We report ultra-low-temperature 23Na NMR measurements on the Ising triangular lattice antiferromagnet Na2BaCo(PO4)2, which precisely resolve the phase diagram under magnetic field applied along the crystalline c axis. With increasing field, the NMR spectra resolve three ordered phases with distinct spin configurations: the Y, up-up-down (UUD), and V phases. The spin-lattice relaxation rate 1/T1 data demonstrate gapless excitations in the Y and V phases, strongly supporting their supersolid nature. However, the phase transitions from the UUD phase to the two supersolid phases exhibit dramatically different behaviors upon cooling. Prior to entering the Y phase, 1/T1 identifies a gapless regime within the UUD phase, suggesting a Berezinskii-Kosterlitz-Thouless phase above a second-order phase transition. In contrast, the coexistence of the UUD and V phases observed in our experiments provides direct evidence of a first-order phase transition between these phases.
@article{arxiv.2504.08570,
title = {NMR study of supersolid phases in the triangular-lattice antiferromagnet Na2BaCo(PO4)2},
author = {Xiaoyu Xu and Zhanlong Wu and Ying Chen and Qing Huang and Ze Hu and Xinyu Shi and Kefan Du and Shuo Li and Rui Bian and Rong Yu and Yi Cui and Haidong Zhou and Weiqiang Yu},
journal= {arXiv preprint arXiv:2504.08570},
year = {2025}
}