Magnetically disordered ground state in the triangular-lattice antiferromagnets Rb3Yb(VO4)2 and Cs3Yb(VO4)2

Quantum spin liquids (QSLs) represent a unique quantum disordered state of matter that hosts long-range quantum entanglement and fractional excitations. However, structural disorder resulting from site mixing between different types of ions usually arises in real QSL candidates, which is considered to be an obstacle to gaining insight into the intrinsic physics. Here, we have synthesized two new rare-earth compounds, ${\mathrm{Rb}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$ and ${\mathrm{Cs}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$. X-ray diffractions reveal a perfect triangular-lattice structure with no detectable disorder. Magnetic susceptibility measurements do not capture any phase transition or spin freezing down to 1.8 K. A fit to low-temperature data indicates dominant antiferromagnetic interactions with a Curie-Weiss temperature of $\ensuremath{-}1.40$ and $\ensuremath{-}0.43$ K for ${\mathrm{Rb}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$ and ${\mathrm{Cs}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$, respectively. Specific-heat results show no sign of long-range magnetic order down to $\ensuremath{\sim}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ either, but only a Schottky anomaly that is continuously mediated by the external magnetic fields. Additionally, inelastic neutron scattering is employed to detect low-energy spin excitations in ${\mathrm{Rb}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$. The absence of magnetic excitation signals as well as static magnetic order down to 97 mK aligns with the results from magnetic susceptibility and specific heat. Collectively, these findings point to a quantum disordered ground state with persistent spin dynamics, reminiscent of QSL behaviors. Our work provides a promising platform for further exploration of quantum magnetism in this new disorder-free system.