5,041 research outputs found
Ironing out primordial temperature fluctuations with polarisation: optimal detection of cosmic structure imprints
Secondary anisotropies of the cosmic microwave background (CMB) can be
detected by using the cross-correlation between the large-scale structure (LSS)
and the CMB temperature fluctuations. In such studies, chance correlations of
primordial CMB fluctuations with the LSS are the main source of uncertainty. We
present a method for reducing this noise by exploiting information contained in
the polarisation of CMB photons. The method is described in general terms and
then applied to our recently proposed optimal method for measuring the
integrated Sachs-Wolfe (ISW) effect. We obtain an expected signal-to-noise
ratio of up to 8.5. This corresponds to an enhancement of the signal-to-noise
by 23 per cent as compared to the standard method for ISW detection, and by 16
per cent w.r.t. our recently proposed method, both for the best-case scenario
of having perfect (noiseless) CMB and LSS data.Comment: added reference to previous work on the topic, accepted by MNRA
The maximum force in a column under constant speed compression
Dynamic buckling of an elastic column under compression at constant speed is
investigated assuming the first-mode buckling. Two cases are considered: (i) an
imperfect column (Hoff's statement), and (ii) a perfect column having an
initial lateral deflection. The range of parameters, where the maximum load
supported by a column exceeds Euler static force is determined. In this range,
the maximum load is represented as a function of the compression rate,
slenderness ratio, and imperfection/initial deflection. Considering the results
we answer the following question: "How slowly the column should be compressed
in order to measure static load-bearing capacity?" This question is important
for the proper setup of laboratory experiments and computer simulations of
buckling.
Additionally, it is shown that the behavior of a perfect column having an
initial deflection differ significantlys form the behavior of an imperfect
column. In particular, the dependence of the maximum force on the compression
rate is non-monotonic. The analytical results are supported by numerical
simulations and available experimental data.Comment: 11 pages, 4 figure
A Review of the Role of Melatonin in Irritable Bowel Syndrome
Irritable bowel syndrome (IBS) is a troubling disease experienced worldwide. The presentation of symptoms varies from patient to patient, and current prescription treatments can be inadequate in resolving symptoms. This article explores the available scientific literature supporting the use of melatonin in alleviating IBS symptoms
A variational approach for calculating Auger electron spectra: going beyond the impurity approximation
We propose a novel variational method to calculate the two-hole propagators
relevant for Auger spectroscopy in transition metal oxides. This method can be
thought of as an intermediary step between the full solution (which is
difficult to generalize to systems with partially filled bands) and the
impurity approximation. Like the former, our solution has full translational
invariance, and like the latter, it can be generalized to certain types of
systems with partially filled bands. Here we compare both our variational
approximation and the impurity approximation against the exact solution for a
simple one-dimensional model with filled bands. We show that when the energies
of the eigenstates residing primarily on the transition metal ions do not
overlap with those of the eigenstates residing primarily on Oxygen ions, both
approximations are valid but the variational approach is superior.Comment: 12 pages, 11 figure
Magnon-mediated interactions between fermions depend strongly on the lattice structure
We propose two new methods to calculate exactly the spectrum of two
spin- charge carriers moving in a ferromagnetic background, at zero
temperature. We find that if the spins are located on a different sublattice
than that on which the fermions move, magnon-mediated effective interactions
are very strong and can bind the fermions into low-energy bipolarons with
triplet character. This never happens in models where spins and charge carriers
share the same lattice, whether they are in the same band or in different
bands. This proves that effective one-lattice models do not describe correctly
the low-energy part of the two-carrier spectrum of a two-sublattice model, even
though they may describe the low-energy single-carrier spectrum appropriately
High-spin polaron in lightly doped CuO planes
We device and investigate numerically a minimal yet detailed spin polaron
model that describes lightly doped CuO layers. The low-energy physics of a
hole is studied by total-spin-resolved exact diagonalization on clusters of up
to 32 CuO unit cells, revealing features missed by previous studies. In
particular, spin-polaron states with total spin 3/2 are the lowest eigenstates
in several regions of the Brillouin zone. In these regions, and also at other
points the quasiparticle weight is identically zero, indicating orthogonal
states to those represented in the one electron Green's function. This
highlights the importance of proper treatment of spin fluctuations in the
many-body background.Comment: To appear in Phys. Rev. Lett. Final version and Supplementary
Materials will be available at the journal's websit
Reply to "Comment on 'High-Spin Polaron in Lightly Doped CuO Planes'"
In arXiv:1108.5413v1, Lee and Lee use several comparisons to argue that the
physics of the three-band model found (Phys. Rev. Lett. 106 036401 (2011)) can
be explained in the one-band model's framework (Phys. Rev. Lett. 91 057001
(2003)). While superficial similarities exist between the two sets of results,
for reasons discussed in this reply, we disagree that they describe the same
physics
The Green's Function of the Holstein Polaron
We present a novel, highly efficient yet accurate analytical approximation
for the Green's function of a Holstein polaron. It is obtained by summing all
the self-energy diagrams, but with each self-energy diagram averaged over the
momenta of its free propagators. The result becomes exact for both zero
bandwidth and for zero electron-phonon coupling, and is accurate everywhere in
the parameter space. The resulting Green's function satisfies exactly the first
six spectral weight sum rules. All higher sum rules are satisfied with great
accuracy, becoming asymptotically exact for coupling both much larger and much
smaller than the free particle bandwidth. Comparison with existing numerical
data also confirms this accuracy. We use this approximation to analyze in
detail the redistribution of the spectral weight as the coupling strength
varies.Comment: 21 pages, 54 figure
Sensitivity of vortex pairing and mixing to initial perturbations in stratified shear flows
The effects of different initial perturbations on the evolution of stratified
shear flows that are subject to Kelvin-Helmholtz instability and vortex pairing
have been investigated through Direct Numerical Simulation (DNS). The effects
of purely random perturbations of the background flow are sensitive to the
phase of the subharmonic component of the perturbation that has a wavelength
double that of the Kelvin-Helmholtz instability. If the phase relationship
between the Kelvin-Helmholtz mode and its subharmonic mode is optimal, or close
to it, vortex pairing occurs. Vortex paring is delayed when there is a phase
difference, and this delay increases with increasing phase difference. In three
dimensional simulations vortex pairing is suppressed if the phase difference is
sufficiently large, reducing the amount of mixing and mixing efficiency. For a
given phase difference close enough to the optimal phase, the response of the
flow to eigenvalues perturbations is very similar to the response to random
perturbations. In addition to traditional diagnostics, we show quantitatively
that a non-modal Fourier component in a random perturbation quickly evolves to
be modal and describe the process of vortex pairing using Lagrangian
trajectories
Graphene-based quantum capacitance wireless vapor sensors
A wireless vapor sensor based upon the quantum capacitance effect in graphene
is demonstrated. The sensor consists of a metal-oxide-graphene variable
capacitor (varactor) coupled to an inductor, creating a resonant oscillator
circuit. The resonant frequency is found to shift in proportion to water vapor
concentration for relative humidity (RH) values ranging from 1% to 97% with a
linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values
extracted from the wireless measurements agree with those determined from
capacitance-voltage measurements, providing strong evidence that the sensing
arises from the variable quantum capacitance in graphene. These results
represent a new sensor transduction mechanism and pave the way for graphene
quantum capacitance sensors to be studied for a wide range of chemical and
biological sensing applications.Comment: 8 pages, 7 figure
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