Magnetic and crystal electric field studies of two Yb$^{3+}$-based triangular lattice antiferromagnets

We present the low-temperature magnetic properties of two Yb$^{3+}$-based triangular lattice compounds NaSrYb(BO$_3$)$_2$ and K$_3$YbSi$_2$O$_7$ via thermodynamic measurements followed by crystal electric field (CEF) calculations. Magnetization and specific heat data as well as the CEF energy levels confirm that the ground state is characterized by the low-lying Kramers' doublet of Yb$^{3+}$ with effective spin-1/2 ($J_{\rm eff} = 1/2$). A small Curie-Weiss temperature and scaling of magnetic isotherms corroborate very weak magnetic correlations among $J_{\rm eff} = 1/2$ spins. The crystal field parameters are calculated using the point charge model and the CEF Hamiltonian is determined for both the compounds. The simulation using the eigenvalues of the CEF Hamiltonian reproduces the experimental susceptibility, magnetic isotherm, and magnetic specific heat data very well. The large separation between the ground state and first excited state doublets implies that the ground state is a Kramers' doublet with $J_{\rm eff} = 1/2$ at low temperatures, endorsing the experimental findings.