1,447 research outputs found
Growth and optical properties of self-assembled InGaAs Quantum Posts
We demonstrate a method to grow height controlled, dislocation-free InGaAs
quantum posts (QPs) on GaAs by molecular beam epitaxy (MBE) which is confirmed
by structural investigations. The optical properties are compared to realistic
8-band k.p calculations of the electronic structure which fully account for
strain and the structural properties of the QP. Using QPs embedded in n-i-p
junctions we find wide range tunability of the interband spectrum and giant
static dipole moments.Comment: Proccedings paper for MSS-13, 7 pages, 4 figure
Nonlinear electromagnetic response of graphene: Frequency multiplication and the self-consistent-field effects
Graphene is a recently discovered carbon based material with unique physical
properties. This is a monolayer of graphite, and the two-dimensional electrons
and holes in it are described by the effective Dirac equation with a vanishing
effective mass. As a consequence, electromagnetic response of graphene is
predicted to be strongly non-linear. We develop a quasi-classical kinetic
theory of the non-linear electromagnetic response of graphene, taking into
account the self-consistent-field effects. Response of the system to both
harmonic and pulse excitation is considered. The frequency multiplication
effect, resulting from the non-linearity of the electromagnetic response, is
studied under realistic experimental conditions. The frequency up-conversion
efficiency is analysed as a function of the applied electric field and
parameters of the samples. Possible applications of graphene in terahertz
electronics are discussed.Comment: 14 pages, 7 figures, invited paper written for a special issue of
JPCM "Terahertz emitters
CEO pay, shareholder returns, and accounting profits
We assess the impact on CEO pay (including salary, cash bonus, and benefits in kind) of changes in both accounting and shareholder returns in 99 British companies in the years 1972-89. After correcting for heterogeneity biases inherent in the standard specifications of the problem, we find a strong positive relationship between CEO pay and within-company changes in shareholder returns, and no statistically significant relationship between CEO pay and within-company changes in accounting returns. Differences between firms in long-term average profitability do appear to have a substantial effect on CEO pay, while differences between firms in shareholder returns add nothing to the within-firm pay dynamics.These findings call into question the rationale for explicitly share-based incentive schemes
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Using a runway paradigm to assess the relative strength of rats' motivations for enrichment objects
Laboratory animals should be provided with enrichment objects in their cages; however, it is first necessary to
test whether the proposed enrichment objects provide benefits that increase the animals’ welfare. The two main
paradigms currently used to assess proposed enrichment objects are the choice test, which is limited to determining
relative frequency of choice, and consumer demand studies, which can indicate the strength of a preference but are complex to design. Here, we propose a third methodology: a runway paradigm, which can be used to assess the strength of an animal’s motivation for enrichment objects, is simpler to use than consumer demand studies, and is faster to complete than typical choice tests. Time spent with objects in a standard choice test was used to rank several enrichment objects in order to compare with the ranking found in our runway paradigm. The rats ran significantly more times, ran faster, and interacted longer with objects with which they had previously spent the most time. It was concluded that this simple methodology is suitable for measuring rats’ motivation to reach enrichment objects. This can be used to assess the preference for different types of enrichment objects or to measure reward system processes
Quantum-Information Processing with Semiconductor Macroatoms
An all optical implementation of quantum information processing with
semiconductor macroatoms is proposed. Our quantum hardware consists of an array
of semiconductor quantum dots and the computational degrees of freedom are
energy-selected interband optical transitions. The proposed quantum-computing
strategy exploits exciton-exciton interactions driven by ultrafast sequences of
multi-color laser pulses. Contrary to existing proposals based on charge
excitations, the present all-optical implementation does not require the
application of time-dependent electric fields, thus allowing for a
sub-picosecond, i.e. decoherence-free, operation time-scale in realistic
state-of-the-art semiconductor nanostructures.Comment: 11 pages, 5 figures, to be published in Phys. Rev. Lett., significant
changes in the text and new simulations (figure 3
Hypervelocity Stars from the Andromeda Galaxy
Hypervelocity stars (HVSs) discovered in the Milky Way (MW) halo are thought
to be ejected from near the massive black hole (MBH) at the galactic centre. In
this paper we investigate the spatial and velocity distributions of the HVSs
which are expected to be similarly produced in the Andromeda galaxy (M31). We
consider three different HVS production mechanisms: (i) the disruption of
stellar binaries by the galactocentric MBH; (ii) the ejection of stars by an
in-spiraling intermediate mass black hole; and (iii) the scattering of stars
off a cluster of stellar-mass black holes orbiting around the MBH. While the
first two mechanisms would produce large numbers of HVSs in M31, we show that
the third mechanism would not be effective in M31. We numerically calculate
1.2*10^6 trajectories of HVSs from M31 within a simple model of the Local Group
and hence infer the current distribution of these stars. Gravitational focusing
of the HVSs by the MW and the diffuse Local Group medium leads to high
densities of low mass (~ solar mass) M31 HVSs near the MW. Within the
virialized MW halo, we expect there to be of order 1000 HVSs for the first
mechanism and a few hundred HVSs for the second mechanism; many of these stars
should have distinctively large approach velocities (< -500 km/s). In addition,
we predict ~5 hypervelocity RGB stars within the M31 halo which could be
identified observationally. Future MW astrometric surveys or searches for
distant giants could thus find HVSs from M31.Comment: 14 pages, 6 figures, changed to match version accepted by MNRA
Quantum Computation with Quantum Dots and Terahertz Cavity Quantum Electrodynamics
A quantum computer is proposed in which information is stored in the two
lowest electronic states of doped quantum dots (QDs). Many QDs are located in a
microcavity. A pair of gates controls the energy levels in each QD. A
Controlled Not (CNOT) operation involving any pair of QDs can be effected by a
sequence of gate-voltage pulses which tune the QD energy levels into resonance
with frequencies of the cavity or a laser. The duration of a CNOT operation is
estimated to be much shorter than the time for an electron to decohere by
emitting an acoustic phonon.Comment: Revtex 6 pages, 3 postscript figures, minor typos correcte
Model studies of dense water overflows in the Faroese Channels Topical Collection on the 5th International Workshop on Modelling the Ocean (IWMO) in Bergen, Norway 17-20 June 2013
The overflow of dense water from the Nordic Seas through the Faroese Channel system was investigated through combined laboratory experiments and numerical simulations using the Massachusetts Institute of Technology General Circulation Model. In the experimental study, a scaled, topographic representation of the Faroe-Shetland Channel, Wyville-Thomson Basin and Ridge and Faroe Bank Channel seabed bathymetry was constructed and mounted in a rotating tank. A series of parametric experiments was conducted using dye-tracing and drogue-tracking techniques to investigate deep-water overflow pathways and circulation patterns within the modelled region. In addition, the structure of the outflowing dense bottom water was investigated through density profiling along three cross-channel transects located in the Wyville-Thomson Basin and the converging, up-sloping approach to the Faroe Bank Channel. Results from the dye-tracing studies demonstrate a range of parametric conditions under which dense water overflow across the Wyville-Thomson Ridge is shown to occur, as defined by the Burger number, a non-dimensional length ratio and a dimensionless dense water volume flux parameter specified at the Faroe-Shetland Channel inlet boundary. Drogue-tracking measurements reveal the complex nature of flow paths and circulations generated in the modelled topography, particularly the development of a large anti-cyclonic gyre in the Wyville-Thompson Basin and up-sloping approach to the Faroe Bank Channel, which diverts the dense water outflow from the Faroese shelf towards the Wyville-Thomson Ridge, potentially promoting dense water spillage across the ridge itself. The presence of this circulation is also indicated by associated undulations in density isopycnals across the Wyville-Thomson Basin. Numerical simulations of parametric test cases for the main outflow pathways and density structure in a similarly-scaled Faroese Channels model domain indicate excellent qualitative agreement with the experimental observations and measurements. In addition, the comparisons show that strong temporal variability in the predicted outflow pathways and circulations have a strong influence in regulating the Faroe Bank Channel and Wyville-Thomson Ridge overflows, as well as in determining the overall response in the Faroese Channels to changes in the Faroe-Shetland Channel inlet boundary conditions. © 2014 Springer-Verlag Berlin Heidelberg
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