Ultrafast photoemission studies of bulk and exfoliated Ta2NiSe5

Event Date:
2024-12-19T10:00:00
2024-12-19T11:00:00
Event Location:
Via Zoom
Speaker:
Sydney Dufresne, PhD candidate
Related Upcoming Events:
Grad Theses
Intended Audience:
Public
Local Contact:

Departmental defense for Sydney Dufresne (sdufresne@phas.ubc.ca)

*Zoom Meeting details:

Zoom link: https://ubc.zoom.us/j/63342923910?pwd=wA4AeGEpGwnNCiwA5ZNtZjHdiOcHsa.1 

  • Meeting ID: 633 4292 3910

  • Passcode: 550355

Event Information:

Abstract:

This thesis details the development of a 6.2 eV laser-based time- and angle-resolved photoemission spectroscopy (TR-ARPES) apparatus with micro-scale spatial resolution for the study of equilibrium and non-equilibrium properties of inhomogeneous and exfoliated samples. To demonstrate the performance of this apparatus, we spatially resolve the sample inhomogeneities giving rise to spectral broadening of the surface state of the topological insulator Bi2Se3 observed when increasing the spot-size of the 6.2 eV source incident on the sample surface.

We then explore the dynamic properties of the correlation-driven ground state of candidate excitonic insulator Ta2NiSe5. Ta2NiSe5 undergoes a semimetal-to-insulator phase transition below 328 K, accompanied by a lattice distortion from an orthorhombic-to-monoclinic structure. Because both electron-phonon and excitonic interactions can give rise to the bandgap-opening, distinguishing between the contributions of both degrees-of-freedom is theoretically and experimentally challenging. The approach presented in this thesis is to examine the change in spectral lineshape width, related to quasiparticle lifetimes, upon photodoping with a 1.55 eV pump-pulse. The experimental results are directly compared to theoretical many-body simulations, revealing that the bandgap of Ta2NiSe5 originates from predominantly electronic contributions with a much smaller, but necessary, contribution from electron-phonon coupling.

In an effort to further elucidate the electronic and lattice contribution, we exfoliate Ta2NiSe5 on Au(111) using an in-situ exfoliation method and probe the sample with our micro-ARPES apparatus. Low-dimensional studies have been shown to induce electronic states that differ from their bulk counterparts due to the confinement, and may enhance exciton binding energies due to reduced screening of the electron-hole Coulomb interaction. In this study, we explore the emergence of an in-gap state, indicating a phase transition of ultrathin Ta2NiSe5 on Au(111) to a metallic state. The study is extended to the time-domain, where we observe the metallic-like response of ultrathin Ta2NiSe5 to a high-fluence 1.55 eV pump-pulse, and directly compare to the more familiar insulating-like region of the sample where we photo-induce a semimetallic phase. Overall, this thesis work explores the tunability of the bandgap of Ta2NiSe5 through photoexcitation, dimensionality, and carrier doping, demonstrating how we can manipulate the electronic properties, and even induce phase transitions, through manipulation of the physical and electrostatic environment of the material. 

Add to Calendar 2024-12-19T10:00:00 2024-12-19T11:00:00 Ultrafast photoemission studies of bulk and exfoliated Ta2NiSe5 Event Information: Abstract: This thesis details the development of a 6.2 eV laser-based time- and angle-resolved photoemission spectroscopy (TR-ARPES) apparatus with micro-scale spatial resolution for the study of equilibrium and non-equilibrium properties of inhomogeneous and exfoliated samples. To demonstrate the performance of this apparatus, we spatially resolve the sample inhomogeneities giving rise to spectral broadening of the surface state of the topological insulator Bi2Se3 observed when increasing the spot-size of the 6.2 eV source incident on the sample surface. We then explore the dynamic properties of the correlation-driven ground state of candidate excitonic insulator Ta2NiSe5. Ta2NiSe5 undergoes a semimetal-to-insulator phase transition below 328 K, accompanied by a lattice distortion from an orthorhombic-to-monoclinic structure. Because both electron-phonon and excitonic interactions can give rise to the bandgap-opening, distinguishing between the contributions of both degrees-of-freedom is theoretically and experimentally challenging. The approach presented in this thesis is to examine the change in spectral lineshape width, related to quasiparticle lifetimes, upon photodoping with a 1.55 eV pump-pulse. The experimental results are directly compared to theoretical many-body simulations, revealing that the bandgap of Ta2NiSe5 originates from predominantly electronic contributions with a much smaller, but necessary, contribution from electron-phonon coupling. In an effort to further elucidate the electronic and lattice contribution, we exfoliate Ta2NiSe5 on Au(111) using an in-situ exfoliation method and probe the sample with our micro-ARPES apparatus. Low-dimensional studies have been shown to induce electronic states that differ from their bulk counterparts due to the confinement, and may enhance exciton binding energies due to reduced screening of the electron-hole Coulomb interaction. In this study, we explore the emergence of an in-gap state, indicating a phase transition of ultrathin Ta2NiSe5 on Au(111) to a metallic state. The study is extended to the time-domain, where we observe the metallic-like response of ultrathin Ta2NiSe5 to a high-fluence 1.55 eV pump-pulse, and directly compare to the more familiar insulating-like region of the sample where we photo-induce a semimetallic phase. Overall, this thesis work explores the tunability of the bandgap of Ta2NiSe5 through photoexcitation, dimensionality, and carrier doping, demonstrating how we can manipulate the electronic properties, and even induce phase transitions, through manipulation of the physical and electrostatic environment of the material.  Event Location: Via Zoom