Spin Hall Effect Research Articles

Emerging Nonlinear Photocurrents in Lead Halide Perovskites for Spintronics.

Lead halide perovskites (LHPs) containing organic parts are emerging optoelectronic materials with a wide range of applications thanks to their high optical absorption, carrier mobility, and easy preparation methods. They possess spin-dependent properties, such as strong spin-orbit coupling (SOC), and are promising for spintronics. The Rashba effect in LHPs can be manipulated by a magnetic field and a polarized light field. Considering the surfaces and interfaces of LHPs, light polarization-dependent optoelectronics of LHPs has attracted attention, especially in terms of spin-dependent photocurrents (SDPs). Currently, there are intense efforts being made in the identification and separation of SDPs and spin-to-charge interconversion in LHP. Here, we provide a comprehensive review of second-order nonlinear photocurrents in LHP in regard to spintronics. First, a detailed background on Rashba SOC and its related effects (including the inverse Rashba-Edelstein effect) is given. Subsequently, nonlinear photo-induced effects leading to SDPs are presented. Then, SDPs due to the photo-induced inverse spin Hall effect and the circular photogalvanic effect, together with photocurrent due to the photon drag effect, are compared. This is followed by the main focus of nonlinear photocurrents in LHPs containing organic parts, starting from fundamentals related to spin-dependent optoelectronics. Finally, we conclude with a brief summary and future prospects.

Tunable quantized spin Hall effect of light in graphene

The spin Hall effect of light is an appealing toolset for metrology that holds incredible potential for precision measurements due to its high sensitivity to the refractive index and nanostructure physical parameters. Here, we theoretically examine the quantized spin Hall effect of light on the surface of a monolayer graphene–substrate system subjected to an external perpendicular magnetic field. We discuss the impact of Landau levels induced by the external magnetic field B and the photon energy on magneto-optical conductivities and Fresnel reflection coefficients. We find that the spin-dependent splittings in graphene exhibit quantized characteristics due to the Landau levels quantization of the magneto-optical conductivities and Fresnel’s reflection coefficients near the Brewster angle. Moreover, the spin-dependent shifts are quantized and show oscillatory behavior with changes in the magnetic field and the photon energy. Our results manifest that the quantized photonic spin-dependent separations are sensitive to variations in the incidence angle, magnetic field, and incident photonic energies. The quantized spin-dependent splitting opens a promising route to find the quantized Hall conductivity and discrete Landau levels in the monolayer graphene by a direct optical weak measurement.

Manipulating chiral spin transport with ferroelectric polarization.

A magnon is a collective excitation of the spin structure in a magnetic insulator and can transmit spin angular momentum with negligible dissipation. This quantum of a spin wave has always been manipulated through magnetic dipoles (that is, by breaking time-reversal symmetry). Here we report the experimental observation of chiral spin transport in multiferroic BiFeO3 and its control by reversing the ferroelectric polarization (that is, by breaking spatial inversion symmetry). The ferroelectrically controlled magnons show up to 18% modulation at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adjacent magnets, with a spin-torque efficiency comparable to the spin Hall effect in heavy metals. Utilizing such controllable magnon generation and transmission in BiFeO3, an all-oxide, energy-scalable logic is demonstrated composed of spin-orbit injection, detection and magnetoelectric control. Our observations open a new chapter of multiferroic magnons and pave another path towards low-dissipation nanoelectronics.

Theoretical proposal for actively manipulating optical differential operation in the quasi-PT symmetric structure

Optical differential operation is an important scheme for image edge detection due to its advantages of high efficiency, real time and low consumption. In this paper, the actively manipulating optical differential operation is proposed in a quasi-PT-symmetric structure containing graphene, and applied to realize edge imaging with tunable contrast. It is found that there are abrupt changes of the in-plane shift and transverse shift in the photonic spin Hall effect at the Fermi energy of 0.4 eV, where the quasi-exceptional points exist according to the degeneracy analysis of scattering matrix eigenvalues. Based on the photonic spin Hall effect near the quasi-exceptional points, we design the optical differential operation whose spatial transfer function is actively manipulated by changing the external electric field to adjust the Fermi energy of graphene. Finally, the edge imaging with different contrast can also be achieved when the optical differential operation is applied to image processing. The actively manipulating optical differential operation opens up new possibilities in microscopic imaging and smart driving.

Highly efficient field-free switching of perpendicular yttrium iron garnet with collinear spin current

Yttrium iron garnet, a material possessing ultralow magnetic damping and extraordinarily long magnon diffusion length, is the most widely studied magnetic insulator in spintronics and magnonics. Field-free electrical control of perpendicular yttrium iron garnet magnetization with considerable efficiency is highly desired for excellent device performance. Here, we demonstrate such an accomplishment with a collinear spin current, whose spin polarization and propagation direction are both perpendicular to the interface. Remarkably, the field-free magnetization switching is achieved not only with a heavy-metal-free material, Permalloy, but also with a higher efficiency as compared with a typical heavy metal, Pt. Combined with the direct and inverse effect measurements, we ascribe the collinear spin current to the anomalous spin Hall effect in Permalloy. Our findings provide a new insight into spin current generation in Permalloy and open an avenue in spintronic devices.

Harnessing Interlayer Magnetic Coupling for Efficient, Field‐Free Current‐Induced Magnetization Switching in a Magnetic Insulator

Owing to the unique features of low Gilbert damping, long spin‐diffusion lengths, and zero Ohmic losses, magnetic insulators are promising candidate materials for next‐generation spintronic applications. However, due to the localized magnetic moments and the complex metal–oxide interface between magnetic insulators and heavy metals, spin‐functional Dzyaloshinskii–Moriya interactions or spin Hall and Edelstein effects are weak, which diminishes the performance of these typical building blocks for spintronic devices. Herein, the exchange coupling between metallic and insulating magnets is exploited for efficient electrical manipulation of heavy metal/magnetic insulator heterostructures. By inserting a thin Co layer, the spin‐orbit torque efficiency is enhanced by more than 20 times, which significantly reduces the switching current density. Moreover, field‐free current‐induced magnetization switching caused by a symmetry‐breaking non‐collinear magnetic texture is demonstrated. This work launches magnetic insulators as an alternative platform for low‐power spintronic devices.

Ultra-high order mode-assisted optical differentiator for edge detection with high tunability

An optical spatial differentiator based on the photonic spin Hall effect (PSHE) with high tunability is presented. By utilizing the characteristics of ultra-high order modes in the symmetrical metal cladding waveguide, the Fresnel reflection coefficient spectrum exhibits a narrow peak width and low trough at the resonant incident angles, resulting in high sensitivity to changes in the incident angle-induced spatial shift caused by the PSHE (the highest ∂(|r s /r p |)/∂θ value can reach 107). After polarization transformation and extinction, the output field demonstrates differential operation with respect to the input field. When applied to edge detection, our differentiator can achieve tunable resolution edge images by adjusting the incident angle. Our proposed edge detection scheme has potential applications for cellular and molecular imaging through two-dimensional extension via the target rotation.

Photonic Spin‐Hall Logic Devices Based on Programmable Spoof Plasmonic Metamaterial

AbstractThe entanglement of the momentum of light with its spin at interfaces or inside structured media, known as the photonic spin‐Hall effect, holds great promise for various applications, such as beam splitting, focusing, and polarization detection. However, the photonic spin‐Hall effect remains unexplored in the field of logic operation. In this work, the photonic spin‐Hall effect of spoof surface plasmon polaritons (SSPPs) in programmable metamaterial is demonstrated. Moreover, photonic spin‐Hall logic devices based on programmable spoof plasmonic metamaterial are designed, enabling the control of energy flow through the utilization of both spin and digital coding, with examples including SSPPs logic gates such as the “AND” gate, the “NIMPLY” gate (A AND NOT B), the “OR” gate, and the “NOT” gate. The findings introduce the combination of digital coding metamaterial with the photonic spin Hall effect, which offers a powerful and flexible platform for controlling electromagnetic waves in information processing.

Photonic spin Hall effect under the action of a full-angle three-dimensional magnetic field in space

In this paper, we have measured the magneto-optical spin Hall effect influenced by an arbitrary oriented magnetic field using CeDyAl thin films on Gd3Ga5O12 (GGG) substrates as reflective materials. Plots of the variation of the magneto-optic spin Hall effect (MOPSHE) with magnetic field strength, the hysteresis line, in three different unit-direction magnetic fields are simulated and experimentally measured. The comparison of the simulation results with the experiments allows the method of weak measurement to determine the MOPSHE under the influence of the magnetic field to be validated. MOPSHE under a three-dimensional magnetic field is measured and the measurement error is within 4 μm.

Momentum-dependent Pancharatnam-Berry phase enabled in- and out-of-plane photonic spin-Hall shifts

The photonic spin-Hall effect (PSHE) is a result of the interaction of spin and orbital angular momentum (OAM). However, previous research on the PSHE has primarily focused on out-of-plane (transverse) shifts. In fact, it encompasses both out-of-plane shifts and relatively weaker in-plane shifts, and the latter of which plays a crucial role in controlling the direction of spin separation. Here, we investigate the in- and out-of-plane shifts of the PSHE based on a full-wave theory, revealing that they have the same physical mechanism. The in- and out-of-plane displacements of the PSHE are determined by a momentum-dependent Pancharatnam-Berry (PB) phase, where the in-plane wavevector kx-dependent phase gradient determines the in-plane shift, and the transverse wavevector ky-dependent phase gradient dictates the out-of-plane shift. We discover that the intrinsic OAM (IOAM) and extrinsic OAM (EOAM) carried by incident off-axis vortex beams can respectively convert into the EOAM and IOAM of abnormal modes in the transmitted beam. The OAM generated during the spin-reversal process, together with the OAM carried by the incident beam, collectively determines the in- and out-of-plane displacements. When off-axis vortex beams are incident, the competition between the IOAM and EOAM is more complex than in those without OAM. Our study unifies in- and out-of-plane shifts into a single theoretical framework, which offers an alternative perspective on the spin–orbit interaction of light.

Growth of thin-film magnetic nanostructures promising for spintronics applications

Multilayered metallic nanostructures are promising for the fabrication of spin valves based on the giant magnetoresistive effect and for studies of the nature of topological magnetism, aimed at the development of new nanoscale data storage and transfer devices, e.g. those based on magnetic skyrmions. It is still an important task to develop methods of synthesis and configuration of thin-film nanostructures and control of spin textures in those nanostructures under electric and spin currents generated as a result of the spin Hall effect in external electric fields. Thin-film polycrystalline ferromagnetic / heavy metal Ru(10nm)/Co(0,8)/Ru(2), Ru(10)/Co(0,8)/Ru(2)/W(4), Pt(5)/Co(0,8)/MgO(2)/Pt(2) and Pt(15)/Co(0,8)/MgO(2)/Pt(2) nanostructures have been synthesized using magnetron sputtering. Electric contacts and Hall structures with different conductive bridge thicknesses have been synthesized on the specimens using electron beam photolithography. Experimental vibration magnetometric data have been utilized to calculate magnetic parameters of the specimens, i.e., saturation magnetization, magnetic anisotropy energy and field and coercive force as functions of ferromagnetic and heavy metal layer types. The domain structure of the specimens has been studied using Kerr microscopy. The electrical resistivity has been simulated and the critical current and current density of the nanostructures have been assessed. We show that all the film specimens exhibit perpendicular magnetic anisotropy and can be used in the studies of current-induced phenomena and spin moment transfer processes in nanostructures.

Intrinsic spin Hall effect in topological semimetals with single Dirac nodal ring

Intrinsic spin Hall effect in topological semimetals with single Dirac nodal ring

Spin Angular Momentum at the Focus of a Superposition of an Optical Vortex and a Plane Wave with Linear Polarizations

In this paper, tight focusing of a superposition of a vortex laser beam with topological charge n with linear polarization and a plane wave with the same linear polarization directed along the horizontal axis is considered. Using the Richards–Wolf formalism, analytical expressions are obtained for the intensity distribution and longitudinal projection of the spin angular momentum in the focal plane. It is shown that for even and odd numbers n, the intensity and the spin angular momentum have different symmetries: for even n they are symmetric about both Cartesian axes, and for odd n they are symmetric only about the vertical axis. The intensity distribution has n local maxima at the focus, and it is nonzero on the optical axis for any n. The distribution of the longitudinal spin angular momentum (spin density) in the focal plane has (n + 2) subwavelength regions with a positive spin angular momentum and (n + 2) regions with a negative spin angular momentum, the centers of which alternately lie on a circle of a certain radius with a center on the optical axis. This spin distribution with different signs demonstrates the spin Hall effect at the focus. Negative and positive spins are mutually compensated, and the total spin is equal to zero at the focus. We have shown that by changing the topological charge of the optical vortex, it is possible to control the spin Hall effect at the focus, that is, to change the number of regions with spins of different signs.

Spin-splitting and spin Hall effect of reflected Airy beams on the surface of topological insulator

We present an analysis of the enhanced spin Hall effect of light (SHEL) resulting from a reflected Airy beam off the interface of an air-topological insulator (TI). Our study suggests that the surface conductivity of TI film and the decay factors of Airy beams are crucial in increasing the spin-splitting distance and the spatial shifts of the left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) components. An apparent spatial shift asymmetry depending on the decay factors can be observed between the LCP and RCP components, where the coupling of intrinsic and extrinsic angular momentum will play a significant impact. The connection among the thickness of TI film, chemical potential and the incident frequency at the fixed decay factors is thoroughly examined to achieve the maximum of spatial shifts. We find that the coupling between the upper and lower surfaces’ conductivity could be responsible for the pronounced SHEL. These conclusions can be extended to other interfaces of comparable structures, such as insulators coated with a layer of two-dimensional Dirac material. Our work provides novel insights into the use of innovative beams incident on the surface of TI film and their potential applications in optical detection.

Quantum-Geometry-Induced Anomalous Hall Effect in Nonunitary Superconductors and Application to Sr_{2}RuO_{4}.

The polar Kerr effect and the closely related anomalous charge Hall effect are among the most distinguishing signatures of the superconducting state in Sr_{2}RuO_{4}, as well as in several other compounds. These effects are often thought to be derived from chiral superconducting pairing, and different mechanisms have been invoked for the explanation. However, the intrinsic mechanisms proposed previously often involve unrealistically strong interband Cooper pairing. We show in this Letter that, even without interband pairing, nonunitary superconducting states can support the intrinsic anomalous charge Hall effect, thanks to the quantum geometric properties of the Bloch electrons. The key here is to have a normal-state spin Hall effect, for which a nonzero spin-orbit coupling is essential. A finite charge Hall effect then naturally arises at the onset of a spin-polarized nonunitary superconducting pairing. It depends on both the spin polarization and the normal-state electron Berry curvature, the latter of which is the imaginary part of the quantum geometric tensor of the Bloch states. Applying our results to the weakly paired Sr_{2}RuO_{4} we conclude that, if the reported Kerr effect is of intrinsic origin, the superconducting state is most likely nonunitary and has odd parity. Our theory may be generalized to other superconductors that exhibit the polar Kerr effect.

Active manipulation of the plasmonic induced asymmetric photonic spin Hall effect

The asymmetric photonic spin Hall effect (APSHE) induced by surface plasmon polaritons in a graphene-based structure is actively manipulated by external magnetic field and electric field. It is revealed that the spin-dependent splitting exhibits spatio-temporal asymmetric property due to the involvement of the anisotropic graphene. The peak of asymmetry degree in APSHE at the position of reflectance valley corresponds toward a smaller incident angle with the increase of magnetic field intensity or Fermi energy, which is attributed to the tunability of reflectance for the graphene-based structure. Based on the asymmetric splitting shift, a potential application is proposed for detecting low concentration gas molecules and the detection resolution can be dynamically tunable by changing the magnetic field intensity and Fermi energy. This study may provide a new reference in the fabrication of graphene-based plasmonic sensor devices.

High-sensitivity determination of ethanol concentration based on weak measurement system with Faraday effect

A high-sensitivity detection method is proposed for ethanol concentration based on weak measurement system with Faraday effect, and the improvement law of the sensitivity is studied. Different from chiral solution determination, the post-selection angle of the system is modified by the Faraday rotation angle of ethanol solution. The amplified photonic spin Hall effect acts as a probe for detecting the ethanol concentration in the system, and the sensitivity of ethanol concentration increases linearly with the increasing magnetic field intensity. Then we use the method to determine the ethanol concentration of commercial liquor, whose determination values are consistent with the label values and Raman spectroscopy results. Compared to other methods such as Raman spectrometry, the method has the advantages of real time, non-invasiveness, low cost and high sensitivity. Finally, the sensitivity of ethanol concentration can be further improved by designing of a suitable BK7-medium structure, and the sensitivity of 9.72 /vol% is achievable for the range 0 vol% ∼100 vol% of ethanol solutions.

High-order photonic spin Hall effect and its application in high-contrast imaging

High-order photonic spin Hall effect and its application in high-contrast imaging

Rashba spin-splitting and spin Hall effect in Janus monolayers Sb2XSX’ (X, X’= S, Se, or Te; X ≠ X’)

The combined influence of spin–orbit coupling and spatial inversion asymmetry leads to an enhancement of electronic properties, including Rashba spin-splittings as well as spin Hall effect. Recent research has shown the possibility to create two-dimensional Janus materials with inherent structural asymmetry. In this work, the structural stability, piezoelectricity, electronic properties, and intrinsic spin Hall conductivity of quintuple-layer atomic Janus Sb2XSX’ (X, X’ = S, Se, Te; X ≠ X’) monolayers are investigated using first-principles calculations within the framework of density functional theory. They demonstrate relatively high in-plane piezoelectric coefficients (d22) and also possess out-of-plane piezoelectric coefficients (d31), which is due to the breaking of inversion symmetry in the crystal structure with the space group P3m1. Large Rashba parameters are obtained in Janus Sb2XSX’ monolayers, especially high for Sb2S2Te (1.62 eV Å) and Sb2SeSTe (1.33 eV Å) due to strong spin–orbit coupling. Moreover, Rashba-like spin-splitting is also observed in the edge-states as well, which is highest for Sb2SeSTe with 2.17 eV Å. Furthermore, Sb2S2Te and Sb2SeSTe monolayers reveal a significantly high Berry curvature (65.59 and 61.05 Bohr2), spin Berry curvature (−118.4 and −120.6 Bohr2), and spin Hall conductivity (1.8 and 1.6 e2/h). Our results suggest that Janus Sb2S2Te and Sb2SeSTe monolayers could be an excellent platform for multifunctional electronic applications.

Evidence of the fractional quantum spin Hall effect in moiré MoTe2.

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap similar to an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities1-6. So far, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G0 = e2/h (with e and h denoting the electron charge and Planck's constant, respectively)7-14. Here we report transport evidence of a fractional QSH insulator in 2.1° twisted bilayer MoTe2, which supports spin-Sz conservation and flat spin-contrasting Chern bands15,16. At filling factor ν = 3 of the moiré valence bands, each edge contributes a conductance with zero anomalous Hall conductivity. The state is probably a time-reversal pair of the even-denominator 3/2-fractional Chern insulators. Furthermore, at ν = 2, 4 and 6, we observe a single, double and triple QSH insulator with each edge contributing a conductance G0, 2G0 and 3G0, respectively. Our results open up the possibility of realizing time-reversal symmetric non-abelian anyons and other unexpected topological phases in highly tunable moiré materials17-19.