Spin-orbit Coupling Mechanism Research Articles

Sparkling Organic Phosphorescence from Fluorinated Tetrathia[7]helicenes: Synthesis and Photophysical, Electrochemical and Computational Studies.

Structure-property correlations in the thiahelicene family are often not trivial beacuse most of the functional groups present on the helical scaffold modify the conjugation size of the π-system. Selecting fluorine-containing groups to provide strong inductive effects without interacting with low-lying orbitals of the system could be the way to overcome the issue. Here we report a study on three fluorine-functionalized tetrathia[7]helicenes, highlighting interesting correlations between the position of the functional groups and the conjugated skeleton properties. Helicenes Heli-F2 and Heli-CF-F2 were prepared by photoinduced isomerization-electrocyclization (the Mallory photocyclization) of the corresponding fluorinated benzodithienyl-ethenes Alk-F2 and Alk-CF-F2, which were prepared in high yields through stereo-conservative Stille reaction. Notably these helicenes were found to display green phosphorescence around 530-550 nm, and the studies suggest an efficient spin-orbit coupling mechanism in these high-energy triplet nonplanar conjugated molecules. Both helicenes and their precursors were thoroughly characterized by means of optical and electrochemical measurements, while DFT calculations enable a rationale on their structure-property correlations to be defined.

Hole spin manipulation in inhomogeneous and nonseparable electric fields

The usual models for electrical spin manipulation in semiconductor quantum dots assume that the confinement potential is separable in the three spatial dimensions and that the AC drive field is homogeneous. However, the electric field induced by the gates in quantum dot devices is not fully separable and displays significant inhomogeneities. Here, we address the electrical manipulation of hole spins in semiconductor heterostructures subject to inhomogeneous vertical electric fields and/or in-plane AC electric fields. We consider Ge quantum dots electrically confined in a Ge/GeSi quantum well as an illustration. We show that the lack of separability between the vertical and in-plane motions gives rise to an additional spin-orbit coupling mechanism (beyond the usual linear and cubic in momentum Rashba terms) that modulates the principal axes of the hole gyromagnetic g-matrix. This non-separability mechanism can be of the same order of magnitude as Rashba-type interactions, and enables spin manipulation when the magnetic field is applied in the plane of the heterostructure even if the dot is symmetric (disk-shaped). More generally, we show that Rabi oscillations in strongly patterned electric fields harness a variety of g-factor modulations. We discuss the implications for the design, modeling and understanding of hole spin qubit devices.

Whirling interlayer fields as a source of stable topological order in moiré CrI3

The moiré engineering of two-dimensional magnets opens unprecedented opportunities to design novel magnetic states with promises for spintronic device applications. The possibility of stabilizing skyrmions in these materials without chiral spin-orbit couplings or dipolar interactions is yet to be explored. Here, we investigate the formation and control of ground state topological spin textures (TSTs) in moiré {Cr}{I}_{3} using stochastic Landau–Lifshitz–Gilbert simulations. We unveil the emergence of interlayer vortex and antivortex Heisenberg exchange fields, stabilizing spontaneous and field-assisted ground state TSTs with various topologies. The developed study accounts for the full bilayer spin dynamics, thermal fluctuations, and intrinsic spin-orbit couplings. By examining the effect of the Kitaev interaction and the next nearest-neighbor Dzyaloshinskii–Moriya interaction, we propose the latter as the unique spin-orbit coupling mechanism compatible with experiments on monolayer and twisted {Cr}{I}_{3}. Our findings contribute to the current knowledge about moiré skyrmionics and uncover the nature of spin-orbit coupling in {Cr}{I}_{3}.

Generation and Detection of Dresselhaus‐Like Spin Current in a Single‐Crystal Ferromagnetic Metal

AbstractDistinct from the existing spin–orbit torque devices to only generate Rashba‐like spin current, it is proposed to generate Dresselhaus‐like spin current based on a single‐crystal ferromagnetic metal. By employing the second harmonic Hall measurement, it is found that the second harmonic Hall signal in single‐crystal ferromagnet exhibits an antisymmetric angular dependence induced by Dresselhaus‐like spin current, opposite to the reported symmetric angular dependence induced by Rashba‐like spin current. Such anomalous Dresselhaus‐like spin current is further confirmed by the current‐induced shift of the hysteresis loop when the magnetic field is swept along the current direction. However, when the ferromagnetic metal exhibits amorphous crystal structure or polycrystal structure, the Dresselhaus‐like spin current is absent. This result complements the missing type of spin currents due to charge‐to‐spin conversion and benefits the formulation of comprehensive spin–orbit coupling mechanism, by considering additional broken symmetry caused by ferromagnetic order and the structure symmetry of the crystalline lattice.

Monte Carlo simulations of spin transport in nanoscale transistors: temperature and size effects

Spin-based metal-oxide-semiconductor field-effect transistors (MOSFETs) with a high-mobility III-V channel are studied using self-consistent quantum corrected ensemble Monte Carlo device simulations of charge and spin transport. The simulations including spin–orbit coupling mechanisms (Dresselhaus and Rashba coupling) examine the electron spin transport in the 25 nm gate length MOSFET. The transistor lateral dimensions (the gate length, the source-to-gate, and the gate-to-drain spacers) are increased to investigate the spin-dependent drain current modulation induced by the gate from room temperature of 300 K down to 77 K. This modulation increases with increasing temperature due to increased Rashba coupling. Finally, an increase of up to 20 nm in the gate length, source-to-gate, or the gate-to-drain spacers increases the spin polarization and enhances the spin-dependent drain current modulation at the drain due to polarization-refocusing effects.

Planar Chirality and Optical Spin-Orbit Coupling for Chiral Fabry-Perot Cavities.

We design, in a most simple way, Fabry–Perot cavities with longitudinal chiral modes by sandwiching between two smooth metallic silver mirrors a layer of polystyrene made planar chiral by torsional shear stress. We demonstrate that the helicity-preserving features of our cavities stem from a spin–orbit coupling mechanism seeded inside the cavities by the specific chiroptical features of planar chirality. Planar chirality gives rise to an extrinsic source of three-dimensional chirality under oblique illumination that endows the cavities with enantiomorphic signatures measured experimentally and simulated with excellent agreement. The simplicity of our scheme is particularly promising in the context of chiral cavity QED and polaritonic asymmetric chemistry.

Variability of Electron and Hole Spin Qubits Due to Interface Roughness and Charge Traps

Semiconductor spin qubits may show significant device-to-device variability in the presence of spin-orbit coupling mechanisms. Interface roughness, charge traps, layout or process inhomogeneities indeed shape the real space wave functions, hence the spin properties. It is, therefore, important to understand how reproducible the qubits can be, in order to assess strategies to cope with variability, and to set constraints on the quality of materials and fabrication. Here we model the variability of single qubit properties (Larmor and Rabi frequencies) due to disorder at the Si/SiO$_2$ interface (roughness, charge traps) in metal-oxide-semiconductor devices. We consider both electron qubits (with synthetic spin-orbit coupling fields created by micro-magnets) and hole qubits (with intrinsic spin-orbit coupling). We show that charge traps are much more limiting than interface roughness, and can scatter Rabi frequencies over one order of magnitude. We discuss the implications for the design of spin qubits and for the choice of materials.

Spin-orbit grating of light in coherent media

We explore the underlying spin-orbit coupling (SOC) mechanism for the interactions between complex structured light fields in a four-level tripod electromagnetically induced transparency (EIT) system. Because one coupling field in the system is a Bessel beam, the susceptibility of the EIT medium can be modulated quasiperiodically in the radial dimension. The paraxial propagation and evolution of a rotating spinor image as the probe field in the medium can be described by a Pauli-like equation with the SOC term. We calculate the spatial distributions of the diffracted patterns of the image in the near field and clearly show the tunable SOC-induced radial splitting of the oppositely polarized pseudospin states. Our scheme may be useful for all-optical processing of spatial multimode signals in coherent media.

Propagation of polarized gravitational waves

The propagation of high-frequency gravitational waves can be analyzed using the geometrical optics approximation. In the case of large but finite frequencies, the geometrical optics approximation is no longer accurate and polarization-dependent corrections at first order in wavelength modify the propagation of gravitational waves, via a spin-orbit coupling mechanism. We present a covariant derivation from first principles of effective ray equations describing the propagation of polarized gravitational waves, up to first-order terms in wavelength, on arbitrary spacetime backgrounds. The effective ray equations describe a gravitational spin Hall effect for gravitational waves and are of the same form as those describing the gravitational spin Hall effect of light, derived from Maxwell's equations.

Tunable and enhanced Rashba spin-orbit coupling in iridate-manganite heterostructures

Tailoring spin-orbit interactions and Coulomb repulsion are the key features to observe exotic physical phenomena such as magnetic anisotropy and topological spin texture at oxide interfaces. Our study proposes a novel platform for engineering the magnetism and spin-orbit coupling at LaMnO3/SrIrO3 (3d-5d oxide) interfaces by tuning the LaMnO3 growth conditions which controls the lattice displacement and spin-correlated interfacial coupling through charge transfer. We report on a tunable and enhanced interface-induced Rashba spin-orbit coupling and Elliot-Yafet spin relaxation mechanism in LaMnO3/SrIrO3 bilayer with change in the underlying magnetic order of LaMnO3. We also observed enhanced spin-orbit coupling strength in LaMnO3/SrIrO3 compared to previously reported SrIrO3 layers. The X-Ray spectroscopy measurement reveals the quantitative valence of Mn and their impact on charge transfer. Further, we performed angle-dependent magnetoresistance measurements, which show signatures of magnetic proximity effect in SrIrO3 while reflecting the magnetic order of LaMnO3. Our work thus demonstrates a new route to engineer the interface induced Rashba spin-orbit coupling and magnetic proximity effect in 3d-5d oxide interfaces which makes SrIrO3 an ideal candidate for spintronics applications.

The dominancy of damping like torque for the current induced magnetization switching in Pt/Co/W multilayers

Two classes of spin-orbit coupling (SOC) mechanisms have been considered as candidate sources for the spin orbit torque (SOT): the spin Hall Effect (SHE) in heavy metals with strong SOC and the Rashba effect arising from broken inversion symmetry at material surfaces and interfaces. In this work, we have investigated the SOT in perpendicularly magnetized Pt/Co/W films, which is compared with the results in Pt/Co/AlOx films. Using the harmonic measurements, we have characterized the effective fields corresponding to the damping like torque and the field like torque. Theoretically, in the case of the asymmetrical Pt/Co/W trilayers with opposite sign of spin Hall angle, both damping like torque and field like torque due to the SHE and the Rashba effect will be enhanced, but we have found the dominancy of damping like torque in the Pt/Co/W films. It is much different from the results in the Pt/Co/AlOx films, in which both the damping like torque and the field like torque are evident.

Quantum spin polarization effect in multi-nanolayer structures

We studied the spin-polarized state transport in Fe–SnO2–Ag and Fe–BeO–Ag three-nanolayer sandwich structures. The exchange-resonance spectra of these sandwich structures are quite specific and different from those observed earlier in other three-nanolayer structures. The presently recorded spectra comprise a set of discrete lines, their width increasing with the sample temperature and also with the Ag layer thickness, for both samples. The linewidth dependences on temperature and Ag layer thickness were studied in detail. The effect of thickness of the intermediate nanolayers of SnO2 and BeO on the linewidth was also explored. To explain the observed line broadening effects, we proposed and developed the spin-orbit (SO) coupling mechanism of the electron spin relaxation. In the frameworks of this mechanism, we assumed that the electron spin of a bonding electron in one of the layers of the sandwich system is coupled by SO interaction with the other layers. We found that the change in phonon densities affects the linewidths of the exchange resonance spectra. We estimated the values of the model parameters from the analysis of the experimental data. To that end, we continue further development of our earlier theoretical model, using it to interpret the current experimental results, including ab initio calculations of the electronic structure. The exchange resonance spectra were simulated using phenomenological model, where the anisotropy of the g-factor was introduced. We performed ab initio simulations of the exchange resonance spectra and their linewidths, using Gaussian-2000 and a homemade FORTRAN code.

Spin-orbit coupling mechanism of singlet oxygen [formula omitted] quenching by solvent vibrations

Degenerate character of the O2(a1Δg) state and of the charge-transfer configurations (CTCs) from solvent to the oxygen open-shell orbitals explains the enhancement of spin-orbit coupling (SOC) which is necessary to overcome spin prohibition during singlet oxygen a1Δg quenching. The former mechanism of non-radiative transition O2(a1Δg)→O2(X3Σg-) based on electronic energy transfer to the solvent vibrational levels (e-v mechanism) is supplemented here by explicit analysis of SOC effects mediated by solvent and O2 vibrations. The SOC matrix element between one component of the initial electronic excited singlet a1Δg state and the final ground triplet X3Σg- state in the oxygen moiety is not equal to zero (as in free O2) in the collision complex with solvent molecule (M) when all possible CTCs of the type O2-…M+ are accounted for. Intermolecular configuration interaction between CTC and locally excited states obeys a simple symmetry selection rule which provides finally the SOC matrix element with a guarantee of large orbital rotation around the molecular oxygen axis creating a torque. The CTCs admixtures into the singlet and triplet wave functions in the collision complex O2…M ensure the SOC enhancement inside the O2 moiety and let the spin-prohibited singlet oxygen a1Δg quenching to become effectively allowed in terms of e-v mechanism. In the new model the solvent is not only a passive “sink” for the singlet oxygen excitation energy but serves as an active perturber of the oxygen open shell and finally – of the whole spin dynamics in the collision system.

Unexpected High Spin Polarization in Cr4 Cluster

Recent X-ray magnetic circular dichroism (XMCD) spectroscopy has demonstrated a total high spin ground state of S = 11/2 in the Cr2+ cluster. The accuracy of these experiments opens the possibility to observe small magnetic particles with greater detail than those obtained in Stern–Gerlach setups. Particularly the chromium clusters, where a high total magnetization can be observed due to their localized d-electrons, are an excellent candidate to be detected in this sophisticated experiments. In this work, the small-sized chromium tetramer has been investigated in detail by means of zero and finite temperature ab initio methods. The dimer and trimer chromium clusters are reported as a benchmark in order to establish a reliable approach. We found the existence of a spin competition between a lower S = 1 and a higher S = 6 spin state in the Cr4 cluster; both spin states are close in geometry, allowing a spin-flip due to an spin–orbit coupling mechanism.

Theoretical Study of the Spin Competition in Small-Sized Al Clusters.

Stern-Gerlach (SG) experiments on aluminum clusters indicate that some small-sized aggregates exhibit a deflection signal consistent with the existence of magnetic moments. However, in the particular case of Al6 and Al8 clusters, electronic structure investigations show ambiguity on the 0 K ground spin state. In this work extensive computations of the electronic structure have been carried out in order to determine the ground state of these structures. Electron correlation has been introduced at MP2, MP4, and CCSD(T) theory level as well as by DFT computations with different density functionals. DFT-based Born-Oppenheimer molecular dynamics results at different simulation temperatures complete this investigation. One of our main conclusions is that singlet spin states are systematically the more stable configuration at 0 K. These Al clusters exhibit almost degenerate electronic structures at singlet and triplet spin states. The geometries are similar, and the paths connecting both structures allow an intersystem crossing through a spin-orbit coupling mechanism, indicating a dynamical interchange of both spin states at finite temperatures.

Investigations of the zero-field splitting with the local tilting angle τFe3+ for Fe3+ in ZnGeP2 crystal

The axial zero-field splitting parameter D for Fe3+ at Ge4+ site of ZnGeP2 chalcopyrite semiconductor is calculated using the method based on the dominant spin–orbit coupling mechanism. In the calculation, the impurity-induced local tilting angle of anion tetrahedron, which is much larger than the corresponding tilting angle in the host ZnGeP2 crystal, is applied. The results indicate the negative sign of D, whereas the magnitude of D is explained reasonably, and the large tilting angle is further confirmed for Fe3+ in ZnGeP2 crystal.

Mechanism of Gadolinium Doping Induced Room-Temperature Phosphorescence from Porphyrin

The spin–orbit coupling mechanism was generally used to explain heavy atom effect induced room-temperature phosphorescence (RTP). Here, we demonstrate that the mechanism of RTP induced by Gd3+ from hematoporphyrin monomethyl ether (HMME) is due to the mixing of singlet (S) and triplet (T) states. The spin-forbidden transition between S and T states was partly allowed due to the states mixing, as indicated by the direct absorption corresponding to transition from S0 to T1, which was observed for the first time from RTP. The quantum yield of T1 was determined to be 0.80, and the percent of each energy transfer process was determined. Meanwhile there is no nonradiative relaxation from HMME to Gd3+ because of the large energy gap between the excited and ground states of Gd3+. The special energy level of Gd3+ as well as the states mixing between S and T states produced the strong phosphorescence emission. The population and deactivation of triplet states in heavy atom induced RTP can be used to analyze the mechanism of phosphorescent emission.

Scattering theory of spin-orbit active adatoms on graphene

The scattering of two-dimensional massless Dirac fermions from local spin-orbit interactions with an origin in dilute concentrations of physisorbed atomic species on graphene is theoretically investigated. The hybridization between graphene and the adatoms' orbitals lifts spin and valley degeneracies of the pristine host material, giving rise to rich spin-orbit coupling mechanisms with features determined by the exact adsorption position on the honeycomb lattice - bridge, hollow or top position - and the adatoms' outer-shell orbital type. Effective graphene-only Hamiltonians are derived from symmetry considerations, while a microscopic tight-binding approach connects effective low-energy couplings and graphene-adatom hybridization parameters. Within the $T$-matrix formalism, a theory for (spin-dependent) scattering events involving graphene's charge carriers, and the spin-orbit active adatoms is developed. Spin currents associated with intravalley and intervalley scattering are found to tend to oppose each other. We establish that under certain conditions, hollow-position adatoms give rise to the spin Hall effect, through skew scattering, while top-position adatoms induce transverse charge currents via trigonal potential scattering. We also identify the critical Fermi energy range where the spin Hall effect is dramatically enhanced, and the associated transverse spin currents can be reversed.

Spin-orbit coupling as a source of long-range triplet proximity effect in superconductor-ferromagnet hybrid structures

We investigate the proximity effect in diffusive superconducting hybrid structures with a spin-orbit (SO) coupling. Our study is focused on the singlet-triplet conversion and the generation of long-range superconducting correlations in ferromagnetic elements. We derive the quasiclassical equations for the Green's functions including the SO coupling terms in form of a background SU(2) field. With the help of these equations, we first present a complete analogy between the spin diffusion process in normal metals and the generation of the triplet components of the condensate in a diffusive superconducting structure in the presence of SO coupling. From this analogy it turns out naturally that the SO coupling is an additional source of the long-range triplet component (LRTC) besides the magnetic inhomogeneities studied in the past. We demonstrate an explicit connection between an inhomogeneous exchange field and SO coupling mechanisms for the generation of the LRTC and establish the conditions for the appearance of the LRTC in different geometries. We also consider a S/F bilayer in contact with normal metal with SO coupling and show that the latter can be used as a source for the LRTC. Our work gives a global description of the singlet-triplet conversion in hybrids structures in terms of generic spin-fields and our results are particularly important for the understanding of the physics underlying spintronics devices with superconductor elements.

Asymmetry in effective fields of spin-orbit torques in Pt/Co/Pt stacks

Measurements of switching via spin-orbit coupling mechanisms are discussed for a pair of inverted Pt/Co/Pt stacks with asymmetrical Pt thicknesses. Taking into account the planar Hall effect contribution, effective fields of spin-orbit torques (SOT) are evaluated using lock-in measurements of the first and second harmonics of the Hall voltage. Reversing the stack structure leads to significant asymmetries in the switching behavior, including clear evidence of a nonlinear current dependence of the transverse effective field. Our results demonstrate potentially complex interplay in devices with all-metallic interfaces utilizing SOT.