Effect Of Rashba Spin-orbit Coupling Research Articles

Anomalous hybridized excitons induced by combined effects of Van der Waals coupling and Rashba spin–orbit coupling

As a typical transition-metal dichalcogenide, MoS2has drawn wide attention due to its good stability and excellent physicochemical properties, making it suitable for visible-region optoelectronic devices. To expand its application, bandgap engineering via heterostructure, thus far, was conventionally employed to tune the band gap. However, this strategy has the disadvantage that energy levels of bands do not show obvious changes compared to the isolated components, limiting the range of applications. Here, we achieve hybridized excitons induced by combined effects of Van der Waals (vdW) coupling and Rashba spin-orbit coupling (SOC), with a small exciton energy of 0.65 eV. For this purpose, we design a MoS2/MoWC heterostructure, where a built-in field (due to the absence of mirror symmetry) induces the Rashba SOC and contributes to the anomalous hybridized states, combined with the vdW coupling. An effective model is proposed to demonstrate the anomalous hybridized states for the heterostructure. Our approach reveals a novel mechanics model for hybridized excitons states, providing new physical ways to achieve infrared-region devices.

Promises and technological prospects of two-dimensional Rashba materials

The Rashba spin–orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating each material with the necessary advancements required to translate theoretical concepts into tangible technologies. Furthermore, we highlight promising avenues for future research and development, drawing from insights gleaned from the current state of the field.

Numerical simulation of Rashba spin–orbit coupling effect on quantum Hall resistivity within tight-binding approximation

A numerical simulation of the effect of Rashba spin–orbit coupling (RSO) on quantum transport properties of electrons confined to a two-dimensional electron gas (2DEG) in the integer quantum Hall regime is presented. The mechanism of quantization and the dependence of the Hall resistivity (HR) on the system built-in and external parameters are studied using the non-equilibrium Green’s function (NEGF) technique. Relevant Green’s functions are computed numerically using spectral decomposition method in combination with recursive techniques. The tight-binding-like representation of the system Hamiltonian including magnetic field is obtained within the finite difference method, using Dirichlet boundary conditions. Corresponding formula linking the Hall resistivity with the transmission function of the system has been derived analytically within the NEGF formalism in a general form, in the sense that the formula can also be used beyond the linear response limit and for nonzero temperatures. In particular, it was shown explicitly that in the presence of the RSO coupling the even- and odd plateaus emerging in the magnetic-field dependence of the quantum Hall resistivity are related to different physical mechanisms.

Bipolar Rashba Semiconductors: A Class of Nonmagnetic Materials for Electrical Spin Manipulation.

The realization of the electrical control of spin is highly desirable. One promising approach is by regulating the Rashba spin-orbit coupling effect of materials through external electric fields. However, this method requires materials to possess either a high electric field response and a large Rashba constant or the simultaneous presence of Rashba splitting and ferroelectric polarization. These stringent requirements result in a scarcity of suitable materials. In order to surpass these limitations and exploit a new prospect for spin manipulation via the Rashba effect, a conceptual class of materials named bipolar Rashba semiconductors (BRS) is proposed, whose valence band and conduction band possess opposite spin texture directions when approaching the Fermi level. The unique electronic structure of BRS makes it feasible to reverse the spin precession by simply applying a gate voltage. The existence of BRS is confirmed through first-principles calculations on the two-dimensional (2D) material AlBiS3.

Over-barrier photoelectron emission with Rashba spin–orbit coupling

We develop a theoretical model to calculate the quantum efficiency (QE) of photoelectron emission from materials with the Rashba spin–orbit coupling (RSOC) effect. In the low temperature limit, an analytical scaling between QE and the RSOC strength is obtained as QE ∝(ℏω−W)2+2ER(ℏω−W)−ER2/3, where ℏω, W, and ER are the incident photon energy, work function, and the RSOC parameter, respectively. Intriguingly, the RSOC effect substantially improves the QE for strong RSOC materials. For example, the QE of Bi2Se3 and Bi/Si(111) increases, by 149% and 122%, respectively, due to the presence of strong RSOC. By fitting to the photoelectron emission characteristics, the analytical scaling law can be employed to extract the RSOC strength, thus offering a useful tool to characterize the RSOC effect in materials. Importantly, when the traditional Fowler–Dubridge model is considered, the extracted results of the prefactor a1 in QE may substantially deviate from the actual values by ∼90%, thus highlighting the importance of employing our model to analyze the photoelectron emission especially for materials with strong RSOC. These findings provide a theoretical foundation for the design of photoemitters using Rashba spintronic materials.

Enhanced Rashba Spin Orbit Coupling and Magnetic Behavior at Oxide Heterointerfaces by Optical Gating.

Complex oxide heterointerfaces have been a hot research spot due to their rich physical phenomena and broad quantum coherence that respond to multiple external stimuli. Among these external stimuli, light is a very powerful one to manipulate properties such as carrier density and spin characteristics. However, achieving a light-magnetic correlation is in high demand for multifield responding devices, and its intrinsic mechanism remains unclear. Here, by illuminating Nd0.86Sr0.14Al0.86Ni0.14O3-SrTiO3 heterointerfaces using 360 nm light, we observe a series of interesting physical phenomena, like enhanced magnetoresistance (MR). More interestingly, a band splitting and strong Rashba spin-orbit coupling (SOC) effect occur after illumination, accompanied by a magnetic feature and thus leading to an anomalous Hall effect (AHE). Upon optical gating, the magnetism can be caused by Rashba SOC induced spin-orbit torque (SOT). The work will be sure to have great importance in both theoretical studies and all-oxide devices.

Ballistic-electron coherent spin precession in a Rashba two-dimensional electron system

In a remarkable manifestation of the Rashba spin-orbit coupling effect, spin polarized carriers injected into a narrow two-dimensional electron system channel have ballistic trajectories and a weakly relativistic Fermi velocity. In the carrier rest frame, the intrinsic electric field in the quantum well transforms as an effective magnetic field, ${H}^{*}$, transverse to the trajectories. Electrons with initial spin momentum and polarization orientations along the channel precess because of ${H}^{*}$ with a wavelength $\ensuremath{\lambda}$. For long mean free paths $l$, spin detection probes at distances $L$ $l$ record conductance oscillations with wavelength $\ensuremath{\lambda}$. Two unique experiments that demonstrate these oscillations are discussed.

Quantized Exciton Motion and Fine Energy-Level Structure of a Single Perovskite Nanowire.

The quantum-confinement effect profoundly influences the exciton energy-level structures and recombination dynamics of semiconductor nanostructures but remains largely unexplored in traditional one-dimensional nanowires mainly due to their poor optical qualities. Here, we show that in defect-tolerant perovskite material of highly luminescent CsPbBr3 nanowires, the exciton's center-of-mass motion perpendicular to the axial direction is severely confined. This is reflected in the two sets of photoluminescence spectra emitted from a single CsPbBr3 nanowire, each of which consists of doublet peaks with linear polarizations perpendicular and parallel to the axial direction. Moreover, different exciton states can be mixed by the Rashba spin-orbit coupling effect, resulting in two single photoluminescence peaks with linear polarizations both along the nanowire axis. The above findings mark the emergence of an ideal platform for the exploration of intrinsic one-dimensional exciton photophysics and optoelectronics, thus bridging the long-missing research gap between the well-studied two- and zero-dimensional semiconductor nanostructures.

Effective spin-orbit gaps in the s and p orbital bands of an artificial honeycomb lattice

Muffin-tin methods have been instrumental in the design of honeycomb lattices that show, in contrast to graphene, separated $s$ and in-plane $p$ bands, a $p$ orbital Dirac cone, and a $p$ orbital flat band. Recently, such lattices have been experimentally realized using the two-dimensional electron gas on Cu(111). A possible next avenue is the introduction of spin-orbit coupling to these systems. Intrinsic spin-orbit coupling is believed to open topological gaps and create a topological flat band. Although Rashba coupling is straightforwardly incorporated in the muffin-tin approximation, intrinsic spin-orbit coupling has only been included either for a very specific periodic system, or only close to the Dirac point. Here, we introduce effective intrinsic and Rashba spin-orbit terms in the Hamiltonian for both periodic and finite-size systems. We observe a strong band opening over the entire Brillouin zone between the $p$ orbital flat band and the Dirac cone hosting a pronounced edge state, robust against the effects of Rashba spin-orbit coupling.

Strain-controlled Rashba spin-orbit coupling effect in SnS and SnSe monolayers

The miniaturization of transistors and the high density of integrated circuits makes conventional transistors reach a size limit, causing leakage currents, unstable performance, and increasing production cost. Spin field effect transistors (SFETs) based on spintronics use electron spin as an information carrier and regulate electron spin degree of freedom for storing and processing information. As a class of spintronics, Rashba spin–orbit coupling (SOC) effect attracts increasing attention because of its electric tunability. SFETs based on Rashba SOC and two-dimensional materials will make a qualitative leap in future semiconductor devices. Based on first-principles calculations, it is found that the applied strains along zigzag (ZZ), armchair (AC), and biaxial direction make the bandgaps of SnS and SnSe monolayers (ML) have an indirect-direct-zero transition. SnS and SnSe ML have a more stable electron spin and stronger Rashba spin splitting than InGaAs/InAlAs heterojunction and Au(111) surface. For SnS ML, 6% tensile strain along ZZ direction not only stabilizes the electron spin but also strengthens Rashba spin splitting to a maximum of 0.76 eV Å. For SnSe ML, 2% tensile strain along biaxial direction makes Rashba spin splitting strength reach a maximum of 1.33 eV Å.

Effects of spin-orbit coupling in superconducting proximity devices: Application to CoSi2/TiSi2 heterostructures

Motivated by the recent findings of unconventional superconductivity in $\mathrm{Co}{\mathrm{Si}}_{2}/\mathrm{Ti}{\mathrm{Si}}_{2}$ heterostructures, we study the effect of interface-induced Rashba spin-orbit coupling on the conductance of a three-terminal T-shaped superconducting device. We calculate the differential conductance for this device within the quasiclassical formalism that includes the mixing of triplet-singlet pairing due to the Rashba spin-orbit coupling. We discuss our result in light of the conductance spectra reported by S.-P. Chiu et al., arXiv:2012.13679 for $\mathrm{Co}{\mathrm{Si}}_{2}/\mathrm{Ti}{\mathrm{Si}}_{2}$ heterostructures.

Multigap superconductivity at an unconventional Lifshitz transition in a three-dimensional Rashba heterostructure at the atomic limit

It is well known that the critical temperature of multi-gap superconducting 3D heterostructures at atomic limit (HAL) made of a superlattice of atomic layers with an electron spectrum made of several quantum subbands can be amplified by a shape resonance driven by the contact exchange interaction between different gaps. The $T_C$ amplification is achieved tuning the Fermi level near the singular nodal point at a Lifshitz transition for opening a neck. Recently high interest has been addressed to the breaking of inversion symmetry which leads to a linear-in-momentum spin-orbit induced spin splitting, universally referred to as Rashba spin-orbit coupling (RSOC) also in 3D layered metals. However the physics of multi-gap superconductivity near unconventional Lifshitz transitions in 3D HAL with RSOC, being in a non-BCS regime, is not known. The key result of this work getting the superconducting gaps by Bogoliubov theory and the 3D electron wave functions by solution of the Dirac equation is the feasibility of tuning multi-gap superconductivity by suitably matching the spin-orbit length with the 3D superlattice period. It is found that the presence of the RSOC amplifies both the k dependent anisotropic gap function and the critical temperature when the Fermi energy is tuned near the circular nodal line. Our results suggest a method to effectively vary the effect of RSOC on macroscopic superconductor condensates via the tuning of the superlattice modulation parameter in a way potentially relevant for spintronics functionalities in several existing experimental platforms and tunable materials needed for quantum devices for quantum computing.

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.

Anisotropy of the spin-polarized edge current in monolayer transition metal dichalcogenide zigzag nanoribbons

We report anisotropic spin polarization of edge currents in ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$ monolayer zigzag nanoribbons (ZNRs) deposited on either nonmagnetic or ferromagnetic insulator substrates. We employ an 11-band tight-binding model to calculate the electronic band structures of transition metal dichalcogenide (TMDC) monolayers and their corresponding nanoribbons in the presence of Rashba spin-orbit coupling (RSOC) and magnetic proximity effect produced by ferromagnetic substrate. We adopt the nonequilibrium Green's function method together with Landauer-B\uttiker formalism to study the quantum transport behavior stemming from the edge states of ZNRs. We demonstrate that the spin-polarized edge current can be generated in both ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$ ZNRs with RSOC. We find that the spin polarization spreads out in all three directions. This is in stark contrast to what occurs in zigzag graphene nanoribbons, for which the polarization only exists in the transverse direction (across the width of ribbons). In addition, the spin polarization direction strongly depends on the strength of the intrinsic SOC component. The interplay of Rashba and intrinsic SOC is crucial for the spin polarization of the currents in any spatial direction. For TMDCs with stronger intrinsic SOC such as in ${\mathrm{WS}}_{2}$ monolayer ZNRs, we observe that the spin polarization along the perpendicular direction to the plane of the ZNR can be as large as $90%$. Moreover, the unusual anisotropy of the spin polarization can be further enhanced by the magnetic proximity effect. These results open up possibilities for the generation of tunable high-spin polarization currents in ZNRs without application of an external magnetic field.

Controlling anisotropic surface group velocity and effective mass in topological crystalline insulator SnTe by Rashba effect

Abstract Rashba effect has become important in the realization of exotic topological features. Herein, we theoretically investigate the Rashba spin–orbit coupling(RSOC) effects on the dispersion energy, group velocity(GV) and effective mass(EM) of Dirac fermions in the topological crystalline insulator SnTe(001) surface. We found that different classes of RSOCs do not change the qualitative behavior of the band structure; its main effect is to break the symmetry of the gapless bands down to a group of discrete rotations and to slightly shift the phase boundaries. Moreover, the inherent anisotropic surface features hold with RSOC. The modulation mechanism of RSOC effects is further analyzed from the orientation-dependent GV and EM at different RSOC values. It is found that the GV and EM of fermions are band-dependent and symmetric concerning the RSOC sign, which plays a decisive role in modifying the optical properties. Our study provides new insight into the designing nanodevices for electric-field-controllable topological electronics.

Role of spin-orbit interactions on the entropy and heat capacity of a quantum dot helium placed in an external magnetic field

The effects of Rashba, Dresselhaus and Coulomb interactions on the entropy and heat capacity of a two-electron quantum dot placed in an external magnetic field are studied at finite temperature. The effect of electron-electron interaction is studied by using the Johnson-Payne harmonic model which is exactly soluble, while the Rashba and Dresselhus spin-orbit coupling effects are dealt with using unitary transformations. It is observed that as a function of the magnetic field, the heat capacity has a double peak structure with the minima between the peaks occurring at the singlet-triplet transition point and the e-e interaction shifts the peaks to lower magnetic fields. It is also observed that the coulomb correlation reduces the heat capacity in the low temperature regime while in the high temperature regime it enhances it. Interestingly, however, in the high field regime, the heat capacity does not seem to be influenced by the electron-electron interaction. The entropy is found to show a single peak as a function of magnetic field at the singlet-triplet transition point. The Rashba spin-orbit interaction reduces the heat capacity and the Dresselhaus interaction enhances it whereas the opposite effect is exhibited by the entropy. The temperature behaviour of both the entropy and heat capacity clearly demonstrates the effect of zero field spin-splitting. • A quantum dot with two electrons is considered in a magnetic field at finite temperature. • Role of Rashba and Dresselhauss interactions (RI & DI) are studied on heat capacity ( C v ) and entropy (S). • Coulomb interaction between electrons is treated by the exactly soluble Johnson-Payne model. • RI reduces C v and DI enhances it while S shows just an opposite behaviour. • Temperature behaviour of C v and S exhibits zero-field splitting.

Rashba spin-orbit coupling enhanced two-photon absorption and its polarization dependence in monolayer black phosphorus

We investigate theoretically the impact of Rashba spin-orbit coupling (RSOC) effect to two-photon absorption (TPA) and its dependence on the polarization direction of the incident light in monolayer black phosphorus (BP) starting from an anisotropic two band k·p model. It is found that the TPA is enhanced several times by RSOC effect which is tuned by the external electric field. And the TPA response shows highly anisotropic, changing periodically with the polarization direction of incident linearly polarized light as the function of cos4θ approximatively. The TPA coefficient reaches its maximum when the polarization direction is aligned along the armchair direction (x-direction), while falls into its minimum along the zigzag direction (y-direction).

Large nonlinear Hall effect in (FeCo)0.67Ge0.33/Ge heterojunctions

The large nonlinear Hall effect was found in (FeCo)[Formula: see text]Ge[Formula: see text]/Ge heterojunctions formed by sputtering amorphous [Formula: see text]-type (FeCo)[Formula: see text]Ge[Formula: see text] magnetic semiconductor films on near intrinsic n-type Ge substrate. It is very interesting that the mechanisms of the large nonlinear Hall effect in (FeCo)[Formula: see text]Ge[Formula: see text]/Ge heterojunctions are different at different temperature ranges. Below 10 K, the Hall resistance of (FeCo)[Formula: see text]Ge[Formula: see text]/Ge heterojunctions is almost the same as the anomalous Hall effect of (FeCo)[Formula: see text]Ge[Formula: see text] ferromagnetic films. While the temperature increased from 10 to 60 K, the nonlinear Hall resistance, longitudinal conductance, and magnetoresistance all increased quickly and reached the maximum at T[Formula: see text]=[Formula: see text]60 K. In this case, thermally excited conducting carriers can tunnel through the interfacial potential barrier in (FeCo)[Formula: see text]Ge[Formula: see text]/Ge heterojunctions. Thus, in the range of 10–60 K, the enhanced nonlinear Hall resistance can be attributed to the anomalous Hall effect which was further enhanced by interfacial Rashba spin–orbit coupling effect. When the temperature further increased from 60 to 250 K, the interfacial potential barrier weakened gradually, and the Hall resistance and magnetoresistance decreased due to the shunting of the Ge substrate. In this case, the nonlinear Hall effect of (FeCo)[Formula: see text]Ge[Formula: see text]/Ge heterojunctions can be explained very well by the two-band model of nonlinear Hall effect.

Orientation of hole quantum Hall nematic phases in an out-of-plane electric field

We present observations of an anisotropic resistance state at Landau level filling factor $\nu=5/2$ in a two-dimensional hole system (2DHS), which occurs for certain values of hole density $p$ and average out-of-plane electric field $E_\perp$. The 2DHS is induced by electric field effect in an undoped GaAs/AlGaAs quantum well, where front and back gates allow independent tuning of $p$ and $E_\perp$, and hence the symmetry of the confining potential. For $p\approx2\times10^{11}$~cm$^{-2}$ and $E_\perp \approx -2 \times10^{5}$~V/m, the magnetoresistance along $\langle01\bar1\rangle$ greatly exceeds that along $\langle011\rangle$, suggesting the formation of a quantum Hall nematic or `stripe' phase. Reversing the sign of $E_\perp$ rotates the stripes by $90^{\circ}$. We suggest this behavior may arise from the mixing of the hole Landau levels and a combination of the Rashba and Dresselhaus spin-orbit coupling effects.

Effects of orientation and strain on the topological characteristics of CdTe/ α -Sn quantum wells

It is known that compressive or tensile planar strain determines the band structure topology of the diamond cubic allotrope of tin ($\ensuremath{\alpha}$-Sn) to be either a Dirac semimetal (DSM) or topological insulator (TI), respectively. Using analytical techniques, we show that quantum confinement in negatively strained CdTe/$\ensuremath{\alpha}$-Sn quantum wells induces a phase transition from the DSM state to a quasi-three-dimensional TI as $\ensuremath{\alpha}$-Sn thickness is decreased. The critical thickness of the transition is found to depend strongly on the orientation of the CdTe, and magnitude of the epitaxial strain, which can be experimentally tuned. This result provides a new rationale for why $\ensuremath{\alpha}$-Sn has been observed in a three-dimensional (3D) TI state when pseudomorphically grown on zincblende substrates possessing smaller lattice constants. Additional impacts of strain and orientation on the electronic properties and spin texture of $\ensuremath{\alpha}$-Sn are explored. In particular, we examine the impact of orientation on the critical length of the Bernevig-Hughes-Zhang transition from a 2D TI to trivial insulator, as well as on the Rashba spin orbit coupling effects of an external electric field.