Magneto-Optical Probe and Control of Lattice, Spin, and Orbital Dynamics in Two-Dimensional van der Waals Quantum Materials

Abstract

Two-dimensional van der Waals materials provide a platform in which collective lattice and spin excitations, phonons and magnons, couple unusually strongly to the electronic and magnetic structure of their host crystal. This is a consequence of the reduced dimensionality and exceptional external tunability inherent to this materials class. This dissertation uses helicity- and polarization-resolved magneto-Raman spectroscopy, complemented by second-harmonic generation, ferromagnetic resonance, and first-principles calculations to identify and deliberately engineer such coupling across four unique material platforms. In monolayer MoS₂, a circularly polarized phonon mode is shown to acquire a giant effective magnetic moment orders of magnitude larger than the Bohr magneton through hybridization with an orbital transition activated by resonant excitation of the A exciton. Additional tuning via nanometer-thick Ni-overlaying and substitutional Fe-doping opens an independent, spatially localized coupling channel to the same phonon without disturbing the intrinsic one. In isotope-engineered graphite, substituting ¹³C for ¹²C introduces a tunable pseudomagnetic field of roughly 0.2~T, splitting Landau level transitions and their selective coupling to graphene's G-band phonon while leaving the electronic Fermi velocity essentially unchanged. In the charge-density-wave material TaTe₂, rotational-anisotropy second-harmonic generation directly images 120°-rotated structural domains in both bulk and few-layer flakes, arising from the reduced symmetry at the cleaved crystal surface, despite a centrosymmetric bulk. Finally, in the van der Waals antiferromagnet CrSBr, a nanometer-thick nickel overlayer is shown to couple selectively to the topmost Cr layer, expanding the in-plane lattice and inducing an antiparallel magnetic moment that suppresses the saturation magnetization while leaving the bulk magnetic anisotropy intact. Magneto-Raman and ferromagnetic resonance measurements further reveal nonreciprocal magnon scattering from Ni-induced interfacial symmetry breaking and a distinct, interface-localized magnon branch. A separate, field-induced optical magnon obeying an antisymmetric, one-magnon-like polarization selection rule, together with a broader set of weaker spectral anomalies, further illustrates how symmetry constrains magnon dynamics in this system. Together, these results establish helicity- and polarization-resolved magneto-Raman spectroscopy as a sensitive probe of, and a practical route to engineering, the coupling between lattice, orbital, and spin degrees of freedom in 2D and layered quantum materials.