Spin-supersolidity induced quantum criticality and magnetocaloric effect in the triangular-lattice antiferromagnet Rb2Co(SeO3)2

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 NMR line splits, and the thermal activation behaviors of the spin-lattice relaxation rate 1/T1 between 2 and 15.8 T provide unambiguous evidence of a gapped up–up–down (UUD) magnetic ordered phase. For fields between 15.8 and 18.5 T, the anomaly in the magnetic susceptibility, the slow saturation of the NMR line spectral ratio with temperature, the decrease of the NMR line split with field, and the power-law temperature dependence of 1/T1 provide evidence consistent with a spin supersolid with gapless spin excitations. Further increasing the field, the Grüneisen ratio, extracted from the MCE data, reveals a continuous quantum phase transition at HC ≈ 19.5 T and a universal quantum critical scaling with the exponents νz ≈ 1. Near HC, 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.