= Bi, Sb)

In non-centrosymmetric materials, spin-orbit coupling induces the momentum-dependent spin splitting of the bands. In 2D systems with in-plane inversion asymmetry, such as monolayer transition metal dichalcogenides (TMDCs), the Zeeman-type spin splitting leads to the valley-contrasting spin polarization[1]. However, the variety of such spin-valley coupling is limited for TMDCs, since the valley configuration is fixed to that for graphene. In this talk, we report the spin-valley coupling is tunable in the layered semimetal BaMnX₂ (X = Bi, Sb), where the distorted X square net layer with in-plane polarization realizes the spin-valley coupled Dirac fermion. Our structural analysis reveals that the lattice distortion for X = Bi is approximately one-tenth of that for X = Sb. Consequently, the first-principles calculation predicts a different configuration of spin-polarized Dirac valleys; X = Sb has two valleys[2], while X = Bi has six. This was experimentally observed as a clear difference in the Shubnikov-de Haas oscillation at high fields between the two materials. Thus, various spin-valley coupling can be controlled by the chemical substitution in BaMnX₂.
References:
[1] Di Xiao et al., Phys. Rev. Lett. 108, 196802 (2012).
[2] H. Sakai et al., Phys. Rev. B 101, 081104(R) (2020).