Observation of charge to spin conversion in Weyl semimetal WTe2 at room temperature

Abstract

Discovery of topological Weyl semimetals has revealed the opportunities to realize several extraordinary physical phenomena in condensed matter physics. Specifically, these semimetals with strong spin-orbit coupling, broken inversion symmetry and novel spin texture are predicted to exhibit a large spin Hall effect that can efficiently convert the charge current to a spin current. Here we report the direct experimental observation of a large spin Hall and inverse spin Hall effects in Weyl semimetal WTe2 at room temperature obeying Onsager reciprocity relation. We demonstrate the detection of the pure spin current generated by spin Hall phenomenon in WTe2 by making van der Waals heterostructures with graphene, taking advantage of its long spin coherence length and spin transmission at the heterostructure interface. These experimental findings well supported by ab initio calculations show a large charge-spin conversion efficiency in WTe2; which can pave the way for utilization of spin-orbit induced phenomena in spintronic memory and logic circuit architectures. Figure S1. Devices with WTe2-graphene heterostructure. (a) Optical micrograph of Device 1, with WTe2graphene heterostructure and ferromagnetic (TiO2 1 nm/Co 60 nm) contacts on graphene for detection and creation of (I)SHE in WTe2. The scale bar is 2 μm. (b) Atomic force microscope (AFM) picture of the heterostructure area (red mark in (a)). The inset is the thickness profile of WTe2 on few-layer graphene along the white dash line showing the WTe2 thickness is 27nm in Device 1. (c) AFM picture and thickness profile of Dev 3 (after the device burnout) showing WTe2 thickness to be 11 nm on monolayer CVD graphene.

Unconventional Charge–Spin Conversion in Weyl‐Semimetal WTe2

“…[ 17 ] In the measured devices, the heterostructures of WTe 2 with graphene show good contact properties with interface resistance in the range of 1–3 kΩ (see Figure S1b, Supporting Information), which is an order of magnitude higher than used for detection of conventional CSC in WTe 2 . [ 12 ] The standard spin injection and detection behavior of ferromagnetic tunnel contacts and spin transport properties of graphene was confirmed, as shown in Figure S2 (Supporting Information).…”

“…Importantly, the detected spin polarization in WTe 2 is found to be different from the conventional spin Hall effect (SHE) and 2D Rashba–Edelstein effect (REE). [ 12 ] Furthermore, the detection of both the unconventional CSC and ICSC prove the robustness of spin polarization in WTe 2 obeying Onsager reciprocity relation and provides a new method for utilization of spin current in graphene for an all‐electrical van der Waals spintronic device at room temperature.…”

“…WTe 2 hosts unique transport phenomena such as chiral anomaly, [ 6,7 ] unconventional quantum oscillations, [ 8 ] colossal magnetoresistance, [ 9 ] spin–orbit torque, [ 10,11 ] substantial spin Hall effect [ 12 ] and quantum spin Hall states in monolayers, [ 13 ] which opens a new era for physics experiments. Most importantly, novel spin textures have been discovered in WSMs by photoemission experiments, showing spin polarization of Fermi pockets in bulk bands and Fermi arc surface states.…”

“…For the measurement of the unconventional charge–spin conversion effects, an electric current is applied vertically through the WTe 2 flake, which generates and injects a spin current into the graphene channel (Figure 1d). To be noted, here, we used a bias current across the WTe 2 –graphene junction, which is different from the conventional CSC measurement configuration reported earlier [ 12 ] (see details in Note S1, Supporting Information). The injected spin polarization from WTe 2 is detected by a ferromagnetic contact (FM) after transport in a graphene channel by a NL measurement method.…”

“…In a WTe 2 –graphene hybrid device, the source of the spin polarization can have several origins, such as the spin Hall effect (SHE) and Edelstein effect (EE) from the bulk WTe 2 , Rashba–Edelstein effect (REE) from the surface states, [ 12 ] and proximity‐induced SHE and REE in graphene. [ 23–25 ] Moreover, some of these effects can induce the in‐plane spin polarization and can be entangled with each other.…”