Generation of spin currents and spin densities in systems with reduced symmetry

Antisymmetric Magnetoresistance in a CrTe2/Bi2Te3/CrTe2 van der Waals Heterostructure Grown by MBE

The magnetoresistance (MR) of spin valves usually displays a symmetric dependence on the magnetic field. An antisymmetric MR phenomenon has been discovered recently that breaks field symmetry and has the potential to realize polymorphic memory. In this work, centimeter-size and high-quality CrTe2/Bi2Te3/CrTe2 van der Waals (vdWs) heterostructure devices have been prepared using molecular beam epitaxy (MBE). By changing the magnetization direction of the top and bottom layers of CrTe2, an antisymmetric MR effect with high, intermediate, and low resistance states has been found and persists up to 75K. The emergence of this antisymmetric MR phenomenon is attributed to the spin Hall effect, which generates spin currents with both spin-up and spin-down orientations on the upper and lower surfaces of Bi2Te3. The spin currents diffuse or reflect at the Bi2Te3/CrTe2 interfaces alongside the additional charge currents induced by the inverse spin Hall effect (ISHE). Through theoretical calculations, the existence of the antisymmetric MR effect has also been confirmed. Our work emphasizes the use of the MBE technology to grow vdWs heterostructures to explore new physical phenomena and potential applications of spin electronic devices in polymorphic solid-state storage.

Control of Charge-Spin Interconversion in van der Waals Heterostructures with Chiral Charge Density Waves

A charge density wave (CDW) represents an exotic state in which electrons are arranged in a long-range ordered pattern in low-dimensional materials. Although the understanding of the fundamental character of CDW is enriched after extensive studies, its practical application remains limited. Here, an unprecedented demonstration of a tunable charge-spin interconversion (CSI) in graphene/1T-TaS2 van der Waals heterostructures is shown by manipulating the distinct CDW phases in 1T-TaS2. Whereas CSI from spins polarized in all three directions is observed in the heterostructure when the CDW phase does not show commensurability, the output of one of the components disappears, and the other two are enhanced when the CDW phase becomes commensurate. The experimental observation is supported by first-principles calculations, which evidence that chiral CDW multidomains in the heterostructure are at the origin of the switching of CSI. The results uncover a new approach for on-demand CSI in low-dimensional systems, paving the way for advanced spin-orbitronic devices.

Time-Reversal-Even Nonlinear Current Induced Spin Polarization

We propose a time-reversal-even spin generation in second order of electric fields, which dominates the current induced spin polarization in a wide class of centrosymmetric nonmagnetic materials, and leads to a novel nonlinear spin-orbit torque in magnets. We reveal a quantum origin of this effect from the momentum space dipole of the anomalous spin polarizability. First-principles calculations predict sizable spin generations in several nonmagnetic hcp metals, in monolayer TiTe_{2}, and in ferromagnetic monolayer MnSe_{2}, which can be detected in experiment. Our work opens up the broad vista of nonlinear spintronics in both nonmagnetic and magnetic systems.

Intrinsic Nonlinear Electric Spin Generation in Centrosymmetric Magnets

We propose an intrinsic nonlinear electric spin generation effect, which can dominate in centrosymmetric magnets. We reveal the band geometric origin of this effect and clarify its symmetry characters. As an intrinsic effect, it is determined solely by the material's band structure and represents a material characteristic. Combining our theory with first-principle calculations, we predict sizable nonlinear spin generation in single-layer MnBi_{2}Te_{4}, which can be detected in experiment. Our theory opens a new route for all-electric controlled spintronics in centrosymmetric magnets which reside outside of the current paradigm based on linear spin response.

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.

Unconventional Charge-Spin Conversion in Weyl-Semimetal WTe2

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe₂ and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba-Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe₂ , which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe₂ due to a reduced crystal symmetry combined with its large spin Berry curvature, spin-orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.

Large Multidirectional Spin-to-Charge Conversion in Low-Symmetry Semimetal MoTe2 at Room Temperature

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration, unusual configurations are expected in low-symmetry transition-metal dichalcogenide semimetals, which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe₂ flakes, which are stacked in graphene lateral spin valves. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration.

Antisymmetric magnetoresistance in van der Waals Fe3GeTe2/graphite/Fe3GeTe2 trilayer heterostructures

With no requirements for lattice matching, van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next-generation spintronic devices. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in vdW heterostructured Fe₃GeTe₂ (FGT)/graphite/FGT devices. Unlike conventional giant MR (GMR), which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high-, intermediate-, and low-resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three-resistance behavior was attributed to a spin momentum locking induced spin-polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures.

Control of Terahertz Emission by Ultrafast Spin-Charge Current Conversion at Rashba Interfaces

We show that a femtosecond spin-current pulse can generate terahertz (THz) transients at Rashba interfaces between two nonmagnetic materials. Our results unambiguously demonstrate the importance of the interface in this conversion process that we interpret in terms of the inverse Rashba Edelstein effect, in contrast to the THz emission in the bulk conversion process via the inverse spin-Hall effect. Furthermore, we show that at Rashba interfaces the THz-field amplitude can be controlled by the helicity of the light. The optical generation of electric photocurrents by these interfacial effects in the femtosecond regime will open up new opportunities in ultrafast spintronics.

Proximity effects of a symmetry-breaking interface on spins of photoexcited electrons

We study reflection of optically spin-oriented hot electrons as a means to probe the semiconductor crystal symmetry and its intimate relation with the spin-orbit coupling. The symmetry breaking by reflection manifests itself by tipping the net-spin vector of the photoexcited electrons out of the light propagation direction. The tipping angle and the pointing direction of the net-spin vector are set by the crystal-induced spin precession, momentum alignment, and spin-momentum correlation of the initial photoexcited electron population. We examine nonmagnetic semiconductor heterostructures and semiconductor-ferromagnet systems and show the unique signatures of these effects.

Proposal for direct measurement of a pure spin current by a polarized light beam

The photon helicity may be mapped to a spin-1/2, whereby we put forward an intrinsic interaction between a polarized light beam as a "photon spin current" and a pure spin current in a semiconductor, which arises from the spin-orbit coupling in valence bands as a pure relativity effect without involving the Rashba or the Dresselhaus effect due to inversion asymmetries. The interaction leads to linear and circular optical birefringence, which are similar to the Voigt effect and the Faraday rotation in magneto-optics but nevertheless involve no net magnetization. The birefringence effects provide a direct, nondemolition measurement of pure spin currents.