Photonic spin Hall effect in topological insulators
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In this paper we theoretically investigate the photonic spin Hall effect (SHE) of a Gaussian beam reflected from the interface between air and topological insulators (TIs). The photonic SHE is attributed to spin-orbit coupling and manifests itself as in-plane and transverse spin-dependent splitting. We reveal that the spin-orbit coupling effect in TIs can be routed by adjusting the axion angle variations. We find that, unlike the transverse spin-dependent splitting, the in-plane one is sensitive to the axion angle. It is shown that the polarization structure in the magneto-optical Kerr effect is significantly altered due to the spin-dependent splitting in the photonic SHE. We theoretically propose a weak measurement method to determine the strength of axion coupling by probing the in-plane splitting of the photonic SHE.Keywords:
Topological insulator
Spin–orbit interaction
We show experimentally that the spin direction of the spin current generated by spin-orbit interactions within a ferromagnetic layer can be reoriented by turning the magnetization direction of this layer. We do this by measuring the field-like component of spin-orbit torque generated by an exchange-biased FeGd thin film and acting on a nearby CoFeB layer. The relative angle of the CoFeB and FeGd magnetic moments is varied by applying an external magnetic field. We find that the resulting torque is in good agreement with predictions that the spin current generated by the anomalous Hall effect from the FeGd layer depends on the FeGd magnetization direction $\hat{m}_{FeGd}$ according to $\vec{\sigma}\propto\left ( \hat{y}\cdot \hat{m}_{FeGd} \right )\hat{m}_{FeGd}$, where $\hat{y}$ is the in-plane direction perpendicular to the applied charge current. Because of this angular dependence, the spin-orbit torque arising from the anomalous Hall effect can be non-zero in a sample geometry for which the spin Hall torque generated by non-magnetic materials is identically zero.
Spinplasmonics
Spin pumping
Spin-transfer torque
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The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator MnBi$_2$Te$_4$ has been predicted to host the axion insulator, but the experimental evidence remains elusive. Specifically, the axion insulator is believed to carry "half-quantized" chiral currents running antiparallel on its top and bottom surfaces. However, it is challenging to measure precisely the half-quantization. Here, we propose a nonlocal surface transport device, in which the axion insulator can be distinguished from normal insulators without a precise measurement of the half-quantization. More importantly, we show that the nonlocal surface transport, as a qualitative measurement, is robust in realistic situations when the gapless side surfaces and disorder come to play. Moreover, thick electrodes can be used in the device of MnBi$_2$Te$_4$ thick films, enhancing the feasibility of the surface measurements. This proposal will be insightful for the search of the axion insulator and axion in topological matter.
Topological insulator
Gapless playback
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A ``spin-guide'' source for generation of electric currents with a high degree of spin polarization, which allows long-distance transmission of the spin polarization, is proposed. In the spin-guide scheme, a nonmagnetic conducting channel is interfaced or surrounded by a grounded magnetic shell that transmits electrons with a particular spin direction preferentially, resulting in net polarization of the current flowing through the channel parallel to the interface. It is argued that this method is more effective than spin-filter-like schemes where the current flows perpendicular to the interface between a ferromagnetic metal to a non-magnetic conducting material. Under certain conditions a spin-guide may generate an almost perfectly spin-polarized current, even when the magnetic material used is not fully polarized. The spin guide is predicted to allow the transport of spin polarization over long distances that may exceed significantly the spin-flip length in the channel. In addition, it readily permits detection and control of the spin polarization of the current. The spin guide may be employed for spin-flow manipulations in spintronic devices.
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We investigate electrically-induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.
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Spin orbit interactions give rise to interesting physics phenomena in solid state materials such as the spin Hall effect (SHE) and topological insulator surface states. Those effects have been extensively studied using various electrical detection methods. However, to date most experiments focus only on characterizing electrons near the Fermi surface, while spin-orbit interaction is expected to be energy dependent. Here we developed a tunneling spectroscopy technique to measure spin Hall materials and topological insulators under finite bias voltages. By electrically injecting spin polarized electrons into spin Hall metals or topological insulators using tunnel junctions and measuring the induced transverse voltage, we are able to study SHE in typical 5d transition metals and the spin momentum locking in topological insulators. For spin Hall effect metals, the magnitude of the spin Hall angle has been a highly controversial topic in previous studies. Results obtained from various techniques can differ by more than an order of magnitude. Our results from this transport measurement turned out to be consistent with the values obtained from spin Hall torque measurements, which can help to address the long debating issue. Besides the magnitude, the voltage dependent spectra from our experiment also provide useful information in distinguishing between different potential mechanisms. Finally, because of the impedance matching capability of tunnel junctions, the spin polarized tunneling technique can also be used as a powerful tool to measure resistive materials such as the topological insulators. Orders of magnitude improvement in the effective spin Hall angle was demonstrated through our measurement
Topological insulator
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Three-dimensional (3D) topological insulators are known for their strong spin–orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for "topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggests that pumped spin current is first greatly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR line width broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.
Topological insulator
Spin pumping
Surface States
Rashba effect
Spin–orbit interaction
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We predict a specific type of charge Hall effect in undoped ferromagnetic graphene that is generated by the spin Hall mechanism in the absence of an external magnetic field. The essential feature is the so-called spin chiral configuration of the spin subbands in such a magnetic material where carriers with opposite spin direction are of different type of electron-like or hole-like. Within the semiclassical theory of spin-orbital dynamics of electrons, we obtain that a longitudinal electric field can produce a spin-orbit transverse current of pure charge with no polarization of the spin and the valley.
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The spin-orbit interaction offers an avenue for the electrical generation and manipulation of electron spin polarization in semiconductors without magnetic materials or magnetic fields. In semiconductor heterostructures, the spin-orbit coupling modifies the electron g factor and introduces momentum-dependent spin splittings. In addition, spin-orbit coupling enables the electrical generation of spin polarization through these spin splittings and the spin Hall effect. Here we present an overview of recent measurements of spin dynamics, spin splittings, and electrically generated spin polarization. We demonstrate manipulation of the spin-orbit coupling using electric and magnetic fields to change the orbital motion of the electrons and using strain and quantum confinement to tune the spin splittings in semiconductor heterostructures.
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It is shown that the spin Hall current generates a nonequilibrium spin polarization in the interior of crystals with reduced symmetry in a way that is drastically different from the previously well-known ``equilibrium'' polarization during the spin relaxation process. The steady state spin polarization value does not depend on the strength of spin-orbit interaction offering possibility to generate relatively high spin polarization even in the case of weak spin-orbit coupling.
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Generating spin currents with controllable spin direction is important to spin-torque technology, which includes spin-based computing and magnetic random-access memory, but it is difficult to produce such currents through conventional means, like the spin Hall effect. This study shows that the anomalous Hall effect can be used to yield spin currents with controllable spin polarization. The magnetization of the generating ferromagnetic layer, and hence the spin polarization, is governed by exchange bias and external field. This result offers a great deal of flexibility in creating spin torque, which should enable innovative device designs.
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Spin-transfer torque
Spin pumping
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