Dissertations, Theses, and Capstone Projects

Date of Degree

6-2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Oleg Berman

Advisor

Godfrey Gumbs

Committee Members

Mirko Amico

Rodrigo Barbosa Capaz

Marcelo Terra Cunha

Gabrielle Grosso

Andrii Iurov

Roman Kezerashvili

German Kolmakov

Subject Categories

Condensed Matter Physics | Quantum Physics

Keywords

Excitons, Transition metal dicalcogenides, Entanglement, Time crystals, superfluids

Abstract

This dissertation is concerned with exploring the properties and applications of excitons and polaritons in two-dimensional (2D) materials and heterostructures. It focuses on their potential to form qubits, time crystals, and superfluids. The work is motivated by the unique electronic and optical properties of 2D materials—specifically, their ability to host strongly bound excitons and hybrid light-matter quasiparticles known as polaritons. By exploiting a combination of theoretical modeling and numerical simulations, this work examines the behavior of these quasiparticles under various physical conditions, including strain-induced pseudomagnetic fields, optical microcavities, and periodic external potentials.

The first part of this dissertation is devoted to investigating quantum entanglement in excitonic systems. We demonstrate that, when coupled to a microcavity, excitons in strained graphene can form robust entangled states suitable for quantum information applications. A Tavis-Cummings model describes the interactions between two excitons and a single cavity mode. The study reveals that under specific conditions, entanglement can be protected from decay and even enhanced by dissipation, delivering promising insights for quantum computing technologies.

Our investigation was extended to multi-qubit systems, where a novel entanglement measure—i.e., the total negativity—is introduced to estimate the overall quantum correlations in multipartite systems. The results indicate that long-lived entanglement can be sustained by coherent photon pumping in the cavity, an effect which generalizes to other qubit implementations, such as superconducting qubits and trapped atoms.

The second part of this dissertation is concerned with time crystals, described as a phase of matter characterized by the spontaneous breaking of time-translation symmetry. This study proposes that exciton and exciton-polariton Bose-Einstein condensates (BECs) in 2D materials can exhibit time-crystalline behavior under periodic external potentials. Two physical settings are analyzed: an exciton-polariton BEC in a spatially curved optical microcavity and a bare exciton BEC in a twisted bilayer transition metal dichalcogenide (TMDC) heterostructure. The study introduces a semiclassical criterion for time crystallization and confirms its validity through numerical simulations. The persistence of time-crystalline order beyond the mean-field approximation is demonstrated by incorporating quantum fluctuations into the system dynamics. These findings suggest potential applications of excitonic and polaritonic time crystals in quantum computers as memory-storing devices.

The final part of the dissertation explores the emergence of dark excitonic superfluidity in strained TMDC heterostructures. It is shown that dark excitons, which arise as a consequence of momentum mismatch between the minimum of the conduction band and the maximum of the valence band, can generate robust, long-lived superfluid phases in double-layer TMDC systems under strain. These dark excitons exhibit significantly longer lifetimes compared to bright excitons due to a more complex decaying process. By analyzing the energy spectrum of collective excitations, we discover the sound velocity and estimate the critical temperature for superfluidity. The results indicate that strain engineering in TMDCs can be used to fine-tune dark excitonic momentum mismatch, opening novel pathways for the development of high-performance excitonic devices.

In conclusion, this dissertation advances our understanding of the fundamental quantum properties of excitons and polaritons in 2D materials and reveals their potential applications in quantum computing, time-dependent phases of matter, and excitonic transport. The theoretical models and numerical approaches developed in this research provide a foundation for future experimental realizations of excitonic and polaritonic quantum technologies.

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