Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers
The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit λsθxy≈1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy≈80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. ; X.W. acknowledges the Agence National pour la Recherche Flagera GRANSPORT funding. ICN2 authors were supported by the European Union Horizon 2020 research and innovation programme under Grant Agreements No. 881603 (Graphene Flagship) and No. 824140 (TOCHA, H2020-FETPROACT-01-2018). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish Ministerio de Ciencia e Innovación (Grants No. SEV-2017-0706 and No. PID2019-111773RB-I00/AEI/10.13039/501100011033). V.M.P. acknowledges the support of the National Research Foundation Singapore under its Medium-Sized Centre Programme.