We use neutron scattering and bulk property measurements to determine the single-ion crystal-field Hamiltonians of delafossites ${\mathrm{KErSe}}_2$ and ${\mathrm{CsErSe}}_2$. These two systems contain planar equilateral triangular Er lattices arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs). Our analysis shows that regardless of the stacking order both compounds exhibit an easy-plane ground-state doublet with large $J_z=1/2$ terms and the potential for significant quantum effects, making them candidates for quantum-spin-liquid or other exotic ground states.

• Received 9 January 2020
• Revised 19 March 2020
• Accepted 8 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.144432