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Dufresne, Sydney K. Y.

University of British Columbia

This thesis investigates the electronic structure and ultrafast dynamics of Ta₂NiSe₅ (TNSe), a candidate excitonic insulator (EI). The EI phase arises from Coulomb interactions between electrons and holes, forming bound electron-hole pairs known as excitons. While electronic interactions play a key role in this ground state, the coupling of charge carriers to lattice vibrations (phonons) complicates distinguishing the origins of the EI phase. Reducing dimensionality offers a means to tune these interactions, as low-dimensional systems can exhibit novel phases distinct from bulk properties and may enhance exciton binding energies through reduced Coulomb screening. Non-equilibrium studies enable disentanglement of these interactions and provide insight into their contributions to emergent physical properties. The first part of this thesis details the development of a 6.2 eV laser-based light source for a time- and angle-resolved photoemission spectroscopy (TR-ARPES) apparatus with micro-scale spatial resolution. The micro-scale spatial resolution aspect enables us to branch out, bringing the study of ultrafast dynamics to inhomogeneous samples and exfoliated samples. The second part of this thesis explores the dynamic properties of the correlation-driven ground state of candidate EI TNSe. TNSe undergoes a semimetal-to-insulator phase transition below 328 K, accompanied by a lattice distortion. The approach presented in this thesis is to use a comparative analysis between TR-ARPES results and results from theoretical many-body simulations to distinguish between the contributions of both degrees, revealing that the ground state of TNSe originates from predominantly electronic contributions. In the final part of this thesis we exfoliate TNSe on Au(111) using an in-situ exfoliation method in an effort to further elucidate the electronic and lattice contribution. We explore the emergence of an in-gap state, indicating a phase transition of ultrathin TNSe on Au(111) to a metallic state, and use TR-ARPES to demonstrate the metallic-like-response of ultrathin TNSe to a high-fluence 1.55 eV pump-pulse. Overall, this thesis work explores the tunability of the bandgap of TNSe through photoexcitation, dimensionality, and carrier doping, demonstrating how we can tune electronic properties, and even induce phase transitions, through manipulation of the physical and electrostatic environment of the material.

Text

2025-04-04

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/90589

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/