Rashba Splitting Research Articles

Elucidating the Interplay between Symmetry Distortions in Passivated MAPbI3 and the Rashba Splitting Effect.

Hybrid organic-inorganic perovskites play a critical role in modern optoelectronic applications, particularly as single photon sources due to their unusual bright ground state. However, the presence of trap states resulting from surface dangling bonds hinders their widespread commercial application. This work uses density functional theory (DFT) to study the effects of various passivating ligands and their binding sites on Rashba splitting, a phenomenon directly linked to the bright ground state. Our results predict that X2- and X4-type ligands that adsorb at acidic oxygen binding sites and zwitterionic binding sites efficiently eliminate trap states introduced by surface iodine vacancies. Furthermore, our results show that distortions from the nominally symmetric cubic structure of the perovskite predominantly determine the presence and magnitude of the Rashba splitting. Specifically, the loss of more symmetry elements consistently leads to Rashba splitting in both the valence band (VB) and the conduction band (CB) with small Rashba splitting coefficients. Conversely, although inversion symmetry breaking alone fails to guarantee the presence of pure Rashba splitting in both the VB and the CB, it significantly increases the degree of splitting. The adsorption of ligands not only mitigates trap states but also plays a critical role in altering the local symmetry, thus influencing Rashba splitting. DFT predicts a distinct Rashba-Dresselhaus splitting in the CB with X2 ligands, causing the largest splitting. The presence of local electric fields causes consistent Rashba splitting of the VB across all studied systems except for the X4 zwitterionic passivated systems (sulfobetaine and lecithin). Electric fields are predicted to cause significant splitting of the CB, particularly for MAPbI3 and SH passivated MAPbI3 surfaces that possess freely rotating ligand binding sites. This study reveals that the wavelength, tunability of Rashba splitting through an applied electric field, and nature of Rashba-Dresselhaus splitting are influenced by the characteristics of the ligand binding site. On the other hand, pure Rashba splitting is predicted to exhibit a greater susceptibility to symmetry distortion than to specific ligand binding sites. These findings elucidate how surface passivating ligands and symmetry distortions influence Rashba splitting, shaping the optoelectronic properties of perovskite nanocrystals.

Field-Free Spin-Orbit Torque Switching in Janus Chromium Dichalcogenides.

We predict a very large spin-orbit torque (SOT) capability of magnetic chromium-based transition-metal dichalcogenide (TMD) monolayers in their Janus forms CrXTe, with X = S, Se. The structural inversion symmetry breaking, inherent to Janus structures is responsible for a large SOT response generated by giant Rashba splitting, equivalent to that obtained by applying a transverse electric field of ∼100 V nm-1 in non-Janus CrTe2, completely out of experimental reach. By performing transport simulations on carefully derived Wannier tight-binding models, Janus systems are found to exhibit an SOT performance comparable to the most efficient two-dimensional materials, while additionally allowing for field-free perpendicular magnetization switching, due to their reduced in-plane symmetry. Altogether, our findings evidence that magnetic Janus TMDs stand as suitable candidates for ultimate SOT-MRAM devices in an ultracompact self-induced SOT scheme.

Nonvolatile Electric Control of Rashba Spin Splitting in Sb/In2Se3 Heterostructure.

Ferroelectric Rashba semiconductors (FRS) are highly demanded for their potential capability for nonvolatile electric control of electron spins. An ideal FRS is characterized by a combination of room temperature ferroelectricity and a strong Rashba effect, which has, however, been rarely reported. Herein, we designed a room-temperature FRS by vertically stacking a Sb monolayer on a room-temperature ferroelectric In2Se3 monolayer. Our first-principles calculations reveal that the Sb/In2Se3 heterostructure exhibits a clean Rashba splitting band near the Fermi level and a strong Rashba effect coupled to the ferroelectric order. Switching the electric polarization direction enhances the Rashba effect, and the flipping is feasible with a low energy barrier of 22 meV. This Rashba-ferroelectricity coupling effect is robust against changes of the heterostructure interfacial distance and external electric fields. Such a nonvolatile electrically tunable Rashba effect at room temperature enables potential applications in next-generation data storage and logic devices operated under small electrical currents.

Evidence for Two-Dimensional Weyl Fermions in Air-Stable Monolayer PtTe1.75.

The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl Fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts is ongoing. Here, we report the realization of 2D Weyl Fermions in monolayer PtTe1.75, which has strong spin-orbit coupling and lacks inversion symmetry, by combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy, second harmonic generation, X-ray photoelectron spectroscopy measurements, and first-principles calculations. The giant Rashba splitting and band inversion lead to the emergence of three pairs of critical Weyl cones. Moreover, monolayer PtTe1.75 exhibits excellent chemical stability in ambient conditions, which is critical for future device applications. The discovery of 2D Weyl Fermions in monolayer PtTe1.75 opens up new possibilities for designing and fabricating novel spintronic devices.

Unveiling the Role of Spin Currents on the Giant Rashba Splitting in Single-Layer WSe2.

The Rashba spin splitting in uniaxial, inversion-asymmetric materials has attracted considerable interest for spintronic applications. The most widely used theoretical framework to model such states is Kohn-Sham density functional theory (DFT) in combination with standard (semi)local exchange-correlation density functional approximations (DFAs). However, in the presence of spin-orbit coupling, DFT misses contributions due to modification of the many-body interaction by spin currents J⃗. Inclusion of the latter effects requires a spin current DFT (SCDFT) formulation, which is seldom considered. We investigate the giant Rashba splitting in single-layer WSe2, and we quantify the effect of including spin currents in DFAs of the SCDFT. Crucially, we show that SCDFT allows fully capturing the giant Rashba band splitting in single-layer WSe2, otherwise previously systematically underestimated by standard (semi)local DFAs within the DFT framework. We find the inclusion of J⃗ on the DFA increases the Rashba splitting by about 20%.

Rashba splitting in polar-nonpolar sandwich heterostructure: a DFT study

In this study, we employ density functional theory based first-principles calculations to investigate the spin-orbit effects in the electronic structure of a polar-nonpolar sandwich heterostructure namely LaAlO3 / SrTiO3 / LaAlO3 . Our focus on the Ti-3d bands reveals an inverted ordering of the SrTiO3 - t2g orbital near the n-type interface, which is consistent with earlier experimental work. In contrast, toward the p-type interface, the orbital ordering aligns with the natural ordering of SrTiO3 orbitals, influenced by crystal field splitting. In the presence of SOC, a notable inter-orbital coupling between t2g and eg orbitals is observed within the tetragonal slab, a phenomenon not reported before in the SrTiO3 -based 2D systems. Additionally, our observations highlight that the cubic Rashba splitting in this system surpasses the linear Rashba splitting, contrary to experimental findings. This comprehensive analysis contributes to a refined understanding of the role of orbital mixing in Rashba splitting in the sandwich oxide heterostructures.

Optical Studies of Doped Two-Dimensional Lead Halide Perovskites: Evidence for Rashba-Split Branches in the Conduction Band.

Two-dimensional (2D) hybrid organic/inorganic perovskites are an emerging materials class for optoelectronic and spintronic applications due to strong excitonic absorption and emission, large spin-orbit coupling, and Rashba spin-splitting effects. For many of the envisioned applications, tuning the majority charge carrier (electron or hole) concentration is desirable, but electronic doping of metal-halide perovskites has proven to be challenging. Here, we demonstrate electron injection into the lower-energy branch of the Rashba-split conduction band of 2D phenethylammonium lead iodide by means of n-type molecular doping at room temperature. The molecular dopant, benzyl viologen (BV), is shown to compensate adventitious p-type impurities and can lead to a tunable Fermi level above the conduction band minimum and increased conductivity in intrinsic samples. The doping-induced carrier concentration is monitored by the observation of free-carrier absorption and intraband optical transitions in the infrared spectral range. These optical measurements allow for an estimation of the Rashba splitting energy ER ≈38 ± 4 meV. Photoinduced quantum beating measurements demonstrate that the excess electron density reduces the electron spin g-factor by ca. 6%. This work demonstrates controllable carrier concentrations in hybrid organic/inorganic perovskites and yields potential for room temperature spin control through the Rashba effect.

The spin-orbital proximity effect induced spin-valley locking and Rashba splitting in heterostructure of BP/MoSe2

The spin-orbital proximity effect induced spin-valley locking and Rashba splitting in heterostructure of BP/MoSe2

Gold intercalation of different 6H-SiC(0001) surface reconstructions

We report on in situ investigation of Au intercalation of different zero-layer graphene surface reconstructions of 6H-SiC(0001): (3×3)R30∘+(63×63)R30∘, (63×63)R30∘+(5×5)R0∘, (63×63)R30∘+(5×5)R0∘+Gr. We show that the most appropriate reconstruction for graphene formation via Au intercalation is a one with some fraction of initial graphene monolayer. A well-ordered epitaxial Au(111) islands on the zero-layer graphene were revealed with the same crystallographic orientation as the SiC surface. Ab initio calculations approve that the value of Rashba splitting in graphene increases with the density of Au atoms per unit cell and with the reduction of average distance between Au and graphene.

Rashba-splitting-induced topological flat band detected by anomalous resistance oscillations beyond the quantum limit in ZrTe5

Topological flat bands — where the kinetic energy of electrons is quenched — provide a platform for investigating the topological properties of correlated systems. Here, we report the observation of a topological flat band formed by polar-distortion-assisted Rashba splitting in the three-dimensional Dirac material ZrTe5. The polar distortion and resulting Rashba splitting on the band are directly detected by torque magnetometry and the anomalous Hall effect, respectively. The local symmetry breaking further flattens the band, on which we observe resistance oscillations beyond the quantum limit. These oscillations follow the temperature dependence of the Lifshitz–Kosevich formula but are evenly distributed in B instead of 1/B at high magnetic fields. Furthermore, the cyclotron mass gets anomalously enhanced about 102 times at fields ~ 20 T. Our results provide an intrinsic platform without invoking moiré or order-stacking engineering, which opens the door for studying topologically correlated phenomena beyond two dimensions.

Current density functional framework for spin-orbit coupling: Extension to periodic systems.

Spin-orbit coupling induces a current density in the ground state, which consequently requires a generalization for meta-generalized gradient approximations. That is, the exchange-correlation energy has to be constructed as an explicit functional of the current density, and a generalized kinetic energy density has to be formed to satisfy theoretical constraints. Herein, we generalize our previously presented formalism of spin-orbit current density functional theory [Holzer et al., J. Chem. Phys. 157, 204102 (2022)] to non-magnetic and magnetic periodic systems of arbitrary dimension. In addition to the ground-state exchange-correlation potential, analytical derivatives such as geometry gradients and stress tensors are implemented. The importance of the current density is assessed for band gaps, lattice constants, magnetic transitions, and Rashba splittings. In the latter, the impact of the current density may be larger than the deviation between different density functional approximations.

Spintronic Phenomena and Applications in Hybrid Organic–Inorganic Perovskites

AbstractThe spin degree of freedom in hybrid organic–inorganic perovskites (HOIPs) has become a rapidly growing research topic in both HOIPs and spintronics fields due to its fundamental scientific significance and tight relevance with optoelectronics. The flourishing achievements of HOIP spintronics call for a timely review to summarize the key progress and guide future developments. In this review, classical spintronic phenomena based on spin‐orbit coupling (SOC) are discussed, especially Rashba splitting and related applications such as spin‐charge conversion. Owing to the unique chirality‐spin coupling, spintronics in chiral HOIPs are particularly focused on, including chirality‐induced spin selectivity (CISS). Based on the complex band structure and carrier/exciton, spin dynamics is also widely investigated and constitutes an indispensable part of HOIP spintronics. Aside from the three main threads, other spintronic phenomena such as magneto‐optical coupling and device exploration are involved as promising opportunities. Despite the continuous breakthroughs in HOIP spintronics, more efforts are required to expand material systems, explore new physics, and optimize device configurations for enhanced performance and high integration. A deep understanding of spin behaviors in HOIPs will create a new platform beyond conventional optoelectronic and photovoltaic applications.

Computational Exploration of Intrinsic Rashba Splitting in Janus Si2SbBi Monolayer Using Density Functional Theory

In this paper, we investigate the electronic structure of Janus Si2SbBi monolayer and compare it with the non-polar systems Si2Bi2 and Si2Sb2 monolayer based on Density Functional Theory (DFT) calculation. According to the first-principles calculation, these systems exhibit semiconductor properties with energy gaps are 0.674 eV, 0.28 eV, and 1.13 eV for Janus Si2SbBi, Si2Bi2, and Si2Sb2, respectively. In addition, the intrinsic Rashba splitting is also observed around the Γ Point on conduction band minimum (CBM) in the electronic structure of Si2SbBi monolayer, which is not found in Si2Sb2 and Si2Bi2 monolayer systems. This Rashba splitting phenomenon we analyze by using the perturbation theory based on symmetry group and get the first-order Rashba Parameter α1=1.84 eVÅ , and α1=1.73 eVÅ , for Γ-K and Γ-M direction, respectively. With the strong intrinsic Rashba Splitting, the Janus Si­2SbBi monolayer has excellent potential to be the candidate of semiconductor material for spintronics devices.

Strain engineering of Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants: an ab initio study

We study the influence of mechanical deformations on the Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants from first principles. In particular, we perform symmetry-adapted density functional theory simulations with spin–orbit coupling to determine the variation in the electronic band structure splittings with axial and torsional deformations. We find significant effects in molybdenum and tungsten nanotubes, for which the Zeeman splitting decreases with increase in strain, going to zero for large enough tensile/shear strains, while the Rashba splitting coefficient increases linearly with shear strain, while being zero for all tensile strains, a consequence of the inversion symmetry remaining unbroken. In addition, the Zeeman splitting is relatively unaffected by nanotube diameter, whereas the Rashba coefficient decreases with increase in diameter. Overall, mechanical deformations represent a powerful tool for spintronics in nanotubes.

The dynamic surface evolution of halide perovskites induced by external energy stimulation.

Tracking the dynamic surface evolution of metal halide perovskite is crucial for understanding the corresponding fundamental principles of photoelectric properties and intrinsic instability. However, due to the volatility elements and soft lattice nature of perovskites, several important dynamic behaviors remain unclear. Here, an ultra-high vacuum (UHV) interconnection system integrated by surface-sensitive probing techniques has been developed to investigate the freshly cleaved surface of CH3NH3PbBr3 in situ under given energy stimulation. On this basis, the detailed three-step chemical decomposition pathway of perovskites has been clarified. Meanwhile, the evolution of crystal structure from cubic phase to tetragonal phase on the perovskite surface has been revealed under energy stimulation. Accompanied by chemical composition and crystal structure evolution, electronic structure changes including energy level position, hole effective mass, and Rashba splitting have also been accurately determined. These findings provide a clear perspective on the physical origin of optoelectronic properties and the decomposition mechanism of perovskites.

Electric Field Controlled Anisotropic Rashba Splitting in Janus Chromium Dichalcogenide Monolayers : A Computational Study based on Density Functional Theory

In this study, we explore the electronic structure of Janus Chromium Dichalcogenide monolayers CrXY (X ≠ Y = S, Se, Te) in the first Brillouin zone using Density Functional Theory. Due to the mirror symmetry break and strong SOC in the crystal structure of Janus Chromium Dichalcogenide monolayer, we discovered Rashba splitting at Γ points in the first Brillouin zone. We analyze this anisotropy of Rashba splitting using k∙p perturbation theory and group symmetry analysis. By giving the effect of external electric field, we manipulate the characteristics of Rashba splitting on the Janus Chromium Dichalcogenide monolayer. The manipulation of Rashba splitting by applying external electric field, shows that the Janus Chromium Dichalcogenide monolayers has the potential for spintronic devices.

Impact of Chiral Spinterfaces on Magneto-Photoluminescence Effects for Chiral Lead Halide Perovskites.

Chiral lead halide perovskites (LHPs) have been widely investigated in chiroptical spintronics due to their significant Rashba spin-orbit coupling (SOC) and chiral-induced spin selectivity (CISS). Ferromagnet/LHP spinterface stems from the orbital hybridization at the interface of the ferromagnet and the nonmagnetic semiconductor, where interfacial density of state is spin-dependent. By far, the impact of the ferromagnet/chiral LHP spinterface on magneto-photoluminescence (Magneto-PL) of chiral LHPs remains unknown. In this work, we find that the negative and tunable Magneto-PL effects for the pristine LHP bulk film can be drastically enhanced by incorporating ferromagnetic/chiral LHP interfaces. A large Magneto-PL magnitude can reach approximately -13% for the Ni/(S-MBA)2PbI4 interface at the field strengths of ±900 mT. With the assistance of circularly polarized PL spectra, anisotropic magneto-resistance, and X-ray photoelectron spectroscopy measurements, we demonstrate that the ferromagnet/chiral LHP interfaces are chirality/spin-dependent and possess ferromagnetic property due to distinct magnetic switching behavior and electronic orbit coupling at interfaces, which boost the Rashba splitting and spin mixing. The comprehensive effects of Rashba-induced exciton states and chiral-induced SOC at chiral spinterfaces with CISS are responsible for the enhanced Magneto-PL of Ni/(R/S-MBA)2PbI4. It is postulated that the chiral spinterfaces play a dominant role for achieving large and tunable magneto-optical effect of chiral LHPs. This work paves the way for chiroptical spintronic applications.

Unraveling the influence of organic cations on tuning electronic structures and spin-splitting in two-dimensional layered organic-inorganic tin-iodine perovskites.

The solar cell and light-emitting device research community is currently focusing on investigating two-dimensional (2D) hybrid perovskite materials owing to their remarkable stability and intriguing optoelectronic characteristics, which hold significant promise for various applications. In general, the introduction of chirality in hybrid perovskites arises from symmetry breaking within their inorganic frameworks. Nevertheless, despite this understanding, the specific factors driving the observed increase in splitting remain obscure due to a lack of comprehensive investigations. Our research delves into the electronic properties of 2D layered hybrid perovskites, considering their behavior with and without spin-orbit coupling. We specifically focus on effect of Rashba splitting and the impact of electronic structure variation across a range of chiral perovskites by introducing chiral organic cations with differing degrees of π-conjugation, resulting in significant changes in spin-splitting magnitude. Systematic first principles investigations confirm that the distortion of the cage and d-spacing of chiral perovskites are crucial design parameters for achieving strong spin-splitting in 2D layered perovskites. Furthermore, our investigation reveals that these systems exhibit remarkable absorption capabilities in the visible light spectrum, as demonstrated by their computed optoelectronic characteristics. The chiral perovskites described in this study, which exhibit substantial spin-splitting, present a distinctive prototype with potential implications for spintronics and photovoltaics.

Layer-dependent spin texture and origins of Rashba splitting quenching in the 2D CsPbI3 perovskite.

The spin-orbit coupling in inorganic perovskite materials containing heavy elements causes interesting electronic characteristics such as Rashba and Dresselhaus effects. Several studies have reported significant band splitting in the presence of asymmetry, while the impacts of the external field strength, surface termination on the electronic structure still need to be resolved. In the current study, a systematic relation between the external parameters and the band splitting in CsPbI3 slabs is clarified through first-principles calculations. Here, we examine the band splitting and spin patterns of CsPbI3 slabs exposed to an external electric field ranging from zero to hundreds of kV cm-1. Our results indicate apparent non-linear behavior of the Rashba coefficient along with a turning point for the band splitting for a definite external field. Here, the origin of this quench in the band splitting is explained in terms of spatial localization of the wave functions and clear change in their center of charge going from low to high electric fields. The findings not only explain the origin of quenching band splitting but also reveal the external field's significance in the spin texture and recombination rate of 2D perovskites. The current research outcome paves the way for atomic scale engineering of perovskite materials for a wide range of applications.

Spin texture evolution of Rashba splitting under pressure: a case study of inorganic nitride perovskite crystals

Spin–orbit coupling (SOC) effects in non-centrosymmetric systems lead to a uniform spin configuration in momentum space known as persistent spin texture (PST).