Effective Spin-1/2 Moments on a Yb$^{3+}$ Triangular Lattice: an ESR Study

We investigated the spin dynamics by electron spin resonance (ESR) of the Yb-based, effective spin-1/2 delafossites NaYbO$_{2}$, AgYbO$_{2}$, LiYbS$_{2}$, NaYbS$_{2}$, and NaYbSe$_{2}$ which all show an absence of magnetic order down to lowest reachable temperatures and thus are prime candidates to host a quantum spin-liquid ground state in the vicinity of long range magnetic order. Clearly resolved ESR spectra allow to obtain well-defined $g$ values which are determined by the crystal field of the distorted octahedral surrounding of the Yb-ions in trigonal symmetry. This local crystal field information provides important input to characterize the effective $S = 1/2$ Kramers doublet as well as the anisotropic exchange coupling between the Yb ions which is crucial for the nature of the groundstate. The ESR linewidth $\Delta B$ is characterised by the spin dynamics and is mainly determined by the anisotropic exchange coupling. We discuss and compare $\Delta B$ of the above mentioned delafossites focussing on the low temperature behaviour which is dominated by the growing influence of spin correlations.