Crystal electric field excitations in the quantum spin liquid candidate NaErS2

The delafossite family of compounds with a triangular lattice of rare earth ions has been recently proposed as a candidate host for quantum spin liquid (QSL) states. To realize QSLs, the crystal electric field (CEF) ground state of the rare earth ions should be composed of a doublet that allows sizable quantum tunneling, but until now the knowledge on CEF states in the delafossite compounds is still limited. Here we employ inelastic neutron scattering (INS) to study the CEF transitions in a powder sample of the delafossite ${\mathrm{NaErS}}₂$, where the large total angular momentum $J=15/2$ of the ${\mathrm{Er}}³⁺$ ions and the resulting plethora of CEF transitions enable an accurate fit of the CEF parameters. Our study reveals nearly isotropic spins with large ${J}_z=\ifmmode\pm\else\textpm\fi{}1/2$ components for the ${\mathrm{Er}}³⁺$ CEF ground states, which might facilitate the development of a QSL state. The scaling of the obtained CEF Hamiltonian to different rare earth ions suggests that sizable ${J}_z=\ifmmode\pm\else\textpm\fi{}1/2$ components are generally present in the CEF ground states, supporting the ternary sulfide delafossites as potential QSL hosts.