Significant Spin-orbit Coupling Research Articles

Insights into the Mechanism of Chiral-Induced Spin Selectivity: The Effect of Magnetic Field Direction and Temperature.

Chiral oligopeptide monolayers are adsorbed on a ferromagnetic surface and their magnetoresistance is measured as a function of the angle between the magnetization of the ferromagnet and the surface normal. These measurements are conducted as a function of temperature for both enantiomers. The angle dependence is found to follow a changing trend with a period of 360°. Quantum simulations reveal that the angular distribution can be obtained only if the monolayer has significant effective spin orbit coupling (SOC), that includes contribution from the vibrations. The model shows that SOC only in the leads cannot reproduce the observed angular dependence. The simulation can reproduce the experiments if it included electron-phonon interactions and dissipation.

Enhancing Fast RISC in Hot‐Exciton Thermally Activated Delayed Fluorescence Emitter Through Fused Ring Modification: A Theoretical Insights

AbstractRecent progress has shown promising advancements in enhancing the Reverse intersystem crossing (RISC) process between high‐energy triplet states (Tn, n ≥ 1) and radiative singlet states (Sm) through the utilization of the classic D‐A‐D' strategy. In this study, 12 molecules employing phenyl carbazole and a peripheral phenanthroimidazole as the donors and fused cores as acceptors are theoretically designed. Additionally, electron‐withdrawing groups such as fluorine and cyano, as well as electron‐donating group like methoxy, are incorporated into anthracene, napthaoxadiazole, and napthothiadiazole derivatives, acting as fused core components. Theoretical analyses reveal that among the molecules examined, eight adhere to the three golden principle rules, demonstrating a large energy gap between S1‐T1, the minimal ΔES1‐T2, and large T2‐T1. Additionally, these molecules exhibit significant Spin‐orbit coupling (SOC) and maintain a strong Hybridized local and charge transfer (HLCT) character, facilitating the fast hRISC from T2 to S1. The auxiliary inclusion of heteroatoms, such as oxygen and sulfur, on the anthracene core, aligns with the prerequisite strategy, ultimately reinforcing the SOC. Furthermore, replacing the methoxy group on the Nz bridge significantly enhances the SOC, resulting in a substantial increase in the hRISC rate (10 +08 s−1). Overall, the findings substantially elevate the photophysical attributes of the designed molecules through the utilization of the hRISC channel.

Strain-tunable ferromagnetism and skyrmions in two-dimensional Janus Cr2XYTe6 (X, Y = Si, Ge, Sn, and X≠Y) monolayers

By using first-principles calculations and micromagnetic simulations, we investigate the electronic structure, magnetic properties, and skyrmions in two-dimensional Janus Cr2XYTe6 (X,Y = Si, Ge, Sn, X ≠ Y) monolayers. Our findings reveal that the Cr2XYTe6 monolayers are ferromagnetic semiconductors with a high Curie temperature (Tc). The bandgap and Tc can be further increased by applying tensile strain. In addition, there is a transition from the ferromagnetic to the antiferromagnetic state at a compressive strain. Both Cr2SiSnTe6 and Cr2SiGeTe6 exhibit a large magnetic anisotropy energy, which are mainly associated with the significant spin–orbit coupling of the nonmagnetic Te atoms rather than that of the magnetic Cr atoms. Interestingly, the Cr2SiSnTe6 monolayer exhibits a significant Dzyaloshinskii–Moriya interaction of 1.12 meV, which facilitates the formation of chiral domain walls and skyrmions. Furthermore, under tensile strain, chiral DWs can be transformed into skyrmions if applying an external magnetic field. These findings suggest that Janus Cr2XYTe6 monolayers hold promise for spintronic nanodevice applications.

Impact of Chiral Spinterfaces on Magneto-Photoluminescence Effects for Chiral Lead Halide Perovskites.

Chiral lead halide perovskites (LHPs) have been widely investigated in chiroptical spintronics due to their significant Rashba spin-orbit coupling (SOC) and chiral-induced spin selectivity (CISS). Ferromagnet/LHP spinterface stems from the orbital hybridization at the interface of the ferromagnet and the nonmagnetic semiconductor, where interfacial density of state is spin-dependent. By far, the impact of the ferromagnet/chiral LHP spinterface on magneto-photoluminescence (Magneto-PL) of chiral LHPs remains unknown. In this work, we find that the negative and tunable Magneto-PL effects for the pristine LHP bulk film can be drastically enhanced by incorporating ferromagnetic/chiral LHP interfaces. A large Magneto-PL magnitude can reach approximately -13% for the Ni/(S-MBA)2PbI4 interface at the field strengths of ±900 mT. With the assistance of circularly polarized PL spectra, anisotropic magneto-resistance, and X-ray photoelectron spectroscopy measurements, we demonstrate that the ferromagnet/chiral LHP interfaces are chirality/spin-dependent and possess ferromagnetic property due to distinct magnetic switching behavior and electronic orbit coupling at interfaces, which boost the Rashba splitting and spin mixing. The comprehensive effects of Rashba-induced exciton states and chiral-induced SOC at chiral spinterfaces with CISS are responsible for the enhanced Magneto-PL of Ni/(R/S-MBA)2PbI4. It is postulated that the chiral spinterfaces play a dominant role for achieving large and tunable magneto-optical effect of chiral LHPs. This work paves the way for chiroptical spintronic applications.

Magnetization switching driven by spin-orbit torque of Weyl semimetal WTe<sub>2</sub>

The Wely semimetal WTe<sub>2</sub> exhibits significant spin-orbit coupling characteristics and can generate unconventional spin current with out-of-plane polarization, which has become a hotspot in recent years. Meanwhile, WTe<sub>2</sub> also has high charge-spin conversion efficiency, allowing perpendicular magnetization to be switched deterministically without the assistance of an external magnetic field, which is critical for the high-density integration of low-power magnetic random-access memories. The purpose of this paper is to review the recent advances in the research on spin orbit torque in heterostructures composed of WTe<sub>2</sub> and ferromagnetic layers, focusing on progress of research on the detection and magnetization switching in the spin orbit torque of heterojunctions composed of WTe<sub>2</sub> prepared by different methods (e.g. mechanical exfoliation and chemical vapor deposition) and ferromagnetic layers such as conventional magnets (e.g, FeNi and CoFeB, etc.) and two-dimensional magnets (e.g. Fe<sub>3</sub>GeTe<sub>2</sub>, etc.). Finally, the prospect of related research is discussed.

Nonequilibrium Magneto-Conductance as a Manifestation of Spin Filtering in Chiral Nanojunctions.

It is generally accepted that spin-dependent electron transmission may appear in chiral systems, even without magnetic components, as long as significant spin-orbit coupling is present in some of its elements. However, how this chirality-induced spin selectivity (CISS) manifests in experiments, where the system is taken out of equilibrium, is still debated. Aided by group theoretical considerations and nonequilibrium DFT-based quantum transport calculations, here we show that when spatial symmetries that forbid a finite spin polarization in equilibrium are broken, a net spin accumulation appears at finite bias in an arbitrary two-terminal nanojunction. Furthermore, when a suitably magnetized detector is introduced into the system, the net spin accumulation, in turn, translates into a finite magneto-conductance. The symmetry prerequisites are mostly analogous to those for the spin polarization at any bias with the vectorial nature given by the direction of magnetization, hence establishing an interconnection between these quantities.

Soft X-ray Fermi surface tomography of palladium and rhodium via momentum microscopy

Fermi surfaces of transition metals, which describe all thermodynamical and transport quantities of solids, often fail to be modeled by one-electron mean-field theory due to strong correlations among the valence electrons. In addition, relativistic spin–orbit coupling pronounced in heavier elements lifts the degeneracy of the energy bands and further modifies the Fermi surface. Palladium and rhodium, two 4d metals attributed to show significant spin–orbit coupling and electron correlations, are ideal for a systematic and fundamental study of the two fundamental physical phenomena and their interplay in the electronic structure. In this study, we explored the Fermi surface of the 4d noble metals palladium and rhodium obtained via high-resolution constant initial state momentum microscopy. The complete 3D-Fermi surfaces of palladium and rhodium were tomographically mapped using soft X-ray photon energies from 34 eV up to 660 eV. To fully capture the orbital angular momentum of states across the Fermi surface, the Fermi surface tomography was performed using p- and s- polarized light. Applicability and limitations of the nearly-free electron final state model in photoemission are discussed using a complex band structure model supported by experimental evidence. The significance of spin–orbit coupling and electron correlations across the Fermi surfaces will be discussed within the context of the photoemission results. State-of-the-art fully relativistic Korringa–Kohn–Rostoker (KKR) calculations within the one-step model of photoemission are used to support the experimental results.

Giant Dzyaloshinskii-Moriya interaction, strong XXZ-type biquadratic coupling, and bimeronic excitations in the two-dimensional CrMnI6 magnet

Two-dimensional magnets have been discovered recently as a new class of quantum matter exhibiting a broad wealth of exotic phenomena, including notably various topological excitations rooted in emergent exchange couplings between the localized magnetic moments. By analyzing the anisotropies in the single-ion magnetization and two-body exchange couplings obtained from first-principles calculations, we reveal coexistence of both giant Dzyaloshinskii–Moriya interaction and strong anisotropic XXZ-type biquadratic coupling in a recently predicted monolayer CrMnI6 magnet. The former is induced by the spontaneous in-plane inversion symmetry breaking in the bipartite system, the latter is inherently tied to the distinct high-spin state of the Mn sublattice, while the large magnitudes of both stem from the significant spin-orbit coupling. Next, we use atomistic magnetics simulations to demonstrate the vital role of Dzyaloshinskii–Moriya interaction in harboring topological bimeronic excitations, and show that the biquadratic coupling favors a Berezinskii–Kosterlitz–Thouless-like transition as the system reduces its temperature from the paramagnetic phase. These findings substantially enrich our understanding of the microscopic couplings in 2D magnets, with appealing application potentials.

Novel Mononuclear Tetrabromonitrosylrhenate(II) Complexes Containing Azole-Type Ligands: Magnetostructural Characterization through Hirshfeld Surfaces Analysis

Our research group has made incursions into the scarcely known coordination chemistry of rhenium(II). The literature shows that Re(II) mononuclear complexes are attractive in molecular magnetism due to high magnetic anisotropy because of a significant spin-orbit coupling, making them a potential source for new molecule-based magnets. In this work, we present the preparation of four novel Re(II) compounds of general formula NBu4[Re(NO)Br4(L)] [NBu4+ = tetra-n-butylammonium: L = imidazole (1), pyrazole (2), 1,2,4-triazole (3) and 1H-tetrazole (4)]. The four compounds were fully characterized by single-crystal X-ray diffraction, infrared spectroscopy, and cryomagnetic measurements in the temperature range of 1.8–300 K. Their crystal structures consist of mononuclear [Re(NO)Br4(L)]− complex anions and NBu4+ cations. Each Re(II) ion is six-coordinate with a linear nitrosyl group and one monodentate nitrogen-donor (L), which are trans-positioned, plus four bromide groups, building a tetragonally distorted octahedral surrounding. The inter-anionic contacts were thoroughly analyzed using Hirshfeld surface analyses (plots over the dnorm, shape index, and 2D fingerprints). Cryomagnetic measurements show that these complexes behave as quasi-magnetically isolated spin doublets with weak antiferromagnetic interactions at low temperatures. The magnetic behavior of Re(II) was modeled by the influence of the ligand field, tetragonal distortion, spin-orbit coupling, and covalence effects. In addition, the antiferromagnetic exchange coupling was correlated to the nature of the intermolecular interactions.

Unraveling the sign reversal of the anomalous Hall effect in ferromagnet/heavy-metal ultrathin films

The sign reversal in the anomalous Hall effect (AHE) that occurs for material offers great prospects for AHE-based spintronic devices design. However, the mechanisms are still controversial in ultrathin ferromagnetic/heavy metal thin film systems due to the complicatedly interfacial effects. Here, we investigate the AHE sign reversal in ultrathin ferromagnetic ${\mathrm{Mn}}_{2}\mathrm{CoAl}/\mathrm{Pd}$ films, a system which has shown unusual AHE, significant spin-orbit coupling, and magnetic texturing. Element-sensitive cross-sectional STEM imaging and the depth-resolved magnetization profile from polarized neutron reflectometry identifies the presence of a second ferromagnetic layer from intermixed Co-Pd. To quantitatively explain the sign reversal of the AHE, we build a model based on two contributions, ferromagnetic ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ and the intermixed CoPd layer. We also clarify that contributions to the AHE from magnetic proximity and spin Hall effect are negligible. Our work demonstrates that interfacial alloying can be a critical factor and provides insightful methods to determine the origins of the AHE in ferromagnet/heavy-metal thin film systems.

Outsourcing Intersystem Crossing without Heavy Atoms: Energy Transfer Dynamics in PyridoneBODIPY-C60 Complexes.

The excited state dynamics in two fully characterized pyridoneBODIPY-fullerene complexes were investigated using time-resolved spectroscopy. Photoexcitation was initially localized on the pyridoneBODIPY chromophore. The energy was rapidly transferred to the fullerene, which subsequently underwent ISC to form a triplet state and returned the energy to the pyridoneBODIPY via triplet-triplet energy transfer. This ping-pong energy transfer mechanism resulted in efficient (>85%) overall conversion of the excited state pyridoneBODIPY constituent despite a complete lack of ISC in the pyridoneBODIPY in the absence of the fullerene partner. The small difference in attachment chemistry for the fullerene did not impact the initial singlet energy transfer. However, the N-methylpyrrolidine bridge did slow both the triplet-triplet energy transfer and the ultimate relaxation rate of the final triplet state when compared to an isoxazole-based bridge. The rates of each step were quantified, and computational predictions were used to complement the proposed mechanism and energetics. The result demonstrated efficient triplet sensitization of a strong chromophore that lacks significant spin-orbit coupling.

Implementing Complex Oxides for Efficient Room‐Temperature Spin–Orbit Torque Switching

AbstractComplex oxides hosting 4d and 5d cations with significant spin–orbit coupling have recently been shown as promising materials for efficient spin‐charge interconversion. Through interfacing 4d and 5d oxides with magnet layers, a large spin–orbit torque (SOT) is reported. However, a room‐temperature SOT switching of perpendicular magnetization by using these oxides, which is essential for spintronic devices, is not demonstrated. Here, this is addressed yet missing aspect by studying heterostructures comprised of two representative complex oxides (4d SrRuO3 and 5d SrIrO3) and a compensated ferrimagnet FeGd with perpendicular magnetic anisotropy. A room temperature current‐induced SOT switching of perpendicular magnetization in both SrRuO3/FeGd and SrIrO3/FeGd bilayers, with the critical switching current density on the order of 106 A cm−2 is demonstrated. The SOT efficiencies of SrRuO3 and SrIrO3 are further quantified by using harmonic Hall voltage measurements. The results suggest that the strongly correlated oxides could be another promising platform for enabling energy‐efficient spin‐orbitronic applications.

Structure, optical and magnetic properties of the pyridinium cobaltate (C6H9N2)2[CoCl4

The reaction of 2-amino-4-methylpyridine (L) with CoCl2·6H2O in hydrochloric acid gave the chlorocobaltate (HL)2[CoCl4] in 82% yield. X-ray diffraction of the blue crystalline material showed isolated [CoCl4]2– tetrahedra in the structure, connected to the aromatic 2-amino-4-methyl pyridinium cations through H bonding while the cations show π-stacking. FT-IR spectroscopy in conjunction with Hirshfeld surface analysis confirmed the importance of the 2-amino function in the cations for the hydrogen bonding. The material is thermally stable up to 270 °C. Intense UV–vis absorptions in the range 200–400 nm were assigned to π–π* transitions in the pyridinium cation, weaker features in the range 600–750 nm were due to ligand-field transitions in the d7 high-spin system [CoCl4]2–. Excitation at 370 nm led to an intense blue-green emission at λem = 473 nm attributable to excited π–π* states. The molar susceptibility measured over a range of 5–300 K reveals paramagnetic behavior at high temperature range with an effective magnetic moment of µeff = 4.74 µB indicating a significant spin–orbit coupling contribution adding to the spin-only value of 3.87 µB expected for Co(II) with 3d7 high spin configuration.

Critical magnetic field for lifting the degeneracy of monochiral domain walls with strong interlayer antiferromagnetic coupling

The addition of spatial inversion asymmetry and significant spin-orbit coupling to synthetic antiferromagnets (SAFs) has led to a lot of fundamental discoveries and technological applications. Here, we experimentally demonstrate that the critical magnetic field for breaking the degeneracy of monochiral domain walls (DWs) to realize the spin-orbit torque (SOT)-induced magnetization switching can be varied in a wide range from field free to $3000\phantom{\rule{4pt}{0ex}}\mathrm{Oe}$ in perpendicularly magnetized CoPt/Ru/CoPt SAFs with strong interlayer antiferromagnetic coupling, which strongly depends on the interfacial Dzyaloshinsky-Moriya interaction (DMI) and SOT at the bottom and top interfaces with multiple inversion asymmetries. Our DW dynamical calculations further reveal that the field-free magnetization switching is caused by SOT-induced left-handed chiral N\'eel DW displacement under the condition that the interfacial DMI of the upper magnetic layer has a significantly smaller value than that of the lower magnetic layer. These findings provide key insights for understanding the role of SOT and DMI in magnetization switching of magnetically coupled chiral systems and pave the way to practical applications of SAFs.

Bad topological semimetals in layered honeycomb compounds

The Mott transition in honeycomb compounds with significant spin-orbit coupling is explored. At finite temperatures we identify a novel semimetallic phase located between a topological insulator and a topological Mott insulator. This semimetal is characterized by having no charge gap, no quasiparticles, and gapped spin excitations with nontrivial topological properties. While charge conduction is incoherent involving mean-free paths violating the Mott-Ioffe-Regel limit, spin excitations can be transported ballistically along the edges giving rise to a quantized spin conductance. Such bad topological semimetal could be searched for close to the Mott transition in certain twisted bilayers of transition metal dicalchogenides.

LaIr3Ga2: A Superconductor Based on a Kagome Lattice of Ir

New materials based on Kagome lattices, predicted to host exotic quantum physics because they can display flat electronic bands, Dirac cones and topologically nontrivial surface states, are strongly desired. Here we report the crystal structure and superconducting properties of LaIr3Ga2, a previously unreported material that is based on a Kagome lattice of the heavy atom Ir. LaIr3Ga2 is a type II superconductor with Tc ~ 5.2 K, mu0Hc1 (0) ~ 7.1 mT and mu0Hc2 (0) ~ 4.7 T. The normalized heat capacity jump at the superconducting transition, (deltaC/gammaTc), 1.41, is within error of the value expected for a weak coupling BCS superconductor (1.43). Strong electron-electron correlation is inferred from the superconducting coherence length. Calculations show that the influence of spin orbit coupling on the electronic structure is significant, that the 5d states of the Ir in the Kagome planes are dominant near the Fermi energy (EF), that a nearly flat band is calculated to occur at about 100 meV below EF, and that two Dirac points are close to EF. This material is of interest for investigating the coupling between topological physics and superconductivity in a system with significant spin orbit coupling.

Angular Harmonic Hall Voltage and Magnetoresistance Measurements of Pt/FeCoB and Pt-Ti/FeCoB Bilayers for Spin Hall Conductivity Determination

Materials with significant spin-orbit coupling enable efficient spin-to-charge interconversion, which can be utilized in novel spin electronic devices. A number of elements, mainly heavy-metals (HM) have been identified to produce a sizable spin current ($j_\mathrm{s}$), while supplied with a charge current ($j$), detected mainly in the neighbouring ferromagnetic (FM) layer. Apart from the spin Hall angle $\theta_\mathrm{SH}$ = $j_\mathrm{s}$/$j$, spin Hall conductivity ($\sigma_\mathrm{SH}$) is an important parameter, which takes also the resistivity of the material into account. In this work, we present a measurement protocol of the HM/FM bilayers, which enables for a precise $\sigma_\mathrm{SH}$ determination. Static transport measurements, including resistivity and magnetization measurements are accompanied by the angular harmonic Hall voltage analysis in a dedicated low-noise rotating probe station. Dynamic characterization includes effective magnetization and magnetization damping measurement, which enable HM/FM interface absorption calculation. We validate the measurement protocol in Pt and Pt-Ti underlayers in contact with FeCoB and present the $\sigma_\mathrm{SH}$ of up to 3.3$\times$10$^5$ S/m, which exceeds the values typically measured in other HM, such as W or Ta.

Higher angular momentum pairings in interorbital shadowed-triplet superconductors: Application to Sr2RuO4

Even-parity interorbital spin-triplet pairing emerges as an intriguing candidate in multiorbital superconductors with significant Hund's and spin-orbit coupling. Within such a state, the pairing is dominated by the intraband pseudospin-singlet component via the spin-orbit coupling, distinguishing it from a pure spin triplet and motivating the name, shadowed triplet. With atomic spin-orbit coupling, it was shown that this pairing exhibits an anisotropic $s$-wave character, while higher angular momentum pairing symmetries such as $d$- or $g$-wave have been proposed based on phenomenological analyses in candidate systems. A natural question is then whether higher angular momentum pairings may arise with this form of spin-triplet pairing. Here, we examine the interplay between spin-orbit coupling and the electronic dispersions in correlated metals and demonstrate how they can be realized. We apply this idea to Sr$_{2}$RuO$_{4}$ and determine the competition among different pairing states as multiple spin-orbit coupling parameters are tuned. The presence of both $d$- and $g$-wave pairings, including a $d+ig$ state, are found when momentum-dependent spin-orbit coupling with $d$-wave character is increased. Implications of the theory and future directions are also discussed.

Magnon modes as a joint effect of surface ferromagnetism and spin–orbit coupling in CoSi chiral topological semimetal

CoSi single crystal is a known realization of a chiral topological semimetal with simultaneously broken mirror and inversion symmetries. In addition to the symmetry-induced spin–orbit coupling, surface ferromagnetism is known in nominally diamagnetic CoSi structures, which appears due to the distorted bonds and ordered vacancies near the surface. We experimentally investigate electron transport through a thin CoSi flake at high current density. Surprisingly, we demonstrate dV/dI(I) curves which are qualitatively similar to ones for ferromagnetic multilayers with characteristic dV/dI magnon peaks and unconventional magnetic field evolution of the peaks’ positions. We understand these observations as a result of current-induced spin polarization due to the significant spin–orbit coupling in CoSi. Scattering of non-equilibrium spin-polarized carriers within the surface ferromagnetic layer is responsible for the precessing spin-wave excitations, so the observed magnon modes are the joint effect of surface ferromagnetism and spin–orbit coupling in a CoSi chiral topological semimetal. Thus, thin CoSi flakes behave as magnetic conductors with broken inversion symmetry, which is important for different spintronic phenomena.

Massive Dirac fermions and strong Shubnikov–de Haas oscillations in single crystals of the topological insulator Bi2Se3 doped with Sm and Fe

Topological insulators (TIs) are emergent materials with unique band structure, which allow the study of quantum effect in solids, as well as contribute to high-performance quantum devices. To achieve the better performance of TIs, here, we present a codoping strategy using synergistic rare-earth (RE) Sm and transition-metal Fe dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ single crystals, which combine the advantages of both a transition-metal-doped TI [high ferromagnetic ordering temperature and observed quantum anomalous Hall effect (QAHE)], and a RE doped TI (large magnetic moments and significant spin-orbit coupling). In the as-grown single crystals, clear evidences of ferromagnetic ordering were observed. The angle-resolve photoemission spectroscopy indicates the ferromagnetism opens a \ensuremath{\sim}44 meV band gap at the surface Dirac point. Moreover, the mobility of the carriers at 3 K is $\ensuremath{\sim}7400\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$, and we thus observed an ultra-strong Shubnikov--de Haas oscillation in the longitudinal resistivity, as well as the Hall steps in transverse resistivity 14 T. Our transport and angular-resolved photoemission spectroscopy results suggest that the RE and transition metal codoping in the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ system is a promising avenue to implement the QAHE, as well as harnessing the massive Dirac fermion in electrical devices.