Large Spin Splitting Research Articles

Enhancing the Rashba spin splitting at X/Au/Si(111) surface by induced intrinsic spin-polarization

Adsorbed/modified surface is usually treated as a prototype to study Rashba spin splitting. Using first-principles calculations and analyses, we first explain the Rashba spin splitting of monovalent alkali metal adsorbed on Au/Si(111) surface as realized in experiment. We clearly identify that Na adatom donates one electron to pristine Au/Si(111) surface, leading to the Fermi level shift upward to the parabolic range of Au d and Si p hybridized states and thus leading to large Rashba spin splitting in Na/Au/Si(111) system. However, for the larger intra-atomic spin-orbit coupling Cs adsorbate, it not only plays the identical role to Na adatom, but induces intrinsic spin-polarization surprisingly, which further enhances the Rashba spin splitting on the Cs/Au/Si(111) surface up to ∼200 meV. Our proposed mechanism that intrinsic spin-polarization significantly enhancing the Rashba spin splitting is generally applicable and can be extended readily to alkali-earth metal as epitaxial layer grown on Au/Si(111) surface.

Controllable and enormous spin splitting in antiferromagnetic MnPSe3 through interfacial coupling with Janus MoSSe

The manipulation of spin and valley degree of freedom in two-dimensional materials is desirable for emerging applications in next-generation electronics. Here, we report a regulation of the spin splitting in monolayer antiferromagnetic MnPSe3 through stacking van der Waals (vdW) heterostructures with transition-metal dichalcogenides (TMDs). Different TMDs including Janus MoSSe, MoSe2, and MoS2 are engaged and the spin and valley dependences on stacking configurations, interfacial atoms, and interlayer spacings are comprehensively investigated based on first-principles calculations. The results show that spin degeneracy of MnPSe3 can be lifted through the interfacial interaction in the vdW heterostructures. Compared with the MnPSe3 interfacing with traditional TMDs (MoSe2 and MoS2), those with Janus TMDs (MoSSe or MoSeS) have larger spin splitting. A maximal spin splitting of 50.8 meV at valence band maximum is achieved in MoSSe/MnPSe3 heterostructure, and such splitting is further enhanced to 243.0 meV when the interlayer spacing is reduced to 2.9 Å. It is found that the different hybridizations between the d-orbitals of the two inequivalent Mn atoms in MnPSe3 and the above TMDs lead to the occurrence of net magnetic moments, and thus induce spin splitting. This work paves a novel way to modify the spin-valley properties of two-dimensional semiconductors.

Tuning anisotropy of an organic DSB semiconductor in a microcavity

Anisotropic organic materials are widely studied in photonic and optoelectronic devices. In this work, we show that by mixing two organic materials trans,trans -1,4-distyrylbenzene (DSB) and 1,4-Bis[4-(di-p-tolylamino)styryl]benzene (NDSB), noticeable anisotropy can be introduced in the DSB/NDSB microplates. Inserted in a planar microcavity, particular photonic modes with synthetic Rashba–Dresselhaus coupled bands can be observed with much larger spin splitting (Δk x = 2.8 μm−1) than other organics or liquid crystal microcavities. Our results offer the study of interesting photon or polariton modes in a synthetic Rashba–Dresselhaus coupled microcavity at room temperature.

Spin-split collinear antiferromagnets: A large-scale ab-initio study

It was recently discovered that, depending on their symmetries, collinear antiferromagnets can actually break the spin degeneracy in momentum space, even in the absence of spin-orbit coupling. Such systems, recently dubbed altermagnets, are signalled by the emergence of a spin-momentum texture set mainly by the crystal and magnetic structure, relativistic effects playing a secondary role. Here we consider all collinear q = 0 antiferromagnetic compounds in the MAGNDATA database allowing for spin-split bands. Based on density-functional calculations for the experimentally reported crystal and magnetic structures, we study more than sixty compounds and introduce numerical measures for the average momentum-space spin splitting. We highlight some compounds that are of particular interest, either due to a relatively large spin splitting, such as CoF2 and FeSO4F, or because of their low-energy electronic structure. The latter include LiFe2F6, which hosts nearly flat spin-split bands next to the Fermi energy, as well as RuO2, CrNb4S8, and CrSb, which are spin-split antiferromagnetic metals.

Hexagonal warping effect in the Janus group-VIA binary monolayers with large Rashba spin splitting and piezoelectricity.

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe2, SeTe2, and Se2Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin-orbit coupling (SOC), the STe2, SeTe2 and Se2Te monolayers exhibit large intrinsic Rashba spin splitting (RSS) at the Γ point with the Rashba parameters 0.19 eV Å, 0.39 eV Å, and 0.34 eV Å, respectively. Interestingly, based on the k·p model via symmetry analysis, the hexagonal warping effect and a nonzero spin projection component Sz arise at a larger constant energy surface due to nonlinear k3 terms. Then, the warping strength λ was obtained by fitting the calculated energy band data. Additionally, in-plane biaxial strain can significantly modulate the band structure and RSS. Furthermore, all these systems exhibit large in-plane and out-of-plane piezoelectricity due to inversion and mirror asymmetry. The calculated piezoelectric coefficients d11 and d31 are about 15-40 pm V-1 and 0.2-0.4 pm V-1, respectively, which are superior to those of most reported Janus monolayers. Because of the large RSS and piezoelectricity, the studied materials have great potential for spintronic and piezoelectric applications.

Spin-orbit splitting and piezoelectric properties of Janus Ge2XY (X ≠ Y = P, As, Sb and Bi).

The coexistence of spin-orbit coupling and piezoelectricity in a single material may have potential application in multifunctional devices, including spintronics, nanorobotics and piezotronics. Spin-orbit coupling provides a new means to manipulate electron's spin without an additional external magnetic field, while piezoelectricity refers to the interplay between mechanical stresses and electric polarization. Using first-principles calculations, the structural, electronic, optical, spin, and piezoelectric properties of the Janus Ge2XY (X ≠ Y = P, As, Sb, and Bi) monolayers were systematically investigated. All the Ge2XY are energetically and dynamically stable in the α phase. At the GW level, Ge2AsSb, Ge2AsBi, and Ge2SbBi have direct fundamental band gaps of 0.65, 0.64, and 0.91 eV. At the GW + BSE level, their optical gaps are 0.42, 0.45, and 0.63 eV, and the optical absorption coefficients can reach about 10-5 cm-1 in the infrared light region, which reveals that they have potential for application in infrared photodetectors. For Ge2PBi, Ge2AsBi, and Ge2SbBi containing the heavy Bi element, the lowermost conduction band and uppermost valence band have large spin splitting along the M-K and K-Γ lines, and the bands near the Fermi level possess Rashba spin splitting at the Γ point. Ge2PBi and Ge2SbBi have both large in-plane piezoelectric coefficients d11 (-0.75 and -3.18 pm V-1) and out-of-plane piezoelectric coefficients d31 (0.37 and 0.30 pm V-1). Our findings are helpful to understand the mechanism of the spin-orbit physics and piezoelectricity of Janus Ge2XY monolayers and guide experiments in exploring novel multifunctional materials.

Spin polarization in quantum point contact based on wurtzite topological quantum well.

Manipulating spin polarization in wide-gap wurtzite semiconductors is crucial for the development of high-temperature spintronics applications. A topological insulator revealed recently in wurtzite quantum wells (QWs) provides a platform to mediate spin-polarized transport through the polarization field-driven topological edges and large Rashba spin-orbit coupling (SOC). Here, we propose a spin-polarized device in a quantum point contact (QPC) structure based on ZnO/CdO wurtzite topological QWs. The results show that the QPC width can sufficiently control the lateral spin-orbit coupling (SOC) as well as the band gap of the edge states through the quantum size effect. As a result, the spin-polarized conductance exhibits oscillation due to the spin precession, which can be controlled by adjusting the voltage imposed on the split gate. The QPC-induced large spin splitting is highly nonlinear and becomes strong close to the gap. The spin splitting of the edge states will be suppressed for QPC widths greater than 50 nm, and thus lead to an extremely long spin precession length. This QPC width-dependent lateral SOC effect provides an emerging electrical approach to manipulate spin-polarized electron transport in topological wurtzite systems.

Gate control of Rashba spin-orbit coupling in multiple-quantum-well structure: From one to multiple subbands

We present in multiple-quantum-well structure a full dynamic picture of Rashba spin-orbit coupling (SOC) with continuous variation from one- to multi-subband occupation, mediated by the external gate potential. Some novel features of Rashba effect can be demonstrated. For intrasubband SOC ανs (ν=1,2,3...is subband index), they show distinct gate dependences which facilitates selective control of ανs with the help of the gate. Specifically, α1 of the lowest subband is always the largest under negative gate potentials, which leads to large spin splitting, desirable for spin detection, while ανs of the highest subband is immune to the gate and keeps zero in a wide gate range, making it possible to suppress electron spin relaxation of these subbands regardless of the gate variation. For intersubband SOC, it is impressive to discover that the intersubband coupling between two nonadjacent subbands with relatively large energy separation can exceed that between two adjacent subbands and dominates the intersubband coupling. Our results should provide new insights for spintronic application based on intra- and intersubband spin-orbit effects, such as spin blocker, topological superconductivity, and all-electron topological insulator.

Strain engineering of electronic and spin properties in SnX (X = P, As, Sb, Bi) monolayers

In this study, the electronic and spin properties of SnX (X = P, As, Sb, Bi) monolayers under biaxial strain were investigated using density functional theory. The electronic and spin properties were studied for two structures of SnX with mirror symmetry and inversion symmetry. We compared the band structure of SnX with and without spin–orbit coupling to determine the importance of spin–orbit interaction for its band structure. The locations of the valleys in the conduction and valence bands were explored for various strains. We found that the bandgap was modified by considering spin–orbit coupling in the presence of strain. In addition, we calculated spin-splitting in the conduction and valence band in the presence of strain for the structure with mirror symmetry. The projected densities of states and band structures clarified the origins of these large spin-splittings. The lack of inversion symmetry and mirror symmetry induced the persistent spin helix. The corresponding spin–orbit strength was explored for compounds at various biaxial strains.

Reversible canted persistent spin textures in two-dimensional ferroelectric bilayer WTe2

The recent discovery of materials hosting persistent spin texture (PST) opens an avenue for the realization of energy-saving spintronics since they support an extraordinarily long spin lifetime. However, the stability of the PST is sensitively affected by symmetry breaking of the crystal induced by external perturbation such as the electric field. In this paper, through first-principles calculations supplemented by symmetry analysis, we report the emergence of the robust and stable PST with large spin splitting in the two-dimensional (2D) ferroelectric bilayer WTe2. Due to the low symmetry of the crystal (Cs point group), we observe a canted PST in the spin-split bands around the Fermi level displaying a unidirectional spin configuration tilted along the yz plane in the first Brillouin zone. Such a typical PST can be effectively reversed by out-of-plane ferroelectric switching induced by interlayer sliding along the in-plane direction. We further demonstrated that the reversible PST is realized by the application of an out-of-plane external electric field. Thus, our findings uncover the possibility of an electrically tunable PST in 2D materials, offering a promising platform for highly efficient and non-volatile spintronic devices.

Thickness-dependent electronic band structure in MBE-grown hexagonal InTe films

Films of the hexagonal InTe with thicknesses from one to three tetralayers (TLs) were synthesized on the bilayer graphene/SiC by molecular beam epitaxy. Valence bands of the one- and two-TL-thick films were found to be flat-like near $\overline{\mathrm{\ensuremath{\Gamma}}}$ point, but become parabolic for the three-TL-thick film and beyond. The band gap of the InTe was found to be equal to 2.1 eV for the single tetralayer and tends to reduce its size with thickness. The band structure calculations revealed a large spin splitting of the InTe single tetralayer lower conduction band with exclusive out-of-plane spin polarization. Bearing in mind inaccessibility of the hexagonal InTe in a bulk form, all above-mentioned findings open up a way for the further study of this perspective material.

Theoretical studies of valleytronic and optical properties in monolayer MoP2X2Y2 (XY=BTe,AlS,andGaS)

Motivated by recent extensive studies on the $M{A}_{2}{Z}_{4}$ family materials, we have predicted three ${\mathrm{MoP}}_{2}{X}_{2}{Y}_{2}$-type monolayers, including ${\mathrm{MoP}}_{2}{\mathrm{B}}_{2}{\mathrm{Te}}_{2}, {\mathrm{MoP}}_{2}{\mathrm{Al}}_{2}{\mathrm{S}}_{2}$, and ${\mathrm{MoP}}_{2}{\mathrm{Ga}}_{2}{\mathrm{S}}_{2}$, which possess excellent dynamic and thermal stabilities and peculiar electronic properties. According to first-principle calculations, the band structures show that the three monolayers are direct band-gap semiconductors with a direct band gap of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The band energy extrema occurs at $K/{K}^{\ensuremath{'}}$ points, giving rise to a pair of degenerate valleys. A large valley spin splitting is found at the valence band under the spin-orbit coupling effect, resulting in valley-contrasting Berry curvature. Moreover, these materials have strong optical absorption intensity in the visible light regions, especially ${\mathrm{MoP}}_{2}{\mathrm{Ga}}_{2}{\mathrm{S}}_{2}$. Our research shows that these three ${\mathrm{MoP}}_{2}{X}_{2}{Y}_{2}$ monolayers, as close relatives of $M{A}_{2}{Z}_{4}$ family materials, have potential valleytronics and photovoltaics applications.

Jacutingaite family: An efficient platform for coexistence of spin valley Hall effects, valley spin-valve realization, and layer spin crossover

Jacutingaite family, a family of naturally occurring exfoliable minerals (${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ and ${\mathrm{Pd}}_{2}{\mathrm{HgSe}}_{3}$) discovered in Brazil, is recently predicted to be the very first large-gap Kane-Mele quantum spin Hall insulators. However, yet to date, the door is locked for realizing versatile spin-valley based phenomena due to inversion symmetry in this family. By exploiting the inversion symmetry using different strategies such as placing the jacutingaite family in the vertical electric field or growing its composites with some suitable well-matched systems, we reveal several promising valley spin based phenomena in the jacutingaite family. Overall, we achieve the coupled spin valley Hall effect, valley spin-valve effect, Rashba spin splitting around the low-symmetric $M$ point, layer crossover accompanied by spin crossover, and selective-excitation of carriers from opposite valleys in this family. More interestingly, swapping of the induced spin splittings, Berry curvatures, and the spin texture between the two valleys occur upon the reversal of electric field from ${E}_{z}>0$ to ${E}_{z}<0$ without destroying the ${\mathcal{Z}}_{2}$ topological hallmarks. Meanwhile, considerable hexagonal trigonal warping effects around the ${K}_{+}$ and ${K}_{\ensuremath{-}}$ valleys are realized. Furthermore, by applying vertical electric field through interfacial coupling with ferroelectric ${\mathrm{III}}_{2}\ensuremath{-}{\mathrm{VI}}_{3}$ film, valley contrasting Berry curvature in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer with a larger spin splitting in the lower conduction band $(\ensuremath{\sim}188\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ is achieved in the $+P$ polarization state. By inverting the polarization state from $+P$ to $\ensuremath{-}P$, the Fermi level shifts down into the valence band of the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer $(\ensuremath{\sim}82\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ revealing a hole doping in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer. This could establish an effective mechanism to control doping levels in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer as desirable for achieving superconductivity at finite doping in the jacutingaite family. Our paper provides a path towards integrating valleytronics and spintronics in multivalley materials with broken inversion symmetry.

Prediction of spin-dependent electronic structures of 3d transition metals doped Hittorf's violet phosphorene towards spintronics

Modulation of spin-dependent properties in semiconductor is of importance in exploring the potential application in spintronics. Here, we theoretically investigated the structural deformation, electronic characters, and magnetic properties of the single-layered Hittorf's violet phosphorene nanosheet with 3 d transition metal atoms (ranging from Sc to Ni) doping and under external electric field. The electronic band structures and magnetic properties was found to be obviously modulated by the dopants: the existence of a large spin splitting of the dopant 3 d states tailors the semiconducting phosphorene into simple magnetic semiconductor or bipolar magnetic semiconductor, except for the case of Sc. The application of an appropriate electric field to V-doped system could induce a transition from bipolar magnetic semiconductor to half metal with the carriers fully spin-polarized in alternative spin channel. Moreover, Hittorf's violet phosphorene-based field-effect transistor was proposed with the aim of realizing gate-voltage control on the carriers' spin orientation. Our findings pointed out the possibilities for realizing spin-dependent field-effect transistor in doped Hittorf's violet phosphorene nanosheet and opened a window for the burgeoning field of multifunctional 2D nanoelectronics and spintronics devices. • Ranging from Ti to Ni dopants can behave (bipolar) magnetic semiconductors. • Sc-doped Hittorfene is a nonmagnetic semiconductor. • The application of the electric field to V-doped Hittorfene can induce half metal transition. • The reversed carriers' spin orientation would be potential for spintronic devices.

Nonlinear optical Hall effect of few-layered NbSe2

${\mathrm{NbSe}}_{2}$ is one of metallic two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials. Because of broken crystal inversion symmetry, large spin splitting is induced by Ising-type spin-orbit coupling in odd-number-layered ${\mathrm{NbSe}}_{2}$, but absent for even-number-layered ${\mathrm{NbSe}}_{2}$ with the inversion symmetry. In this paper we numerically calculate nonlinear optical charge and spin Hall conductivities of few-layered ${\mathrm{NbSe}}_{2}$ based on an effective tight-binding model which includes ${d}_{{z}^{2}}$, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, and ${d}_{xy}$ orbitals of Nb atoms. We show that the nonlinear optical Hall conductivity for the second harmonic generation (SHG) process has a nonvanishing value in odd-number-layered ${\mathrm{NbSe}}_{2}$. Also, we provide a nonlinear optical selection rule in few-layered ${\mathrm{NbSe}}_{2}$ and their polarization dependencies. Furthermore, for the even-number-layered case, the nonlinear optical Hall currents can be generated by applying electric fields which break inversion symmetry. We also discuss that the nonlinear optical Hall effect is expected to occur in TMDC materials in general. Thus, our results will serve to design potential opt-spintronics devices based on 2D materials to generate the spin Hall current by SHG.

Ferroelectric control of pseudospin texture in CuInP2S6 monolayer

Spin–orbit coupling (SOC) plays an important role in condensed matter physics and has potential applications in spintronics devices. In this paper, we study the electronic properties of ferroelectric CuInP2S6 (CIPS) monolayer through first-principles calculations. The result shows that CIPS monolayer is a potential for valleytronics material and we find that the in-plane helical and nonhelical pseudospin texture are induced by the Rashba and Dresselhaus effect, respectively. The chirality of helical pseudospin texture is coupled to the out-of-plane ferroelectric polarization. Furthermore, a large spin splitting due to the SOC effect can be found at K valley, which can be regarded as the Zeeman effect under a valley-dependent pseudomagnetic field. The CIPS monolayer with Rashba et al effects provides a good platform for electrically controlled spin polarization physics.

Prediction of a new two-dimensional valleytronic semiconductor MoGe2P4 with large valley spin splitting.

Recently, MoSi2N4 with large valley spin splitting was experimentally synthesized. However, materials with large valley spin splitting are still rare. We predict a new two-dimensional (2D) MoGe2P4 material. It has large valley spin splitting and excellent optical absorption properties. The results show that 2D MoGe2P4 is a direct semiconductor with a bandgap of 887 meV. Its valley spin splitting (ΔV) at the top of the valence band is 153 meV because of the inversion symmetry breaking and spin-orbit coupling (SOC). 2D MoGe2P4 transforms from a semiconductor to a metal under a biaxial strain of 6%. ΔV increases monotonically from 137 meV to 157 meV under biaxial strain. In addition, the lowest exciton state of 2D MoGe2P4 is near 770 nm, and the optical absorption coefficient in the ultraviolet range is higher than that of MoS2. Our results suggest that 2D MoGe2P4 has excellent potential for applications in valley electronics and optoelectronic devices.

Giant and tunable Rashba spin splitting in MoS2/Bi2Te3 heterostructures

The study of the spin-orbit coupling (SOC) effect is an active research field in physics and it is of great interest for materials with large Rashba spin splitting (RSS) in spintronics applications. In this report, based on first-principles calculations, we propose the heterostructure of MoS2/Bi2Te3 with giant RSS duo to the lack of spatial inversion symmetry and the intrinsic electric field perpendicular to the heterostructure. Furthermore, it is found that the electronic and RSS can be tuned effectively by layers of MoS2 and Bi2Te3. When the Bi2Te3 thickness is less than 2QL, these MoS2/Bi2Te3 heterojunction systems exhibit semiconductor properties. The magnitude of the RSS in these MoS2/Bi2Te3 heterojunctions changes with the thickness, and the largest band splitting is found in the (MoS2)1(Bi2Te3)2 heterostructure with a Rashba coefficient of ~2.5 eV Å, which is a very large value among two-dimensional (2D) materials. While the interlayer distance and charge transfer remain stable at about 3.5 Å and 0.1 e, respectively. Through layer-projected band structure analysis, it is proven that the electronic states of these quantum well states (QWSs) are mainly contributed by Bi2Te3, while the MoS2 layer only plays an inductive role and does not contribute to the QWSs electrons. In addition, strain only plays a minor role in the Rashba effect in the MoS2/Bi2Te3 heterojunction, which is mainly caused by the breaking of the spatial inversion symmetry. The giant and tunable Rashba-type SOC of the MoS2/Bi2Te3 heterostructure suggests that it is a type of promising materials for spintronic applications.

Hot carrier dynamics of BiTeI with large Rashba spin splitting.

We present a time-resolved ultrafast optical spectroscopy study on BiTeI, a noncentrosymmetric semiconductor with large spin–orbit splitting. By tuning the pump photon energy, hot carriers can be excited into different energy bands, and the hot carriers decay dynamics are measured. The hot carriers excited by an 1.544 eV photon induce a positive differential reflectivity following a single exponential decay, while the hot carriers excited by an 1.651 eV photon show a negative reflectivity following two exponential decays, i.e., the hot carriers excited by 1.544 eV and 1.651 eV photons show different decay dynamics. We also investigate hot carrier dynamics in each Rashba splitting band at the 1.544 eV and 1.651 eV photon pump, and there is no difference in hot carrier decay between the left and right Rashba splitting bands for both cases.

A DFT computational prediction of 2H phase W2C monolayer and the effect of O functional groups

MXenes, which are different from the 1T phase, have attracted increasing researches due to their unique physical and chemical properties. In this paper, we investigated the structure stabilities, lattice dynamics, mechanical properties and electronic structures of 2H phase W2C monolayer. Similar to the traditional 1T phase, functionalization also has a significant effect on these properties of 2H phase W2C. The O-functionalization improves the structural stability, thermal conductivity and mechanical properties. In particular, the Young's modulus of 2H phase W2C can be raised to 351 N m−1 after O-functionalized, which is higher than that of graphene. The electronic structures show that there is a metallic-semiconducting transition after O-functionalization when considering spin-orbit coupling effect. 2H phase W2CO2 has semiconductor character with 0.199 eV band gap and a large spin splitting. We hope that our results can further contribute to the development of 2H phase MXenes in experimental synthesis and potential applications.