13,672 research outputs found
An accurate model for genetic hitch-hiking
We suggest a simple deterministic approximation for the growth of the
favoured-allele frequency during a selective sweep. Using this approximation we
introduce an accurate model for genetic hitch-hiking. Only when Ns < 10 (N is
the population size and s denotes the selection coefficient), are discrepancies
between our approximation and direct numerical simulations of a Moran model
noticeable. Our model describes the gene genealogies of a contiguous segment of
neutral loci close to the selected one, and it does not assume that the
selective sweep happens instantaneously. This enables us to compute SNP
distributions on the neutral segment without bias.Comment: 12 pages, 10 figure
Reading between the lines: attitudinal expressions in text
This is a brief overview of the starting points a project currently proposed and under evaluation by funding agencies. We discuss some of the linguistic methodology we plan to employ to idenitify and analyze attitudinal expressions in text, and touch briefly on how to evaluate our future results
Simulation of hydrogenated graphene Field-Effect Transistors through a multiscale approach
In this work, we present a performance analysis of Field Effect Transistors
based on recently fabricated 100% hydrogenated graphene (the so-called
graphane) and theoretically predicted semi-hydrogenated graphene (i.e.
graphone). The approach is based on accurate calculations of the energy bands
by means of GW approximation, subsequently fitted with a three-nearest neighbor
(3NN) sp3 tight-binding Hamiltonian, and finally used to compute ballistic
transport in transistors based on functionalized graphene. Due to the large
energy gap, the proposed devices have many of the advantages provided by
one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff
ratios, reduced band-to-band tunneling, without the corresponding disadvantages
in terms of prohibitive lithography and patterning requirements for circuit
integration
On cloud ice induced absorption and polarisation effects in microwave limb sounding
Microwave limb sounding in the presence of ice clouds was studied by detailed simulations, where clouds and other atmospheric variables varied in three dimensions and the full polarisation state was considered. Scattering particles were assumed to be horizontally aligned oblate spheroids with a size distribution parameterized in terms of temperature and ice water content. A general finding was that particle absorption is significant for limb sounding, which is in contrast to the down-looking case, where it is usually insignificant. Another general finding was that single scattering can be assumed for cloud optical paths below about 0.1, which is thus an important threshold with respect to the complexity and accuracy of retrieval algorithms. The representation of particle sizes during the retrieval is also discussed. Concerning polarisation, specific findings were as follows: Firstly, no significant degree of circular polarisation was found for the considered particle type. Secondly, for the ±45° polarisation components, differences of up to 4 K in brightness temperature were found, but differences were much smaller when single scattering conditions applied. Thirdly, the vertically polarised component has the smallest cloud extinction. An important goal of the study was to derive recommendations for future limb sounding instruments, particularly concerning their polarisation setup. If ice water content is among the retrieval targets (and not just trace gas mixing ratios), then the simulations show that it should be best to observe any of the ±45° and circularly polarised components. These pairs of orthogonal components also make it easier to combine information measured from different positions and with different polarisations
Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause
We perform a theoretical and numerical study of anti-parallel 2D magnetic
reconnection with asymmetries in the density and reconnecting magnetic field
strength in addition to a bulk flow shear across the reconnection site in the
plane of the reconnecting fields, which commonly occurs at planetary
magnetospheres. We predict the speed at which an isolated X-line is convected
by the flow, the reconnection rate, and the critical flow speed at which
reconnection no longer takes place for arbitrary reconnecting magnetic field
strengths, densities, and upstream flow speeds, and confirm the results with
two-fluid numerical simulations. The predictions and simulation results counter
the prevailing model of reconnection at Earth's dayside magnetopause which says
reconnection occurs with a stationary X-line for sub-Alfvenic magnetosheath
flow, reconnection occurs but the X-line convects for magnetosheath flows
between the Alfven speed and double the Alfven speed, and reconnection does not
occur for magnetosheath flows greater than double the Alfven speed. We find
that X-line motion is governed by momentum conservation from the upstream
flows, which are weighted differently in asymmetric systems, so the X-line
convects for generic conditions including sub-Alfvenic upstream speeds. For the
reconnection rate, while the cutoff condition for symmetric reconnection is
that the difference in flows on the two sides of the reconnection site is twice
the Alfven speed, we find asymmetries cause the cutoff speed for asymmetric
reconnection to be higher than twice the asymmetric form of the Alfven speed.
The results compare favorably with an observation of reconnection at Earth's
polar cusps during a period of northward interplanetary magnetic field, where
reconnection occurs despite the magnetosheath flow speed being more than twice
the magnetosheath Alfven speed, the previously proposed suppression condition.Comment: 46 pages, 7 figures, abstract abridged here, accepted to Journal of
Geophysical Research - Space Physic
On the O II ground configuration energy levels
The most accurate way to measure the energy levels for the O II 2p^3 ground
configuration has been from the forbidden lines in planetary nebulae. We
present an analysis of modern planetary nebula data that nicely constrain the
splitting within the ^2D term and the separation of this term from the ground
^4S_{3/2} level. We extend this method to H II regions using high-resolution
spectroscopy of the Orion nebula, covering all six visible transitions within
the ground configuration. These data confirm the splitting of the ^2D term
while additionally constraining the splitting of the ^2P term. The energies of
the ^2P and ^2D terms relative to the ground (^4S) term are constrained by
requiring that all six lines give the same radial velocity, consistent with
independent limits placed on the motion of the O+ gas and the planetary nebula
data.Comment: 20 pages, 3 figures. To be published in Ap
Corrections to scaling in entanglement entropy from boundary perturbations
We investigate the corrections to scaling of the Renyi entropies of a region
of size l at the end of a semi-infinite one-dimensional system described by a
conformal field theory when the corrections come from irrelevant boundary
operators. The corrections from irrelevant bulk operators with scaling
dimension x have been studied by Cardy and Calabrese (2010), and they found not
only the expected corrections of the form l^(4-2x) but also unusual corrections
that could not have been anticipated by finite-size scaling arguments alone.
However, for the case of perturbations from irrelevant boundary operators we
find that the only corrections that can occur to leading order are of the form
l^(2-2x_b) for boundary operators with scaling dimension x_b < 3/2, and l^(-1)
when x_b > 3/2. When x_b=3/2 they are of the form l^(-1)log(l). A marginally
irrelevant boundary perturbation will give leading corrections going as
log(l)^(-3). No unusual corrections occur when perturbing with a boundary
operator.Comment: 8 pages. Minor improvements and updated references. Published versio
Temperature driven to phase-transformation in Ti, Zr and Hf from first principles theory combined with lattice dynamics
Lattice dynamical methods used to predict phase transformations in crystals
typically deal with harmonic phonon spectra and are therefore not applicable in
important situations where one of the competing crystal structures is unstable
in the harmonic approximation, such as the bcc structure involved in the hcp to
bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an
expression for the free energy that does not suffer from such shortcomings, and
we show by self consistent {\it ab initio} lattice dynamical calculations
(SCAILD), that the critical temperature for the hcp to bcc phase transformation
in Ti, Zr and Hf, can be effectively calculated from the free energy difference
between the two phases. This opens up the possibility to study quantitatively,
from first principles theory, temperature induced phase transitions.Comment: 4 pages, 3 figure
Electron correlations in MnGaAs as seen by resonant electron spectroscopy and dynamical mean field theory
After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its
origin is still debated, and many doubts are related to the electronic
structure. Here we report an experimental and theoretical study of the valence
electron spectrum of Mn-doped GaAs. The experimental data are obtained through
the differences between off- and on-resonance photo-emission data. The
theoretical spectrum is calculated by means of a combination of
density-functional theory in the local density approximation and dynamical
mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver.
Theory is found to accurately reproduce measured data, and illustrates the
importance of correlation effects. Our results demonstrate that the Mn states
extend over a broad range of energy, including the top of the valence band, and
that no impurity band splits off from the valence band edge, while the induced
holes seem located primarily around the Mn impurity.Comment: 5 pages, 4 figure
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