Description
It is still a great challenge to explore quantum spin liquid (QSL) state. Here we demonstrate that the van der Waals triangular antiferromagnet CoI$_2$ hosts a remarkable Kitaev interaction. We find the Co$^{2+}$ ion is in the high spin and J$_\mathrm{eff}$ = 1/2 state. The ligand mediated t$_{2g}$-e$_{g}$ hopping gives rise to a strong Kitaev interaction. This bond-dependent frustration together with the geometric frustration determines the experimental helical AF state of CoI$_2$. We then propose to suppress the geometric frustration via a partial Mg substitution for Co, and indeed we find that Co$_{2/3}$Mg$_{1/3}$I$_{2}$ has much weakened geometric frustration but the robust bond dependent frustration. As a result, the overwhelming Kitaev interaction renders Co$_{2/3}$Mg$_{1/3}$I$_{2}$ closest to QSL among several candidate materials.
