Polarized Raman Spectroscopy of the Quantum Antiferromagnet FePS₃

Limitations in scalable quantum computing architecture stimulate research in solid-state materials that demonstrate novel magnetic properties. The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer motivates a thorough Raman spectroscopic study of one such material, FePS₃, a large spin Mott insulator known to be a quasi-2D Ising antiferromagnet. Using Raman spectroscopy, we probe inelastic light scattering from both lattice vibrations (phonons) and magnetic spin-wave excitations (magnons) as a function of temperature, magnetic field, and polarization, providing an accurate measure of its structural and magnetic properties.  We observe emergent features in the Raman spectra below the Néel temperature (T_N ~ 120K) and demonstrate that one of these modes is a magnon. Polarization resolved measurements on FePS₃, as well as reference studies on MoS₂ and Si, highlight symmetry-dependent scattering and clarify how selection rules govern observed Raman modes. Experimental measurements together with modeling of angle-resolved Raman spectra at various numerical apertures further reveal how polarization and optical instrumentation influences Raman intensities. In FePS₃ we resolve the individual nature of several nearly-degenerate A_g/B_g doublet modes. Comparison of our spectra with XRD and predictions from DFT provides insight into the origin and anomalous temperature dependence of additional low-frequency Raman-active modes.