Nematicity from strong quantum spin fluctuations in a two-dimensional XY-type van der Waals magnet*

We study XY-type magnet NiPS₃ to probe spin-induced nematicity in 2D system. We identified three Raman signatures for the static, long-range antiferromagnetic (AFM) order in bulk NiPS₃: suppressed spin fluctuations (revealed by ceased quasi-elastic scattering, QES), broken translational symmetry (revealed by the presence of a folded phonon at 30 cm^-1, P_BTS), and broken rotational symmetry (revealed by degeneracy-lifted phonons at around 180 cm^-1, P_BRS). We then track these three Raman features as the NiPS₃ thickness decreases and found their contrast behaviors in few-layer NiPS₃: the survival of QES, the disappearance of P_BTS, and the presence of P_BRS at 10 K, showing that the static, long-range AFM order in bulk evolves into a spin-induced nematic state in few-layer. We further observed three nematic domain states whose order parameters are rotated by 120^o from one another and are unchanged in different thermal cycles. Our results show the power of strong quantum fluctuations in creating new phases and offer an unprecedented venue for realizing novel 2D magnetic phases.