Rashba Effect in Functional Spintronic Devices

Toward Fully Multiferroic van der Waals SpinFETs: Basic Design and Quantum Calculations

Manipulating spin transport enhances the functionality of electronic devices, allowing them to surpass physical constraints related to speed and power. For this reason, the use of van der Waals multiferroics at the interface of heterostructures offers promising prospects for developing high-performance devices, enabling the electrical control of spin information. Our work focuses primarily on a mechanism for multiferroicity in two-dimensional van der Waals materials that stems from an interplay between antiferromagnetism and the breaking of inversion symmetry in certain bilayers. We provide evidence for spin-electrical couplings that include manipulating van der Waals multiferroic edges via external voltages and the subsequent control of spin transport including for fully multiferroic spin field-effect transistors.

Reconfigurable spintronic logic gate utilizing precessional magnetization switching

In traditional von Neumann computing architecture, the efficiency of the system is often hindered by the data transmission bottleneck between the processor and memory. A prevalent approach to mitigate this limitation is the use of non-volatile memory for in-memory computing, with spin-orbit torque (SOT) magnetic random-access memory (MRAM) being a leading area of research. In this study, we numerically demonstrate that a precise combination of damping-like and field-like spin-orbit torques can facilitate precessional magnetization switching. This mechanism enables the binary memristivity of magnetic tunnel junctions (MTJs) through the modulation of the amplitude and width of input current pulses. Building on this foundation, we have developed a scheme for a reconfigurable spintronic logic gate capable of directly implementing Boolean functions such as AND, OR, and XOR. This work is anticipated to leverage the sub-nanosecond dynamics of SOT-MRAM cells, potentially catalyzing further experimental developments in spintronic devices for in-memory computing.

Spin Gapless Quantum Materials and Devices

Quantum materials, with nontrivial quantum phenomena and mechanisms, promise efficient quantum technologies with enhanced functionalities. Quantum technology is held back because a gap between fundamental science and its implementation is not fully understood yet. In order to capitalize the quantum advantage, a new perspective is required to figure out and close this gap. In this review, spin gapless quantum materials, featured by fully spin-polarized bands and the electron/hole transport, are discussed from the perspective of fundamental understanding and device applications. Spin gapless quantum materials can be simulated by minimal two-band models and could help to understand band structure engineering in various topological quantum materials discovered so far. It is explicitly highlighted that various types of spin gapless band dispersion are fundamental ingredients to understand quantum anomalous Hall effect. Based on conventional transport in the bulk and topological transport on the boundaries, various spintronic device aspects of spin gapless quantum materials as well as their advantages in different models for topological field effect transistors are reviewed.

2D Janus Polarization Functioned by Mechanical Force

2D polarization materials have emerged as promising candidates for meeting the demands of device miniaturization, attributed to their unique electronic configurations and transport characteristics. Although the existing inherent and sliding mechanisms are increasingly investigated in recent years, strategies for inducing 2D polarization with innovative mechanisms remain rare. This study introduces a novel 2D Janus state by modulating the puckered structure. Combining scanning probe microscopy, transmission electron microscopy, and density functional theory calculations, this work realizes force-triggered out-of-plane and in-plane dipoles with distorted smaller warping in GeSe. The Janus state is preserved after removing the external mechanical perturbation, which could be switched by modulating the sliding direction. This work offers a versatile method to break the space inversion symmetry in a 2D system to trigger polarization in the atomic scale, which may open an innovative insight into configuring novel 2D polarization materials.

Obtaining giant Rashba-Dresselhaus spin splitting in two-dimensional chiral metal-organic frameworks

Two-dimensional (2D) nonmagnetic semiconductors with large Rashba-Dresselhaus (R-D) spin splitting at valence or conduction bands are attractive for magnetic-field-free spintronic applications. However, so far, the number of 2D R-D inorganic semiconductors has been quite limited, and the factors that determine R-D spin splitting as well as rational design of giant spin splitting, remain unclear. For this purpose, by exploiting 2D chiral metal-organic frameworks (CMOFs) as a platform, we theoretically develop a three-step screening method to obtain a series of candidate 2D R-D semiconductors with valence band spin splitting up to 97.2 meV and corresponding R-D coupling constants up to 1.37 eV Å. Interestingly, the valence band spin texture is reversible by flipping the chirality of CMOFs. Furthermore, five keys for obtaining giant R-D spin splitting in 2D CMOFs are successfully identified: (i) chirality, (ii) large spin-orbit coupling, (iii) narrow band gap, (iv) valence and conduction bands having the same symmetry at the Γ point, and (v) strong ligand field.

Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Predicting band gaps of ABN3 perovskites: an account from machine learning and first-principle DFT studies

The present paper is primarily focused on predicting the band gaps of nitride perovskites from machine learning (ML) models. The ML models have been framed from the feature descriptors and band gap values of 1563 inorganic nitride perovskites having formation energies <-0.026 eV and band gaps ranging from ∼1.0 to 3.1 eV. Four supervised ML models such as multi-layer perceptron (MLP), gradient boosted decision tree (GBDT), support vector regression (SVR) and random forest regression (RFR) have been considered to predict the band gaps of the said systems. The accuracy of each model has been tested from mean absolute error, root-mean-square error and determination coefficient R2 values. The bivariate plots between the predicted and input band gaps of the compounds for both the training and test datasets have also been estimated. Additionally, two ABN3-type nitride perovskites CeBN3 (B = Mo, W) have been selected and their electronic band structures and optoelectronic properties have been studied from density functional theory (DFT) calculations. The band gap values of the said compounds have been estimated from DFT calculations at PBE, HSE06, G0W0@PBE, G0W0@HSE06 level of theories. The present study will be helpful in exploring the ML models in predicting the band gaps of nitride perovskites which in turn may bear potential applications in photovoltaic cells and optical luminescent devices.

Terahertz linear/non-linear anomalous Hall conductivity of moiré TMD hetero-nanoribbons as topological valleytronics materials

Twisted moiré van der Waals heterostructures hold promise to provide a robust quantum simulation platform for strongly correlated materials and realize elusive states of matter such as topological states in the laboratory. We demonstrated that the moiré bands of twisted transition metal dichalcogenide (TMD) hetero-nanoribbons exhibit non-trivial topological order due to the tendency of valence and conduction band states in K valleys to form giant band gaps when spin-orbit coupling (SOC) is taken into account. Among the features of twisted WS[Formula: see text]/MoS[Formula: see text] and WSe[Formula: see text]/MoSe[Formula: see text], we found that the heavy fermions associated with the topological flat bands and the presence of strongly correlated states, enhance anomalous Hall conductivity (AHC) away from the magic angle. By band analysis, we showed that the topmost conduction bands from the ± K-valleys are perfectly flat and carry a spin/valley Chern number. Moreover, we showed that the non-linear anomalous Hall effect in moiré TMD hetero-nanoribbons can be used to manipulate terahertz (THz) radiation. Our findings establish twisted heterostructures of group-VI TMD nanoribbons as a tunable platform for engineering topological valley quantum phases and THz non-linear Hall conductivity.

Probing Spin-Dependent Ballistic Charge Transport at Single-Nanometer Length Scales

The coherent transport of charge and spin is a key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may significantly reduce the coherence of quantum-mechanical states, thereby affecting the device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's ballistic spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach that utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg2, which is governed by a spin-momentum-locked surface state, we prove that the current direction inverses upon tip magnetization reversal.

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect

The orbital angular momentum (OAM) generation as well as its associated orbital torque is currently a matter of great interest in spin-orbitronics and is receiving increasing attention. In particular, recent theoretical work predicts that the oxidized light metal Cu can serve as an efficient OAM generator through its surface orbital Rashba effect. Here, for the first time, the crucial current-induced magnetic-field-free in-plane magnetization reversal is experimentally demonstrated in CoFeB/CuOx bilayers without any heavy elements. We show that the critical current density can be comparable to that of strong spin-orbit coupling systems with heavy metals (Pt and Ta) and that the magnetization reversal mechanism is governed by coherent rotation in the grains through the second-harmonic and magneto-optical Kerr effect measurements. Our results indicate that light metal oxides can play an equally important role as heavy metals in magnetization reversal, broadening the choice of materials for engineering spintronic devices.

Rashba effect: a chemical physicist's approach

Understanding the mechanisms underlying the emergence of giant spin splitting (GSS) is fundamental in the pursuit of more robust strategies for designing materials with desired spin splitting. This drive for material innovation continues to captivate a burgeoning community of early-career researchers with backgrounds in chemistry and material science. However, new to the field, they are often equipped only with the insight provided by the original Bychkov-Rashba model. Furthermore, daunted by the tight-binding perspective on the non-vanishing orbital angular momentum (OAM), they struggle to accurately account for the atomic spin-orbit interaction (SOI) in the formation of GSS. To address these challenges and equip young chemists with better-suited tools, this review aims to provide a more intuitive perspective on atomic interactions (orbital hybridization), structure symmetry, and atomic SOI in the formation of GSS. In pursuit of this goal, the review explores the Bychkov-Rashba model, its advantages, and limitations. Subsequently, it introduces the orbital framework, wherein GSS is modulated by atomic SOI and the interplay of OAM with the surface electrostatic field. Given the explicit dependence of both these factors on OAM, the review examines why OAM is typically quenched in crystal structures and how chemical bonds involving different orbital types can lead to its non-zero values in the presence of inversion symmetry breaking. Finally, with this chemistry-focused perspective, the review examines the rise of GSS in selected examples.

The tunable anisotropic Rashba spin-orbit coupling effect in Pb-adsorbed Janus monolayer WSeTe

The spin-splitting properties of Pb-adsorbed monolayer Janus WSeTe are investigated based on first-principles calculations. The adsorbed system shows large Rashba splitting (the Rashba parameter is up to 0.75 eV Å), and we find that different adsorption layers (Te/Se adsorption layers) exhibit different significant features under spin-orbit coupling. Zeeman splitting and Rashba splitting co-exist at the high symmetry Γ point of the Te adsorption layer, while the Se adsorption layer exhibits anisotropic Rashba spin-orbit coupling. It was determined using k·p perturbation theory that Pb atom adsorption reduces the initial symmetry of the 2H-WSeTe monolayer and induces a strong spin-orbit coupling effect, so as to induce the anisotropic Rashba effect. Furthermore, the tunability of Rashba splitting was demonstrated by varying the adsorption concentration, adjusting the adsorption distance, and applying biaxial strain. This predicted adsorption system has potential value in spintronic devices.

Large-band-gap non-Dirac quantum spin Hall states and strong Rashba effect in functionalized thallene films

The quantum spin Hall state materials have recently attracted much attention owing to their potential applications in the design of spintronic devices. Based on density functional theory calculations and crystal field theory, we study electronic structures and topological properties of functionalized thallene films. Two different hydrogenation styles (Tl2H and Tl2H2) are considered, which can drastically vary the electronic and topological behaviors of the thallene. Due to the C3v symmetry of the two systems, the px and py orbitals at the Γ point have the non-Dirac band degeneracy. With spin-orbit coupling (SOC), topological nontrivial band gaps can be generated, giving rise to non-Dirac quantum spin Hall states in the two thallium hydride films. The nontrivial band gap for the monolayer Tl2H is very large (855 meV) due to the large on-site SOC of Tl px and py orbitals. The band gap in Tl2H2 is, however, small due to the band inversion between the Tl px/y and pz orbitals. It is worth noting that both the Tl2H and Tl2H2 monolayers exhibit strong Rashba spin splitting effects, especially for the monolayer Tl2H2 (αR = 2.52 eVÅ), rationalized well by the breaking of the structural inversion symmetry. The Rashba effect can be tuned sensitively by applying biaxial strain and external electric fields. Our findings provide an ideal platform for fabricating room-temperature spintronic and topological electronic devices.

Electronic properties, skyrmions and bimerons in Janus CrXY (X, Y = S, Se, Te, Cl, Br, I, and X ≠ Y) monolayers

Using first-principles calculations, we systematically investigate the electronic properties, chiral skyrmions and bimerons in two-dimensional (2D) Janus CrXY (X, Y = S, Se, Te, Cl, Br, I, and X ≠ Y) monolayers. We found that the categories of nonmagnetic atoms (X and Y in CrXY) determine whether CrXY is a ferromagnetic metal or a semiconductor. Unexpectedly, the CrBrS monolayer of these CrXY materials is a room temperature ferromagnetic semiconductor with a Curie temperature of 303 K, and it possesses an off-plane magnetic anisotropy energy of 0.06 meV. Besides, a strong Dzyaloshinskii-Moriya interaction (DMI) of 3.10 meV is found in CrTeI and is mainly induced by the strong spin-orbit coupling of the nonmagnetic atoms Te(I) rather than that of the magnetic Cr atoms. Furthermore, using micromagnetic simulations, skyrmions can be stabilized in CrSeBr without external magnetic fields. More importantly, the bimerons in CrSeCl with in-plane magnetic anisotropy can be transformed into skyrmions or a ferromagnetic state by controlling the direction of external magnetic fields. Our work investigates fourteen kinds of Janus monolayers, serving as guidelines for materials research on DMI, skyrmions and bimerons.

Structure, Stability, and Superconductivity of Two-Dimensional Janus NbSH Monolayers: A First-Principle Investigation

Two-dimensional Janus materials have unique structural characteristics due to their lack of out-of-plane mirror symmetry, resulting in many excellent physical and chemical properties. Using first-principle calculations, we performed a detailed investigation of the possible stable structures and properties of two-dimensional Janus NbSH. We found that both Janus 1T and 2H structures are semiconductors, unlike their metallic counterparts MoSH. Furthermore, we predicted a new stable NbSH monolayer using a particle swarm optimization method combined with first-principle calculations. Interestingly, the out-of-plane mirror symmetry is preserved in this newly found 2D structure. Furthermore, the newly found NbSH is metallic and exhibits intrinsic superconducting behavior. The superconducting critical temperature is about 6.1 K under normal conditions, which is found to be very sensitive to stress. Even under a small compressive strain of 1.08%, the superconducting critical temperature increases to 9.3 K. In addition, the superconductivity was found to mainly originate from Nb atomic vibrations. Our results show the diversity of structures and properties of the two-dimensional Janus transition metal sulfhydrate materials and provide some guidelines for further investigations.

Insights into selected 2D piezo Rashba semiconductors for self-powered flexible piezo spintronics: material to contact properties

The new paradigm in electronics consists in realizing the seamless integration of many properties latent in nanomaterials, such as mechanical flexibility, strong spin-orbit coupling (Rashba spin splitting-RSS), and piezoelectricity. Taking cues from the pointers given on 1D ZnO nanowires (ACS Nano2018121811-20), the concept can be extended to multifunctional two-dimensional (2D) materials, which can serve as an ideal platform in next-generation electronics such as self-powered flexible piezo-spintronic device. However, a microscopically clear understanding reachable from the state-of-the-art density functional theory-based approaches is a prerequisite to advancing this research domain. Atomic-scale insights gained from meticulously performed scientific computations can firmly anchor the growth of this important research field, and that is of undeniable relevance from scientific and technological outlooks. This article reviews the scientific advance in understanding 2D materials hosting all the essential properties, i.e. flexibility, piezoelectricity, and RSS. Important 2D semiconducting monolayers that deserve a special mention, include monolayers of buckled MgX (X = S, Se, Te), CdTe, ZnTe, Janus structures of transition metal trichalcogenides, Janus tellurene and 2D perovskites. van Der Waals multilayers are also built to design multifunctional materials via modulation of the stacking sequence and interlayer coupling between the constituent layers. External electric field, strain engineering and charge doping are perturbations mainly used to tune the spintronic properties. Finally, the contact properties of these monolayers are also crucial for their actual implementation in electronic devices. The nature of the contacts, Schottky/Ohmic, needs to be carefully examined first as it controls the device's performance. In this regard, the rare occurrence of Ohmic contact in graphene/MgS van der Waals hetero bilayer has been presented in this review article.

Bulk Rashba-Type Spin Splitting in Non-Centrosymmetric Artificial Superlattices

Spin current, converted from charge current via spin Hall or Rashba effects, can transfer its angular momentum to local moments in a ferromagnetic layer. In this regard, the high charge-to-spin conversion efficiency is required for magnetization manipulation for developing future memory or logic devices including magnetic random-access memory. Here, the bulk Rashba-type charge-to-spin conversion is demonstrated in an artificial superlattice without centrosymmetry. The charge-to-spin conversion in [Pt/Co/W] superlattice with sub-nm scale thickness shows strong W thickness dependence. When the W thickness becomes 0.6 nm, the observed field-like torque efficiency is about 0.6, which is an order larger than other metallic heterostructures. First-principles calculation suggests that such large field-like torque arises from bulk-type Rashba effect due to the vertically broken inversion symmetry inherent from W layers. The result implies that the spin splitting in a band of such an ABC-type artificial SL can be an additional degree of freedom for the large charge-to-spin conversion.

Metasurface-Controlled Photonic Rashba Effect for Upconversion Photoluminescence

We demonstrate the effect of spin-momentum locking of upconversion photoluminescence emitted from rare-earth doped nanocrystals coupled to a phase-gradient dielectric metasurface. We observe different directionalities for left and right circular polarized light and associate this experimental observation with the photonic Rashba effect realized for upconverted photoluminescence that is manifested in the spin-dependent splitting of emitted light in the momentum space.

Theoretical insight of origin of Rashba-Dresselhaus effect in tetragonal and rhombohedral phases of BiFeO3

The inclusion of the spin-orbit coupling effect in ferroelectric materials with non-centrosymmetry leads to intriguing properties for spintronic applications. In the present work, a comparative study of spin splitting in the bulk electronic energy bands of the tetragonal and rhombohedral phases of BiFeO₃ (BFO) in terms of the Rashba and Dresselhaus effects is carried out through first-principles calculations. The obtained spin splittings, particularly at the conduction band minima, are further supplemented with an effective k·p model analysis. For the tetragonal BFO, a dominating pure bulk-type Rashba effect with helical in-plane spin components shown through diagrams is observed, whereas the rhombohedral BFO shows a significant contribution from the out-of-plane spin components and an interplay between the Rashba and Dresselhaus effects is discussed. In addition, tunability of the Rashba parameters with the application of uniaxial strain (±5%) is obtained in tetragonal BFO, in which the Rashba coefficient (α_R) doubles with a compressive 5% strain, making tetragonal BFO a suitable candidate for spintronic applications. More importantly, full reversal of the in-plane spin texture is obtained for the opposite polarization states in tetragonal BFO with an activation energy barrier of 1.13 eV.

Significant Role of Interfacial Spin-Orbit Coupling in the Spin-to-Charge Conversion in Pt/NiFe Heterostructure

For the spin-to-charge conversion (SCC) in heavy metal/ferromagnet (HM/FM) heterostructure, the contribution of interfacial spin-orbit coupling (SOC) remains controversial. Here, we investigate the SCC process of the Pt/NiFe heterostructure by the spin pumping in YIG/Pt/NiFe/IrMn multilayers. Due to the exchange bias of NiFe/IrMn structure, the NiFe magnetization can be switched between magnetically unsaturated and saturated states by opposite resonance fields of YIG layer. The spin-pumping signal is found to decrease significantly when the NiFe magnetization is changed from the saturated state to the unsaturated state. Theoretical analysis indicates that the interfacial spin absorption is enhanced for the above-mentioned NiFe magnetic state change, which results in the increased and decreased spin flow in the Pt layer and across the Pt/NiFe interface, respectively. These results demonstrate that in our case the interfacial SOC effect at the Pt/NiFe interface is dominant over the bulk inverse spin Hall effect in the Pt layer. Our work reveals a significant role of interfacial SOC in the SCC in HM/FM heterostructure, which can promote the development of high-efficiency spintronic devices through interfacial engineering.

Chiral Rashba Ferroelectrics for Circularly Polarized Light Detection

Direct detection of circularly polarized light (CPL) is a challenging task due to limited materials and ambiguous structure-property relationships that lead to low distinguishability of the light helicities. Perovskite ferroelectric semiconductors incorporating chirality provide new opportunities in dealing with this issue. Herein, a pair of 2D chiral perovskite ferroelectrics is reported, which have enhanced CPL detection performance due to interplays among lattice, photon, charge, spin, and orbit. The chirality-transfer-induced chiral&polar ferroelectric phase enhances the asymmetric nature of the photoactive sublattice and achieves a switchable self-powered detection via the bulk photovoltaic effect. The single-crystal-based device exhibits a CPL-sensitive detection performance under 430 nm with an asymmetric factor of 0.20 for left- and right-CPL differentiation, about two times that of the pure chiral counterparts. The enhanced CPL detection performance is ascribed to the Rashba-Dresselhaus effect that originates from the bulk inversion asymmetry and strong spin-orbit coupling, shown with a large Rashba coefficient, which is demonstrated by density functional theory calculation and circularly polarized light excited photoluminescence measurement. These results provide new perspectives on chiral Rashba ferroelectric semiconductors for direct CPL detection and ferroelectrics-based chiroptics and spintronics.

Enhancement of the Rashba Effect in a Conducting SrTiO3 Surface by MoO3 Capping

Systems having inherent structural asymmetry retain the Rashba-type spin-orbit interaction, which ties the spin and momentum of electrons in the band structure, leading to coupled spin and charge transport. One of the electrical manifestations of the Rashba spin-orbit interaction is nonreciprocal charge transport, which could be utilized for rectifying devices. Further tuning of the Rashba spin-orbit interaction allows additional functionalities in spin-orbitronic applications. In this work, we present our study of nonreciprocal charge transport in a conducting SrTiO3 (001) surface and its significant enhancement by a capping layer. The conductive strontium titanate SrTiO3 (STO) (001) surface was created through oxygen vacancies by Ar+ irradiation, and the nonreciprocal signal was probed by angle- and magnetic field-dependent second harmonic voltage measurement with an AC current. We observed robust directional transport in the Ar+-irradiated sample at low temperatures. The magnitude of the nonreciprocal signal is highly dependent on the irradiation time as it affects the depth of the conducting layer and the impact of the topmost conducting layer. Moreover, the nonreciprocal resistance was significantly enhanced by simply adding a MoO3 capping layer on the conductive STO surface. These results show a simple methodology for tuning and investigating the Rashba effect in a conductive STO surface, which could be adopted for various two-dimensional (2D) conducting layers for spin-orbitronic applications.

The Giant Spin-to-Charge Conversion of the Layered Rashba Material BiTeI

Rashba spin-orbit coupling (SOC) could facilitate an efficient interconversion between spin and charge currents. Among various systems, BiTeI holds one of the largest Rashba-type spin splittings. Unlike other Rashba systems (e.g., Bi/Ag and Bi₂Se₃), an experimental investigation of the spin-to-charge interconversion in BiTeI remains to be explored. Through performing an angle-resolved photoemission spectroscopy (ARPES) measurement, such a large Rashba-type spin splitting with a Rashba parameter α_R = 3.68 eV Å is directly identified. By studying the spin pumping effect in the BiTeI/NiFe bilayer, we reveal a very large inverse Rashba-Edelstein length λ_IREE ≈ 1.92 nm of BiTeI at room temperature. Furthermore, the λ_IREE monotonously increases to 5.00 nm at 60 K, indicating an enhanced Rashba SOC at low temperature. These results suggest that BiTeI films with the giant Rashba SOC are promising for achieving efficient spin-to-charge interconversion, which could be implemented for building low-power-consumption spin-orbitronic devices.

Liquid-Phase Exfoliation of Bismuth Telluride Iodide (BiTeI): Structural and Optical Properties of Single-/Few-Layer Flakes

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a two-dimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonication-assisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m^-1 and a Hildebrandt parameter between 19 and 25 MPa^0.5. The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of ∼3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |χ⁽²⁾| ∼ 1 nm V^-1, which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS₂).

Spin Swapping Effect of Band Structure Origin in Centrosymmetric Ferromagnets

We theoretically demonstrate the spin swapping effect of band structure origin in centrosymmetric ferromagnets. It is mediated by an orbital degree of freedom but does not require inversion asymmetry or impurity spin-orbit scattering. Analytic and tight-binding models reveal that it originates mainly from k points where bands with different spins and different orbitals are nearly degenerate, and thus it has no counterpart in normal metals. First-principle calculations for centrosymmetric 3d transition-metal ferromagnets show that the spin swapping conductivity of band structure origin can be comparable in magnitude to the intrinsic spin Hall conductivity of Pt. Our theory generalizes transverse spin currents generated by ferromagnets and emphasizes the important role of the orbital degree of freedom in describing spin-orbit-coupled transport in centrosymmetric materials.

Giant tunable spin Hall angle in sputtered Bi2Se3 controlled by an electric field

Finding an effective way to greatly tune spin Hall angle in a low power manner is of fundamental importance for tunable and energy-efficient spintronic devices. Recently, topological insulator of Bi₂Se₃, having a large intrinsic spin Hall angle, show great capability to generate strong current-induced spin-orbit torques. Here we demonstrate that the spin Hall angle in Bi₂Se₃ can be effectively tuned asymmetrically and even enhanced about 600% reversibly by applying a bipolar electric field across the piezoelectric substrate. We reveal that the enhancement of spin Hall angle originates from both the charge doping and piezoelectric strain effet on the spin Berry curvature near Fermi level in Bi₂Se₃. Our findings provide a platform for achieving low power consumption and tunable spintronic devices.

Controlling the spin Hall angle is significant to tunable and energy-efficient spintronic devices. Here, the authors demonstrate that the spin Hall angle in Bi₂Se₃ can be tuned and even enhanced about 600% reversibly by the electric field.

Two-Dimensional Janus Antimony Selenium Telluride with Large Rashba Spin Splitting and High Electron Mobility

Janus two-dimensional materials with large Rashba spin splitting and high electron mobility are rarely reported but highly desired for nanoscale spintronics. Herein, using density functional theory calculations, we predicated Janus Sb₂Se _x Te_3-x (x = 1 or 2) monolayers simultaneously harboring these fascinating properties. The predicated monolayers are indirect semiconductors with great dynamical, thermal, and mechanical stability. The spin-orbital coupling (SOC) and the out-of-plane asymmetry lead to Rashba spin splitting at the conduction band minimum (CBM), which can be effectively tuned by the small uniaxial strain. The strong band dispersion at the CBM leads to small electron effective mass, consequently enabling a high electron mobility that reaches up to 6816.63 cm² V^-1 s^-1. Moreover, Janus Sb₂Se _x Te_3-x monolayers possess great light absorption capability within the visible and infrared regions of solar light. Our findings highlight promising candidates for high-speed spintronic devices and may motivate more research efforts on carrier transport and SOC effects in Janus group V and VI monolayers.

Anisotropic Rashba splitting in Pt-based Janus monolayers PtXY (X,Y = S, Se, or Te)

Recent studies have demonstrated the feasibility of synthesizing two-dimensional (2D) Janus materials which possess intrinsic structural asymmetry. Hence, we performed a systematic first-principles study of 2D Janus transition metal dichalcogenide (TMD) monolayers based on PtXY (X,Y = S, Se, or Te). Our calculated formation energies show that these monolayer Janus structures retain the 1T phase. Furthermore, phonon spectral calculations confirm that these Janus TMD monolayers are thermodynamically stable. We found that PtSSe, PtSTe, and PtSeTe exhibit an insulating phase with indirect band gaps of 2.108, 1.335, and 1.221 eV, respectively, from hybrid functional calculations. Due to the breaking of centrosymmetry in the crystal structure, the spin-orbit coupling (SOC)-induced anisotropic Rashba splitting is observed around the M point. The calculated Rashba strengths from M to Γ (α ^M-Γ _R) are 1.654, 1.103, and 0.435 eV Å^-1, while the calculated values from M to K (α ^M-K _R) are 1.333, 1.244, and 0.746 eV Å^-1, respectively, for PtSSe, PtSTe, and PtSeTe. Interestingly, the spin textures reveal that the spin-splitting is mainly attributed to the Rashba effect. However, a Dresselhaus-like contribution also plays a secondary role. Finally, we found that the band gaps and the strength of the Rashba effect can be further tuned through biaxial strain. Our findings indeed show that Pt-based Janus TMDs demonstrate the potential for spintronics applications.

Two-Dimensional Van Der Waals Materials for Spin-Orbit Torque Applications

Spin-orbit torque (SOT) provides an efficient approach to control the magnetic state and dynamics in different classes of materials. Recent years, the crossover between two-dimensional van der Waals (2D vdW) materials and SOT opens a new prospect to push SOT devices to the 2D limit. In this mini-review, we summarize the latest progress in 2D vdW materials for SOT applications, highlighting the comparison of the performance between devices with various structures. It is prospected that the large family of 2D vdW materials and numerous combinations of heterostructures will widely extend the material choices and bring new opportunities to SOT devices in the future.

Thermally Assisted Rashba Splitting and Circular Photogalvanic Effect in Aqueously Synthesized 2D Dion-Jacobson Perovskite Crystals

Recently, a two-dimensional Dion-Jacobson (DJ) perovskite (AMP)PbI₄ (AMP = 4-(aminomethyl)piperidinium) is emerging with remarkable Rashba effect and ferroelectricity. However, the origin of the giant Rashba splitting remains elusive and the current synthetic strategy via slow cooling is time- and power-consuming, hindering its future applications. Here, we report on an economical aqueous method to obtain (AMP)PbI₄ crystals and clarify the origin of the giant Rashba effect by temperature- and polarization-dependent photoluminescence (PL) spectroscopy. The strong temperature-dependent PL helicity indicates the thermally assisted structural distortion as the main origin of the Rashba effect, suggesting that valley polarization still preserves at high temperatures. The Rashba effect was further confirmed by the circular photogalvanic effect near the indirect bandgap. Our study not only optimizes the synthetic strategies of this DJ perovskite but also sheds light on its potential applications in room/high-temperature spintronics and valleytronics.