Spintronics with low-symmetry materials

Felix Casanova, CIC nanoGUNE

Two-dimensional materials are an exciting new material family in which the proximity effect is especially important and opens ways to transfer useful spintronic properties from one 2D material into another. For instance, transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene. The spin-orbit proximity in such graphene/TMD van der Waals heterostructures leads to spin-to-charge conversion (SCC) of out-of-plane spins due to spin Hall effect (SHE), first observed by our group using MoS2 as the TMD [1]. The combination of long-distance spin transport and SHE in the same material gives rise to an unprecedented figure of merit (product of spin Hall angle and spin diffusion length) of 40 nm in graphene proximitized with WSe2, which is also gate tunable [2].
The low symmetry present in many of these low-dimensional materials allows the creation of spin polarizations in unconventional directions and enables new fundamental effects and configurations for devices. In this regard, chiral systems are the ultimate expression of broken symmetry, lacking inversion and mirror symmetry. One way to achieve this is by twisting a graphene/TMD heterostructure. We use twisted graphene/WSe2 to observe SCC arising from Rashba-Edelstein effect (REE) from spins not only perpendicular to the current (conventional configuration), but also parallel to the current (unconventional configuration) [3]. Furthermore, we can tune the twist angle between graphene and WSe2 to control the helicity of the Rashba spin texture, which even changes sign, in excellent agreement with theoretical predictions [4].
Another way to exploit chirality is by directly using materials with a chiral crystal structure, such as elemental tellurium (Te), a 1D van der Waals material. We have recently demonstrated a gate-tunable chirality-dependent charge-to-spin conversion in Te, [5], detected by recording a large unidirectional magnetoresistance (up to 7%). The orientation of the electrically generated spin polarization is determined by the crystal handedness, while its magnitude can be manipulated by an electrostatic gate.
Our results pave the way for the development of chirality-based spintronic devices.

References
[1] C. K. Safeer, FC et al., Nano Lett. 19, 1074 (2019).
[2] F. Herling, FC et al. APL Mater. 8, 071103 (2020).
[3] H. Yang, FC et al. submitted.
[4] S. Lee, FC et al. Phys. Rev. B 106, 165420 (2022).
[5] F. Calavalle, FC et al., Nat. Mater. 21, 526 (2022).

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