Collective Excitations in Low-Dimensional Magnetic Materials

Abstract

Low-dimensional quantum materials provide a versatile platform for exploring collective excitations such as phonons, magnons, and topological spin textures. Advances in synthesis techniques, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and mechanical exfoliation (ME), have enabled the discovery and investigation of atomically thin materials and oxide heterostructures that exhibit rich quantum phenomena. These low-dimensional systems can host collective excitations, facilitate the interplay among lattice, charge, and spin degrees of freedom, and enable the study of fundamental quantum phenomena that govern energy dissipation, information transport, and phase transitions in condensed matter.

Understanding and controlling these collective excitations is essential for advancing quantum information technologies, spintronics, and energy-efficient computing. However, the manipulation of such excitations through external stimuli, including optical excitation, magnetic interactions, and interfacial strain, remains incompletely understood. This dissertation investigates collective excitations in three forms, lattice vibrations (phonons), topological spin textures, and spin dynamics (magnons), in two-dimensional transition-metal dichalcogenides (TMDCs) and epitaxial oxide thin films.

First, the optical excitation of chiral phonons is investigated in monolayer MoS2 as a route to probing phonon-driven magnetic phenomena. TMDCs host chiral phonons at high-symmetry points of the Brillouin zone, providing an ideal platform to study their interaction with the valley degree of freedom. Helicity-resolved magneto-Raman spectroscopy reveals a doubly degenerate chiral phonon mode at the Brillouin zone center. Wavelength- and temperature-dependent measurements show that the chiral phonon is activated through resonant excitation of the A exciton. The observed linear Zeeman splitting is reproduced by theoretical calculations based on morphic effects in nonmagnetic crystals.

Second, topological spin textures are explored in proximity-coupled Fe-doped monolayer WSe2 (Fe:WSe2) synthesized by CVD on a Pt Hall-bar device. Electrical transport measurements reveal antisymmetric humps in the Hall resistivity. Temperature-dependent measurements further identify additional antisymmetric peak features that are distinct from the intrinsic anomalous Hall effect. These features are attributed to the topological Hall effect arising from non-coplanar spin configurations, indicating the presence of topologically nontrivial magnetic textures.

Third, magnon--phonon coupling is investigated in epitaxial La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) heterostructures using ferromagnetic resonance (FMR) spectroscopy. The FMR spectra reveal strong coupling between the Kittel magnon mode and transverse acoustic phonons, evidenced by anticrossing gaps in the Kittel dispersion. Vibrating sample magnetometry (VSM) and second-harmonic generation (SHG) measurements show that the STO substrate undergoes a cubic-to-tetragonal structural phase transition, which introduces interfacial strain. This strain lifts the degeneracy of the Kittel mode and splits it into multiple bands. An analytical model incorporating magnon--phonon coupling reproduces the strain-induced magnon splitting, establishing a platform for a tunable hybrid magnon--phonon cavity system.

To support these studies, QuDAP (Quantum Materials Data Acquisition and Processing), an open-source Python-based software package for automated instrument control and real-time data visualization based on Quantum Design physical property measurement systems (PPMS), was developed and implemented. Together, these results demonstrate multiple pathways for manipulating collective excitations in low-dimensional quantum materials while advancing experimental capabilities for quantum materials research.