We present the ground state properties of a new quantum antiferromagnet, YbBO 3 , in which the isotropic Yb 3+ triangular layers are separated by a nonmagnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin J eff = 1 2 state of Yb 3+ ions at low temperatures, interacting antiferromagnetically with an intralayer coupling J/k B 0.53 K. The absence of oscillations and a 1/3 tail in the zero-field muon asymmetries rules out the onset of magnetic long-range order as well as spin freezing down to 20 mK. An anomalous broad maximum in the temperature-dependent heat capacity with an unusually reduced value and a broad anomaly in the zero-field muon depolarization rate centered at T * 0.7J/k B provide compelling evidence for a wide fluctuating regime (0.2 T /J 1.5) with slow relaxation. We infer that the fluctuating regime is a universal feature of highly frustrated triangular-lattice antiferromagnets while the absence of magnetic long-range order is due to perfect two-dimensionality of the spin lattice protected by nonmagnetic site disorder.
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