Prof. Liuyan Zhao (University of Michigan)

Abstract: Due to the reduced dimensionality, the impact of fluctuations is significantly enhanced in the two dimensions (2D) as compared to in the three dimensions (3D), with consequences captured by the Mermin-Wagner theorem and the Berezinskii–Kosterlitz–Thouless theory, etc. Taking 2D magnets as an example, a mainstream effort has been fighting against such fluctuations so as to maintain the magnetically ordered phases in the 2D limit. In this talk, we will try to take a different perspective that is to explore and exploit enhanced fluctuations in 2D magnets for realizing new magnetic states and phenomena. Here, we choose NiPS₃ as a material platform, because of its ferromagnetic-antiferromagnetic competitions and weak spin anisotropies that promote the fluctuations. We will show the emergence of a three-state Potts nematicity, which is one type of vestigial states, as NiPS₃ is thinned down, and demonstrate its presence over a wide temperature-thickness range in the phase diagram. We will further discuss the magnetic excitations in the context of this temperature-thickness phase diagram. The combination of evolutions in the magnetic ground state and the magnetic collective excitations is anticipated to provide a comprehensive picture for the role of fluctuations.