We report a comprehensive experimental investigation of the structural, thermodynamic, static, and dynamic properties of a triangular lattice antiferromagnet ${\mathrm{Rb}}_3\mathrm{Yb}$(${\mathrm{VO}}_4{)}_2$. Through the analysis of magnetic susceptibility, magnetization, and specific heat, complemented by crystal electric field (CEF) calculations, we confirm the Kramers' doublet with effective spin $J_{\mathrm{eff}}=1/2$ ground state. Magnetic susceptibility and isothermal magnetization analysis reveal a weak antiferromagnetic interaction among the $J_{\mathrm{eff}}=1/2$ spins, characterized by a small Curie-Weiss temperature (${\theta }_{\text{CW}}^{\text{LT}}\simeq -0.26$ K) or a reduced exchange coupling ($J/k_{\mathrm{B}}\simeq 0.18$ K). The ${}^{51}\mathrm{V}$ NMR spectra and spin-lattice relaxation rate ($1/T_1$) show no evidence of magnetic long-range-order down to 1.6 K but reflect strong influence of CEF excitations in the intermediate temperatures. At low temperatures, $1/T_1\left(T\right)$ shows pronounced frequency dependence and $1/T_1$ vs field at different temperatures follows the scaling behavior, highlighting the role of paramagnetic fluctuations. The CEF calculations using the point charge approximation divulge a large energy gap ($\sim 18.61$ meV) between the lowest and second lowest energy doublets, further establishing Kramers' doublet as the ground state. Our calculations also reproduce the experimental magnetization and specific heat data and indicate an in-plane magnetic anisotropy. These findings position ${\mathrm{Rb}}_3\mathrm{Yb}$(${\mathrm{VO}}_4{)}_2$ as an ideal candidate to explore intrinsic quantum fluctuations and possible quantum spin-liquid physics in a ${\mathrm{Yb}}^{3+}$-based triangular lattice antiferromagnet.

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• Exchange interaction
• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Nuclear magnetic resonance