Tunable anisotropic crystal spin transport in two-dimensional altermagnetic transition metal oxychalcogenides

Enhanced and tunable direction-dependent spin transport is highly desirable for high-performance spintronics applications; yet achieving this remains a significant challenge. Altermagnets, governed by specific crystal symmetries, have emerged as promising candidates, offering novel opportunities to manipulate direction dependent crystal spin transport. Here, we theoretically and computationally explore how tunable anisotropic spin transport can be realized in two-dimensional (2D) altermagnetic transition metal oxychalcogenides TM₂Ch₂O (where TM = V, Cr, Ni, Nb, Mo, Ta, and W; Ch = S and Se), which exhibit intrinsic spin-valley coupling. Taking monolayer Nb₂S₂O as an example, we demonstrate that enhanced direction-dependent crystal spin transport can be achieved by breaking the crystal symmetry. Specifically, applying uniaxial strain to monolayer Nb₂S₂O induces a longitudinal conductivity σ_xx in the spin-up channel that significantly exceeds σ_yy, whereas σ_yy dominates in the spin-down channel. Moreover, the application of external electric fields, in bilayer Nb₂S₂O, effectively yields distinct anisotropic spin transport profiles in each spin channel, with orthogonal principal directions between the two spin channels, thus facilitating tunable direction-dependent spin transport. Our findings not only deepen the understanding of transport physics in 2D altermagnets but also pave the way for further fundamental research and potential applications in device technologies.