We discovered a new rare-earth-based quantum magnet ${\mathrm{Rb}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$, in which the ${\mathrm{Yb}}^{3+}$ ions form an equilateral triangular lattice (space group $\mathrm{P}\overline{3}\mathrm{m}$ 1), without detectable antisite disorder. Magnetic susceptibility, magnetization, specific heat, and electron spin resonance were measured. The results reveal weak antiferromagnetic interactions between the ${\mathrm{Yb}}^{3+}$ ions with Curie-Weiss temperature ${\theta }_{\text{CW}}\sim -0.45$ K and the absence of spin glass or long-range magnetic ordering down to 1.8 K. The ground state of the system is well separated from the first excited Kramers doublet by an energy gap of 23.65 meV (274 K), leading to an effective spin-orbit entangled spin of $J_{\text{eff}}=1/2$ below a characteristic temperature of 80 K. The easy-plane type of anisotropy with $g_{ab}=2.68$ ($ab$ plane) and $g_c=2.30$ ($c$ axis) and gapless excitation are also established. These features are further supported by the crystal electric-field calculations and fittings of the magnetic data. Our findings make ${\mathrm{Rb}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$ a promising candidate for further exploration of spin-orbit entangled nature and potential spin-liquid state in the rare-earth triangular-lattice quantum magnets.

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• Magnetic anisotropy
• Magnetic interactions
• Magnetic susceptibility
• Spin-orbit coupling
• Crystal-field theory
• Electron spin resonance