Charge Spin Conversion in Topological Materials and Heterostructures INVITED

Abstract

An outstanding feature of topological quantum materials is their unique spin topology in the electronic band structures with novel charge to spin conversion effects. Here, I will present the novel charge to spin conversion phenomenon in topological insulators, Weyl semimetals, and its van der Waals heterostructures with graphene.The discovery of topological Weyl semimetals has revealed opportunities to realize several extraordinary physical phenomena in condensed matter physics. Weyl semimetals with strong spin orbit coupling, broken inversion symmetry, and novel spin textures are predicted to exhibit a charge to spin conversion effect that can efficiently convert the charge current to a spin current. We report a direct experimental observation of charge-spin conversion and its inverse effect in semimetal WTe2 at room temperature as shown in Fig. 1 (Ref. 1). We also observe an evolution of the charge to spin conversion signals from WTe2 and graphene hybrid device with a geometrical design (Ref. 2). The spin precession measurements of the signal at different gate voltages and ferromagnet magnetization show the robustness of the charge to spin conversion in WTe2 at room temperature. These results can be useful for designing heterostructure devices and in the architectures of 2D spintronic circuits.We could also detect an unconventional charge to spin conversion in WTe2, which is different from the conventional spin Hall and Rashba Edelstein effects (Ref. 3). Such a large and unconventional 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 in the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization in WTe2 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 Weyl materials WTe2 as nonmagnetic spin sources in all electrical van der Waals spintronic circuits and high performance nonvolatile spintronic technologies.Unique electronic spin textures in topological insulators are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to enhancing their performance, electrical gate tunability, interference from trivial bulk states, and heterostructure interfaces. We address some challenges by integrating 2D graphene with a 3D topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity induced spin orbit phenomena (Ref. 4). In these heterostructures, we experimentally demonstrate a gate tunable spin galvanic effect (SGE) at room temperature, allowing for efficient conversion of a nonequilibrium spin polarization into a transverse charge current (Ref. 5). Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin to charge conversion in the graphene and TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin galvanic signal. These findings provide an efficient route for realizing all electrical and gate tunable spin orbit technology using TIs and graphene in heterostructures.Fig. 1. Charge to spin conversion in WTe2 at room temperature. a. Device geometry and Hanle measurement of conventional charge to spin conversion in WTe2 (Ref. 1). B. Measurement of unconventional charge to spin conversion in WTe2. The measurement of spin valve and Hanle signals both in parallel and antiparallel configurations at room temperature show the unconventional nature of charge to spin conversion (Ref 3).Fig. 2. Spin galvanic effect in graphene and topological insulator heterostructure at room temperature. a. A schematic representing the spin galvanic effect, where spin polarized carriers diffuse in the graphene and TI heterostructure, acquire a transverse momentum and produce a charge current. b. A scanning electron microscopy picture of a graphene and TI hybrid device and measurement scheme. c. Schematics of band structures of proximitized graphene develop Rashba spin split bands in the valence and conduction bands. d. The SGE signal R measured with the inplane magnetic field Bx. e. The SGE was detected via spin precession with the out of plane field Bz at different gate voltages in the valence and conduction band of proximitized graphene (Ref 5). **