First-principles derivation of magnetic interactions in the triangular quantum spin liquid candidates KYbCh2 (Ch=S,Se,Te)

The $A\mathrm{Yb}C{h}_{2}$ ($A=\text{alkali}\phantom{\rule{4.pt}{0ex}}\text{metal}, Ch=\text{chalcogen}$) delafossites are a family of crystals which have a triangular lattice of effective $S=1/2$ moments on their Yb atoms. They hold great promise for the realization of the triangular quantum spin liquid state because of their defect-minimized growth and the ability to interchange chemical constituencies among the family. Here we use ab initio computations to evaluate the exchange couplings of four realized (and one theoretical) rhombohedral delafossite structures and examine the influence of chemical substitution on promoting the development of a quantum spin liquid state. We find good agreement with experiment regarding the antiferromagnetic nearest-neighbor exchange ${J}_{1}$, but our calculations underestimate ${J}_{2}$.