Optical Study of Symmetry Breaking in Two-dimensional Antiferromagnets

Abstract

The advent of two-dimensional (2D) materials has paved the way for the discovery and investigation of unique properties that arise from their low dimensionality. But, it also provided an avenue to study and test traditional, theoretical ideas related to long-range ordering in reduced dimensions. In particular, two-dimensional antiferromagnetism underlies many interesting and pertinent phenomena in modern condensed matter physics. However, the absence of net magnetization in systems exhibiting 2D antiferromagnetic order poses significant experimental challenges for their probing. This thesis delves into the captivating emergent optical behaviors in two-dimensional zigzag antiferromagnetic materials of the transition metal phosphorus trichalcogenide family that can be used to study 2D antiferromagnetic behavior. We first showcase zigzag antiferromagnetic order-induced excitons in NiPS3. The photoluminescence (PL) signal exhibits remarkable characteristics, including an exceptionally narrow linewidth and high linear polarization. Intriguingly, the PL intensity and polarization mirror the behavior of the zigzag antiferromagnetic order, vanishing above the Néel temperature. Our findings reveal a direct link between the exciton properties and the presence of antiferromagnetic order. Expanding our optical investigation, we demonstrate the ubiquitous presence of a linear dichroism response in the MPX3 zigzag antiferromagnets, exemplified by FePS3. The reduction of three-fold rotational symmetry to two-fold, induced by magnetic order, manifests as two-fold anisotropy in the optical response. Lastly, we demonstrate the remarkable ability to reversibly tune the three-state Potts nematicity in FePSe3 using strain. Through in-situ strain measurements, we elucidate the electronic nematic phase as the origin of the linear dichroism response. We also unveil strain-controlled nematic domain populations and the manipulation of the nematic phase transition nature. This strain control of the nematic phase provides a window into the nematic susceptibility near the magnetic ordering temperature, offering deeper insights into the behavior of these correlated materials. Through detailed optical experimental investigations, we provide a comprehensive understanding of the interplay between magnetic order, symmetry breaking, and emergent properties in zigzag antiferromagnets.