Chapter 1 THE WESTMINSTER MODEL AND THE UK POLITICAL SYSTEM BEFORE BREXIT

Westminster model; UK politics; Brexi

Chapter 2 UNDERSTANDING THE BREXIT EFFECT

Westminster model; UK politics; Brexi

Exciton Control in a Room-Temperature Bulk Semiconductor with Coherent Strain Pulses

The coherent manipulation of excitons in bulk semiconductors via the lattice degrees of freedom is key to the development of acousto-optic and acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit strongly bound excitons that are interesting for applications in the deep-ultraviolet, but their properties have remained elusive due to the lack of efficient generation and detection schemes in this spectral range. Here, we perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO$_2$ single crystals at room temperature, and reveal a dramatic modulation of the exciton peak amplitude due to coherent acoustic phonons. This modulation is comparable to those of nanostructures where exciton-phonon coupling is enhanced by quantum confinement, and is accompanied by a giant exciton shift of 30-50 meV. We model these results by many-body perturbation theory and show that the deformation potential coupling within the nonlinear regime is the main mechanism for the generation and detection of the coherent acoustic phonons. Our findings pave the way to the design of exciton control schemes in the deep-ultraviolet with propagating strain pulses

Chapter 2 UNDERSTANDING THE BREXIT EFFECT

Westminster model; UK politics; Brexi

Chapter 1 THE WESTMINSTER MODEL AND THE UK POLITICAL SYSTEM BEFORE BREXIT

Westminster model; UK politics; Brexi

Giant exciton Mott density in anatase TiO2

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO$_2$ and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators

Elucidating Piezoelectricity and Strain in Monolayer MoS2 at the Nanoscale Using Kelvin Probe Force Microscopy

Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in twodimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.This research was primarily supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement Nos. DMR-1720595 and DMR-2308817. This work was performed in part at the University of Texas Microelectronics Research Center, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-2025227), and using the facilities and instrumentation supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement Nos. DMR-1720595 and DMR-2308817 and NSF Major Research Instrumentation (MRI) program DMR- 2019130. E.B. acknowledges support from the National Science Foundation under grant DMR-2308817 (X.P.) and the Robert A. Welch Foundation under grant F-2092- 20220331 (F.Y.G.).Center for Dynamics and Control of Material