NMR Evidence of Spin Supersolid and Pomeranchuk Effect Behaviors in Triangular-Lattice Antiferromagnet Rb 2 Ni 2 (SeO 3 ) 3

Abstract We perform 85 Rb nuclear magnetic resonance (NMR) measurements on the S = 1 bilayer triangular-lattice antiferromagnet Rb 2 Ni 2 (SeO 3 ) 3 in magnetic fields up to 26 T. In the field range from 3 to 26 T, the NMR spectral lines split, and their respective spectral weight ratios reveal the existence of the magnetic up-up-down (UUD) phase, although the 1/3-plateau phase is only attained in fields above 16 T. Two distinct gapless regimes are further identified: one at low fields and low temperatures, and the other at high fields and high temperatures, which are consistent with spin supersolid Y and V phases. Notably, the UUD-V phase boundary exhibits a negative slope in d T /d H , where the supersolid phase is located at temperatures above those of the solid phase because of strong low-energy spin fluctuations.