Rashba Splitting Research Articles

Isolated fourfold fermion in BiTeI

We use angle-resolved photoemission spectroscopy to revisit the electronic structure of BiTeI, previously identified as a polar semiconductor with giant bulk Rashba splitting. We propose an alternative description, which is based on the experimentally determined crystal structure and agrees well with resistivity, quantum oscillations,and optical measurements. BiTeI emerges as a topological 3D Dirac semimetal hosting only two, well isolated from each other and rest of the band structure, Dirac points. Properly doped bulk material or the controlled synthesis of iodine-terminated surface of the pristine material promise to become a canonical condensed matter system whose physical properties are completely defined by the behavior of fourfold fermions.

Magnetic Doping Induced Strong Circularly Polarized Light Emission and Detection in 2D Layered Halide Perovskite

AbstractThe generation, manipulation, and detection of polarized light are the foundations of spin optoelectronics. Polarized light has a wide range of applications in new‐generation information transmission, storage, and quantum communications. Hybrid organic–inorganic halide perovskites have attracted extensive attention recently because of their strong spin–orbit coupling, Rashba splitting, spin‐dependent optical selection, and have been regarded as promising candidates for application in spin optoelectronic devices. Herein, the authors report the successful synthesis of 2D layered lead halide perovskites (PEA)2PbI4 (PEI) doped with magnetic metal Co2+ and the observation of strong circularly polarized photoluminescence and photoresponse. The introduction of magnetic metals in perovskites can induce an analogous Zeeman effect in the system, which can increase its energy degeneracy, giving rise to an imbalanced population of electronic spin states, thereby enhancing the spin polarization in doped samples. Thus, a maximum circularly polarized PL of 35% is observed at room temperature. Moreover, the achieved Co2+‐doped (PEA)2PbI4 also exhibits a selective photoresponse with circularly polarized light (CPL) emission, and an outstanding anisotropy factor of 0.41 for photocurrent is achieved. The experimental results lay the foundation for the controlled synthesis of magnetically doped perovskites as well as applications for direct CPL detection in spin optoelectronics.

Progress and challenges in layered two-dimensional hybrid perovskites

Dimensionality is the game-changer property of a material. The optical and electronic properties of a compound get dramatically influenced by confining dimensions from 3D to 2D. The bulk 3D perovskite materials have shown remarkable up-gradation in the power conversion efficiency, hence grabbing worldwide attention. But instability against moisture, temperature, and ion migration are the factors constantly back-stabbing and hindering from full-scale commercialization. 2D perovskite material has emerged as an excellent bridging entity between structural-chemical stability, and viable commercialization. Organic–inorganic 2D perovskite materials come with a layered structure in which a large organic cation layer as a spacer is sandwiched between two inorganic metal halide octahedra layers. Moreover, hydrophobic spacer cations are employed which isolate inorganic octahedral layers from water molecules. Hydrophobic spacer cations protect the authentic structure from being degraded. These layered structures occur in two phases namely the Ruddlesden–Popper phase and Dion–Jacobson phase, depending on the spacer cation types. Alternating inorganic and organic layers form multiple quantum wells naturally, along with spin–orbit-coupling gives Rashba splitting. 2D perovskite materials are coming up with interesting chemical, physical properties like exciton dynamics, charge carrier transport, and electron–phonon coupling as a result of the quantum confinement effect. Despite appreciable stability, limited charge transport and large bandgap are limiting the application of 2D perovskite materials in solar cells. These limitations can be overcome by using the concept of 2D/3D multidimensional hybrid perovskites, which includes the long-term stability of 2D perovskite and the high performance of 3D perovskite at the same time. Here in this perspective, we have given brief insight on structural versatility, synthesis techniques, some of the unique photophysical properties, potential device fabrication, and recent advancements in the 2D structure to stand against degradation. Certain shortcomings and future outlooks are also discussed to make the perspective more informative.

Rashba and Dresselhaus effects in two-dimensional Pb-I-based perovskites

Bulk hybride halide perovskites are governed by significant Rashba and Dresselhaus splitting. This indicates that such effects will not only affect their optoelectronic properties, but also those of their two-dimensional layered relatives. This work aims at understanding how different ways of symmetry breaking influence these effects in those materials. For this purpose, model structures are adopted where the organic compounds are replaced by Cs atoms. Disregarding possible distortions in the inorganic layers, results in structures with composition ${\mathrm{Cs}}_{n+1}{\mathrm{Pb}}_{n}{\mathrm{I}}_{3n+1}$. Using the all-electron full-potential density-functional-theory code exciting, the impact of atomic displacement on the band structure is systematically studied for $n=1$, 2, 3, and $\ensuremath{\infty}$. The displacement patterns that yield Rashba or Dresselhaus splitting are identified, and the amount of the splitting is determined as a function of displacement. Furthermore, the spin textures in the electronic states around the band gap are analyzed to differentiate between Rashba and Dresselhaus effects. This study reveals in-plane Pb displacements as the origin of the strongest effects.

Rashba Spin Splitting in HgCdTe Quantum Wells with Inverted and Normal Band Structures.

In quantum wells (QWs) formed in HgCdTe/CdHgTe heterosystems with a variable composition of Cd(Hg), Shubnikov-de-Haas (SdH) oscillations are investigated to characterize the Rashba-type spin-orbit coupling in QWs with both a normal and inverted band structure. Several methods of extracting the Rashba spin-splitting at zero magnetic field and their magnetic field dependences from the beatings of SdH oscillations are used for greater reliability. The large and similar Rashba splitting (25–27 meV) is found for different kinds of spectrum, explained by a significant fraction of the p-type wave functions, in both the E1 subband of the sample with a normal spectrum and the H1 subband for the sample with an inverted one.

Theoretical study of spin-orbit coupling in Janus monolayer MA2Z4

Abstract In this paper, based on first-principles calculations, we investigate the energy valley and spin-orbit coupling properties of Janus monolayer MA2Z4. The stability of different structures is illustrated. Due to the breaking of mirror symmetry, Rashba splitting occurs at the Γ point in the band structures of Janus monolayers WSiGeN4 and WSi2(NP)2. The relationship between Rashba spin-orbit coupling strength and potential energy gradient and d-orbital composition is explored. Janus monolayer WSi2(NP)2 has stronger Rashba effect than WSiGeN4 due to the strong asymmetry of the d orbital of W atom. These results help to promote the application of two-dimensional materials in spintronics and valleytronics.

Rashba splitting in organic\u2013inorganic lead\u2013halide 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.

Giant valley-polarized spin splittings in magnetized Janus Pt dichalcogenides

We reveal giant proximity-induced magnetism and valley-polarization effects in Janus Pt dichalcogenides (such as SPtSe), when bound to the Europium oxide (EuO) substrate. Using first-principles simulations, it is surprisingly found that the charge redistribution, resulting from proximity with EuO, leads to the formation of two K and K$^{'}$valleys in the conduction bands. Each of these valleys displays its own spin polarization and a specific spin-texture dictated by broken inversion and time-reversal symmetries, and valley-exchange and Rashba splittings as large as hundreds of meV. This provides a platform for exploring novel spin-valley physics in low-dimensional semiconductors, with potential spin transport mechanisms such as spin-orbit torques much more resilient to disorder and temperature effects.

Controllable spin direction in nonmagnetic BX/MX2 (M = Mo or W; X = S, Se and Te) van der Waals heterostructures by switching between the Rashba splitting and valley polarization

A schematic of the spin FET based on the BSe/MoSe2 heterostructure. The Rashba-type spin splitting can be evoked by a positive electric field and the spin/valley Hall effect is controlled by a negative electric field.

Molybdenum as a versatile dopant in SnTe: a promising material for thermoelectric application

Electronic structure engineering of SnTe by doping with molybdenum results in an increase in the band gap, valence band convergence, introduction of resonance levels, Rashba splitting and multiband transport, leading to enhanced thermoelectric performance.

Pseudo Janus based on rhombohedral homobilayer transition metal dichalcogenides

Out-of-plane mirror symmetry breaking has enabled pseudo Janus structures as a platform to explore the coupling between electric polarization, piezoelectricity, spin–orbit coupling, and Rashba splitting in a two-dimensional (2D) system.

Nanoscale phase separation in the oxide layer at GeTe (111) surfaces.

As a semiconductor ferroelectric, GeTe has become a focus of renewed attention due to the recent discovery of giant Rashba splitting. It already has a wide range of applications, from thermoelectricity to data storage. Its stability in ambient air, as well as the structure and properties of an oxide layer, define the processing media for device production and operation. Here, we studied a reaction between the GeTe (111) surface and molecular oxygen for crystals having solely inversion domains. We evaluated the reaction kinetics both ex situ and in situ using NAP XPS. The structure of the oxide layer is extensively discussed, where, according to HAADF-STEM and STEM-EDX, nanoscale phase separation of GeO2 and Te is observed, which is unusual for semiconductors. We believe that such behaviour is closely related to the ferroelectric properties and the domain structure of GeTe.

Manipulation of Rashba splitting and thermoelectric performance of MTe (M = Ge, Sn, Pb) via Te off-centering distortion.

The Rashba effect is an essential phenomenon in asymmetrical ferroelectric materials with local dipole fields. Rashba spin splitting due to spin–orbit coupling in the asymmetrical 2D ferroelectricity MTe (M = Ge, Sn, Pb) has provided a promising arena for achieving an ultra-high power factor to improve their thermoelectric performance. Herein, we show that the hidden Rashba effect may exist in centrosymmetric rock-salt MTe because of the emerging macroscopic electric dipoles caused by the local Te off-centering distortion. Using first-principles calculations, we prove that Te off-centering causes a change in the atomic reciprocal displacement and induces a ferroelectric Rashba effect in rock-salt MTe. Further analyses of the energy evaluation, lattice vibration and chemical orbital confirm that the atomic off-centering behavior manipulates Rashba spin splitting and stereochemical irregularities of the s2 lone electron pair of the cation, leading to the formation of strong anharmonic bonds in the original high-symmetry crystalline solids. The changes of the phonon dispersion and electronic structure also affect the electron–phonon scattering and scattering mechanism of MTe, which then manipulates the electrical transport properties. This work unravels the underlying physics on how the local Te off-centering distortion manipulates Rashba spin splitting and improves the thermoelectric performance of MTe.

Giant rashba splitting of confined Te chains in nanotubes: the size-, chirality-, and type- effects of nanotubes

Understanding the coupling between one-dimensional (1D) materials and their protective materials is essential for developing nanodevices. Herein, we investigate the effect of the size, chirality, and type of nanotubes [such as carbon/boron nitride nanotubes (CNTs/BNNTs)] on the atomic and electronic structures of confined Te chains using density functional theory. We find that the optimal configurations of the Te chains confined in CNTs/BNNTs depend strongly on the size of the nanotubes but weakly on their chirality and type. Furthermore, the Te@BNNTs exhibit giant Rashba splitting with a Rashba constant of up to 2.65 eV Å, while the Te@CNTs show no splitting. This is mainly due to the large bandgap of the BNNTs, as well as the enhanced symmetry breaking of the Te chains when confined. Our findings provide a basis for the design of nano spin devices through protective materials.

Enhanced Rashba Indirect Exciton Emission in 2D Dion–Jacobson Perovskite Microplates via Efficient Photon Recycling

Abstract2D Dion–Jacobson perovskite (4AMP)PbI4 (4AMP = 4‐(aminomethyl)piperidinium) (DJP) has recently attracted considerable attention due to its remarkable Rashba effect. Upon Rashba splitting in the momentum space of DJP, it is significant to identify the momentum‐indirect excitons (IXs) through photoluminescence spectroscopy and explore the photophysics influenced by the Rashba effect. In this work, a space‐confined aqueous method to synthesize single‐crystalline DJP microplates with uniform rectangular morphology is developed. Functioning as whispering‐gallery‐mode cavities, the as‐grown microplates exhibit greatly enhanced Rashba IX emission at all temperatures investigated through efficient photon recycling in the Rashba indirect gap. The efficient photon recycling can substantially extend the exciton transport length, which is conducive to light‐harvesting devices. This study not only provides a convenient method to observe Rashba states but also sheds light on the future applications of DJP in optoelectronics and optospintronics.

Externally controlled and switchable two-dimensional electron gas at the Rashba interface between ferroelectrics and heavy d metals

Strong spin-orbit coupling in noncentrosymmetric materials and interfaces results in remarkable physical phenomena, such as nontrivial spin textures, which may exhibit Rashba, Dresselhaus, and other intricated configurations. This provides a promising basis for nonvolatile spintronic devices and further implications. Here, we simulate from first principles a two-dimensional electron gas in ultrathin platinum and palladium layers grown on ferroelectric PbTiO$_3$(001). The latter allows, in principle, to switch and control the spin-to-charge conversion by the polarization reversal. We show how the band structure and its Rashba splitting differ in the Pt and Pd overlayers and how these electronic features change with increasing the overlayer thickness and upon reversal of polarization. Besides, for both overlayers, we simulated their current-voltage ($I-V$) characteristics, the resistance of which upon the polarization reversal changes between 20% and several hundred percent. The reported findings can be used to model directly the Rashba-Edelstein effect.

Partial gap in two-leg ladders with Rashba effect and its experimental signatures in Si(553)-Au

We study the effects of Rashba splitting on two-leg ladders with weakly screened Coulomb interactions. Past research has shown that in this class of systems the two backscattering channels with the largest amplitude favor ordering of canonically conjugated collective fields which effectively renders the system gapless. Here we show that the band-dependent Rashba spin-orbit interaction breaks this symmetry of scattering channels, leading to a new fixed point with yet unexplored instabilities. Exotic properties can be found, for instance, the mixing of the magnetism with the charge-density wave. We then investigate the physical consequences of this partial spectral gap opening. We find a striking difference in signatures of order observed in single-particle (spectral-function) and two-body (susceptibilities) experimental probes. We conclude this paper by comparing theoretical and experimental results obtained on the Au-Si(553) platform. In STM measurements, we identify the lowest-lying soliton excitation as a hallmark of the gapped sine-Gordon model in an apparently metallic system. This implies the presence of partial spectral gaps opening. Furthermore, by ARPES measurements we confirm the expected temperature dependence of the outer bands backfolding. These two findings constitute the experimental evidence of the many-body physics proposed here.

Spin-charge conversion in ferromagnetic Rashba states

We show here theoretically and experimentally that a Rashba-split electron state inside a ferromagnet can efficiently convert a dynamical spin accumulation into an electrical voltage. The effect is understood to stem from the Rashba splitting but with a symmetry linked to the magnetization direction. It is experimentally measured by spin pumping in a $\mathrm{CoFeB}|\mathrm{MgO}$ structure where it is found to be as efficient as the inverse spin Hall effect at play when Pt replaces MgO, with the extra advantage of not affecting the magnetic damping in the ferromagnet.

Nonreciprocal Transport in a Rashba Ferromagnet, Delafossite PdCoO2.

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO2, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 ± 0.3 eV Å and argue that our results qualify delafossites as a new family of oxides for nanospintronics and spin-orbitronics, beyond perovskite materials.

First‐Principles Study of the Electronic and Optical Properties of Bi2Se3/MoSe2 Heterojunction

MoSe2 and Bi2Se3 are two kinds of 2D materials that are gradually receiving more attention because of their unique electronic and optical properties. Herein, a Bi2Se3/MoSe2 van der Waals heterojunction (vdWH) is constructed and the electronic and optical properties of the heterojunction are calculated using first‐principles calculations. The effects of the external electric field and the interlayer distance on the electronic properties of the heterojunction are also studied. The calculated results show that the inherent Bi2Se3/MoSe2 vdWH has a type‐2 band alignment with a very small indirect bandgap (28 meV). It also has strong spin–orbit coupling effects, and the characteristics of Zeeman splitting and Rashba splitting are observed at the same time. Both applying an external electric field and changing the interlayer distance can effectively modulate the band structure of the heterojunction; these modulation methods can change the band alignment of the vdWH from type‐2 to type‐3 or even type‐1, and the bandgap type can be changed from indirect to direct. The infrared absorption of the heterojunction is much higher than that of Bi2Se3 and MoSe2. All the calculation results show that the Bi2Se3/MoSe2 vdWH has good application prospects in optoelectronic and spintronic devices.