327 research outputs found
A first-principles approach to closing the "10-100 eV gap" for charge-carrier thermalization in semiconductors
The present work is concerned with studying accurately the energy-loss
processes that control the thermalization of hot electrons and holes that are
generated by high-energy radiation in wurtzite GaN, using an ab initio
approach. Current physical models of the nuclear/particle physics community
cover thermalization in the high-energy range (kinetic energies exceeding ~100
eV), and the electronic-device community has studied extensively carrier
transport in the low-energy range (below ~10 eV). However, the processes that
control the energy losses and thermalization of electrons and holes in the
intermediate energy range of about 10-100 eV (the "10-100 eV gap") are poorly
known. The aim of this research is to close this gap, by utilizing density
functional theory (DFT) to obtain the band structure and dielectric function of
GaN for energies up to about 100 eV. We also calculate charge-carrier
scattering rates for the major charge-carrier interactions (phonon scattering,
impact ionization, and plasmon emission), using the DFT results and first-order
perturbation theory. With this information, we study the thermalization of
electrons starting at 100 eV using the Monte Carlo method to solve the
semiclassical Boltzmann transport equation. Full thermalization of electrons
and holes is complete within ~1 and 0.5 ps, respectively. Hot electrons
dissipate about 90% of their initial kinetic energy to the electron-hole gas
(90 eV) during the first ~0.1 fs, due to rapid plasmon emission and impact
ionization at high energies. The remaining energy is lost more slowly as phonon
emission dominates at lower energies (below ~10 eV). During the thermalization,
hot electrons generate pairs with an average energy of ~8.9 eV/pair (11-12
pairs per hot electron). Additionally, during the thermalization, the maximum
electron displacement from its original position is found to be on the order of
100 nm.Comment: 23 pages, 20 figures. This LaTex file uses RevTex4.2 from AP
Signatures of High-Intensity Compton Scattering
We review known and discuss new signatures of high-intensity Compton
scattering assuming a scenario where a high-power laser is brought into
collision with an electron beam. At high intensities one expects to see a
substantial red-shift of the usual kinematic Compton edge of the photon
spectrum caused by the large, intensity dependent, effective mass of the
electrons within the laser beam. Emission rates acquire their global maximum at
this edge while neighbouring smaller peaks signal higher harmonics. In
addition, we find that the notion of the centre-of-mass frame for a given
harmonic becomes intensity dependent. Tuning the intensity then effectively
amounts to changing the frame of reference, going continuously from inverse to
ordinary Compton scattering with the centre-of-mass kinematics defining the
transition point between the two.Comment: 25 pages, 16 .eps figure
Automated distribution of quantum circuits
Quantum algorithms are usually described as monolithic circuits, becoming
large at modest input size. Near-term quantum architectures can only manage a
small number of qubits. We develop an automated method to distribute quantum
circuits over multiple agents, minimising quantum communication between them.
We reduce the problem to hypergraph partitioning and then solve it with
state-of-the-art optimisers. This makes our approach useful in practice, unlike
previous methods. Our implementation is evaluated on five quantum circuits of
practical relevance.Comment: 12 pages, 10 figure
Do UK universities communicate their brands effectively through their websites?
This paper attempts to explore the effectiveness of UK universities’ websites. The area of branding in higher education has received increasing academic investigation, but little work has researched how universities demonstrate their brand promises through their websites. The quest to differentiate through branding can be challenging in the university context, however. It is argued that those institutions that have a strong distinctive image will be in a better position to face a changing future. Employing a multistage methodology, the web pages of twenty UK universities were investigated by using a combination of content and multivariable analysis. Results indicated ‘traditional values’ such as teaching and research were often well communicated in terms of online brand but ‘emotional values’ like social responsibility and the universities’ environments were less consistently communicated, despite their increased topicality. It is therefore suggested that emotional values may offer a basis for possible future online differentiation
Three particles in an external trap: Nature of the complete J=0 spectrum
Three bosonic, spin-polarized atoms in a spherical oscillator potential
constitutes the simplest nontrivial Bose-Einstein condensate (BEC). The present
paper develops the tools needed to understand the nature of the complete J=0
energy spectrum for this prototype system, assuming a sum of two-body
potentials. The resulting spectrum is calculated as a function of the two-body
scattering length a_sc, which documents the evolution of certain many-body
levels that evolve from BEC-type to molecular-type as the scattering length is
decreased. Implications for the behavior of the condensate excited-state
spectrum and for condensate formation and decay are elucidated. The energy
levels evolve smoothly, even through the regime where the number of two-body
bound states N_b increases by 1, and a_{sc} switches from -infinity to
infinity. We point out the possibility of suppressing three-body recombination
by tuning the two-body scattering length to values that are larger than the
size of the condensate ground state. Comparisons with mean-field treatments are
presented
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How predation and landscape fragmentation affect vole population dynamics
Background: Microtine species in Fennoscandia display a distinct north-south gradient from regular cycles to stable
populations. The gradient has often been attributed to changes in the interactions between microtines and their predators.
Although the spatial structure of the environment is known to influence predator-prey dynamics of a wide range of species,
it has scarcely been considered in relation to the Fennoscandian gradient. Furthermore, the length of microtine breeding
season also displays a north-south gradient. However, little consideration has been given to its role in shaping or generating
population cycles. Because these factors covary along the gradient it is difficult to distinguish their effects experimentally in
the field. The distinction is here attempted using realistic agent-based modelling.
Methodology/Principal Findings: By using a spatially explicit computer simulation model based on behavioural and
ecological data from the field vole (Microtus agrestis), we generated a number of repeated time series of vole densities
whose mean population size and amplitude were measured. Subsequently, these time series were subjected to statistical
autoregressive modelling, to investigate the effects on vole population dynamics of making predators more specialised, of
altering the breeding season, and increasing the level of habitat fragmentation. We found that fragmentation as well as the
presence of specialist predators are necessary for the occurrence of population cycles. Habitat fragmentation and predator
assembly jointly determined cycle length and amplitude. Length of vole breeding season had little impact on the
oscillations.
Significance: There is good agreement between our results and the experimental work from Fennoscandia, but our results
allow distinction of causation that is hard to unravel in field experiments. We hope our results will help understand the
reasons for cycle gradients observed in other areas. Our results clearly demonstrate the importance of landscape
fragmentation for population cycling and we recommend that the degree of fragmentation be more fully considered in
future analyses of vole dynamics
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