Impact of strong quantum spin fluctuations on the magnetism, the excitons, and the spin waves in a two-dimensional XY-type magnetic system*

The newly emergent two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform to understand the interplay between quantum fluctuations and symmetry breaking orders. Among them, the antiferromagnet NiPS₃ has drawn tremendous attention due to the possible realization of 2D XY model in its 2D limit, as well as its exceptionally sharp exciton mode. In this talk, I will show our recent optical spectroscopy results on bulk and few-layer NiPS₃. First, I will use Raman signatures of spin fluctuations, magnetism-induced broken translational symmetry and broken rotational symmetry to show the dimensionality crossover of the magnetic state in NiPS₃ from the long-range antiferromagnetic order in bulk to the spin-induced nematic order in few-layers. Second, I will correlate the thickness, temperature, and spatial dependencies of magnetism to the evolution of the intriguing excitons in NiPS₃. Finally, I will comment on the spin waves in NiPS₃ and their thickness dependence.