Spin-split Band Research Articles

Unconventional broadening of Rashba spin splitting in a Au2Sb surface alloy with periodic structural defects

Most Rashba spin splitting experimentally studied so far has ideal lattice with inversion symmetry broken, which limits the possibility to minimize the presence of spin-degenerate carriers near the Fermi level. Here, we report a novel 2D Au2Sb surface alloy decorated with periodic structural defects that exhibits modulation on the Rashba spin-orbit coupling band. Spin- and angle-resolved photoemission spectroscopy reveals a Rashba spin-split band with antiparallel spin polarization, significantly broadened compared to the Au₂Sn surface alloy. From the good agreement between the experimental results and DFT calculations, we identify that the broadening of the Rashba bands comes from variations in Sb atom corrugation induced by the periodic three-pointed star-shaped defects. These periodic defects can shift the energy position of the Rashba bands without breaking the in-plane rotational and mirror symmetries. Our findings highlight the potential to tune spin-dependent properties in 2D materials for spintronic applications.

Surface Circular Photogalvanic Effect in Tl-Pb Monolayer Alloys on Si(111) with Giant Rashba Splitting.

We have found that surface superstructures made of "monolayer alloys" of Tl and Pb on Si(111), having giant Rashba effect, produce nonreciprocal spin-polarized photocurrent via circular photogalvanic effect (CPGE) by obliquely shining circularly polarized near-infrared (IR) light. CPGE is here caused by the injection of in-plane spin into spin-split surface-state bands, which is observed only on Tl-Pb alloy layers but not on single-element Tl nor Pb layers. In the Tl-Pb monolayer alloys, despite their monatomic thickness, the magnitude of CPGE is comparable to or even larger than the cases of many other spin-split thin-film materials. A model analysis has provided the relative permittivity ε* of the monolayer alloys to be ∼1.0, which is because the monolayer exists at a transition region between vacuum and the substrate. The present result opens the possibility that we can optically manipulate the spins of electrons even on monolayer materials.

Altermagnetism: A Chemical Perspective.

Altermagnets have been recently introduced as a classification of collinear, spin compensated magnetic materials that host net-zero magnetization yet display some electronic behaviors typically associated with noncompensated magnetic materials like ferromagnets. The emergence of such properties are a consequence of spin-split bands that arise under specific symmetry conditions in the limit of zero spin-orbit coupling. In this Perspective, we summarize the fundamental criteria for realizing an altermagnetic phase and present a qualitative electronic band structure derivation and symmetry analysis through chemical principles. We then discuss the properties that make altermagnets distinctive candidates for charge-to-spin conversion elements in spintronic devices and provide a brief review of some altermagnetic candidate materials. Finally, we discuss future directions for altermagnetism and highlight opportunities for chemists to advance this emerging field.

Coexistence of altermagnetism and robust ferroelectricity in a bulk MnO wurtzite structure.

Altermagnetism is a new class of material with zero net magnetization, but having a nonrelativistic spin-split band structure. Here, we investigate the multifunctional properties of the hexagonal wurtzite MnO (w-MnO). w-MnO has a direct band gap of 0.39 eV and the altermagnetic behavior is achieved with a spin splitting of up to 188 meV. The bulk w-MnO has an in-plane magnetic anisotropy energy of -0.17 meV per cell along the [100] direction and temperature-dependent magnetization reveals a Néel temperature of around 180 K. Moreover, a maximum spin Hall conductivity of -285(ℏ/e) S cm-1 is obtained at a chemical potential of -0.24 eV. Along with the altermagnetism, we also find a large out-of-plane spontaneous polarization of 76.25 μC cm-2. w-MnO exhibits a low energy barrier of 0.26 eV per formula unit (f.u.) for polarization switching and this is further decreased to 0.15 eV per f.u. under 5% epitaxial tensile strain. The molecular dynamics simulations suggest that w-MnO retains its ferroelectric order below 2100 K indicating excellent thermal stability. These findings suggest that the hexagonal w-MnO can be a promising candidate for multiferroic applications particularly in spintronic and ferroelectric devices with high thermal stability.

Nonrelativistic Spin Splitting at the Brillouin Zone Center in Compensated Magnets.

The nonrelativistic spin-splitting (NRSS) of electronic bands in "altermagnets" has sparked renewed interest in antiferromagnets (AFMs) that have no net magnetization. However, altermagnets with collinear and compensated magnetism are not the only type of NRSS AFMs. In this Letter, we identify the symmetry conditions and characteristic signatures of a distinct group of NRSS AFMs that go beyond the description of altermagnets. These compounds exhibit a broken spin degeneracy among the spin-polarized bands at the Γ point in the absence of spin-orbit coupling. We use density functional theory calculations to validate these models in ternary magnetic nitrides, specifically MnXN_{2} (X=Si, Ge, Sn), and their cation ordered variants. By removing the previous spin degenerate constraint at Γ, these compounds may facilitate the generation of spin currents without cancellation arising from the alternating spin polarizations. Our findings expand the scope of NRSS eligible materials.

Vacancies Engineering in Molybdenum Boride MBene Nanosheets to Activate Room-Temperature Ferromagnetism.

The rapid development of low energy dissipation spintronic devices has stimulated the search for air-stable 2D nanomaterials possessing room-temperature ferromagnetism. Here the experimental realization of 2D Mo4/3B2 nanosheets is reported with intrinsic room-temperature ferromagnetic characteristics by vacancy engineering. These nanosheets are synthesized by etching the bulk MAB phase (Mo2/3Y1/3)2AlB2 into Mo4/3B2 nanosheets in ZnCl2 molten salt. The Mo4/3B2 nanosheets show robust intrinsic ferromagnetic properties, with a saturation magnetic moment of 0.044emug-1 at 300K, while vacancy-free MoB MBene exhibits paramagnetism. It is elucidated that the Mo-vacancy defect generates large density of states near the Fermi surface and spontaneously spin-split bands through first-principles calculations, which contributes to the non-zero magnetic moment in Mo4/3B2 nanosheets. This work lays the groundwork for activating the magnetic properties of MBene nanosheets by vacancy engineering, offering the possibilities for development of practical spintronic devices.

Detection of antiferromagnetic order in a RuO2/Pt bilayer by spin Hall magnetoresistance

Altermagnets have spin-split band structures that correspond to the rotational symmetry of the two sublattices in real space. Theoretically, their unique band structures are expected to exhibit intriguing transport phenomena, depending on their magnetic structures. Anomalous Hall effect (AHE) measurement is a method by which to electrically detect magnetic structure and has been reported for typical altermagnets, such as RuO2 and MnTe. However, AHE measurements are limited to specific cases. Thus, it is important to apply other methods by which to determine functionality based on magnetic structure. In this study, we report the spin Hall magnetoresistance (SMR) in a RuO2 (1 nm)/Pt (10 nm) system. A negative SMR signal is clearly observed, indicating the spin-flop antiferromagnetic structure of RuO2. Interestingly, a negative SMR was observed, even at 1 T, which is much smaller than the estimated spin-flop field reported in a previous study. This reflects the thinner film of RuO2 in our study, suggesting that thickness control is effective in adjusting the magnetic anisotropy of RuO2. In addition, the temperature-dependent SMR measurement revealed the Néel temperature of 1 nm thick RuO2 to be 70 ± 9 K. Our results show that SMR measurement can serve as an efficient tool to explore the magnetic features in an altermagnet.

Halogen-Dependent Circular Dichroism and Magneto-Photoluminescence Effects in Chiral 2D Lead Halide Perovskites.

Chiral lead halide perovskites (chiral LHPs) have emerged as one of the best candidates for opto-spintronics due to their large spin-orbit coupling (SOC) and unique chirality-induced spin selectivity (CISS) even in the absence of a magnetic field. Here, we report the impact of halide composition on circular dichroism (CD) and magneto-photoluminescence (PL) effects of chiral 2D LHPs (R/S-MBA)2PbBrxI4-x (MBA = C6H5CH2(CH3)NH3). By tuning the mixing ratio of Br/I halide anions, we find that (R/S-MBA)2PbBrxI4-x thin films exhibit tunable and wide wavelength range CD signals. Simultaneously, the main CD signals near the exciton absorption band gradually blue shift until they disappear. Moreover, the halogen-dependent negative magneto-PL effects of (R/S-MBA)2PbBrxI4-x thin films excited by left/right circularly polarized light can be detected at room temperature. We demonstrated that the halide composition can effectively modulate exciton splitting and chirality transfer in (R/S-MBA)2PbBrxI4-x owing to the chirality-induced SOC and crystalline structure transition, which lead to the adjustable CD signals. The interplay of Rashba-type band spin splitting and spin mixing among bright triplet exciton states is responsible for the halogen-dependent magneto-PL effect of chiral 2D LHPs. This study enables chiral 2D LHPs with CISS to be a new class of promising opto-spintronics materials for exploring high-performance spin-light-emitting diodes by halide engineering.

Exchange-enhanced spin-orbit splitting and its density dependence for electrons in monolayer transition metal dichalcogenides

We show that spin-orbit splitting (SOS) in monolayers of semiconducting transition metal dichalcogenides (TMDs) is substantially enhanced by an electron-electron interaction. This effect, similar to the exchange enhancement of the electron g factor, is most pronounced for conduction band electrons (in particular, in MoS2), and it has a nonmonotonic dependence on the carrier sheet density n. That is, the SOS enhancement is peaked at the onset of filling of the higher-energy spin-split band by electrons, n*, which also separates the regimes of slow (at n<n*) and fast (for n>n*) spin and valley relaxation of charge carriers. Moreover, this density itself is determined by the enhanced SOS value, making the account of exchange renormalization important for the analysis of the spintronic performance of field-effect transistors based on two-dimensional TMDs. Published by the American Physical Society 2024

Spin-Polarized Antiferromagnetic Metals

Spin-polarized antiferromagnets have recently gained significant interest because they combine the advantages of both ferromagnets (spin polarization) and antiferromagnets (absence of net magnetization) for spintronics applications. In particular, spin-polarized antiferromagnetic metals can be useful as active spintronics materials because of their high electrical and thermal conductivities and their ability to host strong interactions between charge transport and magnetic spin textures. We review spin and charge transport phenomena in spin-polarized antiferromagnetic metals in which the interplay of metallic conductivity and spin-split bands offers novel practical applications and new fundamental insights into antiferromagnetism. We focus on three types of antiferromagnets: canted antiferromagnets, noncollinear antiferromagnets, and collinear altermagnets. We also discuss how the investigation of spin-polarized antiferromagnetic metals can open doors to future research directions.

Interface defect state induced spin injection in organic magnetic tunnel junctions

This article analytically explores defect assisted spin injection in organic magnetic tunnel junctions (MTJs) [x/rubrene/Co, x = La2O3, LaMnO3, La0.7Ca0.3MnO3 (LCMO), La0.7Sr0.3MnO3 (LSMO)] employing nonequilibrium Green’s function (NEGF). Spin precession at ferromagnet (FM)/organic semiconductor (OSC) interface defect states have been considered while modeling the MTJ devices. Variations in voltage dependent parallel (RP) and antiparallel (RAP) resistances have been attributed to modified spin dependent scattering at modified spin resolved density of states of magnetic electrodes. Moreover, change in distribution of defect state depths at a spin injection interface has also been observed to modify RP/RAP, and hence, tunnel magnetoresistance (TMR) across the devices. Localization of defect state distribution due to a high spin split band may have resulted in large TMR for La2O3 devices. Nonlinear spin transfer torque (STT) in devices other than LSMO indicates compensation of spin damping, resulting in a high TMR response across the devices. Hence, the localization of defect state distribution and the choice of magnetic electrodes with high spin split bands may be exercised to realize spintronic devices for low power spin memory applications.

Chiral Split Magnon in Altermagnetic MnTe.

Altermagnetism is a newly recognized magnetic class named after the alternating spin polarizations in both real and reciprocal spaces. Like the spin splitting of electronic bands, the magnon bands in altermagnets are predicted to exhibit alternating chiral splitting. In this work, by performing inelastic neutron scattering on α-MnTe, we directly observed the altermagnetic magnon splitting. The lifted degeneracy of magnons is well explained by a symmetric-exchange origin. Further calculation based on the obtained spin-wave model demonstrates the magnons are chiral split as well. In addition, the g-wave magnetism was experimentally identified in MnTe.

Giant resistance switch in twisted transition metal dichalcogenide tunnel junctions

Abstract Resistance switching in multilayer structures are typically based on materials possessing ferroic orders. Here we predict an extremely large resistance switching based on the relative spin-orbit splitting in twisted transition metal dichalcogenide (TMD) monolayers tunnel junctions. Because of the valence band spin splitting which depends on the valley index in the Brillouin zone, the perpendicular electronic transport through the junction depends on the relative reciprocal space overlap of the spin-dependent Fermi surfaces of both layers, which can be tuned by twisting one layer. Our quantum transport calculations reveal a switching resistance larger than 106% when the relative alignment of TMDs goes from 0º to 60º and when the angle is kept fixed at 60º and the Fermi level is varied. By creating vacancies, we evaluate how inter-valley scattering affects the efficiency and find that the resistance switching remains large (104%) for typical values of vacancy concentration. Not only should this resistance switching be observed at room temperature due to the large spin splitting, but our results also show how twist angle engineering and control of van der Waals heterostructures could be used for next-generation memory and electronic applications.

Giant spin seebeck effect with highly polarized spin current generation and piezoelectricity in flexible V2SeTeO altermagnet at room temperature

Studies on altermagnetic materials are attracting extensive research efforts owing to their directional dependent spin split band structure. However, it is rare to find reports on the possibility of multifunctionality in altermagnetic systems. Here, we explore the spin dependent transport properties and piezoelectricity of two-dimensional V2SeTeO altermagnet. The V2SeTeO system has a direct band gap of 0.32 eV with a Neel temperature of 510 K. We find a giant effective Seebeck coefficient of 0.64 mV/K at 300 K. This is several times larger than that found in bulk and other two-dimensional materials. Moreover, the effective Seebeck effect is entirely determined by either only spin-up or spin-down component. This feature implies that we can generate highly spin polarized current by temperature gradient at room temperature. We attribute this pure spin current generation to the directional dependent spin split band structure. Along with the spin dependent transport properties, we also find that the Janus V2SeTeO altermagnet shows outstanding flexibility and piezoelectric response with out-of-plane piezoelectric coefficient of d31=0.245pm/V. Overall, we propose that the V2SeTeO altermagnet system exhibits multifunctional physical properties at room temperature, and this can be utilized for potential spintronics and flexible piezoelectric applications simultaneously.

Valley-Related Multipiezo Effect and Noncollinear Spin Current in an Altermagnet Fe2Se2O Monolayer.

An altermagnet exhibits many novel physical phenomena because of its intrinsic antiferromagnetic coupling and natural band spin splitting, which are expected to give rise to new types of magnetic electronic components. In this study, an Fe2Se2O monolayer is proven to be an altermagnet with out-of-plane magnetic anisotropy, and its Néel temperature is determined to be 319 K. The spin splitting of the Fe2Se2O monolayer reaches 860 meV. Moreover, an Fe2Se2O monolayer presents a pair of energy valleys, which can be polarized and reversed by applying uniaxial strains along different directions, resulting in a piezovalley effect. Under the strain, the net magnetization can be induced in the Fe2Se2O monolayer by doping with holes, thereby realizing a piezomagnetic property. Interestingly, noncollinear spin current can be generated by applying an in-plane electric field on an unstrained Fe2Se2O monolayer doped with 0.2 hole/formula unit. These excellent physical properties make the Fe2Se2O monolayer a promising candidate for multifunctional spintronic and valleytronic devices.

Emerging flat bands and spin polarization in nanodiamond island superlattices with varying carrier effective masses

For the first time, a set of regular 2D sp2/sp3 hybrid superlattices of nanodiamond islands confined between two graphene sheets (NDI-c2G) were proposed and explored using Density Functional Theory Periodic Boundary Conditions simulations. The fusion involves coronene and ovalene molecules with top and bottom graphene lattices, which can be treated with electron beam irradiation and atomic hydrogen at the nanodiamond site in three different configurations: (111)CD, (0001)HD, and (2¯110)HD. This creates distorted sp3 nanodiamond sublattices within the sp2 graphene fragments, serving as regions with confined electrons, making the charge carriers’ effective masses heavier. The presence of spin-splitting valence spin-up and conduction spin-down flat bands near the Fermi level in cubic ((111)CD-I and (111)CD-II) and hexagonal ((2¯110)HD)-II) superlattices resulted in magnetic moments of 3.99μB, 3.99μB, and 1.99μB per unit cell, respectively. Various configurations significantly impacted their electronic properties, resulting in spin-polarized and non-spin-polarized systems with flat bands near the Fermi level, exhibiting a semiconducting nature. The averaged spin moments per carbon atom were 0.16μB, 0.14μB, and 0.08μB, localized at the sp2/sp3 interfaces of the nanodiamond sites. Analysis of the electronic states, band structure, charge carrier effective masses, and spin density distribution suggests NDI-c2G shares similar electronic nature to strongly correlated twisted bilayer graphenes at small magic angles.

Alternating spin splitting of electronic and magnon bands in two-dimensional altermagnetic materials

Abstract Unconventional antiferromagnetism dubbed as altermagnetism was firstly discovered in rutile structured magnets, which is featured by spin splitting even without the spin-orbit coupling effect. This interesting phenomenon has led to the discovery of more altermagnetic materials. In this work, we explore two-dimensional altermagnetic materials by studying two series of two-dimensional magnets, including MF4 with M covering all 3d and 4d transition metal elements, as well as TS2 with T = V, Cr, Mn, Fe. Through the magnetic symmetry operation of the RuF4 and MnS2, it was verified that breaking the time inversion is a necessary condition for spin splitting. Based on symmetry analysis and first-principles calculations, we found that the electronic bands and magnon dispersion experience alternating spin splitting along the same path. This work paves the way for exploring altermagnetism in two-dimensional materials.

Strain-tuned electronic and valley-related properties in Janus monolayers of SWSiX2 (X = N, P, As)

Abstract Exploring novel two-dimensional (2D) valleytronic materials has an essential impact on the design of spintronic and valleytronic devices. Our first principles calculation results reveal that the Janus SWSiX2 (X = N, P, As) monolayer has excellent dynamical and thermal stability. Owing to strong spin-orbit coupling (SOC), the SWSiX2 monolayer exhibits a valence band spin splitting of up to 0.49 eV, making it promising 2D semiconductor for valleytronic applications. The opposite Berry curvatures and optical selection rules lead to the coexistence of valley and spin Hall effects in the SWSiX2 monolayer. Moreover, the optical transition energies can be remarkably modulated by the in-plane strains. Large tensile (compressive) inplane strains can achieve spin flipping in the SWSiN2 monolayer, and induce both SWSiP2 and SWSiAs2 monolayers transit from semiconductor to metal. Our research provides new 2D semiconductor candidates for designing high-performance valleytronic devices.

Search for an antiferromagnetic Weyl semimetal in (MnTe) m (Sb2Te3) n and (MnTe) m (Bi2Te3) n superlattices

The interaction between topology and magnetism can lead to novel topological materials including Chern insulators, axion insulators, and Dirac and Weyl semimetals. In this work, a family of van der Waals layered materials using MnTe and Sb2Te3 or Bi2Te3 superlattices as building blocks are systematically examined in a search for antiferromagnetic Weyl semimetals, preferably with a simple node structure. The approach is based on controlling the strength of the exchange interaction as a function of layer composition to induce the phase transition between the topological and the normal insulators. Our calculations, utilizing a combination of first-principles density functional theory and tight-binding analyses based on maximally localized Wannier functions, clearly indicate a promising candidate for a type-I magnetic Weyl semimetal. This centrosymmetric material, Mn10Sb8Te22 (or (MnTe) m (Sb2Te3) n with m = 10 and n = 4), shows ferromagnetic intralayer and antiferromagnetic interlayer interactions in the antiferromagnetic ground state. The obtained electronic bandstructure also exhibits a single pair of Weyl points in the spin-split bands consistent with a Weyl semimetal. The presence of Weyl nodes is further verified with Berry curvature, Wannier charge center, and surface state (i.e. Fermi arc) calculations. Other combinations of the MnSbTe-family materials are found to be antiferromagnetic topological or normal insulators on either side of the Mn:Sb ratio, respectively, illustrating the topological phase transition as anticipated. A similar investigation in the homologous (MnTe) m (Bi2Te3) n system produces mostly nontrivial antiferromagnetic insulators due to the strong spin–orbit coupling. When realized, the antiferromagnetic Weyl semimetals in the simplest form (i.e. a single pair of Weyl nodes) are expected to provide a promising candidate for low-power spintronic applications.

Orientation-dependent Andreev reflection in an altermagnet/altermagnet/superconductor junction

Altermagnets, an emerging class of magnetic materials distinguished by a significant non-relativistic spin splitting band structure but zero net macroscopic magnetization, have recently received considerable attention. Here, we present a theoretical study focusing on the orientation-dependent conductance induced by Andreev reflection (AR) at an altermagnet/altermagnet/superconductor junction. Conventional and equal-spin AR processes occur when the Néel vector directions of the AMs are non-collinear and controllable by the altermagnet’s orientation θ and Néel vector direction α. The conductance spectra resemble ferromagnet/ferromagnet/superconductor junctions with parallel or antiparallel magnetization configurations, depending on θ and α. Both the anisotropic altermagnetic state and the equal-spin AR signature can be probed by studying conductance spectra. These findings suggest that altermagnet/superconductor junctions are promising platforms for future superconducting spintronics applications.