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

Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite

Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the order parameter are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe$_3$O$_4$) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions

Anomalous anisotropic exciton temperature dependence in rutile TiO2{\mathrm{TiO}}_{2}

Elucidating the details of electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO2, revealing a strong anisotropic polarization dependence with increasing temperature, namely, an anomalous blue shift for light polarized along the a axis and a conventional red shift for light polarized along the c axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blue shift of the a-axis exciton. Our approach paves the way to a complete ab initio treatment of the electron-phonon interaction and of its influence on the optical spectra of polar materials

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

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

Gleam: the GLAST Large Area Telescope Simulation Framework

This paper presents the simulation of the GLAST high energy gamma-ray telescope. The simulation package, written in C++, is based on the Geant4 toolkit, and it is integrated into a general framework used to process events. A detailed simulation of the electronic signals inside Silicon detectors has been provided and it is used for the particle tracking, which is handled by a dedicated software. A unique repository for the geometrical description of the detector has been realized using the XML language and a C++ library to access this information has been designed and implemented.Comment: 10 pages, Late