7,317 research outputs found
Channeling 5-min photospheric oscillations into the solar outer atmosphere through small-scale vertical magnetic flux tubes
We report two-dimensional MHD simulations which demonstrate that photospheric
5-min oscillations can leak into the chromosphere inside small-scale vertical
magnetic flux tubes. The results of our numerical experiments are compatible
with those inferred from simultaneous spectropolarimetric observations of the
photosphere and chromosphere obtained with the Tenerife Infrared Polarimeter
(TIP) at 10830 A. We conclude that the efficiency of energy exchange by
radiation in the solar photosphere can lead to a significant reduction of the
cut-off frequency and may allow for the propagation of the 5 minutes waves
vertically into the chromosphere.Comment: accepted by ApJ
Spectroscopy of doubly charmed baryons
We study the mass spectrum of baryons with two and three charmed quarks. For
double charm baryons the spin splitting is found to be smaller than standard
quark-model potential predictions. This splitting is not influenced either by
the particular form of the confining potential or by the regularization taken
for the contact term of the spin-spin potential. We consistently predict the
spectra for triply charmed baryons.Comment: 6 pages, 1 figure, accepted for publication in Phys. Rev.
Canonical Quantization of the Maxwell-Chern-Simons Theory in the Coulomb Gauge
The Maxwell-Chern-Simons theory is canonically quantized in the Coulomb gauge
by using the Dirac bracket quantization procedure. The determination of the
Coulomb gauge polarization vector turns out to be intrincate. A set of quantum
Poincar\'e densities obeying the Dirac-Schwinger algebra, and, therefore, free
of anomalies, is constructed. The peculiar analytical structure of the
polarization vector is shown to be at the root for the existence of spin of the
massive gauge quanta.The Coulomb gauge Feynman rules are used to compute the
M\"oller scattering amplitude in the lowest order of perturbation theory. The
result coincides with that obtained by using covariant Feynman rules. This
proof of equivalence is, afterwards, extended to all orders of perturbation
theory. The so called infrared safe photon propagator emerges as an effective
propagator which allows for replacing all the terms in the interaction
Hamiltonian of the Coulomb gauge by the standard field-current minimal
interaction Hamiltonian.Comment: 21 pages, typeset in REVTEX, figures not include
Analyzing and Predicting Verification of Data-Aware Process Models – a Case Study with Spectrum Auctions
Verification techniques play an essential role in detecting undesirable behaviors in many applications like spectrum auctions. By verifying an auction design, one can detect the least favorable outcomes, e.g., the lowest revenue of an auctioneer. However, verification may be infeasible in practice, given the vast size of the state space on the one hand and the large number of properties to be verified on the other hand. To overcome this challenge, we leverage machine-learning techniques. In particular, we create a dataset by verifying properties of a spectrum auction first. Second, we use this dataset to analyze and predict outcomes of the auction and characteristics of the verification procedure. To evaluate the usefulness of machine learning in the given scenario, we consider prediction quality and feature importance. In our experiments, we observe that prediction models can capture relationships in our dataset well, though one needs to be careful to obtain a representative and sufficiently large training dataset. While the focus of this article is on a specific verification scenario, our analysis approach is general and can be adapted to other domains
Ground-state phase diagram of the spin-1/2 square-lattice J1-J2 model with plaquette structure
Using the coupled cluster method for high orders of approximation and Lanczos
exact diagonalization we study the ground-state phase diagram of a quantum
spin-1/2 J1-J2 model on the square lattice with plaquette structure. We
consider antiferromagnetic (J1>0) as well as ferromagnetic (J1<0)
nearest-neighbor interactions together with frustrating antiferromagnetic
next-nearest-neighbor interaction J2>0. The strength of inter-plaquette
interaction lambda varies between lambda=1 (that corresponds to the uniform
J1-J2 model) and lambda=0 (that corresponds to isolated frustrated 4-spin
plaquettes). While on the classical level (s \to \infty) both versions of
models (i.e., with ferro- and antiferromagnetic J1) exhibit the same
ground-state behavior, the ground-state phase diagram differs basically for the
quantum case s=1/2. For the antiferromagnetic case (J1 > 0) Neel
antiferromagnetic long-range order at small J2/J1 and lambda \gtrsim 0.47 as
well as collinear striped antiferromagnetic long-range order at large J2/J1 and
lambda \gtrsim 0.30 appear which correspond to their classical counterparts.
Both semi-classical magnetic phases are separated by a nonmagnetic quantum
paramagnetic phase. The parameter region, where this nonmagnetic phase exists,
increases with decreasing of lambda. For the ferromagnetic case (J1 < 0) we
have the trivial ferromagnetic ground state at small J2/|J1|. By increasing of
J2 this classical phase gives way for a semi-classical plaquette phase, where
the plaquette block spins of length s=2 are antiferromagnetically long-range
ordered. Further increasing of J2 then yields collinear striped
antiferromagnetic long-range order for lambda \gtrsim 0.38, but a nonmagnetic
quantum paramagnetic phase lambda \lesssim 0.38.Comment: 10 pages, 15 figure
Wave propagation in two-dimensional periodic lattices
International audiencePlane wave propagation in infinite two-dimensional periodic lattices is investigated using Floquet-Bloch principles. Frequency bandgaps and spatial filtering phenomena are examined in four representative planar lattice topologies: hexagonal honeycomb, Kagomé lattice, triangular honeycomb, and the square honeycomb. These topologies exhibit dramatic differences in their long-wavelength deformation properties. Long-wavelength asymptotes to the dispersion curves based on homogenization theory are in good agreement with the numerical results for each of the four lattices. The slenderness ratio of the constituent beams of the lattice (or relative density) has a significant influence on the band structure. The techniques developed in this work can be used to design lattices with a desired band structure. The observed spatial filtering effects due to anisotropy at high frequencies (short wavelengths) of wave propagation are consistent with the lattice symmetries
One-dimensional metallic behavior of the stripe phase in LaSrCuO
Using an exact diagonalization method within the dynamical mean-field theory
we study stripe phases in the two-dimensional Hubbard model. We find a
crossover at doping from diagonal stripes to vertical
site-centered stripes with populated domain walls, stable in a broad range of
doping, . The calculated chemical potential shift and the doping dependence of the magnetic incommensurability are in
quantitative agreement with the experimental results for doped
LaSrCuO. The electronic structure shows one-dimensional
metallic behavior along the domain walls, and explains the suppression of
spectral weight along the Brillouin zone diagonal.Comment: 4 pages, 4 figure
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study
We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.Comment: 10 pages, 7 figure
On the Mass of Dense Star Clusters in Starburst Galaxies from Spectro-Photometry
The mass of unresolved young star clusters derived from spectro-photometric
data may well be off by a factor of 2 or more once the migration of massive
stars driven by mass segregation is accounted for. We quantify this effect for
a large set of cluster parameters, including variations in the stellar IMF, the
intrinsic cluster mass, and mean mass density. Gas-dynamical models coupled
with the Cambridge stellar evolution tracks allow us to derive a scheme to
recover the real cluster mass given measured half-light radius, one-dimensional
velocity dispersion and age. We monitor the evolution with time of the ratio of
real to apparent mass through the parameter eta. When we compute eta for rich
star clusters, we find non-monotonic evolution in time when the IMF stretches
beyond a critical cutoff mass of 25.5 solar mass. We also monitor the rise of
color gradients between the inner and outer volume of clusters: we find trends
in time of the stellar IMF power indices overlapping well with those derived
for the LMC cluster NGC 1818 at an age of 30 Myr. We argue that the core region
of massive Antennae clusters should have suffered from much segregation despite
their low ages. We apply these results to a cluster mass function, and find
that the peak of the mass distribution would appear to observers shifted to
lower masses by as much as 0.2 dex. The star formation rate (SFR) derived for
the cluster population is then underestimated by from 20 to 50 per cent.Comment: 20 pages, 16 figures, accepted for publication in MNRA
Lo-fi prototyping to design interactive-tabletop applications for children
Interactive tabletops are an exiting new platform for supporting children's collaboration. With design guidelines and standardized interaction principles still immature, there is a considerable need for iterative prototyping to define the task and interface. Lo-fi prototypes-using cardboard, paper, etc.- are easy to develop, flexible to adjust during design sessions, and intuitive for users to manipulate. Using them can be a valuable step in designing tabletop applications.
In this paper, we detail the design process of two tabletop applications, concentrating on the role of lo-fi prototyping. TransTime is a pattern game for 5-6 year olds to engage how time progresses. OurSpace is a design tool for 7-9 year olds to arrange desks and assign seats for students in their classroom. By comparing the experiences, we arrive at a better understanding of the benefits, challenges, and limits of using lo-fi prototypes to design interactive-tabletop applications for children
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