Abstract
Recent advances in condensed matter physics have revealed a wide range of novel quantum phenomena in two-dimensional (2D) materials. The discovery of unique topological, magnetic, and optical properties in solids has greatly expanded our understanding of quantum phases in electronic structures. As more material systems are proposed theoretically and realized experimentally, our research focuses on uncovering and explaining these phenomena through effective models and first-principles calculations. First, we explore topological mosaic patterns in moiré superlattices—structures composed of periodic, spatially distinct domains, each with different topological properties. These domains give rise to periodic edge states along domain walls. We explore the interaction between these topological domains and predict a global topological phase transition at the charge neutrality point, driven by the size of the domain walls and the moiré period. A phase diagram is provided to illustrate how the twist angle and the geometry of the mosaic pattern influence the emergence of nontrivial topological phases. Second, we investigate the photogalvanic effect, a second-order nonlinear optical response, in multiferroic breathing kagome materials. Using monolayer Nb$_3$I$_8$ as a case study, we show that the shift current—linked to real-space electron–hole displacement—is predominantly unaffected by magnetic order. In contrast, the injection current, associated with the quantum metric dipole in momentum space,is closely related to valley polarization which can be tuned by magnetic field. Both types of photocurrent can be reversed by applying an out-of-plane electric field that modulates the breathing distortion of the lattice. These results suggest that breathing kagome structures are promising platforms for multifunctional optoelectronic devices and sensors. Lastly, we explore the optical properties of altermagnets, a recently proposed magnetic phase distinguished by its unique spin symmetry. Altermagnets exhibit spin-split electronic structures even in the absence of both net magnetization and spin–orbit coupling, setting them apart from conventional ferromagnets and antiferromagnets. Using first-principles many-body perturbation theory, we study valley-resolved optical and excitonic properties in monolayer Mn$_2$WS$_4$, a 2D $d$-wave altermagnet. We discover valley-selective linear dichroism arising from the interplay of spin symmetry and orbital character, along with a spin–valley-dependent optical selection rule that enables linearly polarized light to selectively excite spin-polarized excitons. Additionally, we show that uniaxial strain lifts the valley degeneracy and allows for selective excitation of excitons with highly anisotropic wavefunctions. These findings highlight the potential of altermagnets for next-generation spintronic and valleytronic applications.
Committee Chair
Li Yang
Committee Members
Alexander Seidel; Erik Henriksen; Rohan Mishra; Shaffique Adam
Degree
Doctor of Philosophy (PhD)
Author's Department
Physics
Document Type
Dissertation
Date of Award
5-8-2025
Language
English (en)
DOI
https://doi.org/10.7936/jd77-cy11
Author's ORCID
https://orcid.org/0000-0003-2558-6637
Recommended Citation
Wang, Haonan, "Topological and Optical Properties of Two-Dimensional Materials" (2025). Arts & Sciences Theses and Dissertations. 3565.
The definitive version is available at https://doi.org/10.7936/jd77-cy11