2,207 research outputs found
Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells
We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2Ă10(10) to 5Ă10(12) cm(â2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic
PickCells: A Physically Reconfigurable Cell-composed Touchscreen
Touchscreens are the predominant medium for interactions with digital services; however, their current fixed form factor narrows the scope for rich physical interactions by limiting interaction possibilities to a single, planar surface. In this paper we introduce the concept of PickCells, a fully reconfigurable device concept composed of cells, that breaks the mould of rigid screens and explores a modular system that affords rich sets of tangible interactions and novel acrossdevice relationships. Through a series of co-design activities â involving HCI experts and potential end-users of such systems â we synthesised a design space aimed at inspiring future research, giving researchers and designers a framework in which to explore modular screen interactions. The design space we propose unifies existing works on modular touch surfaces under a general framework and broadens horizons by opening up unexplored spaces providing new interaction possibilities. In this paper, we present the PickCells concept, a design space of modular touch surfaces, and propose a toolkit for quick scenario prototyping
Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots
Polaron dephasing processes are investigated in InAs/GaAs dots using
far-infrared transient four wave mixing (FWM) spectroscopy. We observe an
oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant
excitation of the lowest energy conduction band transition due to coherent
acoustic phonon generation. The subsequent single exponential decay yields long
intraband dephasing times of 90 ps. We find excellent agreement between our
measured and calculated FWM dynamics, and show that both real and virtual
acoustic phonon processes are necessary to explain the temperature dependence
of the polarization decay.Comment: 10 pages, 4 figures, submitted to Phys Rev Let
Molecular Tracers of the Central 12 pc of the Galactic Center
We have used the BIMA array to image the Galactic Center with a 19-pointing
mosaic in HCN(1-0), HCO+(1-0), and H 42-alpha emission with 5 km/s velocity
resolution and 13'' x 4'' angular resolution. The 5' field includes the
circumnuclear ring (CND) and parts of the 20 and 50 km/s clouds. HCN(1-0) and
HCO+ trace the CND and nearby giant molecular clouds while the H 42-alpha
emission traces the ionized gas in Sgr A West. We find that the CND has a
definite outer edge in HCN and HCO+ emission at ~45'' radius and appears to be
composed of two or three distinct streams of molecular gas rotating around the
nucleus. Outside the CND, HCN and HCO+ trace dense clumps of high-velocity gas
in addition to optically thick emission from the 20 and 50 km/s clouds. A
molecular ridge of compressed gas and dust, traced in NH3 emission and
self-absorbed HCN and HCO+, wraps around the eastern edge of Sgr A East. Just
inside this ridge are several arcs of gas which have been accelerated by the
impact of Sgr A East with the 50 km/s cloud. HCN and HCO+ emission trace the
extension of the northern arm of Sgr A West which appears to be an independent
stream of neutral and ionized gas and dust originating outside the CND. Broad
line widths and OH maser emission mark the intersection of the northern arm and
the CND. Comparison to previous NH3 and 1.2mm dust observations shows that HCN
and HCO+ preferentially trace the CND and are weaker tracers of the GMCs than
NH3 and dust. We discuss possible scenarios for the emission mechanisms and
environment at the Galactic center which could explain the differences in these
images.Comment: 24 pages, including 17 figures; to appear in The Astrophysical
Journa
Constraining Born-Infeld models of dark energy with CMB anisotropies
We study the CMB constraints on two Dark Energy models described by scalar
fields with different Lagrangians, namely a Klein-Gordon and a Born-Infeld
field. The speed of sound of field fluctuations are different in these two
theories, and therefore the predictions for CMB and structure formation are
different. Employing the WMAP data on CMB, we make a likelihood analysis on a
grid of theoretical models. We constrain the parameters of the models and
compute the probability distribution functions for the equation of state. We
show that the effect of the different sound speeds affects the low multipoles
of CMB anisotropies, but is at most marginal for the class of models studied
here.Comment: 5 pages, 4 figures, uses revtex
The use of 3D printing in the development of gaseous radiation detectors
Fused Deposition Modelling has been used to produce a small, single wire, Iarocci-style drift tube to demonstrate the feasibility of using the Additive Manufacturing technique to produce cheap detectors, quickly. Recent technological developments have extended the scope of Additive Manufacturing, or 3D printing, to the possibility of fabricating Gaseous Radiation Detectors, such as Single Wire Proportional Counters and Time Projection Chambers. 3D printing could allow for the production of customisable, modular detectors; that can be easily created and replaced and the possibility of printing detectors on-site in remote locations and even for outreach within schools.
The 3D printed drift tube was printed using Polylactic acid to produce a gas volume in the shape of an inverted triangular prism; base length of 28 mm, height 24.25 mm and tube length 145 mm. A stainless steel anode wire was placed in the centre of the tube, mid-print. P5 gas (95% Argon, 5% Methane) was used as the drift gas and a circuit was built to capacitively decouple signals from the high voltage. The signal rate and average pulse height of cosmic ray muons were measured over a range of bias voltages to characterise and prove correct operation of the printed detector
User-centered development of a Virtual Research Environment to support collaborative research events
This paper discusses the user-centred development process within the Collaborative Research Events on the Web (CREW) project, funded under the JISC Virtual Research Environments (VRE) programme. After presenting the project, its aims and the functionality
of the CREW VRE, we focus on the user engagement approach, grounded in the method of co-realisation. We describe the different research settings and requirements of our three embedded user groups and the respective activities conducted so far. Finally we elaborate on
the main challenges of our user engagement approach and end with the projectâs next steps
Could we find any signal of the stratosphere-ionosphere coupling in Antarctica?
An investigation searching for a possible coupling between the lower ionosphere and the middle atmosphere in Antarctica is here performed on the basis of stratospheric vertical temperature profiles and ionospheric absorption data observed at the Antarctic Italian Base of Terra Nova Bay (74.69S, 164.12E) during local summer time. The result obtained by applying a multi-regression analysis and a Superimposed Epoch Analysis (SEA) shows a statistically significant ionosphere-stratosphere interaction. In particular, by selecting stratospheric temperature maxima occurring at different heights as the referring epoch for the SEA approach, the ionospheric absorption is found to show a positive and/or negative trend (several days) around it. The tendency for an increasing/decreasing absorption is obtained for temperature maxima occurring below/above the stratospheric level of about 17-19 km, respectively
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