Rashba Spin-orbit Coupling Strength Research Articles

Light-induced giant enhancement of nonreciprocal transport at KTaO3-based interfaces

Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and epitaxial CaZrO3/KTaO3 (111) interfaces. The nonreciprocal transport coefficient undergoes a giant increase with three orders of magnitude up to 105 A−1 T−1. Furthermore, a strong Rashba spin-orbit coupling effective field of 14.7 T is achieved with abundant high-mobility photocarriers under ultraviolet illumination, which accounts for the giant enhancement of nonreciprocal transport coefficient. Our first-principles calculations further disclose the stronger Rashba spin-orbit coupling strength and the longer relaxation time in the photocarrier excitation process, bridging the light-property quantitative relationship. Our work provides an alternative pathway to boost nonreciprocal transport in noncentrosymmetric systems and facilitates the promising applications in opto-rectification devices and spin-orbitronic devices.

Controlling the spin current around the rectangular cavities in two-dimensional topological insulators.

Controlling spin current in topological insulators (TIs) is a crucial requirement for applications in quantum computing and spintronics. Using the non-equilibrium Keldysh Green's function formalism, we demonstrate that such control can be established around rectangular cavities of two-dimensional TIs by breaking their time reversal symmetry via exchange magnetic fields and magnetic defects. In the presence of magnetic defects with xy symmetry or Ising symmetry, the density of states is localized, and the spin current forms a current loop around the rectangular cavity in TIs interfacing with two ferromagnetic stripes. We also observe that the spin direction of the traveling electrons is inverted under the reversal of bias and gate voltages. The change in the spin-polarized current around the cavities is predicted by varying the strength of Rashba spin-orbit coupling. This result allows for the creation and control of nearly fully spin-polarized currents with various spatial patterns around the cavities in TIs, and the design of tunable spin diodes for highly integrated spintronics.

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.

Shot noise in strained WSe2 multiple barrier structure under spin–orbit interaction

The Rashba spin–orbit coupling (RSOC) effect on the polarization and shot noise properties in a strained WSe2 multiple barrier structure is studied by transfer-matrix technique. The results indicate that the spin-dependent Fano factor depends on the strain and RSOC strength with oscillatory evolution. Thus, the value of Fano factor with spin rotation can be tuned mechanically and by an external electric field. Also, Fano factor shows high sensitivity to the number of electrostatic barriers in strained WSe2 multiple barrier structure. Meanwhile, in a strained WSe2 multiple barrier structure both sign and value of the valley and spin polarizations could be adjusted by tuning both RSOC and strain strength. These findings show that a WSe2 multiple barrier structure under strain and RSOC can be useful for future spintronics, straintronic and valleytronic devices.

A controllable spin flip and filter in zigzag graphene nanoribbons with triangular defect

Based on the recursive Green-function method together with Landauer-Büttiker formalism, we theoretically study the controllable spin-dependent electron transport properties by using zigzag graphene nanoribbons (ZGNRs) with triangular nanoconstrictions in the presence of the ferromagnetic strips and spatial modulation of Rashba spin-orbit coupling (RSOC). In this structure, the peaks of spin-flip and spin-conserved conductance can be modulated by the strength of RSOC and exchange magnetic field. The distribution of the local density of states (LDOS) around triangular nanoconstrictions is predicted. The spin filtering effect of ZGNRs with triangular and rectangular nanoconstrictions are compared by spin conductance from left (L) to right (R) lead and R to L lead, respectively. The results might be useful in designing a bidirectional spin converter and spin filter with the modulation of the multi-parameter.

Light-induced giant Rashba spin-orbit coupling at superconducting KTaO3 (110) heterointerfaces.

The two-dimensional electron system (2DES) at KTaO3 surface or heterointerface with 5d orbitals hosts extraordinary physical properties including the stronger Rashba spin-orbit coupling (RSOC), higher superconducting transition temperature, and potential of topological superconductivity. Herein, we report a huge enhancement of RSOC under light illumination achieved at a superconducting amorphous-Hf0.5 Zr0.5 O2 /KTaO3 (110) heterointerfaces. The superconducting transition is observed with Tc = 0.62 K and temperature dependent upper critical field reveals the interaction between spin-orbit scattering and superconductivity. A strong RSOC with Bso =1.9 T is revealed by weak antilocalization in normal state, which undergoes sevenfold enhancement under light illumination. Furthermore, RSOC strength develops a dome-shaped dependence of carrier density with the maximum of Bso =12.6 T achieved near the Lifshitz transition point nc ∼ 4.1×1013 cm-2 . The highly tunable giant RSOC at KTaO3 (110)-based superconducting interfaces show great potential for spintronics. This article is protected by copyright. All rights reserved.

Reduction of carrier density and enhancement of the bulk Rashba spin-orbit coupling strength in Bi2Te3/GeTe superlattices

Featured with the large Rashba constant (αR ∼ 4.2 eVÅ) coupled with the ferroelectricity, GeTe has attracted much interest, proposing novel energy-efficient spintronic devices. However, those approaches are hampered by the high hole density of GeTe around 1020-1021 cm−3, which deteriorates both the ferroelectricity and the bulk Rashba effect of GeTe. To solve this problem, we have investigated the superlattices composed of Bi2Te3 and GeTe ([BT|GT] SL), the former of which is known as a topological insulator (TI) with typically n-type conduction and strong spin-orbit coupling. We have investigated the magnetotransport properties of 25 [BTx|GTy]z SLs with varying thicknesses (x, y: the thickness in the multiples of the unit cell of the respective layer) of each layer and the repetition number (z) systematically. We have observed a minimum carrier density of 5.7 × 1019 cm−3 in [BT6|GeTe6]6 superlattice sample smaller than that (∼4.4 ×1020 cm−3) of GeTe single film by an order. In addition, we have found that some [BT|GT] SLs with reduced carrier density have a larger Rashba constant compared to that of a single GeTe film. This is explained by the change of the spin polarization which can be modulated by the Fermi level in the Rashba band. These results provide a viable way to realize robust ferroelectricity and strong SOC in GeTe-related material systems simultaneously.

Electrical tuning of valley polarization in monolayer transition metal dichalcogenides

The valley degree of freedom in two-dimensional crystalline materials is another promising candidate for designing dissipationless information devices in addition to the spin degree of freedom. The primary challenge for the application of valley degree of freedom is to achieve a significant and controllable valley polarization. Here, we discover an electrical tunable mechanism for generating valley polarization caused by the backscattering induced by Rashba spin-orbit coupling in monolayer nanoribbon of transition-metal dichalcogenides. Since the Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides can be induced by the external electric field, the valley polarization here can be dynamically controlled by means of the external gating. The results of the valley-dependent transport calculations show that a pronounced valley polarization can be generated when the external electric field-induced Rashba spin-orbit coupling is present in the central scattering region of the nanoribbon. Moreover, the efficiency and direction of the valley polarization can be controlled by tuning the Fermi energy and the strength of Rashba spin-orbit coupling. The calculated results also confirm that the valley polarization remains significant in the presence of atomic vacancy defects. This finding opens up a potential possibility for realizing electrical control of the valleytronics devices.

Conductance matrix symmetries of multiterminal semiconductor-superconductor devices

Nonlocal tunneling spectroscopy of multiterminal semiconductor-superconductor hybrid devices is a powerful tool to investigate the Andreev bound states below the parent superconducting gap. We examine how to exploit both microscopic and geometrical symmetries of the system to extract information on the normal and Andreev transmission probabilities from the multiterminal electric or thermoelectric differential conductance matrix under the assumption of an electrostatic potential landscape independent of the bias voltages, as well as the absence of leakage currents. These assumptions lead to several symmetry relations on the conductance matrix. Next, by considering a numerical model of a proximitized semiconductor wire with spin-orbit coupling and two normal contacts at its ends, we show how such symmetries can be used to identify the direction and relative strength of Rashba versus Dresselhaus spin-orbit coupling. Finally, we study how a voltage-bias-dependent electrostatic potential as well as quasiparticle leakage break the derived symmetry relations and investigate characteristic signatures of these two contributions.

Optical manipulation of Rashba-split 2-dimensional electron gas

In spintronics, the two main approaches to actively control the electrons’ spin involve static magnetic or electric fields. An alternative avenue relies on the use of optical fields to generate spin currents, which can bolster spin-device performance, allowing for faster and more efficient logic. To date, research has mainly focused on the optical injection of spin currents through the photogalvanic effect, and little is known about the direct optical control of the intrinsic spin-splitting. To explore the optical manipulation of a material’s spin properties, we consider the Rashba effect. Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we demonstrate that an optical excitation can tune the Rashba-induced spin splitting of a two-dimensional electron gas at the surface of Bi2Se3. We establish that light-induced photovoltage and charge carrier redistribution - which in concert modulate the Rashba spin-orbit coupling strength on a sub-picosecond timescale - can offer an unprecedented platform for achieving optically-driven spin logic devices.

Theoretical study of spin-orbit coupling in Janus monolayer MA2Z4

Abstract In this paper, based on first-principles calculations, we investigate the energy valley and spin-orbit coupling properties of Janus monolayer MA2Z4. The stability of different structures is illustrated. Due to the breaking of mirror symmetry, Rashba splitting occurs at the Γ point in the band structures of Janus monolayers WSiGeN4 and WSi2(NP)2. The relationship between Rashba spin-orbit coupling strength and potential energy gradient and d-orbital composition is explored. Janus monolayer WSi2(NP)2 has stronger Rashba effect than WSiGeN4 due to the strong asymmetry of the d orbital of W atom. These results help to promote the application of two-dimensional materials in spintronics and valleytronics.

Numerical study of PbTe-Pb hybrid nanowires for engineering Majorana zero modes

Epitaxial semiconductor-superconductor (SM-SC) hybrid nanowires are potential candidates for implementing Majorana qubits. Recent experimental and theoretical works show that charged impurities in SM remain a major problem in all existing hybrid nanowires, in which the SM is either InAs or InSb while the SC is mainly Al. Here, we theoretically validate the recently proposed PbTe-Pb hybrid nanowire as a potential candidate for Majorana devices. By studying the electrostatic and electronic properties of PbTe nanowires, we demonstrate that the huge dielectric constant of PbTe endows itself a high tolerance of charged impurity, which is a potential advantage over InAs and InSb nanowires. Moreover, we find that the effective axial Land\'e $g$ factor and Rashba spin-orbit coupling strength of PbTe nanowires are comparable to those of InAs nanowires. The conceivable merits of using Pb as the SC are (i) Pb has a larger superconducting gap, higher critical temperature, and higher parallel critical magnetic field than those of Al; (ii) a superconducting gap comparable with those of InAs-Al and InSb-Al can be induced in PbTe-Pb even by a weak coupling between Pb and PbTe, which simultaneously relieves the adverse renormalization and induced disorder effects on SM from SC; and (iii) Pb film can be grown on PbTe with a thin buffer CdTe layer in between, solving the lattice mismatch problem as an important source of disorder. In the presence of a parallel magnetic field, we show that the typical BdG energy spectrum and tunneling spectroscopy of PbTe-Pb resemble those of InAs and InSb based hybrid nanowires exposed to a tilting magnetic field, as a result of the highly anisotropic Land\'e $g$ factors of PbTe nanowires. The calculated topological phase diagrams of PbTe-Pb indicate that the multivalley character of PbTe makes it easier than InAs and InSb to access topological superconducting phases. Our results could facilitate the experimental realization of PbTe-Pb hybrid nanowires and inspire further theoretical works.

Photon-assisted spin transport characteristics of A-B interferometer embedded with three quantum dots

We design an A-B interferometer with a “single quantum dot atom” embedded in the upper arm and a “double quantum dot molecule” embedded in the lower arm. The transmission coefficient and average current characteristics are studied as the time-dependent external field is introduced. When the interdot coupling strength is weak, the change of Rashba spin orbit coupling strength has a strong influence on the spin transmission coefficient. When the interdot coupling strength and Rashba spin orbit coupling strength take appropriate values, the spin up and spin down transmission coefficients completely coincide, and a resonance band emerges at the Fermi level. The spin polarization can be close to +1 or −1 by adjusting the Zeeman magnetic field, the interdot coupling strength, and the gate voltage. In the presence of photon assistance, the spin polarization changes periodically in a certain proportion, so as to realize the spin-polarized transport with controllable time-dependent external field. These findings provide a theoretical basis for the spin polarized function of A-B interferometer.

Multitude of topological phase transitions in bipartite dice and Lieb lattices with interacting electrons and Rashba coupling

We report the results of a Hartree-Fock study applied to interacting electrons moving in two different bipartite lattices: the dice and the Lieb lattices, at half-filling. Both lattices develop ferrimagnetic order in the phase diagram $U\text{\ensuremath{-}}\ensuremath{\lambda}$, where $U$ is the Hubbard on-site repulsion and $\ensuremath{\lambda}$ is the Rashba spin-orbit coupling strength. Our main result is the observation of an unexpected multitude of topological phases for both lattices. All these phases are ferrimagnetic, but they differ among themselves in their set of six Chern numbers (six numbers because the unit cells have three atoms). The Chern numbers $|C|$ observed in our study range from 0 to 3, showing that large Chern numbers can be obtained by the effect of electronic correlations, adding to the recently discussed methodologies to increase $|C|$ based on extending the hopping range in tight-binding models, using sudden quenches, or photonic crystals, all without including electronic interactions.

Topological phases in few-layer silicene nanoribbon induced by normally applied electric field and Rashba spin-orbit coupling

Edge states of a few-layer silicene nanoribbon (FSNR) have potential applications in spintronic and nanodevices. Quantum transport properties of zigzag superlattice FSNR exposed to vertical electric field and Rashba spin-orbit coupling (RSOC) are studied within the tight-binding based non-equilibrium Green's function approach. In addition, the topological insulator-band insulator phase transition of FSNR and the influence of adding a new layer on edge states of conduction electrons have been investigated. Calculations demonstrate that the external electric field applied perpendicularly to the FSNR device, results in gap opening and metal-semiconductor and metal-semimetal phase transition. Further, we found that different topological phase transitions, quasi-topological insulator-band insulator and topological insulator-band insulator by applying a vertical electric field. Meanwhile, results reveal that by tuning the RSOC strength, we can control the spin current. This controllability of spin and quantum transport in FSNR may contribute to potential applications in silicene-based devices and the development of spintronic.

Shot noise of spin-polarized electrons in a single-channel magnetic tunnel junctions

Based on the free electron approximation and Egues’ shot noise theory, the shot noise of spin-polarized electrons tunneling in ferromagnetic/semiconductor/ferromagnetic tunnel junctions is studied. Considering the matching of conduction band between ferromagnetic and semiconductor layers, our results show that the Fano factors of spin-polarized electrons have resonant tunneling characteristics when the semiconductor thickness and Rashba spin–orbit coupling strength are increased. When the magnetic moments in two ferromagnetic layers are parallel, with the increase of the molecular field in the ferromagnets, the Fano factor for spin-up electron decreases to zero and then increases exponentially and the Fano factor for spin-down electron is always linear. But when the magnetic moments are antiparallel, the Fano factors for different spin directions tend to be the same. In addition, the Fano factors for different spin directions are almost zero when the incident electron energy is located in the low energy region, but exhibit irregular oscillation when the incident electron energy is located in the high energy region. At the same time, with the variations of the angle of the magnetic moments in two ferromagnetic layers, the electrons Fano factors for different spin orientations show obvious separation characteristics. On the other hand, the conduction band mismatch between ferromagnetic and semiconductor layers is considered, the Fano factors of electrons with different spin directions show obvious difference compared with the results of conduction band matching.

Manipulation of the Rashba spin-orbit coupling of a distorted 1T-phase Janus WSSe monolayer: Dominant role of charge transfer and orbital components

Janus monolayer materials, as the thinnest materials that may possess strong Rashba spin-orbit coupling, are helpful for the miniaturization of the charge-spin conversion devices. Using first-principles calculations, we investigated the structural stability and Rashba spin-orbit coupling of distorted $1T\text{-phase}$ Janus monolayer WSSe. Although the W atoms rotating around the Se atom are dynamically unstable in the stress-free state, the imaginary frequency disappears by applying a small compressive strain. From the perspectives of macroscopic charge transfer and microscopic atomic orbital compositions, the mechanism of the Rashba spin-orbit coupling strength of the distorted $1T\text{-phase}$ Janus monolayer WSSe was analyzed. The coupling strength can be greatly manipulated when \ensuremath{-}5 to 5% biaxial strain is applied. The charge transfer caused by the larger lattice constant and the rotation of the W atoms reduce the gradient of the potential so that the greater the electric polarization, the weaker the spin-orbit coupling. The coupling strength of different electronic states is determined by the proportion of the out-of-plane atomic orbitals, where ${d}_{\mathrm{x}z}$ and ${d}_{yz}$ orbits play the dominant role. These results contribute to the design of materials with greater Rashba spin-orbit coupling and the understanding of its mechanism.

Spin-orbit coupling assisted magnetoanisotropic Josephson effect in ferromagnetic graphene Josephson junctions

The spin-flipping effect can be induced by the Rashba spin-orbit coupling (RSOC), leading to triplet equal-spin pairs in a superconducting hybrid structure. Herein, by combining the Dirac--Bogoliubov--de Gennes equation and the Furusaki-Tsukada formalism at a finite temperature, we theoretically investigate the Josephson effect in graphene-based superconductor-ferromagnet-R-superconductor junctions, where R refers to a region with the RSOC. It is demonstrated that as a result of the RSOC, one $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition can be attained by tuning the orientation of the exchange field $\stackrel{P\vec}{h}$, which is determined by its magnitude $h$ that could be periodically taken, and the periodical $0\text{\ensuremath{-}}\ensuremath{\pi}$ transitions are also caused by manipulating $h$ during a considerable scope of the orientation. More interestingly, although varying the RSOC strength $\ensuremath{\lambda}$ cannot give rise to the $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition in itself, not only is it a necessary condition for the $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition induced by modulating the orientation of $\stackrel{P\vec}{h}$ but also can produce the shift of the crossover point. Furthermore, two different kinds of anomalous Josephson current effect are exhibited by controlling the orientation of $\stackrel{P\vec}{h}$. Particularly, the out-of- and in-plane magnetoanisotropic Josephson currents always exist, varying monotonically with $\ensuremath{\lambda}$ while nonmonotonically with $h$. The characteristics may provide more insights into the proximity-induced RSOC and pave the way to a new class of tunable superconducting spintronic devices based on large-scale graphene.

Effects of atomic substitutional doping on electronic structure of monolayer Janus WSeTe

Based on the first principles calculations, the effects of substitutional doping of nitrogen, halogen and 3d transition metal elements on the electronic structure of monolayer Janus transition metal dichalcogenides WSeTe are studied in this paper, where the VASP software package is used based on density functional theory to perform calculations through using both the projector augmented wave method and the GGA-PBE functional method. A monolayer WSeTe hexagonal crystal system with 4 × 4 supercells is established, which contains 48 atoms. When VA (VIIA) element substitutes for monolayer WSeTe, one of the Se atoms is replaced with a nitrogen (halogen) atom; when the 3d transition metal element substitutes for monolayer WSeTe, one of the W atoms is replaced with a transition metal atom. Through the analysis of band structure, charge transfer and magnetism, it is found that VA (VIIA) nonmetallic elements doped monolayer WSeTe due to the introduction of the hole (electronic) doped, makes the Fermi level shift downward (upward), thus transforming into a p(n) type semiconductor. The Ti and V element substitutional doped monolayer WSeTe will present semiconductor-metal transformation. A doping for each of Cr, Co, Mn, Fe element doesn’t lead semiconductor material properties to change, but the each of Co, Mn, Fe element doped monolayer WSeTe can create a band gap of less than 20 meV. The VIIA (VA) non-metallic element and 3d transition metal element doped monolayer WSeTe will not have a huge influence on the original geometric structure of the material. Due to the charge transfer and doped atoms on the top of the valence band hybridization phenomenon, the Rashba spin splitting intensity near the <i>Γ</i> point of the top valence band increases with the increase of the atomic number of the doped atoms in the same main group when VIIA and VA non-metallic elements are doped. Moreover, the increase in atomic number and charge transfer have a greater influence on the strength of Rashba spin-orbit coupling than the change in electronegativity. The 3d transition metal element substitution doped single-layer WSeTe has obvious spin polarization phenomenon, which produces valley polarization near the Fermi level and introduces magnetism. In particular, since Cr-doped WSeTe retains the original semiconductor properties of WSeTe and has a large energy valley polarization, it may have a wide range of applications, such as in the field of spintronic devices. The monolayer WSeTe doped separately with Cr, Mn and Fe element produces an impurity band with fully polarized spin electrons in the band gap. The results are of great significance in systematically understanding the properties of monolayer WSeTe doping model and can provide theoretical reference for designing the monolayer WSeTe based electronic devices.

Determining Rashba spin-orbit coupling strength in InSb nanowire quantum dots: Influence of temperature and nuclear environment⋆

We identify theoretically the possibility to determine the Rashba spin-orbit coupling strength in InSb nanowire quantum dots by measuring the magnetic susceptibility of the two-electron system in a double quantum dot. To realistically describe such a system, we study the role of finite temperature and the interaction of the double quantum dot spins with the nuclear environment. Magnetic susceptibility, a quantity dependent on the relative angle between the magnetic and the spin-orbit field, reaches a maximal value when the spin-orbit and magnetic field are parallel/antiparallel to each other. We show that the dependence of the peak with respect to the magnetic field orientation, being a controllable quantity, can be used to extract the value of the Rashba parameter in materials with strong spin-orbit coupling. Since the strength of the Rashba parameter is independent of the confinement along the nanowire axis, the determined value can be used in configurations with different gating potential profiles and the number of electrons trapped inside the quantum dot system.