Spin-orbit Coupling Research Articles

Mathematical analysis of spin transport in topological insulators to optimize memory devices performance using numerical simulations

Understanding spin transport in topological insulators (TIs) is crucial for the development of spin-based technologies, such as magnetic memory, sensors, and quantum bits. To optimize memory device performance, we developed a comprehensive mathematical model that describes the evolution of spin density, as a function of space and time, taking into account spin-momentum locking, spin-orbit coupling, spin dynamics, and diffusive transport processes (including phonon and impurity scattering). Using numerical simulations (finite element method and Backward Euler Method) implemented in Python, we analyzed the spin transport in TIs. Our study demonstrated a critical dependence of spin transport efficiency on device length, with a maximum efficiency of 75% at a length of 8 micrometers. Beyond a critical length of 10 micrometers, efficiency recovered, reaching 80% at 14 micrometers. We observed oscillatory behavior in spin polarization, with amplitude modulation indicating constructive and destructive interference patterns. The inclusion of spin-orbit coupling and Dirac terms in our model revealed a non-uniform spin polarization distribution, with a 30% increase in polarization near the center. Defects and boundaries significantly impacted spin transport, reducing polarization by 40% near the defect region. Through optimization, we achieved a 25% increase in spin transport efficiency and a 20% enhancement in memory device performance. Our results demonstrate the crucial role of optimizing device length, material quality, and interface engineering in achieving efficient spin transport and improved memory device performance, paving the way for the development of high-performance spin-based technologies.

Separation of Inverse Altermagnetic Spin-Splitting Effect from Inverse Spin Hall Effect in RuO_{2}.

Recently, a significant amount of attention has been attracted toward a third classification of magnetism, altermagnetism, due to the unique physical properties of altermagnetic materials, which are compensated collinear antiferromagnets that host time-reversal symmetry-breaking phenomena like a ferromagnet. In an altermagnetic material, through the nonrelativistic altermagnetic spin-splitting effect (ASSE), a transverse spin current is generated upon charge current injection. However, it is very challenging to experimentally establish the ASSE since it is inevitably mixed with the spin Hall effect due to the relativistic spin-orbit coupling of the material. Additionally, the dependence on the hard-to-probe and hard-to-control Néel vectors makes it even more difficult to observe and establish the ASSE. In this Letter, we utilize the thermal spin injection from the ferrimagnetic insulator yttrium iron garnet and detect an inverse altermagnetic spin-splitting effect (IASSE) in the high-quality epitaxial altermagnetic RuO_{2} thin films. We observe an opposite sign for the spin-to-charge conversion through the IASSE compared to the inverse spin Hall effect (ISHE). The efficiency of the IASSE is approximately 70% of the ISHE in RuO_{2}. Moreover, we demonstrate that the ASSE or IASSE effect is observable only when the Néel vectors are well aligned. By modifying the Néel vector domains via RuO_{2} crystallinity, we study the ASSE or IASSE unequivocally and quantitatively. Our Letter provides significant insight into the spin-splitting effect in altermagnetic materials.

Efficient calculation of magnetocrystalline anisotropy energy using symmetry-adapted Wannier functions

Magnetocrystalline anisotropy, a crucial factor in magnetic properties and applications like magnetoresistive random-access memory, often requires extensive k-point mesh in first-principles calculations. In this study, we develop a Wannier orbital tight-binding model incorporating crystal and spin symmetries and utilize time-reversal symmetry to divide magnetization components. This model enables efficient computation of magnetocrystalline anisotropy. Applying this method to L10 FePt and FeNi, we calculate the dependence of the anisotropic energy on k-point mesh size, chemical potential, spin-orbit interaction, and magnetization direction. The results validate the practicality of the models to the energy order of 100[μeV/f.u.] for these systems.

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study.

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule.

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin-orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

Novel Halogenated Curcumin-Mediated Photodynamic Inactivation for the Preservation of Small Yellow Croaker (Larimichthys polyactis).

A novel class of halogenated curcumin, X-Cur (X = F, Cl, or Br), was synthesized, and its photosensitivity was evaluated. The results showed that Br-Cur with the highest singlet oxygen (1O2) generation capacity exhibited a better photodynamic inactivation (PDI) effect on the small yellow croaker (Larimichthys polyactis) than curcumin. This was attributed to the heavy atom effect of Br, which resulted in Br-Cur having the smallest singlet-triplet energy difference ΔEst(S1-T3) (0.140 eV) and the largest spin-orbit coupling value (0.642262 cm-1). When L. polyactis was treated with 0.025 wt % Br-Cur and exposed to blue LED irradiation (450 nm, 20 mW/cm2) for 20 min, the increase in the total volatile basic nitrogen content (28.23 ± 2.38 mg/100 g on day 6), pH, and total viable count (6.13 ± 0.06 log CFU/g on day 6) could be effectively controlled. Accordingly, Br-Cur is a promising photosensitizer for PDI preservation.

Color Polymorphism and Room Temperature Phosphorescence of 4-Bromo-2,7-Di-Tert-Butyl- 9-Methoxypyrene.

Color polymorphism combined with crystal packing-dependent luminescence properties of polymorphs reflects differences in intermolecular interactions in different molecular arrangements. The title compound has two polymorphic crystal structures having strikingly different absorption and luminescence spectra that result from different packing motifs in the crystal lattice. The polymorph with brick wall-like packing of molecules is white and shows very weak violet fluorescence whereas the second polymorph, where molecules are arranged in columnar stacks, is bright yellow and displays intense green fluorescence with maximum at 487 nm (20530 cm-1). In the white polymorph, where the distance between neighboring chromophores is increased, absorption and fluorescence spectra are similar to those of monomer in solution, and intersystem crossing to triplet manifold is the dominant pathway of relaxation. In the yellow polymorph, molecules within the columnar stacks are rotated which mitigates the steric hindrance and leads to closer π-stacking of the pyrene cores. That increases the ππ overlap and strengthens intermolecular interactions decreasing energy of the excited states. This affects emission spectra and photophysical processes-fluorescence yield grows whereas triplet formation yield decreases when S1 is lowered below higher triplet states and conditions for effective vibronic spin-orbit coupling are not favorable. The effect is not observed for other similar pyrene derivatives, testifying the uniqueness of the phenomenon.

Local Excitation of Kagome Spin Ice Magnetism Seen by Scanning Tunneling Microscopy.

The kagome spin ice can host frustrated magnetic excitations by flipping its local spin. Under an inelastic tunneling condition, the tip in a scanning tunneling microscope can flip the local spin, and we apply this technique to kagome metal HoAgGe with a long-range ordered spin ice ground state. Away from defects, we discover a pair of pronounced dips in the local tunneling spectrum at symmetrical bias voltages with negative intensity values, serving as a striking inelastic tunneling signal. This signal disappears above the spin ice formation temperature and has a dependence on the magnetic fields, demonstrating its intimate relation with the spin ice magnetism. We provide a two-level spin-flip model to explain the tunneling dips considering the spin ice magnetism under spin-orbit coupling. Our results uncover a local emergent excitation of spin ice magnetism in a kagome metal, suggesting that local electrical field induced spin flip climbs over a barrier caused by spin-orbital locking.

Conductance, spin and valley polarizations through 8-Pmmn borophene magnetic barriers

Conductance, spin and valley polarizations through 8-Pmmn borophene barriers in the presence of external Rashba spin–orbit interaction (RSOI) and magnetic field are investigated theoretically. The spin-dependent conductance shows a strong dependence on the direction of the 8-Pmmn borophene barriers, the strength of RSOI and value of the magnetic field strength. For two barriers unlike to one barrier, the spin-dependent conductance exhibits a prominent oscillatory behavior. For the parallel configuration, the dependence of conductance on the magnetic field is more than for the antiparallel configuration. Transmission probability without and with spin rotation decreases drastically with the increase in the value of the magnetic field, so that for sufficiently large magnetic fields, the transmission probability is zero for most electron incidence angles. The size and direction of the spin and valley polarization can be tuned by the strength of RSOI, direction of the 8-Pmmn borophene barriers, size of the magnetic field and type of parallel or antiparallel configuration. In certain conditions, full valley polarization can be achieved in the proposed structure.

Multifunctional single-component photosensitizers as metal-free ferroptosis inducers for enhanced photodynamic immunotherapy

Immunotherapy can enhance primary tumor efficacy, restrict distant growth, and combat lung metastasis. Unfortunately, it remains challenging to effectively activate the immune response. Here, tertiary butyl, methoxy, and triphenylamine (TPA) were utilized as electron donors to develop multifunctional photosensitizers (PSs). CNTPA-TPA, featuring TPA as the donor (D) and cyano as the acceptor (A), excelled in reactive oxygen species (ROS) generation due to its smaller singlet-triplet energy gap (ΔES-T) and larger spin-orbit coupling constant (SOC). Additionally, cyano groups reacted with glutamate (Glu) and glutathione (GSH), reducing intracellular GSH levels. This not only enhanced PDT efficacy but also triggered redox dyshomeostasis-mediated ferroptosis. The positive effects of photodynamic therapy (PDT) and ferroptosis promoted immunogenic cell death (ICD) and immune activation. By further combining anti-programmed cell death protein ligand-1 (anti-PD-L1) antibody, the powerful treatments of ferroptosis-assisted photodynamic immunotherapy significantly eradicated the primary tumors, inhibited the growth of distant tumors, and suppressed lung metastasis. In this study, a three-pronged approach was realized by single-component CNTPA-TPA, which simultaneously served as metal-free ferroptosis inducers, type-I photosensitizers, and immunologic adjuvants for near-infrared fluorescence imaging (NIR FLI)-guided multimodal phototheranostics of tumor. Statement of significance(1) CNTPA-TPA shared the smallest singlet-triplet energy gap and the largest spin-orbit coupling constant, which boosted intersystem crossing for efficient type-I photodynamic therapy (PDT);(2) Special reactions between cyano groups with glutamate and glutathione in mild conditions restricted the biosynthesis of intracellular GSH. GSH-depletion efficiently induced glutathione peroxidase 4 inactivation and lipid peroxide, resulting in ferroptosis of tumor cells;(3) The combination treatments of ferroptosis-assisted photodynamic immunotherapy induced by single-component CNTPA-TPA with the participation of anti-PD-L1 antibody resulted in increased T-cell infiltration and profound suppression of both primary and distant tumor growth, as well as lung metastasis.

G-factor symmetry and topology in semiconductor band states

The [Formula: see text] tensor, which determines the reaction of Kramers-degenerate states to an applied magnetic field, is of increasing importance in the current design of spin qubits. It is affected by details of heterostructure composition, disorder, and electric fields, but it inherits much of its structure from the effect of the spin-orbit interaction working at the crystal-lattice level. Here, we uncover interesting symmetry and topological features of [Formula: see text] for important valence and conduction bands in silicon, germanium, and gallium arsenide. For all crystals with high (cubic) symmetry, we show that large departures from the nonrelativistic value [Formula: see text] are guaranteed by symmetry. In particular, considering the spin part [Formula: see text], we prove that the scalar function [Formula: see text] must go to zero on closed surfaces in the Brillouin zone, no matter how weak the spin-orbit coupling is. We also prove that for wave vectors [Formula: see text] on these surfaces, the Bloch states [Formula: see text] have maximal spin-orbital entanglement. Using tight-binding calculations, we observe that the surfaces [Formula: see text] exhibit many interesting topological features, exhibiting Lifshitz critical points as understood in Fermi-surface theory.

Electronic band structure and optical properties of BGaAsBi/GaAs using 16 band kp Hamiltonian

The numerical simulations of optical and electronic properties of BGaAsBi/GaAs quantum wells (QWs) are obtained by diagonalizing the 16-band kp Hamiltonian with distinct boron and bismuth compositions that exhibit multifaceted potential in 1.3–1.5 μm. It has been observed that under strain-relaxed conditions for a doping concentration of 12.5 % (B, Bi), the anticipated values of spin-orbit (SO) coupling energy (ΔSO) and the bandgap (Eg) are 302 meV and 0.952 eV, respectively. Additionally, strain interaction on band structure decreases the band gap from (Eg = 0.92eV–0.85eV), increasing dopant concentration from 8 % to 12.5 %. The consequence of incorporating both Bi and B impurity is a reduction in electron effective mass of BGaAsBi by ∼1.3 times concerning the host GaAs, enhancing the optical properties of BGaAsBi/GaAs quantum-confined heterostructures. Moreover, an increase in well-width introduced a red shift and reduced the peak amplitude of the gain spectra. In addition, for solar cell application, the onset of the fundamental absorption edge is depicted at an energy of about 0.96 eV. For laser applications, the optimum threshold current density and quality factor of BGaAsBi/GaAs quantum well achieved at room temperature are 1089 A/cm2 and 3653, respectively, at well width (w = 2.0 nm), cavity length (L = 0.5 mm), and average thickness of the active region (d = 45 nm). The variation in power density is studied with injected current density, while the quality factor is calculated with well width. The outcomes could be beneficial for future use in optical devices.

Josephson Diode Effect in Parallel-Coupled Double-Quantum Dots Connected to Unalike Majorana Nanowires.

We study theoretically the Josephson diode effect (JDE) when realized in a system composed of parallel-coupled double-quantum dots (DQDs) sandwiched between two semiconductor nanowires deposited on an s-wave superconductor surface. Due to the combined effects of proximity-induced superconductivity, strong Rashba spin-orbit interaction, and the Zeeman splitting inside the nanowires, a pair of Majorana bound states (MBSs) may possibly emerge at opposite ends of each nanowire. Different phase factors arising from the superconductor substrate can be generated in the coupling amplitudes between the DQDs and MBSs prepared at the left and right nanowires, and this will result in the Josephson current. We find that the critical Josephson currents in positive and negative directions are different from each other in amplitude within an oscillation period with respect to the magnetic flux penetrating through the system, a phenomenon known as the JDE. It arises from the quantum interference effect in this double-path device, and it can hardly occur in the system of one QD coupled to MBSs. Our results also show that the diode efficiency can reach up to 50%, but this depends on the overlap amplitude between the MBSs, as well as the energy levels of the DQDs adjustable by gate voltages. The present model is realizable within current nanofabrication technologies and may find practical use in the interdisciplinary field of Majorana and Josephson physics.

Spin-Charge Conversion in Chiral Polymers with Hopping Conduction.

Organic and biological materials are often chiral. Chiral polymers, as recent experiments indicate, facilitate spin-charge conversion: a charge current results in a spin polarization and vice versa, dubbed chirality-induced spin selectivity (CISS) and inverse CISS (ICISS). While CISS/ICISS in crystalline chiral systems such as tellurium can be understood in terms of their chirality- and spin-dependent band structure, such a picture becomes inapplicable to disordered chiral polymers, where carrier transport is via hopping rather than band conduction. Here, we develop a microscopic theory to describe CISS and ICISS in disordered chiral organics, in which chirality-induced geometric spin-orbit coupling leads to a purely geometric spin-dependent Berry phase in electron hops involving triads, whose orientations are dictated by the material's chirality. Our theory reveals a central role of spin-flip hopping, which suppresses CISS but enables ICISS.

Dwell time and spin polarization for electron in single ferromagnetic-stripe device modulated by spin–orbit couplings

Considering both Zeeman effect and spin–orbit coupling, we calculate dwell time for electron in single ferromagnetic-stripe device (SFSD), which can be constructed by patterning a nanosized ferromagnetic stripe on the surface of GaAs/AlxGa1-xAs heterostructure. Due to an intrinsic symmetry in the SFSD, dwell time is independent of electron spins, if only Zeeman effect is involved. However, the intrinsic symmetry is broken by spin–orbit coupling, which gives rise to spin-dependent dwell time for electron in the SFSD. As a result, electron spins can be separated in time dimension, which induces an obvious electron-spin polarization effect in the SFSD. Spin polarization ratio can be efficaciously modified by interfacial confining electric-field or strain engineering, which attributes to the dependence of effective potential felt by electron in the SFSD on spin–orbit couplings. Thus, the SFSD can act as a controllable temporal electron-spin splitter, a class of electron-spin polarized sources in semiconductor spintronics.

Spin-momentum properties of the spin–orbit interactions of light at optical interfaces

The spin–orbit interaction (SOI) of light manifests as the generation of spin-dependent vortex beams when a spin-polarized beam strikes an optical interface normally. However, the spin-momentum nature of this SOI process remains elusive, which impedes further manipulation. Here, we systematically investigate the spin-momentum properties of the transmitted beam in this SOI process using a full-wave theory. The transmitted beam has three components, a spin-maintained normal mode, a spin-reversed abnormal mode, and a longitudinal component. By decomposing the total spin angular momentum (SAM) into the transverse SAM (T-SAM) and the helicity dependent longitudinal SAM (L-SAM), we demonstrate that the L-SAM dominates the total SAM of the normal mode, while the T-SAM dictates that of the abnormal mode. The underlying physics is that the normal mode exhibits a much larger weight than the longitudinal field, while the abnormal mode has a weight comparable to the longitudinal field. This study enriches the understanding of the spin-momentum nature of light’s SOI and offers new opportunities for manipulating light’s angular momentum.

Realization of Yin–Yang kagome bands and tunable quantum anomalous Hall effect in monolayer V3Cl6

Kagome materials serve as crucial platforms for investigating the quantum anomalous Hall effect (QAHE) due to the presence of kagome bands in their electronic structures. However, despite the theoretical predictions being proposed, kagome band material realizations have been limited. In this work, through tight-binding (TB) model analysis, by setting the nearest-neighbor hopping integrals with opposite signs, we propose a Yin–Yang kagome band structure characterized by two stable enantiomorphic kagome bands. Furthermore, we design a monolayer V3Cl6 to confirm the TB model. Three V atoms are located in different coordination environments in V3Cl6, so opposite signs of the hopping integrals between two of their orthogonal d orbitals can be achieved, which is the key to realize Yin–Yang kagome band structures. The calculated band structures obtained from first principles are consistent with those from the TB model. Additionally, we find that the two enantiomorphic flat bands in monolayer V3Cl6 possess opposite Chern number after spin–orbit coupling is considered, which can also be confirmed from symmetry index analysis. The Chern numbers as well as the topological properties can be modulated by doping hole or adjusting the magnetization directions, so the QAHE can be tuned in monolayer V3Cl6. Our results provide a practicable pathway for realizing Yin–Yang kagome band structures and achieving tunable QAHE in them.

Enhancement of spin Hall angle in semimetallic materials α -Sn under voltage regulation

Utilizing current-induced spin–orbit torque (SOT) to control magnetization is essential for the advancement of spintronics. SOT offers high energy efficiency and rapid operation speed. The ideal SOT material should have a high charge-to-spin conversion efficiency and excellent electrical conductivity. Recently, there has been a focus on topological insulator materials with topological surface states in SOT research due to their controllability in spin–orbit coupling, conductivity, and energy band topology. While topological Dirac semimetallic materials show promise for SOT applications, research on voltage regulation of their spin Hall angle is still in its early stages. This paper investigates the multilayer structure of a Dirac semimetallic material. In an α-Sn/Ag bilayer, the voltage regulation effect can increase the spin Hall angle by five times by adjusting the strain on the Fermi level. Experiments explore the role of a silver layer as a transport layer in the electric field control of multilayer films. This material system can enhance its effects under electric field regulation and offer insight for achieving regulation in new spintronic devices.

Underluminous tidal disruption events

Abstract We have evidence of X-ray flares in several galaxies consistent with a a star being tidally disrupted by a supermassive black hole (MBH). If the star starts on a nearly parabolic orbit relative to the MBH, one can derive that the fallback rate follows a t−5/3 decay. Depending on the penetration factor, β, a star will be torn apart differently, and relativistic effects play a role. We have modified the standard version of the smoothed-particle hydrodynamics (SPH) code Gadget to include a relativistic treatment of the gravitational forces between the gas particles of a main-sequence (MS) star and a MBH. We include non-spinning post-Newtonian corrections to incorpore the periapsis shift and the spin-orbit coupling up to next-to-lowest order. We find that tidal disruptions around MBHs in the relativistic cases are underluminous for values starting at β≧2.25; i.e. the fallback curves produced in the relativistic cases are progressively lower compared to the Newtonian simulations as the penetration parameter increases. While the Newtonian cases display a total disruption, we find that all relativistic counterparts feature a survival core for penetration factors going to values as high as 12.05. We perform a additional dynamical numerical study which shows that the geodesics of the elements in the star converge at periapsis. We confirm these findings with an analytical study of the geodesic separation equation. The luminosity of TDEs must be lower than predicted theoretically due to the fact that the star will partially survive when relativistic effects are taken into account. A survival core should consistently emerge from any TDE with β≧2.25.

Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2.

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.