We performed high-field magnetization, magnetocaloric effect (MCE), and NMR measurements on the Ising triangular-lattice antiferromagnet Rb₂Co (SeO₃) ₂. The observations of the 1/3-magnetization plateau, the split NMR lines, and the thermal activation behaviors of the spin-lattice relaxation rate 1/T₁ between 2 T and 15. 8 T provide unambiguous evidence of a gapped up-up-down (UUD) magnetic ordered phase. For fields between 15. 8 T and 18. 5 T, the anomaly in the magnetic susceptibility, the slow saturation of the NMR line spectral ratio with temperature, and the power-law temperature dependence of 1/T₁ suggest the ground state to be a spin supersolid with gapless spin excitations. With further increasing the field, the Grüneisen ratio, extracted from the MCE data, reveals a continuous quantum phase transition at H ₂ 19. 5 T and a universal quantum critical scaling with the exponents νz~~1. Near H ₂, the large high-temperature MCE signal and the broad peaks in the NMR Knight shift and 1/T₁, manifest the strong spin fluctuations driven by both magnetic frustration and quantum criticality. These results establish Rb₂Co (SeO₃) ₂ as a candidate platform for cryogenic magnetocaloric cooling.
Cui et al. (Tue,) studied this question.