Giant Rashba-type spin splitting in bulk BiTeI

Revealing hidden spin polarization in centrosymmetric van der Waals materials on ultrafast timescales

One of the key challenges for spintronic and quantum technologies is to achieve active control of the spin angular momentum of electrons in nanoscale materials on ultrafast, femtosecond timescales. While conventional ferromagnetic materials and materials supporting spin texture suffer both from conceptional limitations in miniaturization and inefficiency of optical and electronic manipulation, non-magnetic centrosymmetric layered materials with hidden spin polarization may offer an alternative pathway to manipulate the spin degree of freedom by external stimuli. Here we demonstrate an approach for generating transient spin polarization on a femtosecond timescale in the otherwise spin-unpolarized band structure of the centrosymmetric 2H-stacked group VI transition metal dichalcogenide WSe2. Using ultrafast optical excitation of a fullerene layer grown on top of WSe2, we trigger an ultrafast interlayer electron transfer from the fullerene layer into the WSe2 crystal. The resulting transient charging of the C60/WSe2 interface leads to a substantial interfacial electric field that by means of spin-layer-valley locking ultimately creates ultrafast spin polarization without the need of an external magnetic field. Our findings open a novel pathway for true optical engineering of spin functionalities such as the sub-picosecond generation and manipulation of ultrafast spin currents in 2D heterostructures.

Enhanced Nonlinear Response by Manipulating the Dirac Point at the (111) LaTiO_{3}/SrTiO_{3} Interface

Tunable spin-orbit interaction (SOI) is an important feature for future spin-based devices. In the presence of a magnetic field, SOI induces an asymmetry in the energy bands, which can produce nonlinear transport effects (V∼I^{2}). Here, we focus on such effects to study the role of SOI in the (111) LaTiO_{3}/SrTiO_{3} interface. This system is a convenient platform for understanding the role of SOI since it exhibits a single-band Hall response through the entire gate-voltage range studied. We report a pronounced rise in the nonlinear longitudinal resistance at a critical in-plane field H_{cr}. This rise disappears when a small out-of-plane field component is present. We explain these results by considering the location of the Dirac point formed at the crossing of the spin-split energy bands. An in-plane magnetic field pushes this point outside of the Fermi contour, and consequently changes the symmetry of the Fermi contours and intensifies the nonlinear transport. An out-of-plane magnetic field opens a gap at the Dirac point, thereby significantly diminishing the nonlinear effects. We propose that magnetoresistance effects previously reported in interfaces with SOI could be comprehended within our suggested scenario.

First principles predictions of structural, electronic and topological properties of two-dimensional Janus Ti2N2XI (X = Br, Cl) structures

Motivated by the report of the giant Rashba effect in ternary layered compounds BiTeX, we consider two Janus structured compounds Ti2N2XI (X = Br, Cl) of the same ternary family exhibiting a 1 : 1 : 1 stoichiometric ratio. Broken inversion symmetry in the Janus structure, together with its unique electronic structure exhibiting anti-crossing states formed between Ti-d states and strong spin-orbit coupled I-p states, generates large Rashba cofficients of 2-3 eV Å for these compounds, classifying them as strong Rashba compounds. The anti-crossing features of the first-principles calculated electronic structure also result in non-trivial topology, combining two quantum phenomena - Rashba effect and non-trivial topology - in the same materials. This makes Janus TiNI compounds candidate materials for two-dimensional composite quantum materials. The situation becomes further promising by the fact that the properties are found to exhibit extreme sensitivity and tunability upon application of uniaxial strain.

Observation of plaid-like spin splitting in a noncoplanar antiferromagnet

Spatial, momentum and energy separation of electronic spins in condensed-matter systems guides the development of new devices in which spin-polarized current is generated and manipulated1-3. Recent attention on a set of previously overlooked symmetry operations in magnetic materials4 leads to the emergence of a new type of spin splitting, enabling giant and momentum-dependent spin polarization of energy bands on selected antiferromagnets5-10. Despite the ever-growing theoretical predictions, the direct spectroscopic proof of such spin splitting is still lacking. Here we provide solid spectroscopic and computational evidence for the existence of such materials. In the noncoplanar antiferromagnet manganese ditelluride (MnTe2), the in-plane components of spin are found to be antisymmetric about the high-symmetry planes of the Brillouin zone, comprising a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an unconventional spin pattern, further found to diminish at the high-temperature paramagnetic state, originates from the intrinsic AFM order instead of spin-orbit coupling (SOC). Our finding demonstrates a new type of quadratic spin texture induced by time-reversal breaking, placing AFM spintronics on a firm basis and paving the way for studying exotic quantum phenomena in related materials.

Altermagnetic lifting of Kramers spin degeneracy

Lifted Kramers spin degeneracy (LKSD) has been among the central topics of condensed-matter physics since the dawn of the band theory of solids1,2. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology3-7 to topological quantum matter8-14. Traditionally, LKSD has been considered to originate from two possible internal symmetry-breaking mechanisms. The first refers to time-reversal symmetry breaking by magnetization of ferromagnets and tends to be strong because of the non-relativistic exchange origin15. The second applies to crystals with broken inversion symmetry and tends to be comparatively weaker, as it originates from the relativistic spin-orbit coupling (SOC)16-19. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic20,21, that allows for LKSD without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of LKSD generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization20-23. Our observation of the altermagnetic LKSD can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors20,21, that have been either identified recently or perceived for many decades as conventional antiferromagnets21,24,25.

Nonlinear transport and radio frequency rectification in BiTeBr at room temperature

Materials showing second-order nonlinear transport under time reversal symmetry can be used for Radio Frequency (RF) rectification, but practical application demands room temperature operation and sensitivity to microwatts level RF signals in the ambient. In this study, we demonstrate that BiTeBr exhibits a giant nonlinear response which persists up to 350 K. Through scaling and symmetry analysis, we show that skew scattering is the dominant mechanism. Additionally, the sign of the nonlinear response can be electrically switched by tuning the Fermi energy. Theoretical analysis suggests that the large Rashba spin-orbit interactions (SOI), which gives rise to the chirality of the Bloch electrons, provide the microscopic origin of the observed nonlinear response. Our BiTeBr rectifier is capable of rectifying radiation within the frequency range of 0.2 to 6 gigahertz at room temperature, even at extremely low power levels of -15 dBm, and without the need for external biasing. Our work highlights that materials exhibiting large Rashba SOI have the potential to exhibit nonlinear responses at room temperature, making them promising candidates for harvesting high-frequency and low-power ambient electromagnetic energy.

A Novel Ferroelectric Rashba Semiconductor

Fast, reversible, and low-power manipulation of the spin texture is crucial for next generation spintronic devices like non-volatile bipolar memories, switchable spin current injectors or spin field effect transistors. Ferroelectric Rashba semiconductors (FERSC) are the ideal class of materials for the realization of such devices. Their ferroelectric character enables an electronic control of the Rashba-type spin texture by means of the reversible and switchable polarization. Yet, only very few materials are established to belong to this class of multifunctional materials. Here, Pb1- x Gex Te is unraveled as a novel FERSC system down to nanoscale. The ferroelectric phase transition and concomitant lattice distortion are demonstrated by temperature dependent X-ray diffraction, and their effect on electronic properties are measured by angle-resolved photoemission spectroscopy. In few nanometer-thick epitaxial heterostructures, a large Rashba spin-splitting is exhibiting a wide tuning range as a function of temperature and Ge content. This work defines Pb1- x Gex Te as a high-potential FERSC system for spintronic applications.

Lone-pair-induced formation of intrinsic one-dimensional SbSX (X = Cl, Br, I) helix chain materials

Intrinsic one-dimensional (1D) helix chain materials are extremely rare in inorganic chemistry due to their novel structural features and complex syntheses. Herein, we report a class of inborn 1D helix chains, namely 1D SbSX (X = Cl, Br, I), that can exist stably. Through ab initio calculations, we demonstrate that the formation of this helical feature is facilitated by the lone pairs in antimony atoms. Owing to the different chemical bonds induced by the lone pairs, a phase transition between different helix chain phases can occur by applying extra elongation strain. More importantly, 1D SbSX helix chains possess superior flexibility. Under large elongation strains, the elastic energy is stored via bond angle redistributions, while the average bond lengths can remain invariant. Our work not only enriches the family of intrinsic 1D helical materials, but also provides a novel avenue for the diversification of low-dimensional phase change and flexible materials.

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.

A closer look at how symmetry constraints and the spin-orbit coupling shape the electronic structure of Bi(111)

Fully relativistic density-functional-theory calculations of Bi(111) thin films are analyzed to revisit their two metallic surface-states branches. We first contrast these metallic branches with surface states arising at gaps in the valence band opened by the spin-orbit coupling (SOC). We find that the two metallic branches alongΓM‾do not overlap with the bulk band at the zone boundary,M. We show that the spin texture observed in such states cannot be traced to the lifting of Kramers' degeneracy. Instead, we track them to themj=±1/2-mj=±3/2SOC splitting, the potential anisotropy for in-plane and out-of-plane states, and the coupling between the opposite surfaces of a slab occurring nearM, which is driven by a spatial redistribution of the four metallic states composing the two metallic branches. Each of these branches appears to be non-degenerate at the tested surface, yet each is degenerate with another state of opposite spin at the other surface. Nevertheless, the four metallic states bear some contribution on both surfaces of the film because of their spatial redistribution nearM. The overlapping among these states nearM, afforded by their spatial redistribution on both surfaces, causes a hybridization that perpetuates the splitting between the two branches, makes the film's electronic structure thickness dependent nearM, extinguishes the magnetic moment of the metallic states avoiding the magnetic-moment discontinuity atM, and denies the need or expectancy of the metallic branches becoming degenerate atM. We propose that theoppositespin polarization observed for the two metallic branches occurs because the surface atoms retain their covalent bonds and thus cannot afford magnetic polarization. We show that the Rashba-splitting of the metallic states for inversion-asymmetric films does not have a fixed magnitude but can be tuned by changing the perturbation breaking inversion symmetry.

Spin-valley Rashba monolayer laser

Direct-bandgap transition metal dichalcogenide monolayers are appealing candidates to construct atomic-scale spin-optical light sources owing to their valley-contrasting optical selection rules. Here we report on a spin-optical monolayer laser by incorporating a WS2 monolayer into a heterostructure microcavity supporting high-Q photonic spin-valley resonances. Inspired by the creation of valley pseudo-spins in monolayers, the spin-valley modes are generated from a photonic Rashba-type spin splitting of a bound state in the continuum, which gives rise to opposite spin-polarized ±K valleys due to emergent photonic spin-orbit interaction under inversion symmetry breaking. The Rashba monolayer laser shows intrinsic spin polarizations, high spatial and temporal coherence, and inherent symmetry-enabled robustness features, enabling valley coherence in the WS2 monolayer upon arbitrary pump polarizations at room temperature. Our monolayer-integrated spin-valley microcavities open avenues for further classical and non-classical coherent spin-optical light sources exploring both electron and photon spins.

Highly Durable Spin Filter Switching Based on Self-Assembled Chiral Molecular Motor

Chiral molecules have recently received renewed interest as highly efficient sources of spin-selective charge emission known as chiral-induced spin selectivity (CISS), which potentially offers a fascinating utilization of organic chiral materials in novel solid-state spintronic devices. However, a practical use of CISS remains far from completion, and rather fundamental obstacles such as (i) external controllability of spin, (ii) function durability, and (iii) improvement of spin-polarization efficiency have not been surmounted to date. In this study, these issues are addressed by developing a self-assembled monolayer (SAM) of overcrowded alkene (OCA)-based molecular motor. With this system, it is successfully demonstrated that the direction of spin polarization can be externally and repeatedly manipulated in an extremely stable manner by switching the molecular chirality, which is achieved by a formation of the covalent bonds between the molecules and electrode. In addition, it is found that a higher stereo-ordering architecture of the SAM of OCAs tailored by mixing them with simple alkanethiols considerably enhances the efficiency of spin polarization per a single OCA molecule. All these findings provide the creditable feasibility study for strongly boosting development of CISS-based spintronic devices that can simultaneously fulfill the controllability, durability, and high spin-polarization efficiency.

Order-disorder phase transition driven by interlayer sliding in lead iodides

A variety of phase transitions have been found in two-dimensional layered materials, but some of their atomic-scale mechanisms are hard to clearly understand. Here, we report the discovery of a phase transition whose mechanism is identified as interlayer sliding in lead iodides, a layered material widely used to synthesize lead halide perovskites. The low-temperature crystal structure of lead iodides is found not 2H polytype as known before, but non-centrosymmetric 4H polytype. This undergoes the order-disorder phase transition characterized by the abrupt spectral broadening of valence bands, taken by angle-resolved photoemission, at the critical temperature of 120 K. It is accompanied by drastic changes in simultaneously taken photocurrent and photoluminescence. The transmission electron microscopy is used to reveal that lead iodide layers stacked in the form of 4H polytype at low temperatures irregularly slide over each other above 120 K, which can be explained by the low energy barrier of only 10.6 meV/atom estimated by first principles calculations. Our findings suggest that interlayer sliding is a key mechanism of the phase transitions in layered materials, which can significantly affect optoelectronic and optical characteristics.

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.

Current Understanding of Band-Edge Properties of Halide Perovskites: Urbach Tail, Rashba Splitting, and Exciton Binding Energy

The band-edge structure of halide perovskites, derived from the hybridization of atomic orbitals, plays a fundamental role in determining their optical and electronic properties. Several important concepts have been frequently discussed to describe the influence of band-edge structure on their optoelectronic properties, including Urbach tail, Rashba splitting, and exciton binding energy. In this Perspective, we provide a fundamental understanding of these concepts, with the focus on their dependence on composition, structure, or dimensionality. Subsequently, the implications for material optimization and device fabrication are discussed. Furthermore, we highlight the Rashba effect on the exciton fine structure in perovskite nanocrystals (PNCs), which explains the unique emissive properties. Finally, we discuss the potential influence of band-edge properties on the light emission process. We hope that this Perspective can inspire the investigation of band-edge properties of halide perovskites for light-emitting diodes, lasers, and spin electronics.

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.

Observation of spin-momentum locked surface states in amorphous Bi2Se3

Crystalline symmetries have played a central role in the identification and understanding of quantum materials. Here we investigate whether an amorphous analogue of a well known three-dimensional strong topological insulator has topological properties in the solid state. We show that amorphous Bi₂Se₃ thin films host a number of two-dimensional surface conduction channels. Our angle-resolved photoemission spectroscopy data are consistent with a dispersive two-dimensional surface state that crosses the bulk gap. Spin-resolved photoemission spectroscopy shows this state has an anti-symmetric spin texture, confirming the existence of spin-momentum locked surface states. We discuss these experimental results in light of theoretical photoemission spectra obtained with an amorphous topological insulator tight-binding model, contrasting it with alternative explanations. The discovery of spin-momentum locked surface states in amorphous materials opens a new avenue to characterize amorphous matter, and triggers the search for an overlooked subset of quantum materials outside of current classification schemes.

A study of the Rashba effect in two-dimensional ternary compounds ABC monolayers (A = Sb, Bi; B = Se, Te; C = Br; I)

The structure and electronic and spintronic properties of two-dimensional (2D) ternary compounds ABC (A = Sb, Bi; B = Se, Te; C = Br; I) monolayers are investigated using the first-principles method. The ABC monolayers possess typical Janus structures with a considerable potential gradient normal to the surface, inducing intrinsic Rashba spin splitting (RSS) at the conduction band minimum near the Γ point. Among them, the splitting strength of the BiSeI monolayer is the largest and its Rashba coefficient can reach 1.84 eV Å. The projected energy band of the BiSeI monolayer suggests that the RSS state is mainly rooted in the Bi-p_z orbital. The RSS strength can be modulated by applying the in-plane strain. The tensile strain can improve the RSS strength, which is ascribed to the increase of the potential gradient normal to the surface. These results indicate that these 2D ternary compounds have great potential for application in tunable spintronic devices.

Topological properties of Xene tuned by perpendicular electric field and exchange field in the presence of Rashba spin-orbit coupling

Xene (X=Si, Ge, Sn) is a typical and promising two-dimensional topological insulator with many novel topological properties. Here, we investigate the topological properties of Xene tuned by a perpendicularly applied electric field, exchange field, and Rashba spin-orbit coupling (RSOC) using the tight-binding (TB) method. We show that in the presence of RSOC, the system can be converted from a quantum spin Hall (QSH) insulator into a conventional band insulator (BI) by a weak perpendicular electric field or into a quantum anomalous Hall (QAH) insulator by a weak exchange field. Additionally, a suitable combination of electric and exchange fields can give rise to a valley-polarized metallic (VPM) state. Furthermore, we explore the competition between the electric field and exchange field in tuning the topological states owing to the Rashba coupling effect. When the electric field is stronger than the exchange field, the system tends to be in a topologically trivial BI state; otherwise, it will be a QAH insulator. More intriguingly, for a fixed exchange field and RSOC, as the perpendicular electric field increase continuously from zero, the system undergoes multiphase (e.g. QSH-VPM-BI) transitions. This paves the way for designing multiphase transition devices through external single-field regulation.

Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2xalloy

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS_1.4Se_0.6alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of thek-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 ± 0.7 eV Å. The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS_1.4Se_0.6alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this 2D material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

Two-dimensional BiTeI as a novel perovskite additive for printable perovskite solar cells

Hybrid organic-inorganic perovskite solar cells (PSCs) are attractive printable, flexible, and cost-effective optoelectronic devices constituting an alternative technology to conventional Si-based ones. The incorporation of low-dimensional materials, such as two-dimensional (2D) materials, into the PSC structure is a promising route for interfacial and bulk perovskite engineering, paving the way for improved power conversion efficiency (PCE) and long-term stability. In this work, we investigate the incorporation of 2D bismuth telluride iodide (BiTeI) flakes as additives in the perovskite active layer, demonstrating their role in tuning the interfacial energy-level alignment for optimum device performance. By varying the concentration of BiTeI flakes in the perovskite precursor solution between 0.008 mg mL^-1 and 0.1 mg mL^-1, a downward shift in the energy levels of the perovskite results in an optimal alignment of the energy levels of the materials across the cell structure, as supported by device simulations. Thus, the cell fill factor (FF) increases with additive concentration, reaching values greater than 82%, although the suppression of open circuit voltage (V _oc) is reported beyond an additive concentration threshold of 0.03 mg mL^-1. The most performant devices delivered a PCE of 18.3%, with an average PCE showing a +8% increase compared to the reference devices. This work demonstrates the potential of 2D-material-based additives for the engineering of PSCs via energy level optimization at perovskite/charge transporting layer interfaces.

MX family: an efficient platform for topological spintronics based on Rashba and Zeeman-like spin splittings

Taking various combinations of M = (Mo, W) and X = (C, S, Se) as examples, we propose that MX (M = transition metals, X = IV,V or VI elements) family can establish an excellent platform for both conventional and topological spintronics applications based on anisotropic Rashba-like and non-magnetic Zeeman-type spin splittings with electrically tunable nature. In particular, we observe sizeable Zeeman-like and Rashba-like spin splittings with an anisotropic nature. Meanwhile, they exhibit Rashba-like and topologically robust helical edge states when grown in ferroelectric and paraelectric phases, respectively. These MX monolayers are realized to be quantum valley Hall insulators due to valley contrasting Berry curvatures. The carriers in these MX monolayers can be selectively excited from opposite valleys depending on the polarity of circularly polarized light. The amplitude of the spin splitting can be further tuned by applying external means such as strain, electric field or alloy engineering. Furthermore, considering graphene sheet over the WC monolayer as a prototype example, we show that these MX monolayers can boost the relativistic effect by coupling with the systems exhibiting extremely weak spin-orbit coupling (SOC). Depending on the surface of WC monolayer in contact with the graphene sheet, graphene over WC monolayer passes through the transformation from the semiconducting junction to the Shotcky barrier-free contact. Finally, we reveal that these MX monolayers could also be grown on the substrates such as WS2(001)and GaTe (001) with type-II band alignment, where electron and hole become layer splitted across the interface. Our analysis should be fairly applied to other systems with strong SOC and an equivalent geometrical structure to the MX monolayers.

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).

Ultrafast Thermalization Pathways of Excited Bulk and Surface States in the Ferroelectric Rashba Semiconductor GeTe

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics.

Defect-gradient-induced Rashba effect in van der Waals PtSe2 layers

Defect engineering is one of the key technologies in materials science, enriching the modern semiconductor industry and providing good test-beds for solid-state physics. While homogenous doping prevails in conventional defect engineering, various artificial defect distributions have been predicted to induce desired physical properties in host materials, especially associated with symmetry breakings. Here, we show layer-by-layer defect-gradients in two-dimensional PtSe₂ films developed by selective plasma treatments, which break spatial inversion symmetry and give rise to the Rashba effect. Scanning transmission electron microscopy analyses reveal that Se vacancies extend down to 7 nm from the surface and Se/Pt ratio exhibits linear variation along the layers. The Rashba effect induced by broken inversion symmetry is demonstrated through the observations of nonreciprocal transport behaviors and first-principles density functional theory calculations. Our methodology paves the way for functional defect engineering that entangles spin and momentum of itinerant electrons for emerging electronic applications.

Materials with strong Rashba-type spin-orbit coupling hold promise for spintronic applications and the investigation of topological phases of matter. Here, the authors report a method to generate layer-by-layer defect gradients in a van der Waals material, inducing broken spatial inversion symmetry and Rashba effect in the engineered layers.

Gate-tuneable and chirality-dependent charge-to-spin conversion in tellurium nanowires

Chiral materials are an ideal playground for exploring the relation between symmetry, relativistic effects and electronic transport. For instance, chiral organic molecules have been intensively studied to electrically generate spin-polarized currents in the last decade, but their poor electronic conductivity limits their potential for applications. Conversely, chiral inorganic materials such as tellurium have excellent electrical conductivity, but their potential for enabling the electrical control of spin polarization in devices remains unclear. Here, we demonstrate the all-electrical generation, manipulation and detection of spin polarization in chiral single-crystalline tellurium nanowires. By recording a large (up to 7%) and chirality-dependent unidirectional magnetoresistance, we show that the orientation of the electrically generated spin polarization is determined by the nanowire handedness and uniquely follows the current direction, while its magnitude can be manipulated by an electrostatic gate. Our results pave the way for the development of magnet-free chirality-based spintronic devices.

High-throughput inverse design and Bayesian optimization of functionalities: spin splitting in two-dimensional compounds

The development of spintronic devices demands the existence of materials with some kind of spin splitting (SS). In this Data Descriptor, we build a database of ab initio calculated SS in 2D materials. More than that, we propose a workflow for materials design integrating an inverse design approach and a Bayesian inference optimization. We use the prediction of SS prototypes for spintronic applications as an illustrative example of the proposed workflow. The prediction process starts with the establishment of the design principles (the physical mechanism behind the target properties), that are used as filters for materials screening, and followed by density functional theory (DFT) calculations. Applying this process to the C2DB database, we identify and classify 358 2D materials according to SS type at the valence and/or conduction bands. The Bayesian optimization captures trends that are used for the rationalized design of 2D materials with the ideal conditions of band gap and SS for potential spintronics applications. Our workflow can be applied to any other material property.

Measurement(s)	Spin polarized and spin-orbit coupling band structures • Spin-splitting type at the valence and/or conduction bands
Technology Type(s)	Density functional theory • Bayesian optimization and High-throughput calculations
Factor Type(s)	Atomic composition and stoichiometry of two-dimensional compounds • Crystalline structure of two-dimensional compounds

Ferroelectric control of pseudospin texture in CuInP2S6monolayer

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 atKvalley, which can be regarded as the Zeeman effect under a valley-dependent pseudomagnetic field. The CIPS monolayer with Rashbaet aleffects provides a good platform for electrically controlled spin polarization physics.

Rashba splitting in organic-inorganic lead-halide perovskites revealed through two-photon absorption spectroscopy

The Rashba splitting in hybrid organic-inorganic lead-halide perovskites (HOIP) is particularly promising and yet controversial, due to questions surrounding the presence or absence of inversion symmetry. Here we utilize two-photon absorption spectroscopy to study inversion symmetry breaking in different phases of these materials. This is an all-optical technique to observe and quantify the Rashba effect as it probes the bulk of the materials. In particular, we measure two-photon excitation spectra of the photoluminescence in 2D, 3D, and anionic mixed HOIP crystals, and show that an additional band above, but close to the optical gap is the signature of new two-photon transition channels that originate from the Rashba splitting. The inversion symmetry breaking is believed to arise from ionic impurities that induce local electric fields. The observation of the Rashba splitting in the bulk of HOIP has significant implications for the understanding of their spintronic and optoelectronic device properties.

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.

Spin-Orbit Coupling in 2D Semiconductors: A Theoretical Perspective

This theoretical Perspective reviews spin-orbit coupling (SOC), including the Rashba effect and Dresselhaus effect, in two-dimensional (2D) semiconductors. We first introduce the origin of the Rashba effect and Dresselhaus effect using the Hamiltonian models; we then summarize 2D Rashba semiconductors predicted by first-principles density functional theory (DFT) calculations, including AB binary monolayers, Janus monolayers, 2D perovskites, and so on. We also review various manipulating techniques of the Rashba effect on 2D semiconductors, such as external electric field, strain engineering, charge doping, interlayer interactions, proximity effect of substrates, and external magnetic field. We then briefly summarize the applications of SOC, including the generation, detection, and manipulation of spin currents in spin Hall effect transistors and spin field effect transistors. Finally, we conclude this Perspective and propose three promising research fields of SOC in low-dimensional semiconductors, including the nonlinear SOC Hamiltonian model, 2D ferroelectric SOC semiconductors, and 1D Rashba model and semiconductors. This theoretical Perspective enriches the fundamental understanding of SOC in 2D semiconductors and will help in the design of new types of spintronic devices in future experiments.

Non-monotonic variation of the Kramers point band gap with increasing magnetic doping in BiTeI

Polar Rashba-type semiconductor BiTeI doped with magnetic elements constitutes one of the most promising platforms for the future development of spintronics and quantum computing thanks to the combination of strong spin-orbit coupling and internal ferromagnetic ordering. The latter originates from magnetic impurities and is able to open an energy gap at the Kramers point (KP gap) of the Rashba bands. In the current work using angle-resolved photoemission spectroscopy (ARPES) we show that the KP gap depends non-monotonically on the doping level in case of V-doped BiTeI. We observe that the gap increases with V concentration until it reaches 3% and then starts to mitigate. Moreover, we find that the saturation magnetisation of samples under applied magnetic field studied by superconducting quantum interference device (SQUID) magnetometer has a similar behaviour with the doping level. Theoretical analysis shows that the non-monotonic behavior can be explained by the increase of antiferromagnetic coupled atoms of magnetic impurity above a certain doping level. This leads to the reduction of the total magnetic moment in the domains and thus to the mitigation of the KP gap as observed in the experiment. These findings provide further insight in the creation of internal magnetic ordering and consequent KP gap opening in magnetically-doped Rashba-type semiconductors.

Novel optimization perspectives for thermoelectric properties based on Rashba spin splitting: a mini review

The energy problem has recently become increasingly more serious, therefore the rational use of heat energy and conversion into electrical energy is particularly important. The thermoelectric (TE) field is closely related to human life, as heat from automobiles, heat dissipation from high-power electrical appliances, or other electrical products that produce a lot of heat, can all be transformed with TE materials. The search for TE materials with an excellent performance and effective TE optimization strategies (STs) has attracted significant attention owing to the fact that thermal energy can be directly converted into electric energy. In contrast to the common TE-optimized STs, such as constructing point defects or reducing dimensionality, spin-related optimization STs have emerged from previous published research, such as the spin Seebeck effect or the Rashba effect, in which the Rashba effect shows an effective method to break through the bottleneck of ZT optimization. In this review, typical high ZT materials, common traditional optimized STs, Rashba-type TE materials and their corresponding ZT values are comprehensively discussed. The TE performance of Rashba-type materials is analysed, such as BiTeX (X = I, Br), GeTe, BiSbSeTe₂, and the BiSb monolayer. Moreover, the TE optimization mechanisms (band engineering, phonon engineering, and Rashba spin-split engineering) are summarised. Finally, the development and challenges of Rashba spin-split combined with TE in breaking the bottleneck in ZT optimization are highlighted.

Theory of unidirectional magnetoresistance and nonlinear Hall effect

We study the unidirectional magnetoresistance (UMR) and the nonlinear Hall effect (NLHE) in the ferromagnetic Rashba model. For this purpose we derive expressions to describe the response of the electric current quadratic in the applied electric field. We compare two different formalisms, namely the standard Keldysh nonequilibrium formalism and the Moyal-Keldysh formalism, to derive the nonlinear conductivities of UMR and NLHE. We find that both formalisms lead to identical numerical results when applied to the ferromagnetic Rashba model. The UMR and the NLHE nonlinear conductivities tend to be comparable in magnitude according to our calculations. Additionally, their dependencies on the Rashba parameter and on the quasiparticle broadening are similar. The nonlinear zero-frequency response considered here is several orders of magnitude higher than the one at optical frequencies that describes the photocurrent generation in the ferromagnetic Rashba model. Additionally, we compare our Keldysh nonequilibrium expression in the independent-particle approximation to literature expressions of the UMR that have been obtained within the constant relaxation time approximation of the Boltzmann formalism. We find that both formalisms converge to the same analytical formula in the limit of infinite relaxation time. However, remarkably, we find that the Boltzmann result does not correspond to the intraband term of the Keldysh expression. Instead, the Boltzmann result corresponds to the sum of the intraband term and an interband term that can be brought into the form of an effective intraband term due to thef-sum rule.

Coexisting unconventional Rashba- and Zeeman-type spin splitting in Pb-adsorbed monolayer WSe2

Based on first-principles calculations, the unconventional Rashba- and Zeeman-type spin splitting can simultaneously coexist in the Pb-adsorbed monolayer WSe₂system. The first two adsorption configurationst₁andt₂show remarkable features under the spin-orbit coupling, in which two split energy branches show same spin states at the left or right side of Γ, and the spin polarization is reversed for both Rashba band branches. For the second adsorption configuration, an energy gap was observed near the unconventional spin polarization caused by the repelled Rashba bands for avoid crossing, and this gap can produce non-dissipative spin current by applying the voltage. The results fort₂configuration with spin reversal show that the repel band gap and Rashba parameter can be effectively regulated within the biaxial strain range of -8% to 6%. By changing the adsorption distancedbetween Pb and the neighboring Se atom layer, the reduceddcaused the transfer from Rashba-type to Zeeman-type spin splitting. This predicted adsorption system would be promising for spintronic applications.

Berry curvature induced magnetotransport in 3D noncentrosymmetric metals

We study the magnetoelectric and magnetothermal transport properties of noncentrosymmetric metals using semiclassical Boltzmann transport formalism by incorporating the effects of Berry curvature (BC) and orbital magnetic moment (OMM). These effects impart quadratic-Bdependence to the magnetoelectric and magnetothermal conductivities, leading to intriguing phenomena such as planar Hall effect, negative magnetoresistance (MR), planar Nernst effect and negative Seebeck effect. The transport coefficients associated with these effects show the usual oscillatory behavior with respect to the angle between the applied electric field and magnetic field. The bands of noncentrosymmetric metals are split by Rashba spin-orbit coupling except at a band touching point (BTP). For Fermi energy below (above) the BTP, giant (diminished) negative MR is observed. This difference in the nature of MR is related to the magnitudes of the velocities, BC and OMM on the respective Fermi surfaces, where the OMM plays the dominant role. The absolute MR and planar Hall conductivity show a decreasing (increasing) trend with Rashba coupling parameter for Fermi energy below (above) the BTP.

Synthetic Rashba spin-orbit system using a silicon metal-oxide semiconductor

The spin-orbit interaction (SOI), mainly manifesting itself in heavy elements and compound materials, has been attracting much attention as a means of manipulating and/or converting a spin degree of freedom. Here, we show that a Si metal-oxide- semiconductor (MOS) heterostructure possesses Rashba-type SOI, although Si is a light element and has lattice inversion symmetry resulting in inherently negligible SOI in bulk form. When a strong gate electric field is applied to the Si MOS, we observe spin lifetime anisotropy of propagating spins in the Si through the formation of an emergent effective magnetic field due to the SOI. Furthermore, the Rashba parameter α in the system increases linearly up to 9.8 × 10^-16 eV m for a gate electric field of 0.5 V nm^-1; that is, it is gate tuneable and the spin splitting of 0.6 μeV is relatively large. Our finding establishes a family of spin-orbit systems.

Spin-Current Modulation in Hexagonal Buckled ZnTe and CdTe Monolayers for Self-Powered Flexible-Piezo-Spintronic Devices

The next-generation spintronic device demands the gated control of spin transport across the semiconducting channel through the replacement of the external gate voltage source by the piezo potential, as experimentally demonstrated in Zhu et al. ACS Nano, 2018, 12 (2), 1811-1820. Consequently, a high level of out-of-plane piezoelectricity together with a large Rashba spin splitting is sought after in semiconducting channel materials. Inspired by this experiment, a new hexagonal buckled two-dimensional (2D) semiconductor, ZnTe, and its iso-electronic partner, CdTe, are proposed herewith. These 2D materials show a strong spin-orbit coupling (SOC), which is evidenced by a large Rashba constant of 1.06 and 1.27 eV·Å, respectively, in ZnTe and CdTe monolayers. Moreover, these Rashba semiconductors exhibit a giant out-of-plane piezoelectric coefficient (d₃₃) = 88.68 and 172.61 pm/V, and can thereby generate a high piezo potential for gating purposes in spin field-effect transistors (spin-FETs). While the low elastic stiffness implies the mechanical flexibility or stretchability in these monolayers. The Rashba constants are found to be effectively modulated via external perturbations, such as strain and electric field. The wide band gap provides ample room for modulation in its electronic properties via external perturbations. Such scope is severely limited in previously reported narrow band gap Rashba semiconductors. The fascinating results found in this work indicate their great potential for applications in next-generation self-powered flexible-piezo-spintronic devices. Moreover, a new class of hexagonal buckled ZnX (X: S, Se, or Te) monolayers is proposed herein based on their previously synthesized bulk counterparts, while their electronic, mechanical, piezoelectric, and thermal properties have been thoroughly investigated using the state-of-art density functional theory (DFT).

Kramers nodal line metals

Recently, it was pointed out that all chiral crystals with spin-orbit coupling (SOC) can be Kramers Weyl semimetals (KWSs) which possess Weyl points pinned at time-reversal invariant momenta. In this work, we show that all achiral non-centrosymmetric materials with SOC can be a new class of topological materials, which we term Kramers nodal line metals (KNLMs). In KNLMs, there are doubly degenerate lines, which we call Kramers nodal lines (KNLs), connecting time-reversal invariant momenta. The KNLs create two types of Fermi surfaces, namely, the spindle torus type and the octdong type. Interestingly, all the electrons on octdong Fermi surfaces are described by two-dimensional massless Dirac Hamiltonians. These materials support quantized optical conductance in thin films. We further show that KNLMs can be regarded as parent states of KWSs. Therefore, we conclude that all non-centrosymmetric metals with SOC are topological, as they can be either KWSs or KNLMs.

Rashba valleys and quantum Hall states in few-layer black arsenic

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin-orbit coupling (SOC)¹ interplays dynamically with Coulomb interactions^2,3, band topology^4,5 and external modulating forces^6-8. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle-hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with p_z symmetry, coexisting with doubly degenerate D valleys of p_x origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the p_x bands, which produces spin-valley-flavoured D^± valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the p_z symmetry. Intriguingly, the giant Stark effect shows the same p_x-orbital selectiveness, collectively shifting the valence band maximum of the D^± Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.

Conflux of tunable Rashba effect and piezoelectricity in flexible magnesium monochalcogenide monolayers for next-generation spintronic devices

The coupling of piezoelectric properties with Rashba spin-orbit coupling (SOC) has proven to be the limit breaker that paves the way for a self-powered spintronic device (ACS Nano, 2018, 12, 1811-1820). For further advancement in next-generation devices, a new class of buckled, hexagonal magnesium-based chalcogenide monolayers (MgX; X = S, Se, Te) have been predicted which are direct band gap semiconductors satisfying all the stability criteria. The MgTe monolayer shows a strong SOC with a Rashba constant of 0.63 eV Å that is tunable to the extent of ±0.2 eV Å via biaxial strain. Also, owing to its broken inversion symmetry and buckling geometry, MgTe has a very large in-plane as well as out-of-plane piezoelectric coefficient. These results indicate its prospects for serving as a channel semiconducting material in self-powered piezo-spintronic devices. Furthermore, a prototype for a digital logic device can be envisioned using the ac pulsed technology via a perpendicular electric field. Heat transport is significantly suppressed in these monolayers as observed from their intrinsic low lattice thermal conductivity at room temperature: MgS (9.32 W m-1 K-1), MgSe (4.93 W m-1 K-1) and MgTe (2.02 W m-1 K-1). Further studies indicate that these monolayers can be used as photocatalytic materials for the simultaneous production of hydrogen and oxygen on account of having suitable band edge alignment and high charge carrier mobility. This work provides significant theoretical insights into both the fundamental and applied properties of these new buckled MgX monolayers, which are highly suitable for futuristic applications at the nanoscale in low-power, self-powered multifunctional electronic and spintronic devices and solar energy harvesting.

Transverse Transport in Two-Dimensional Relativistic Systems with Nontrivial Spin Textures

Using multiple scattering theory, we show that the generally accepted expression of transverse resistivity in magnetic systems that host skyrmions, given by the linear superposition of the ordinary, the anomalous, and the topological Hall effect, is incomplete and must be amended by an additional term, the "noncollinear" Hall effect (NHE). Its angular form is determined by the magnetic texture, the spin-orbit field of the electrons, and the underlying crystal structure, allowing us to disentangle the NHE from the various other Hall contributions. Its magnitude is proportional to the spin-orbit interaction strength. The NHE is an essential term required for decoding two- and three-dimensional spin textures from transport experiments.

Quasiparticle interference and quantum confinement in a correlated Rashba spin-split 2D electron liquid

Exploiting inversion symmetry breaking (ISB) in systems with strong spin-orbit coupling promises control of spin through electric fields-crucial to achieve miniaturization in spintronic devices. Delivering on this promise requires a two-dimensional electron gas with a spin precession length shorter than the spin coherence length and a large spin splitting so that spin manipulation can be achieved over length scales of nanometers. Recently, the transition metal oxide terminations of delafossite oxides were found to exhibit a large Rashba spin splitting dominated by ISB. In this limit, the Fermi surface exhibits the same spin texture as for weak ISB, but the orbital texture is completely different, raising questions about the effect on quasiparticle scattering. We demonstrate that the spin-orbital selection rules relevant for conventional Rashba system are obeyed as true spin selection rules in this correlated electron liquid and determine its spin coherence length from quasiparticle interference imaging.

Angle, Spin, and Depth Resolved Photoelectron Spectroscopy on Quantum Materials

The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal "hidden" spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.

Switchable Enhanced Spin Photocurrent in Rashba and Cubic Dresselhaus Ferroelectric Semiconductors

Generating and controlling spin current (SC) are of central interest in spin physics and applications. To date, the spin-orbit interaction (SOI) is an established pathway to generate SC through the spin-charge current conversion. We predict an efficient spin-light conversion via the Rashba and higher-order cubic Dresselhaus SOIs in ferroelectrics. Different from the known Edelstein effect, where SC is created by the nonequilibrium spin density, our predicted spin-polarized current is from direct interactions between light and unique spin textures generated by SOI in ferroelectrics. Using first-principles simulations, we demonstrate these concepts by calculating the DC spin photocurrent in a prototypical Rashba ferroelectric, α-GeTe. The photoinduced SC is about 2 orders of magnitude larger than the charge photocurrent. More importantly, we can conveniently switch the direction of SC by an applied electric field via inverting the spin textures. These predictions give hope to generating and controlling light-driven SC via a nonvolatile electric field.