the magnetic properties of an Mn-doped GaP compound. We discuss the emerged global magnetic moments and the robustness of half-metallicity by varying the Mn composition in the GaP compound. Using GGA + U, the results of the density of states demonstrate that the incorporation of Mn develops a half-metallic state in the GaP compound with an engendered band gap at the Fermi level (EF) in the spin–down state. Accordingly, the half-metallic feature is produced through the hybridization of Mn-d and P-p orbitals. However, the half-metallic character is present at a low x composition with the GGA procedure. The produced magnetic state occurs in these materials, which is a consequence of the exchange interactions between the Mn-element and the host GaP system. For the considered alloys, we estimated the X-ray absorption spectra at the K edge of Mn. A thorough clarification of the pre-edge peaks is provided via the results of the theoretical absorption spectra. It is inferred that the valence state of Mn in Ga1−xMnxP alloys is +3. The predicted theoretical determinations surmise that the Mn-incorporated GaP semiconductor could inevitably be employed in spintronic devices