Defect chemistry of Ti and Fe impurities and aggregates in Al2O3

We report a theoretical evaluation of the properties of iron and titanium impurities in sapphire (corundum structured α-Al2O3). Calculations using analytical force fields have been performed on the defect structure with the metals present in isolated, co-doped and tri-cluster configurations. Crystal field parameters have been calculated with good agreement to available experimental data. When titanium and iron are present in neighbouring face and edge-sharing orientations, the overlap of the d-orbitals facilitates an intervalence charge transfer (FeIII/TiIII → FeII/TiIV) with an associated optical excitation energy of 1.85 eV and 1.76 eV in the respective configurations. Electronic structure calculations based on density functional theory confirm that FeIII/TiIII is the ground-state configuration for the nearest-neighbour pairs, in contrast to the often considered FeII/TiIV pair. Homonuclear intervalence charge transfer energies between both FeIII/FeII and TiIV/TiIII species have also been calculated, with the energy lying in the infra-red region. Investigation of multiple tri-clusters of iron and titanium identified one stable configuration, TiIII–(TiIV/FeII), with the energy of electron transfer remaining unchanged

Participation in domestic energy retrofit programmes: key spatio-temporal drivers

The Canadian government created the EcoENEGY Retrofit for Homes programme (2007–12) to improve residential energy efficiency and reduce emissions produced through energy use. The uptake of retrofits varied both spatially and temporally. This research examines spatio-temporal patterns of retrofit adoption to understand the drivers behind participation in the grant programme and assess how future grant-based programmes might improve the uptake of efficiency measures. Temporal analysis demonstrated continued growth of programme participation over its original period of availability, and this accelerated once the programme was extended for an additional year after its original closure date. However, some spatial correlations weakened, which may be attributable to changes in programme design during the extension period. Seasonal variation was also observed, with spikes in retrofit activity occurring in winter. A regression analysis for conversion rates in Ontario and British Columbia displayed significant positive correlations for high shelter costs (>30% of household income) and households occupied by usual residents (regular occupants). Population density, median property value (only in Ontario) and units that were recently occupied demonstrated negative correlations. Spatial variation at both the city and neighbourhood levels suggests a greater degree of programme customisation is required to ensure uniform building stock improvement

Terahertz Waveguiding in Silicon-Core Fibers

We propose the use of a silicon-core optical fiber for terahertz (THz) waveguide applications. Finite-difference time-domain simulations have been performed based on a cylindrical waveguide with a silicon core and silica cladding. High-resistivity silicon has a flat dispersion over a 0.1 - 3 THz range, making it viable for propagation of tunable narrowband CW THz and possibly broadband picosecond pules of THz radiation. Simulations show the propagation dynamics and the integrated intensity, from which transverse mode profiles and absorption lengths are extraced. It is found that for 140 - 250 micron core diameters the mode is primarily confined to the core, such that the overall absorbance is only slightly less than in bulk polycrystalline silicon.Comment: 6 pages, 3 figures, journal submissio

Aircraft aerodynamic prediction method for V/STOL transition including flow separation

A numerical procedure was developed for the aerodynamic force and moment analysis of V/STOL aircraft operating in the transition regime between hover and conventional forward flight. The trajectories, cross sectional area variations, and mass entrainment rates of the jets are calculated by the Adler-Baron Jet-in-Crossflow Program. The inviscid effects of the interaction between the jets and airframe on the aerodynamic properties are determined by use of the MCAIR 3-D Subsonic properties are determined by use of the MCAIR 3-D Subsonic Potential Flow Program, a surface panel method. In addition, the MCAIR 3-D Geometry influence Coefficient Program is used to calculate a matrix of partial derivatives that represent the rate of change of the inviscid aerodynamic properties with respect to arbitrary changes in the effective wing shape

Coherent control of injection currents in high-quality films of Bi2Se3

Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.Comment: 11 pages, 3 figure, journal articl

Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3

Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi2Se3 revealed a strong dependence of the carrier relaxation time on the film thickness. For thicker films the relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films

Brood Season Habitat Selection by Montezuma Quail in Southeastern Arizona

Habitat conditions during brood season can affect Montezuma quail (Cyrtonyx montezumae) population levels in Arizona, and land use practices can affect these habitat conditions. General habitat affinities of Montezuma quail are known, however, information on specific habitat selection patterns is limited. We investigated seasonal habitat selection by Montezuma quail in the foothills of the Huachuca and Santa Rita mountains in southeastern Arizona. We used pointing dogs to locate quail during brood seasons (Aug–Oct) of 1998 and 1999. We measured habitat components at 60 flush sites and 60 associated (100 m) random plots. Compared to random plots, quail used areas with higher grass and forb species richness, and more trees (P \u3c 0.10). Low level (\u3c= 50 cm) visual obstruction, usually associated with bunchgrass cover, was greater (P \u3c 0.10) at flush sites than at random plots. Optimum brood season habitat for Montezuma quail should contain \u3e= 6 species of forbs/0.01 ha, tree canopy cover between 10 and 50%, and grass canopy cover between 50 and 85% with a minimum average height of 25cm. Maintaining these habitat characteristics could minimize negative impacts of land-use practices on Montezuma quail

Signatures of four-particle correlations associated with exciton-carrier interactions in coherent spectroscopy on bulk GaAs

Transient four-wave mixing studies of bulk GaAs under conditions of broad bandwidth excitation of primarily interband transitions have enabled four-particle correlations tied to degenerate (exciton-exciton) and nondegenerate (exciton-carrier) interactions to be studied. Real two-dimensional Fourier-transform spectroscopy (2DFTS) spectra reveal a complex response at the heavy-hole exciton emission energy that varies with the absorption energy, ranging from dispersive on the diagonal, through absorptive for low-energy interband transitions to dispersive with the opposite sign for interband transitions high above band gap. Simulations using a multilevel model augmented by many-body effects provide excellent agreement with the 2DFTS experiments and indicate that excitation-induced dephasing (EID) and excitation-induced shift (EIS) affect degenerate and nondegenerate interactions equivalently, with stronger exciton-carrier coupling relative to exciton-exciton coupling by approximately an order of magnitude. These simulations also indicate that EID effects are three times stronger than EIS in contributing to the coherent response of the semiconductor