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f7ae5cb8-b453-4d27-9e1f-eb9b993da2fd | f7ae5cb8-b453-4d27-9e1f-eb9b993da2fd | f7ae5cb8-b453-4d27-9e1f-eb9b993da2fd | human | null | null | none | abstracts | Compton Scattering of Fe K alpha Lines in Magnetic Cataclysmic Variables | null | Compton scattering of X-rays in the bulk flow of the accretion column in
magnetic cataclysmic variables (mCVs) can significantly shift photon energies.
We present Monte Carlo simulations based on a nonlinear algorithm demonstrating
the effects of Compton scattering on the H-like, He-like and neutral Fe K alpha
lines produced in the post-shock region of the accretion column. The peak line
emissivities of the photons in the post-shock flow are taken into consideration
and frequency shifts due to Doppler effects are also included. We find that
line profiles are most distorted by Compton scattering effects in strongly
magnetized mCVs with a low white dwarf mass and high mass accretion rate and
which are viewed at an oblique angle with respect to the accretion column. The
resulting line profiles are most sensitive to the inclination angle. We have
also explored the effects of modifying the accretion column width and using a
realistic emissivity profile. We find that these do not have a significant
overall effect on the resulting line profiles. A comparison of our simulated
line spectra with high resolution Chandra/HETGS observations of the mCV GK Per
indicates that a wing feature redward of the 6.4 keV line may result from
Compton recoil near the base of the accretion column.
|
da7be0aa-1ee7-41ac-9064-10d871fbd27e | da7be0aa-1ee7-41ac-9064-10d871fbd27e | da7be0aa-1ee7-41ac-9064-10d871fbd27e | human | null | null | none | abstracts | The Limits of Special Relativity | null | The Special Theory of Relativity and the Theory of the Electron have had an
interesting history together. Originally the electron was studied in a non
relativistic context and this opened up the interesting possibility that lead
to the conclusion that the mass of the electron could be thought of entirely in
electromagnetic terms without introducing inertial considerations. However the
application of Special Relativity lead to several problems, both for an
extended electron and the point electron. These inconsistencies have, contrary
to popular belief not been resolved satisfactorily today, even within the
context of Quantum Theory. Nevertheless these and subsequent studies bring out
the interesting result that Special Relativity breaks down within the Compton
scale or when the Compton scale is not neglected. This again runs contrary to
an uncritical notion that Special Relativity is valid for point particles.
|
3c565bc8-6771-461f-98c1-e34db17ff997 | 3c565bc8-6771-461f-98c1-e34db17ff997 | 3c565bc8-6771-461f-98c1-e34db17ff997 | human | null | null | none | abstracts | Singular Energy Distributions in Granular Media | null | We study the kinetic theory of driven granular gases, taking into account
both translational and rotational degrees of freedom. We obtain the high-energy
tail of the stationary bivariate energy distribution, depending on the total
energy E and the ratio x=sqrt{E_w/E} of rotational energy E_w to total energy.
Extremely energetic particles have a unique and well-defined distribution f(x)
which has several remarkable features: x is not uniformly distributed as in
molecular gases; f(x) is not smooth but has multiple singularities. The latter
behavior is sensitive to material properties such as the collision parameters,
the moment of inertia and the collision rate. Interestingly, there are
preferred ratios of rotational-to-total energy. In general, f(x) is strongly
correlated with energy and the deviations from a uniform distribution grow with
energy. We also solve for the energy distribution of freely cooling Maxwell
Molecules and find qualitatively similar behavior.
|
bbdaf766-145e-4d8f-a95b-900742050954 | bbdaf766-145e-4d8f-a95b-900742050954 | bbdaf766-145e-4d8f-a95b-900742050954 | human | null | null | none | abstracts | Cosmological Shock Waves in the Large Scale Structure of the Universe:
Non-gravitational Effects | null | Cosmological shock waves result from supersonic flow motions induced by
hierarchical clustering of nonlinear structures in the universe. These shocks
govern the nature of cosmic plasma through thermalization of gas and
acceleration of nonthermal, cosmic-ray (CR) particles. We study the statistics
and energetics of shocks formed in cosmological simulations of a concordance
$\Lambda$CDM universe, with a special emphasis on the effects of
non-gravitational processes such as radiative cooling, photoionization/heating,
and galactic superwind feedbacks. Adopting an improved model for gas
thermalization and CR acceleration efficiencies based on nonlinear diffusive
shock acceleration calculations, we then estimate the gas thermal energy and
the CR energy dissipated at shocks through the history of the universe. Since
shocks can serve as sites for generation of vorticity, we also examine the
vorticity that should have been generated mostly at curved shocks in
cosmological simulations. We find that the dynamics and energetics of shocks
are governed primarily by the gravity of matter, so other non-gravitational
processes do not affect significantly the global energy dissipation and
vorticity generation at cosmological shocks. Our results reinforce scenarios in
which the intracluster medium and warm-hot intergalactic medium contain
energetically significant populations of nonthermal particles and turbulent
flow motions.
|
5bf0c29c-55ea-4a3c-8548-fae83c3947fa | 5bf0c29c-55ea-4a3c-8548-fae83c3947fa | 5bf0c29c-55ea-4a3c-8548-fae83c3947fa | human | null | null | none | abstracts | Outstanding Issues in Our Understanding of L, T, and Y Dwarfs | null | Since the discovery of the first L dwarf 19 years ago and the discovery of
the first T dwarf 7 years after that, we have amassed a large list of these
objects, now numbering almost six hundred. Despite making headway in
understanding the physical chemistry of their atmospheres, some important
issues remain unexplained. Three of these are the subject of this paper: (1)
What is the role of "second parameters" such as gravity and metallicity in
shaping the emergent spectra of L and T dwarfs? Can we establish a robust
classification scheme so that objects with unusual values of log(g) or [M/H],
unusual dust content, or unresolved binarity are easily recognized? (2) Which
physical processes drive the unusual behavior at the L/T transition? Which
observations can be obtained to better confine the problem? (3) What will
objects cooler than T8 look like? How will we know a Y dwarf when we first
observe one?
|
ec57ae69-9bc8-4a2b-ae47-a8b495446f86 | ec57ae69-9bc8-4a2b-ae47-a8b495446f86 | ec57ae69-9bc8-4a2b-ae47-a8b495446f86 | human | null | null | none | abstracts | The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A
tidally-evolved binary Centaur | null | Hubble Space Telescope observations of Uranus- and Neptune-crossing object
(65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a
close binary system. The mutual orbit has a period of 9.554 +/- 0.011 days and
a semimajor axis of 1840 +/- 48 km. These values enable computation of a system
mass of (5.41 +/- 0.42) 10^18 kg. Spitzer Space Telescope observations of
thermal emission at 24 and 70 microns are combined with visible photometry to
constrain the system's effective radius (109 +10/-11 km) and geometric albedo
(0.084 +0.021/-0.014). We estimate the average bulk density to be 1.37
+0.66/-0.32 g cm^-3, consistent with ice plus rocky and/or carbonaceous
materials. This density contrasts with lower densities recently measured with
the same technique for three other comparably-sized outer Solar System binaries
(617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to
the density of the saturnian irregular satellite Phoebe. The mutual orbit of
Ceto and Phorcys is nearly circular, with an eccentricity <= 0.015. This
observation is consistent with calculations suggesting that the system should
tidally evolve on a timescale shorter than the age of the solar system.
|
d0875b17-51fc-4831-9891-447e737aa64e | d0875b17-51fc-4831-9891-447e737aa64e | d0875b17-51fc-4831-9891-447e737aa64e | human | null | null | none | abstracts | GLRT-Optimal Noncoherent Lattice Decoding | null | This paper presents new low-complexity lattice-decoding algorithms for
noncoherent block detection of QAM and PAM signals over complex-valued fading
channels. The algorithms are optimal in terms of the generalized likelihood
ratio test (GLRT). The computational complexity is polynomial in the block
length; making GLRT-optimal noncoherent detection feasible for implementation.
We also provide even lower complexity suboptimal algorithms. Simulations show
that the suboptimal algorithms have performance indistinguishable from the
optimal algorithms. Finally, we consider block based transmission, and propose
to use noncoherent detection as an alternative to pilot assisted transmission
(PAT). The new technique is shown to outperform PAT.
|
98188352-7836-4418-aa3d-bfdb6646dcef | 98188352-7836-4418-aa3d-bfdb6646dcef | 98188352-7836-4418-aa3d-bfdb6646dcef | human | null | null | none | abstracts | Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon
from IRTF/SpeX observations | null | We present 0.8 to 2.4 micron spectral observations of uranian satellites,
obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the
first time the presence of CO2 ice on the surfaces of Umbriel and Titania, by
means of 3 narrow absorption bands near 2 microns. Several additional, weaker
CO2 ice absorptions have also been detected. No CO2 absorption is seen in
Oberon spectra, and the strengths of the CO2 ice bands decline with
planetocentric distance from Ariel through Titania. We use the CO2 absorptions
to map the longitudinal distribution of CO2 ice on Ariel, Umbriel, and Titania,
showing that it is most abundant on their trailing hemispheres. We also examine
H2O ice absorptions in the spectra, finding deeper H2O bands on the leading
hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon.
Potential mechanisms to produce the observed longitudinal and planetocentric
distributions of the two ices are considered.
|
1a4f1b24-3141-4472-86cf-c877ea16b4bd | 1a4f1b24-3141-4472-86cf-c877ea16b4bd | 1a4f1b24-3141-4472-86cf-c877ea16b4bd | human | null | null | none | abstracts | Extremely strong-coupling superconductivity and anomalous lattice
properties in the beta-pyrochlore oxide KOs2O6 | null | Superconducting and normal-state properties of the beta-pyrochlore oxide
KOs2O6 are studied by means of thermodynamic and transport measurements. It is
shown that the superconductivity is of conventional s-wave type and lies in the
extremely strong-coupling regime. Specific heat and resistivity measurements
reveal that there are characteristic low-energy phonons that give rise to
unusual scattering of carriers due to strong electron-phonon interactions. The
entity of the low-energy phonons is ascribed to the heavy rattling of the K ion
confined in an oversized cage made of OsO6 octahedra. It is suggested that this
electron-rattler coupling mediates the Cooper pairing, resulting in the
extremely strong-coupling superconductivity.
|
5632b735-1ba3-49bc-9a27-e496fbf56e44 | 5632b735-1ba3-49bc-9a27-e496fbf56e44 | 5632b735-1ba3-49bc-9a27-e496fbf56e44 | human | null | null | none | abstracts | On the exact formula for neutrino oscillation probability by Kimura,
Takamura and Yokomakura | null | The exact formula for the neutrino oscillation probability in matter with
constant density, which was discovered by Kimura, Takamura and Yokomakura, has
been applied mostly to the standard case with three flavor neutrino so far. In
this paper applications of their formula to more general cases are discussed.
It is shown that this formalism can be generalized to various cases where the
matter potential have off-diagonal components, and the two non-trivial examples
are given: the case with magnetic moments and a magnetic field and the case
with non-standard interactions. It is pointed out that their formalism can be
applied also to the case in the long baseline limit with matter whose density
varies adiabatically as in the case of solar neutrino.
|
12690fdd-dc99-4831-9a69-f5765365c2ba | 12690fdd-dc99-4831-9a69-f5765365c2ba | 12690fdd-dc99-4831-9a69-f5765365c2ba | human | null | null | none | abstracts | Absolute measurement of the nitrogen fluorescence yield in air between
300 and 430 nm | null | The nitrogen fluorescence induced in air is used to detect ultra-high energy
cosmic rays and to measure their energy. The precise knowledge of the absolute
fluorescence yield is the key quantity to improve the accuracy on the cosmic
ray energy. The total yield has been measured in dry air using a 90Sr source
and a [300-430 nm] filter. The fluorescence yield in air is 4.23 $\pm$ 0.20
photons per meter when normalized to 760 mmHg, 15 degrees C and with an
electron energy of 0.85 MeV. This result is consistent with previous
experiments made at various energies, but with an accuracy improved by a factor
of about 3. For the first time, the absolute continuous spectrum of nitrogen
excited by 90Sr electrons has also been measured with a spectrometer. Details
of this experiment are given in one of the author's PhD thesis [32].
|
807b0276-5255-456e-8808-5d42b83aaa0f | 807b0276-5255-456e-8808-5d42b83aaa0f | 807b0276-5255-456e-8808-5d42b83aaa0f | human | null | null | none | abstracts | The Phase-resolved High Energy Spectrum of the Crab Pulsar | null | We present a modified outer gap model to study the phase-resolved spectra of
the Crab pulsar. A theoretical double peak profile of the light curve
containing the whole phase is shown to be consistent with the observed light
curve of the Crab pulsar by shifting the inner boundary of the outer gap
inwardly to $\sim 10$ stellar radii above the neutron star surface. In this
model, the radial distances of the photons corresponding to different phases
can be determined in the numerical calculation. Also the local electrodynamics,
such as the accelerating electric field, the curvature radius of the magnetic
field line and the soft photon energy, are sensitive to the radial distances to
the neutron star. Using a synchrotron self-Compton mechanism, the
phase-resolved spectra with the energy range from 100 eV to 3 GeV of the Crab
pulsar can also be explained.
|
18e78a04-3046-4736-8e80-37daad6418b8 | 18e78a04-3046-4736-8e80-37daad6418b8 | 18e78a04-3046-4736-8e80-37daad6418b8 | human | null | null | none | abstracts | Critical Current of Type-II Superconductors in a Broken Bose Glass State | null | The tilt modulus of a defective Abrikosov vortex lattice pinned by material
line defects is computed using the boson analogy. It tends to infinity at long
wavelength, which yields a Bose glass state that is robust to the addition of
weak point-pinning centers, and which implies a restoring force per vortex line
for rigid translations about mechanical equilibrium that is independent of
magnetic field. It also indicates that the Bose glass state breaks into pieces
along the direction of the correlated pinning centers if the latter have finite
length. The critical current is predicted to crossover from two dimensional to
three dimensional behavior as a function of sample thickness along the
correlated pinning centers in such case. That crossover notably can occur at a
film thickness that is much larger than that expected from point pins of
comparable strength. The above is compared to the dependence on thickness shown
by the critical current in certain films of high-temperature superconductors
currently being developed for wire technology.
|
3481949f-c6a7-46f9-9dde-254dd9dea75f | 3481949f-c6a7-46f9-9dde-254dd9dea75f | 3481949f-c6a7-46f9-9dde-254dd9dea75f | human | null | null | none | abstracts | Rounding of first-order phase transitions and optimal cooperation in
scale-free networks | null | We consider the ferromagnetic large-$q$ state Potts model in complex evolving
networks, which is equivalent to an optimal cooperation problem, in which the
agents try to optimize the total sum of pair cooperation benefits and the
supports of independent projects. The agents are found to be typically of two
kinds: a fraction of $m$ (being the magnetization of the Potts model) belongs
to a large cooperating cluster, whereas the others are isolated one man's
projects. It is shown rigorously that the homogeneous model has a strongly
first-order phase transition, which turns to second-order for random
interactions (benefits), the properties of which are studied numerically on the
Barab\'asi-Albert network. The distribution of finite-size transition points is
characterized by a shift exponent, $1/\tilde{\nu}'=.26(1)$, and by a different
width exponent, $1/\nu'=.18(1)$, whereas the magnetization at the transition
point scales with the size of the network, $N$, as: $m\sim N^{-x}$, with
$x=.66(1)$.
|
280b3279-045d-47f8-a048-c3e1abe5d5b6 | 280b3279-045d-47f8-a048-c3e1abe5d5b6 | 280b3279-045d-47f8-a048-c3e1abe5d5b6 | human | null | null | none | abstracts | A New Monte Carlo Method and Its Implications for Generalized Cluster
Algorithms | null | We describe a novel switching algorithm based on a ``reverse'' Monte Carlo
method, in which the potential is stochastically modified before the system
configuration is moved. This new algorithm facilitates a generalized
formulation of cluster-type Monte Carlo methods, and the generalization makes
it possible to derive cluster algorithms for systems with both discrete and
continuous degrees of freedom. The roughening transition in the sine-Gordon
model has been studied with this method, and high-accuracy simulations for
system sizes up to $1024^2$ were carried out to examine the logarithmic
divergence of the surface roughness above the transition temperature, revealing
clear evidence for universal scaling of the Kosterlitz-Thouless type.
|
451599a6-f9e9-4b95-85b2-42f285bc04b9 | 451599a6-f9e9-4b95-85b2-42f285bc04b9 | 451599a6-f9e9-4b95-85b2-42f285bc04b9 | human | null | null | none | abstracts | The Orbifolds of Permutation-Type as Physical String Systems at
Multiples of $c=26$ III. The Spectra of $\hat{c}=52$ Strings | null | In the second paper of this series, I obtained the twisted BRST systems and
extended physical-state conditions of all twisted open and closed $\hat{c} =
52$ strings. In this paper, I supplement the extended physical-state conditions
with the explicit form of the extended (twisted) Virasoro generators of all
$\hat{c} = 52$ strings, which allows us to discuss the physical spectra of
these systems. Surprisingly, all the $\hat{c}=52$ spectra admit an equivalent
description in terms of generically-unconventional Virasoro generators at
$c=26$. This description strongly supports our prior conjecture that the
$\hat{c}=52$ strings are free of negative-norm states, and moreover shows that
the spectra of some of the simpler cases are equivalent to those of ordinary
untwisted open and closed $c=26$ strings.
|
ac68d72a-9f57-4bad-b842-144af16e2c3d | ac68d72a-9f57-4bad-b842-144af16e2c3d | ac68d72a-9f57-4bad-b842-144af16e2c3d | human | null | null | none | abstracts | Quasiparticles in Neon using the Faddeev Random Phase Approximation | null | The spectral function of the closed-shell Neon atom is computed by expanding
the electron self-energy through a set of Faddeev equations. This method
describes the coupling of single-particle degrees of freedom with correlated
two-electron, two-hole, and electron-hole pairs. The excitation spectra are
obtained using the Random Phase Approximation, rather than the Tamm-Dancoff
framework employed in the third-order algebraic diagrammatic contruction
[ADC(3)] method. The difference between these two approaches is studied, as
well as the interplay between ladder and ring diagrams in the self-energy.
Satisfactory results are obtained for the ionization energies as well as the
energy of the ground state with the Faddeev-RPA scheme that is also appropriate
for the high-density electron gas.
|
f4bb666b-7cd3-4c09-af23-2bc4d3e31da6 | f4bb666b-7cd3-4c09-af23-2bc4d3e31da6 | f4bb666b-7cd3-4c09-af23-2bc4d3e31da6 | human | null | null | none | abstracts | Discrete Nonholonomic Lagrangian Systems on Lie Groupoids | null | This paper studies the construction of geometric integrators for nonholonomic
systems. We derive the nonholonomic discrete Euler-Lagrange equations in a
setting which permits to deduce geometric integrators for continuous
nonholonomic systems (reduced or not). The formalism is given in terms of Lie
groupoids, specifying a discrete Lagrangian and a constraint submanifold on it.
Additionally, it is necessary to fix a vector subbundle of the Lie algebroid
associated to the Lie groupoid. We also discuss the existence of nonholonomic
evolution operators in terms of the discrete nonholonomic Legendre
transformations and in terms of adequate decompositions of the prolongation of
the Lie groupoid. The characterization of the reversibility of the evolution
operator and the discrete nonholonomic momentum equation are also considered.
Finally, we illustrate with several classical examples the wide range of
application of the theory (the discrete nonholonomic constrained particle, the
Suslov system, the Chaplygin sleigh, the Veselova system, the rolling ball on a
rotating table and the two wheeled planar mobile robot).
|
af2ec52a-6370-4b9f-b9ea-c5456b4acf3e | af2ec52a-6370-4b9f-b9ea-c5456b4acf3e | af2ec52a-6370-4b9f-b9ea-c5456b4acf3e | human | null | null | none | abstracts | AFM Imaging of SWI/SNF action: mapping the nucleosome remodeling and
sliding | null | We propose a combined experimental (Atomic Force Microscopy) and theoretical
study of the structural and dynamical properties of nucleosomes. In contrast to
biochemical approaches, this method allows to determine simultaneously the DNA
complexed length distribution and nucleosome position in various contexts.
First, we show that differences in the nucleo-proteic structure observed
between conventional H2A and H2A.Bbd variant nucleosomes induce quantitative
changes in the in the length distribution of DNA complexed with histones. Then,
the sliding action of remodeling complex SWI/SNF is characterized through the
evolution of the nucleosome position and wrapped DNA length mapping. Using a
linear energetic model for the distribution of DNA complexed length, we extract
the net wrapping energy of DNA onto the histone octamer, and compare it to
previous studies.
|
bd962af4-c2f6-4393-a62d-523f9055a91b | bd962af4-c2f6-4393-a62d-523f9055a91b | bd962af4-c2f6-4393-a62d-523f9055a91b | human | null | null | none | abstracts | Saturated actions by finite dimensional Hopf *-algebras on C*-algebras | null | If a finite group action $\alpha$ on a unital $C^*$-algebra $M$ is saturated,
the canonical conditional expectation $E:M\to M^\alpha$ onto the fixed point
algebra is known to be of index finite type with $Index(E)=|G|$ in the sense of
Watatani. More generally if a finite dimensional Hopf $*$-algebra $A$ acts on
$M$ and the action is saturated, the same is true with $Index (E)=\dim(A)$. In
this paper we prove that the converse is true. Especially in case $M$ is a
commutative $C^*$-algebra $C(X)$ and $\alpha$ is a finite group action, we give
an equivalent condition in order that the expectation $E:C(X)\to C(X)^\alpha$
is of index finite type, from which we obtain that $\alpha$ is saturated if and
only if $G$ acts freely on $X$.
Actions by compact groups are also considered to show that the gauge action
$\gamma$ on a graph $C^*$-algebra $C^*(E)$ associated with a locally finite
directed graph $E$ is saturated.
|
edc9043b-c1c8-44e0-b783-84557c5eab40 | edc9043b-c1c8-44e0-b783-84557c5eab40 | edc9043b-c1c8-44e0-b783-84557c5eab40 | human | null | null | none | abstracts | The electronic structures, the equilibrium geometries and finite
temperature properties of Na_n (n=39-55) | null | Density-functional theory has been applied to investigate systematics of
sodium clusters Na_n in the size range of n= 39-55. A clear evolutionary trend
in the growth of their ground-state geometries emerges. The clusters at the
beginning of the series (n=39-43) are symmetric and have partial icosahedral
(two-shell) structure. The growth then goes through a series of disordered
clusters (n=44-52) where the icosahedral core is lost. However, for n>52 a
three shell icosahedral structure emerges. This change in the nature of the
geometry is abrupt. In addition, density-functional molecular dynamics has been
used to calculate the specific heat curves for the representative sizes n= 43,
45, 48 and 52. These results along with already available thermodynamic
calculations for n= 40, 50, and 55 enable us to carry out a detailed comparison
of the heat capacity curves with their respective geometries for the entire
series. Our results clearly bring out strong correlation between the evolution
of the geometries and the nature of the shape of the heat capacities. The
results also firmly establish the size-sensitive nature of the heat capacities
in sodium clusters.
|
1423dacf-54fe-4b7f-a085-f3a1c86097b5 | 1423dacf-54fe-4b7f-a085-f3a1c86097b5 | 1423dacf-54fe-4b7f-a085-f3a1c86097b5 | human | null | null | none | abstracts | Quantum Zeno Effect in the Decoherent Histories | null | The quantum Zeno effect arises due to frequent observation. That implies the
existence of some experimenter and its interaction with the system. In this
contribution, we examine what happens for a closed system if one considers a
quantum Zeno type of question, namely: "what is the probability of a system,
remaining always in a particular subspace". This has implications to the
arrival time problem that is also discussed. We employ the decoherent histories
approach to quantum theory, as this is the better developed formulation of
closed system quantum mechanics, and in particular, dealing with questions that
involve time in a non-trivial way. We get a very restrictive decoherence
condition, that implies that even if we do introduce an environment, there will
be very few cases that we can assign probabilities to these histories, but in
those cases, the quantum Zeno effect is still present.
|
92265c3a-28a7-48c4-80a7-4926801890a7 | 92265c3a-28a7-48c4-80a7-4926801890a7 | 92265c3a-28a7-48c4-80a7-4926801890a7 | human | null | null | none | abstracts | Green function theory versus Quantum Monte Carlo calculations for thin
magnetic films | null | In this work we compare numerically exact Quantum Monte Carlo (QMC)
calculations and Green function theory (GFT) calculations of thin ferromagnetic
films including second order anisotropies. Thereby we concentrate on easy plane
systems, i.e. systems for which the anisotropy favors a magnetization parallel
to the film plane. We discuss these systems in perpendicular external field,
i.e. B parallel to the film normal. GFT results are in good agreement with QMC
for high enough fields and temperatures. Below a critical field or a critical
temperature no collinear stable magnetization exists in GFT. On the other hand
QMC gives finite magnetization even below those critical values. This indicates
that there occurs a transition from non-collinear to collinear configurations
with increasing field or temperature. For slightly tilted external fields a
rotation of magnetization from out-of-plane to in-plane orientation is found
with decreasing temperature.
|
389710bd-e989-410a-a75f-dae1c55dc610 | 389710bd-e989-410a-a75f-dae1c55dc610 | 389710bd-e989-410a-a75f-dae1c55dc610 | human | null | null | none | abstracts | Galaxy evolution in the infra-red: comparison of a hierarchical galaxy
formation model with SPITZER data | null | We present predictions for the evolution of the galaxy luminosity function,
number counts and redshift distributions in the IR based on the Lambda-CDM
cosmological model. We use the combined GALFORM semi-analytical galaxy
formation model and GRASIL spectrophotometric code to compute galaxy SEDs
including the reprocessing of radiation by dust. The model, which is the same
as that in Baugh et al (2005), assumes two different IMFs: a normal solar
neighbourhood IMF for quiescent star formation in disks, and a very top-heavy
IMF in starbursts triggered by galaxy mergers. We have shown previously that
the top-heavy IMF seems to be necessary to explain the number counts of faint
sub-mm galaxies. We compare the model with observational data from the SPITZER
Space Telescope, with the model parameters fixed at values chosen before
SPITZER data became available. We find that the model matches the observed
evolution in the IR remarkably well over the whole range of wavelengths probed
by SPITZER. In particular, the SPITZER data show that there is strong evolution
in the mid-IR galaxy luminosity function over the redshift range z ~ 0-2, and
this is reproduced by our model without requiring any adjustment of parameters.
On the other hand, a model with a normal IMF in starbursts predicts far too
little evolution in the mid-IR luminosity function, and is therefore excluded.
|
ffb4d25e-0962-4e0e-ab7e-8ee7ce3405cd | ffb4d25e-0962-4e0e-ab7e-8ee7ce3405cd | ffb4d25e-0962-4e0e-ab7e-8ee7ce3405cd | human | null | null | none | abstracts | Use of Triangular Elements for Nearly Exact BEM Solutions | null | A library of C functions yielding exact solutions of potential and flux
influences due to uniform surface distribution of singularities on flat
triangular and rectangular elements has been developed. This library, ISLES,
has been used to develop the neBEM solver that is both precise and fast in
solving a wide range of problems of scientific and technological interest. Here
we present the exact expressions proposed for computing the influence of
uniform singularity distributions on triangular elements and illustrate their
accuracy. We also present a study concerning the time taken to evaluate these
long and complicated expressions \textit{vis a vis} that spent in carrying out
simple quadratures. Finally, we solve a classic benchmark problem in
electrostatics, namely, estimation of the capacitance of a unit square plate
raised to unit volt. For this problem, we present the estimated values of
capacitance and compare them successfully with some of the most accurate
results available in the literature. In addition, we present the variation of
the charge density close to the corner of the plate for various degrees of
discretization. The variations are found to be smooth and converging. This is
in clear contrast to the criticism commonly leveled against usual BEM solvers.
|
f58b55c4-db60-40fe-b073-ad028039eb78 | f58b55c4-db60-40fe-b073-ad028039eb78 | f58b55c4-db60-40fe-b073-ad028039eb78 | human | null | null | none | abstracts | One-way permutations, computational asymmetry and distortion | null | Computational asymmetry, i.e., the discrepancy between the complexity of
transformations and the complexity of their inverses, is at the core of one-way
transformations. We introduce a computational asymmetry function that measures
the amount of one-wayness of permutations. We also introduce the word-length
asymmetry function for groups, which is an algebraic analogue of computational
asymmetry. We relate boolean circuits to words in a Thompson monoid, over a
fixed generating set, in such a way that circuit size is equal to word-length.
Moreover, boolean circuits have a representation in terms of elements of a
Thompson group, in such a way that circuit size is polynomially equivalent to
word-length. We show that circuits built with gates that are not constrained to
have fixed-length inputs and outputs, are at most quadratically more compact
than circuits built from traditional gates (with fixed-length inputs and
outputs). Finally, we show that the computational asymmetry function is closely
related to certain distortion functions: The computational asymmetry function
is polynomially equivalent to the distortion of the path length in Schreier
graphs of certain Thompson groups, compared to the path length in Cayley graphs
of certain Thompson monoids. We also show that the results of Razborov and
others on monotone circuit complexity lead to exponential lower bounds on
certain distortions.
|
9f1acc0d-2a78-4622-864c-024a6ef82de4 | 9f1acc0d-2a78-4622-864c-024a6ef82de4 | 9f1acc0d-2a78-4622-864c-024a6ef82de4 | human | null | null | none | abstracts | A method for the direct determination of the surface gravities of
transiting extrasolar planets | null | We show that the surface gravity of a transiting extrasolar planet can be
calculated from only the spectroscopic orbit of its parent star and the
analysis of its transit light curve. This does not require additional
constraints, such as are often inferred from theoretical stellar models or
model atmospheres. The planet's surface gravity can therefore be measured
precisely and from only directly observable quantities. We outline the method
and apply it to the case of the first known transiting extrasolar planet, HD
209458b. We find a surface gravity of g_p = 9.28 +/- 0.15 m/s, which is an
order of magnitude more precise than the best available measurements of its
mass, radius and density. This confirms that the planet has a much lower
surface gravity that that predicted by published theoretical models of gas
giant planets. We apply our method to all fourteen known transiting extrasolar
planets and find a significant correlation between surface gravity and orbital
period, which is related to the known correlation between mass and period. This
correlation may be the underlying effect as surface gravity is a fundamental
parameter in the evaporation of planetary atmospheres.
|
e06e6fa3-b96b-40ea-8902-5dc9b27d5ca1 | e06e6fa3-b96b-40ea-8902-5dc9b27d5ca1 | e06e6fa3-b96b-40ea-8902-5dc9b27d5ca1 | human | null | null | none | abstracts | On restrictions of balanced 2-interval graphs | null | The class of 2-interval graphs has been introduced for modelling scheduling
and allocation problems, and more recently for specific bioinformatic problems.
Some of those applications imply restrictions on the 2-interval graphs, and
justify the introduction of a hierarchy of subclasses of 2-interval graphs that
generalize line graphs: balanced 2-interval graphs, unit 2-interval graphs, and
(x,x)-interval graphs. We provide instances that show that all the inclusions
are strict. We extend the NP-completeness proof of recognizing 2-interval
graphs to the recognition of balanced 2-interval graphs. Finally we give hints
on the complexity of unit 2-interval graphs recognition, by studying
relationships with other graph classes: proper circular-arc, quasi-line graphs,
K_{1,5}-free graphs, ...
|
af5b540c-e50e-4910-ac00-a418932156f7 | af5b540c-e50e-4910-ac00-a418932156f7 | af5b540c-e50e-4910-ac00-a418932156f7 | human | null | null | none | abstracts | Exchange parameters from approximate self-interaction correction scheme | null | The approximate atomic self-interaction corrections (ASIC) method to density
functional theory is put to the test by calculating the exchange interaction
for a number of prototypical materials, critical to local exchange and
correlation functionals. ASIC total energy calculations are mapped onto an
Heisenberg pair-wise interaction and the exchange constants J are compared to
those obtained with other methods. In general the ASIC scheme drastically
improves the bandstructure, which for almost all the cases investigated
resemble closely available photo-emission data. In contrast the results for the
exchange parameters are less satisfactory. Although ASIC performs reasonably
well for systems where the magnetism originates from half-filled bands, it
suffers from similar problems than those of LDA for other situations. In
particular the exchange constants are still overestimated. This reflects a
subtle interplay between exchange and correlation energy, not captured by the
ASIC.
|
7d35e373-1b63-4966-a785-7175993953dd | 7d35e373-1b63-4966-a785-7175993953dd | 7d35e373-1b63-4966-a785-7175993953dd | human | null | null | none | abstracts | Generalized sqrt(epsilon)-law. The role of unphysical source terms in
resonance line polarization transfer and its importance as an additional test
of NLTE radiative transfer codes | null | Context. A derivation of a generalized sqrt(epsilon)-law for nonthermal
collisional rates of excitation by charged perturbers is presented. Aims. Aim
of this paper is to find a more general analytical expression for a surface
value of the source function which can be used as an addtional tool for
verification of the non-LTE radiative transfer codes. Methods. Under the impact
approximation hypothesis, static, one-dimensional, plane-parallel atmosphere,
constant magnetic field of arbitrary strength and direction, two-level atom
model with unpolarized lower level and stimulated emission neglected, we
introduce the unphysical terms into the equations of statistical equilibrium
and solve the appropriate non-LTE integral equations. Results. We derive a new
analytical condition for the surface values of the source function components
expressed in the basis of irreducible spherical tensors.
|
2e9dff8b-ed1a-450e-a0b1-adf560614e9a | 2e9dff8b-ed1a-450e-a0b1-adf560614e9a | 2e9dff8b-ed1a-450e-a0b1-adf560614e9a | human | null | null | none | abstracts | Retarded electric and magnetic fields of a moving charge: Feynman's
derivation of Li\'enard-Wiechert potentials revisited | null | Retarded electromagnetic potentials are derived from Maxwell's equations and
the Lorenz condition. The difference found between these potentials and the
conventional Li\'{e}nard-Wiechert ones is explained by neglect, for the latter,
of the motion-dependence of the effective charge density. The corresponding
retarded fields of a point-like charge in arbitary motion are compared with
those given by the formulae of Heaviside, Feynman, Jefimenko and other authors.
The fields of an accelerated charge given by the Feynman are the same as those
derived from the Li\'{e}nard-Wiechert potentials but not those given by the
Jefimenko formulae. A mathematical error concerning partial space and time
derivatives in the derivation of the Jefimenko equations is pointed out.
|
338e6f13-25cd-4687-a802-347f1eabce83 | 338e6f13-25cd-4687-a802-347f1eabce83 | 338e6f13-25cd-4687-a802-347f1eabce83 | human | null | null | none | abstracts | Proper motion L and T dwarf candidate members of the Pleiades | null | We present the results of a deep optical-near-infrared multi-epoch survey
covering 2.5 square degrees of the Pleiades open star cluster to search for new
very-low-mass brown dwarf members. A significant (~ 5 year) epoch difference
exists between the optical (CFH12k I-, Z-band) and near infrared (UKIRT WFCAM
J-band) observations. We construct I,I-Z and Z,Z-J colour magnitude diagrams to
select candidate cluster members. Proper motions are computed for all candidate
members and compared to the background field objects to further refine the
sample. We recover all known cluster members within the area of our survey. In
addition, we have discovered 9 new candidate brown dwarf cluster members. The 7
faintest candidates have red Z-J colours and show blue near-infrared colours.
These are consistent with being L and T-type Pleiads. Theoretical models
predict their masses to be around 11 Jupiter masses.
There is 1 errata for this paper
|
9ce49892-4e8f-4aa5-8067-9efc3286a04b | 9ce49892-4e8f-4aa5-8067-9efc3286a04b | 9ce49892-4e8f-4aa5-8067-9efc3286a04b | human | null | null | none | abstracts | Optical implementation and entanglement distribution in Gaussian valence
bond states | null | We study Gaussian valence bond states of continuous variable systems,
obtained as the outputs of projection operations from an ancillary space of M
infinitely entangled bonds connecting neighboring sites, applied at each of $N$
sites of an harmonic chain. The entanglement distribution in Gaussian valence
bond states can be controlled by varying the input amount of entanglement
engineered in a (2M+1)-mode Gaussian state known as the building block, which
is isomorphic to the projector applied at a given site. We show how this
mechanism can be interpreted in terms of multiple entanglement swapping from
the chain of ancillary bonds, through the building blocks. We provide optical
schemes to produce bisymmetric three-mode Gaussian building blocks (which
correspond to a single bond, M=1), and study the entanglement structure in the
output Gaussian valence bond states. The usefulness of such states for quantum
communication protocols with continuous variables, like telecloning and
teleportation networks, is finally discussed.
|
2e6d4bb0-f79b-466b-a0d0-44b986ebf2a5 | 2e6d4bb0-f79b-466b-a0d0-44b986ebf2a5 | 2e6d4bb0-f79b-466b-a0d0-44b986ebf2a5 | human | null | null | none | abstracts | Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8 | null | Mn3V2O8 is a magnetic system in which S = 5/2 Mn2+ is found in the kagome
staircase lattice. Here we report the magnetic phase diagram for temperatures
above 2 K and applied magnetic fields below 9 T, characterized by measurements
of the magnetization and specific heat with field along the three unique
lattice directions. At low applied magnetic fields, the system first orders
magnetically below Tm1 ~ 21 K, and then shows a second magnetic phase
transition at Tm2 ~ 15 K. In addition, a phase transition that is apparent in
specific heat but not seen in magnetization is found for all three applied
field orientations, converging towards Tm2 as H -> 0. The magnetic behavior is
highly anisotropic, with critical fields for magnetic phase boundaries much
higher when the field is applied perpendicular to the Kagome staircase plane
than when applied in-plane. The field-temperature (H - T) phase diagrams are
quite rich, with 7 distinct phases observed.
|
c2fa60e9-2e25-476a-9527-d57199a49045 | c2fa60e9-2e25-476a-9527-d57199a49045 | c2fa60e9-2e25-476a-9527-d57199a49045 | human | null | null | none | abstracts | Can One Estimate The Unconditional Distribution of Post-Model-Selection
Estimators? | null | We consider the problem of estimating the unconditional distribution of a
post-model-selection estimator. The notion of a post-model-selection estimator
here refers to the combined procedure resulting from first selecting a model
(e.g., by a model selection criterion like AIC or by a hypothesis testing
procedure) and then estimating the parameters in the selected model (e.g., by
least-squares or maximum likelihood), all based on the same data set. We show
that it is impossible to estimate the unconditional distribution with
reasonable accuracy even asymptotically. In particular, we show that no
estimator for this distribution can be uniformly consistent (not even locally).
This follows as a corollary to (local) minimax lower bounds on the performance
of estimators for the distribution; performance is here measured by the
probability that the estimation error exceeds a given threshold. These lower
bounds are shown to approach 1/2 or even 1 in large samples, depending on the
situation considered. Similar impossibility results are also obtained for the
distribution of linear functions (e.g., predictors) of the post-model-selection
estimator.
|
1c5eeb88-56b8-40a3-8146-2a0505dcafea | 1c5eeb88-56b8-40a3-8146-2a0505dcafea | 1c5eeb88-56b8-40a3-8146-2a0505dcafea | human | null | null | none | abstracts | Fixed Phase Quantum Search Algorithm | null | Building quantum devices using fixed operators is a must to simplify the
hardware construction. Quantum search engine is not an exception. In this
paper, a fixed phase quantum search algorithm that searches for M matches in an
unstructured search space of size N will be presented. Selecting phase shifts
of 1.91684\pi in the standard amplitude amplification will make the technique
perform better so as to get probability of success at least 99.58% in
O(sqrt(N/M)) better than any know fixed operator quantum search algorithms. The
algorithm will be able to handle either a single match or multiple matches in
the search space. The algorithm will find a match in O(sqrt(N/M)) whether the
number of matches is known or not in advance.
|
c26d5449-1485-408d-9fb1-5edf8a36125e | c26d5449-1485-408d-9fb1-5edf8a36125e | c26d5449-1485-408d-9fb1-5edf8a36125e | human | null | null | none | abstracts | Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN | null | Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we
study correlations between X-ray spectral features. The sample consists of 350
time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is
fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to
the central black hole that is reprocessed and absorbed by material around the
black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW) and the underlying power law index (Gamma). The
data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2,
but finds a weak anti-correlation for steeper photon indices. We propose that
this relationship is driven by dilution of a disk spectrum (which includes the
narrow iron line) by a beamed jet component and, hence, could be used as a
diagnostic of jet-dominance. In addition, our sample shows a strong correlation
between the reflection fraction (R) and Gamma, but we find that it is likely
the result of modeling degeneracies. We also see the X-ray Baldwin effect (an
anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample
as a whole, but not for the individual galaxies and galaxy types.
|
54ab5059-b70e-4128-9ac6-7c3b27c5b749 | 54ab5059-b70e-4128-9ac6-7c3b27c5b749 | 54ab5059-b70e-4128-9ac6-7c3b27c5b749 | human | null | null | none | abstracts | Constraints on the Very Early Universe from Thermal WIMP Dark Matter | null | We investigate the relic density n_\chi of non-relativistic long-lived or
stable particles \chi in non-standard cosmological scenarios. We calculate the
relic abundance starting from arbitrary initial temperatures of the
radiation-dominated epoch, and derive the lower bound on the initial
temperature T_0 \geq m_\chi/23, assuming that thermally produced \chi particles
account for the dark matter energy density in the universe; this bound holds
for all \chi annihilation cross sections. We also investigate cosmological
scenarios with modified expansion rate. Even in this case an approximate
formula similar to the standard one is capable of predicting the final relic
abundance correctly. Choosing the \chi annihilation cross section such that the
observed cold dark matter abundance is reproduced in standard cosmology, we
constrain possible modifications of the expansion rate at T \sim m_\chi/20,
well before Big Bang nucleosynthesis.
|
1e795a73-420f-46e3-9260-4162b8acc093 | 1e795a73-420f-46e3-9260-4162b8acc093 | 1e795a73-420f-46e3-9260-4162b8acc093 | human | null | null | none | abstracts | Flavour-Dependent Type II Leptogenesis | null | We reanalyse leptogenesis via the out-of-equilibrium decay of the lightest
right-handed neutrino in type II seesaw scenarios, taking into account
flavour-dependent effects. In the type II seesaw mechanism, in addition to the
type I seesaw contribution, an additional direct mass term for the light
neutrinos is present. We consider type II seesaw scenarios where this
additional contribution arises from the vacuum expectation value of a Higgs
triplet, and furthermore an effective model-independent approach. We
investigate bounds on the flavour-specific decay asymmetries, on the mass of
the lightest right-handed neutrino and on the reheat temperature of the early
universe, and compare them to the corresponding bounds in the type I seesaw
framework. We show that while flavour-dependent thermal type II leptogenesis
becomes more efficient for larger mass scale of the light neutrinos, and the
bounds become relaxed, the type I seesaw scenario for leptogenesis becomes more
constrained. We also argue that in general, flavour-dependent effects cannot be
ignored when dealing with leptogenesis in type II seesaw models.
|
f16ded04-fc69-4245-a087-c0e8a869d92c | f16ded04-fc69-4245-a087-c0e8a869d92c | f16ded04-fc69-4245-a087-c0e8a869d92c | human | null | null | none | abstracts | Orbital currents in the Colle-Salvetti correlation energy functional and
the degeneracy problem | null | Popular density functionals for the exchange-correlation energy typically
fail to reproduce the degeneracy of different ground states of open-shell
atoms. As a remedy, functionals which explicitly depend on the current density
have been suggested. We present an analysis of this problem by investigating
functionals that explicitly depend on the Kohn-Sham orbitals. Going beyond the
exact-exchange approximation by adding correlation in the form of the
Colle-Salvetti functional we show how current-dependent terms enter the
Colle-Salvetti expression and their relevance is evaluated. A very good
description of the degeneracy of ground-states for atoms of the first and
second row of the periodic table is obtained.
|
a2ac8160-282b-4cee-aefe-6cacfd93d033 | a2ac8160-282b-4cee-aefe-6cacfd93d033 | a2ac8160-282b-4cee-aefe-6cacfd93d033 | human | null | null | none | abstracts | Numerical estimation of critical parameters using the Bond entropy | null | Using a model of spinless fermions in a lattice with nearest neighbor and
next-nearest neighbor interaction we show that the entropy of the reduced two
site density matrix (the bond entropy) can be used as an extremely accurate and
easy to calculate numerical indicator for the critical parameters of the
quantum phase transition when the basic ordering pattern has a two-site
periodicity. The actual behavior of the bond entropy depends on the particular
characteristics of the transition under study. For the Kosterlitz-Thouless type
phase transition from a Luttinger liquid phase to a charge density wave state
the bond entropy has a local maximum while in the transition from the Luttinger
liquid to the phase separated state the derivative of the bond entropy has a
divergence due to the cancelation of the third eigenvalue of the two-site
reduced density matrix.
|
ffd2f037-7cef-4c6d-9523-56a6527d99ef | ffd2f037-7cef-4c6d-9523-56a6527d99ef | ffd2f037-7cef-4c6d-9523-56a6527d99ef | human | null | null | none | abstracts | The VIMOS VLT Deep Survey. The Assembly History of the Stellar Mass in
Galaxies: from the Young to the Old Universe | null | We present a detailed analysis of the Galaxy Stellar Mass Function of
galaxies up to z=2.5 as obtained from the VVDS. We estimate the stellar mass
from broad-band photometry using 2 different assumptions on the galaxy star
formation history and show that the addition of secondary bursts to a
continuous star formation history produces systematically higher (up to 40%)
stellar masses. At low redshift (z=0.2) we find a substantial population of
low-mass galaxies (<10^9 Msun) composed by faint blue galaxies (M_I-M_K=0.3).
In general the stellar mass function evolves slowly up to z=0.9 and more
significantly above this redshift. Conversely, a massive tail is present up to
z=2.5 and have extremely red colours (M_I-M_K=0.7-0.8). We find a decline with
redshift of the overall number density of galaxies for all masses (59+-5% for
M>10^8 Msun at z=1), and a mild mass-dependent average evolution
(`mass-downsizing'). In particular our data are consistent with mild/negligible
(<30%) evolution up to z=0.7 for massive galaxies (>6x10^10 Msun). For less
massive systems the no-evolution scenario is excluded. A large fraction (>=50%)
of massive galaxies have been already assembled and converted most of their gas
into stars at z=1, ruling out the `dry mergers' as the major mechanism of their
assembly history below z=1. This fraction decreases to 33% at z=2. Low-mass
systems have decreased continuously in number and mass density (by a factor up
to 4) from the present age to z=2, consistently with a prolonged mass assembly
also at z<1.
|
df859f2c-43bf-4501-8e68-bb5ba25cca66 | df859f2c-43bf-4501-8e68-bb5ba25cca66 | df859f2c-43bf-4501-8e68-bb5ba25cca66 | human | null | null | none | abstracts | Measuring energy dependent polarization in soft gamma-rays using Compton
scattering in PoGOLite | null | Linear polarization in X- and gamma-rays is an important diagnostic of many
astrophysical sources, foremost giving information about their geometry,
magnetic fields, and radiation mechanisms. However, very few X-ray polarization
measurements have been made, and then only mono-energetic detections, whilst
several objects are assumed to have energy dependent polarization signatures.
In this paper we investigate whether detection of energy dependent polarization
from cosmic sources is possible using the Compton technique, in particular with
the proposed PoGOLite balloon-experiment, in the 25-100 keV range. We use
Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus
X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6
and 35 hours were simulated, corresponding to a standard and a long duration
flight respectively. Both smooth and sharp energy variations of the
polarization are investigated and compared to constant polarization signals
using chi-square statistics. We can reject constant polarization, with energy,
for the Cygnus X-1 spectrum (in the hard state), if the reflected component is
assumed to be completely polarized, whereas the distinction cannot be made for
weaker polarization. For the accreting pulsar, constant polarization can be
rejected in the case of polarization in a narrow energy band with at least 50%
polarization, and similarly for a negative step distribution from 30% to 0%
polarization.
|
28a8a517-4c5f-409d-9a7c-a059d0e68259 | 28a8a517-4c5f-409d-9a7c-a059d0e68259 | 28a8a517-4c5f-409d-9a7c-a059d0e68259 | human | null | null | none | abstracts | Asteroseismic Signatures of Stellar Magnetic Activity Cycles | null | Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.
|
c5986474-abab-4672-bf51-833c0052619b | c5986474-abab-4672-bf51-833c0052619b | c5986474-abab-4672-bf51-833c0052619b | human | null | null | none | abstracts | Burgers Turbulence | null | The last decades witnessed a renewal of interest in the Burgers equation.
Much activities focused on extensions of the original one-dimensional
pressureless model introduced in the thirties by the Dutch scientist J.M.
Burgers, and more precisely on the problem of Burgers turbulence, that is the
study of the solutions to the one- or multi-dimensional Burgers equation with
random initial conditions or random forcing. Such work was frequently motivated
by new emerging applications of Burgers model to statistical physics,
cosmology, and fluid dynamics. Also Burgers turbulence appeared as one of the
simplest instances of a nonlinear system out of equilibrium. The study of
random Lagrangian systems, of stochastic partial differential equations and
their invariant measures, the theory of dynamical systems, the applications of
field theory to the understanding of dissipative anomalies and of multiscaling
in hydrodynamic turbulence have benefited significantly from progress in
Burgers turbulence. The aim of this review is to give a unified view of
selected work stemming from these rather diverse disciplines.
|
361826ab-989a-44bf-af52-23a1c10d3014 | 361826ab-989a-44bf-af52-23a1c10d3014 | 361826ab-989a-44bf-af52-23a1c10d3014 | human | null | null | none | abstracts | Modelling the Galactic bar using OGLE-II Red Clump Giant Stars | null | Red clump giant stars can be used as distance indicators to trace the mass
distribution of the Galactic bar. We use RCG stars from 44 bulge fields from
the OGLE-II microlensing collaboration database to constrain analytic tri-axial
models for the Galactic bar. We find the bar major axis is oriented at an angle
of 24 - 27 degrees to the Sun-Galactic centre line-of-sight. The ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane x_0,
y_0, and vertical bar scale length z_0, is x_0 : y_0 : z_0 = 10 : 3.5 : 2.6,
suggesting a slightly more prolate bar structure than the working model of
Gerhard (2002) which gives the scale length ratios as x_0 : y_0 : z_0 = 10 : 4
: 3 .
|
2a80de8e-eed8-4968-b109-3c46a04d713c | 2a80de8e-eed8-4968-b109-3c46a04d713c | 2a80de8e-eed8-4968-b109-3c46a04d713c | human | null | null | none | abstracts | Dynamical Coupled-Channel Model of $\pi N$ Scattering in the W $\leq$ 2
GeV Nucleon Resonance Region | null | As a first step to analyze the electromagnetic meson production reactions in
the nucleon resonance region, the parameters of the hadronic interactions of a
dynamical coupled-channel model, developed in {\it Physics Reports 439, 193
(2007)}, are determined by fitting the $\pi N$ scattering data. The channels
included in the calculations are $\pi N$, $\eta N$ and $\pi\pi N$ which has
$\pi\Delta$, $\rho N$, and $\sigma N$ resonant components. The non-resonant
meson-baryon interactions of the model are derived from a set of Lagrangians by
using a unitary transformation method. One or two bare excited nucleon states
in each of $S$, $P$, $D$, and $F$ partial waves are included to generate the
resonant amplitudes in the fits. The parameters of the model are first
determined by fitting as much as possible the empirical $\pi N$ elastic
scattering amplitudes of SAID up to 2 GeV. We then refine and confirm the
resulting parameters by directly comparing the predicted differential cross
section and target polarization asymmetry with the original data of the elastic
$\pi^{\pm} p \to \pi^{\pm} p$ and charge-exchange $\pi^- p \to \pi^0 n$
processes. The predicted total cross sections of $\pi N$ reactions and $\pi
N\to \eta N$ reactions are also in good agreement with the data. Applications
of the constructed model in analyzing the electromagnetic meson production data
as well as the future developments are discussed.
|
2424e857-3a2a-42a9-b0c8-5650d6173f2f | 2424e857-3a2a-42a9-b0c8-5650d6173f2f | 2424e857-3a2a-42a9-b0c8-5650d6173f2f | human | null | null | none | abstracts | High-resolution study of a star-forming cluster in the Cep-A HW2 region | null | Due to its relatively small distance (725 pc), the Cepheus A East
star-forming region is an ideal laboratory to study massive star formation
processes. Based on its morphology, it has been suggested that the flattened
molecular gas distribution around the YSO HW2 may be a 350-AU-radius massive
protostellar disk. Goal of our work is to ascertain the nature of this
structure. We have employed the Plateau de Bure Interferometer to acquire
(sub-)arcsecond-resolution imaging of high-density and shock tracers, such as
methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position. On
the 1-arcsecond (about 725 AU) scale, the flattened distribution of molecular
gas around HW2 appears to be due to the projected superposition, on the plane
of the sky, of at least three protostellar objects, of which at least one is
powering a molecular outflow at a small angle with respect to the line of
sight. The presence of a protostellar disk around HW2 is not ruled out, but
such structure is likely to be detected on a smaller spatial scale, or using
different molecular tracers.
|
81d26198-50c0-4408-9711-2422e0350075 | 81d26198-50c0-4408-9711-2422e0350075 | 81d26198-50c0-4408-9711-2422e0350075 | human | null | null | none | abstracts | A Renormalization group approach for highly anisotropic 2D Fermion
systems: application to coupled Hubbard chains | null | I apply a two-step density-matrix renormalization group method to the
anisotropic two-dimensional Hubbard model. As a prelude to this study, I
compare the numerical results to the exact one for the tight-binding model. I
find a ground-state energy which agrees with the exact value up to four digits
for systems as large as $24 \times 25$. I then apply the method to the
interacting case. I find that for strong Hubbard interaction, the ground-state
is dominated by magnetic correlations.
These correlations are robust even in the presence of strong frustration.
Interchain pair tunneling is negligible in the singlet and triplet channels and
it is not enhanced by frustration. For weak Hubbard couplings, interchain
non-local singlet pair tunneling is enhanced and magnetic correlations are
strongly reduced. This suggests a possible superconductive ground state.
|
cb2be37c-6539-4bc2-ab84-9dc9c277be3d | cb2be37c-6539-4bc2-ab84-9dc9c277be3d | cb2be37c-6539-4bc2-ab84-9dc9c277be3d | human | null | null | none | abstracts | Proper Motion Dispersions of Red Clump Giants in the Galactic Bulge:
Observations and Model Comparisons | null | Red clump giants in the Galactic bulge are approximate standard candles and
hence they can be used as distance indicators. We compute the proper motion
dispersions of RCG stars in the Galactic bulge using the proper motion
catalogue from the second phase of the Optical Gravitational Microlensing
Experiment (OGLE-II, Sumi et al. 2004) for 45 fields. The proper motion
dispersions are measured to a few per cent accuracy due to the large number of
stars in the fields. The observational sample is comprised of 577736 stars.
These observed data are compared to a state-of-the-art particle simulation of
the Galactic bulge region. The predictions are in rough agreement with
observations, but appear to be too anisotropic in the velocity ellipsoid. We
note that there is significant field-to-field variation in the observed proper
motion dispersions. This could either be a real feature, or due to some unknown
systematic effect.
|
0ca4c1c2-2148-4fac-9b65-a95279c835e4 | 0ca4c1c2-2148-4fac-9b65-a95279c835e4 | 0ca4c1c2-2148-4fac-9b65-a95279c835e4 | human | null | null | none | abstracts | Intramolecular long-range correlations in polymer melts: The segmental
size distribution and its moments | null | Presenting theoretical arguments and numerical results we demonstrate
long-range intrachain correlations in concentrated solutions and melts of long
flexible polymers which cause a systematic swelling of short chain segments.
They can be traced back to the incompressibility of the melt leading to an
effective repulsion $u(s) \approx s/\rho R^3(s) \approx ce/\sqrt{s}$ when
connecting two segments together where $s$ denotes the curvilinear length of a
segment, $R(s)$ its typical size, $ce \approx 1/\rho be^3$ the ``swelling
coefficient", $be$ the effective bond length and $\rho$ the monomer density.
The relative deviation of the segmental size distribution from the ideal
Gaussian chain behavior is found to be proportional to $u(s)$. The analysis of
different moments of this distribution allows for a precise determination of
the effective bond length $be$ and the swelling coefficient $ce$ of
asymptotically long chains. At striking variance to the short-range decay
suggested by Flory's ideality hypothesis the bond-bond correlation function of
two bonds separated by $s$ monomers along the chain is found to decay
algebraically as $1/s^{3/2}$. Effects of finite chain length are considered
briefly.
|
981582c4-f898-434e-babe-755eea93d1f2 | 981582c4-f898-434e-babe-755eea93d1f2 | 981582c4-f898-434e-babe-755eea93d1f2 | human | null | null | none | abstracts | Stability Properties of Strongly Magnetized Spine Sheath Relativistic
Jets | null | The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.
|
c56d4b63-4193-4b86-8bd0-946277be4166 | c56d4b63-4193-4b86-8bd0-946277be4166 | c56d4b63-4193-4b86-8bd0-946277be4166 | human | null | null | none | abstracts | A Systematic Scan for 7-colourings of the Grid | null | We study the mixing time of a systematic scan Markov chain for sampling from
the uniform distribution on proper 7-colourings of a finite rectangular
sub-grid of the infinite square lattice, the grid. A systematic scan Markov
chain cycles through finite-size subsets of vertices in a deterministic order
and updates the colours assigned to the vertices of each subset. The systematic
scan Markov chain that we present cycles through subsets consisting of 2x2
sub-grids and updates the colours assigned to the vertices using a procedure
known as heat-bath. We give a computer-assisted proof that this systematic scan
Markov chain mixes in O(log n) scans, where n is the size of the rectangular
sub-grid. We make use of a heuristic to compute required couplings of
colourings of 2x2 sub-grids. This is the first time the mixing time of a
systematic scan Markov chain on the grid has been shown to mix for less than 8
colours. We also give partial results that underline the challenges of proving
rapid mixing of a systematic scan Markov chain for sampling 6-colourings of the
grid by considering 2x3 and 3x3 sub-grids.
|
9bc9ecb7-5aba-43e7-80cc-84ccbbe09b2f | 9bc9ecb7-5aba-43e7-80cc-84ccbbe09b2f | 9bc9ecb7-5aba-43e7-80cc-84ccbbe09b2f | human | null | null | none | abstracts | Detection of single electron spin resonance in a double quantum dot | null | Spin-dependent transport measurements through a double quantum dot are a
valuable tool for detecting both the coherent evolution of the spin state of a
single electron as well as the hybridization of two-electron spin states. In
this paper, we discuss a model that describes the transport cycle in this
regime, including the effects of an oscillating magnetic field (causing
electron spin resonance) and the effective nuclear fields on the spin states in
the two dots. We numerically calculate the current flow due to the induced spin
flips via electron spin resonance and we study the detector efficiency for a
range of parameters. The experimental data are compared with the model and we
find a reasonable agreement.
|
1bc35458-8b6f-4a19-87ff-9bdff1991486 | 1bc35458-8b6f-4a19-87ff-9bdff1991486 | 1bc35458-8b6f-4a19-87ff-9bdff1991486 | human | null | null | none | abstracts | Exciting the Magnetosphere of the Magnetar CXOU J164710.2-455216 in
Westerlund 1 | null | We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days
subsequent to two remarkable events that were detected with Swift on 2006
September 21 from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms
burst with an energy of 1e37 erg (15-150 keV), and (2) a rapid spin-down
(glitch) with a fractionap period change of 1e-4. We find that the luminosity
of the pulsar increased by a factor of 100 in the interval between
observations, from 1e33 to 1e35 erg/s (0.5-8.0 keV), and that its spectrum
hardened. The pulsed count rate increased by a factor of 10 (0.5-8.0 keV), but
the fractional rms amplitude of the pulses decreased from 65 to 11 per cent,
and their profile changed from being single-peaked to exhibiting three peaks.
Similar changes have been observed from other magnetars in response to
outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic
deformation of the neutron star's crust induced a very slight twist in the
external magnetic field, which in turn generated currents in the magnetosphere
that were the direct cause of the X-ray outburst.
|
a9c3894d-5a60-45f7-a0f1-deda025b1c48 | a9c3894d-5a60-45f7-a0f1-deda025b1c48 | a9c3894d-5a60-45f7-a0f1-deda025b1c48 | human | null | null | none | abstracts | Further Evidence that the Redshifts of AGN Galaxies May Contain
Intrinsic Components | null | In the decreasing intrinsic redshift (DIR) model galaxies are assumed to be
born as compact objects that have been ejected with large intrinsic redshift
components, z_(i), out of the nuclei of mature AGN galaxies. As young AGN
(quasars) they are initially several magnitudes sub-luminous to mature galaxies
but their luminosity gradually increases over 10^8 yrs, as z_(i) decreases and
they evolve into mature AGN (Seyferts and radio galaxies). Evidence presented
here that low- and intermediate-redshift AGN are unquestionably sub-luminous to
radio galaxies is then strong support for this model and makes it likely that
the high-redshift AGN (quasars) are also sub-luminous, having simply been
pushed above the radio galaxies on a logz-m_(v) plot by the presence of a large
intrinsic component in their redshifts. An increase in luminosity below z =
0.06 is also seen. It is associated in the DIR model with an increase in
luminosity as the sources mature but, if real, is difficult to interpret in the
cosmological redshift (CR) model since at this low redshift it is unlikely to
be associated with a higher star formation rate or an increase in the material
used to build galaxies. Whether it might be possible in the CR model to explain
these results by selection effects is also examined.
|
a91300bd-0f88-4515-9f53-6b822ded5abf | a91300bd-0f88-4515-9f53-6b822ded5abf | a91300bd-0f88-4515-9f53-6b822ded5abf | human | null | null | none | abstracts | Light Curves of Dwarf Plutonian Planets and other Large Kuiper Belt
Objects: Their Rotations, Phase Functions and Absolute Magnitudes | null | (Abridged) I report new light curves and determine the rotations and phase
functions of several large Kuiper Belt objects, including the dwarf planet Eris
(2003 UB313). (120348) 2004 TY364 shows a light curve which if double-peaked
has a period of 11.70+-0.01 hours and peak-to-peak amplitude of 0.22+-0.02
magnitudes. (84922) 2003 VS2 has a well defined double-peaked light curve of
7.41+-0.02 hours with a 0.21+-0.02 magnitude range. (126154) 2001 YH140 shows
variability of 0.21+-0.04 magnitudes with a possible 13.25+-0.2 hour
single-peaked period. The seven new KBOs in the sample which show no
discernible variations within the uncertainties on short rotational time scales
are 2001 UQ18, (55565) 2002 AW197, (119979) 2002 WC19, (120132) 2003 FY128,
(136108) Eris 2003 UB313, (90482) Orcus 2004 DW, and (90568) 2004 GV9. The
three medium to large sized Kuiper Belt objects 2004 TY364, Orcus and 2004 GV9
show fairly steep linear phase curves (~0.18 to 0.26 mags per degree) between
phase angles of 0.1 and 1.5 degrees. The extremely large dwarf planet Eris
(2003 UB313) shows a shallower phase curve (0.09+-0.03 mags per degree) which
is more similar to the other known dwarf planet Pluto. It appears the surface
properties of the largest dwarf planets in the Kuiper Belt maybe different than
the smaller Kuiper Belt objects. This may have to do with the larger objects
ability to hold more volatile ices as well as sustain atmospheres. The absolute
magnitudes obtained using the measured phase slopes are a few tenths of
magnitudes different from those given by the MPC.
|
6579bc7b-693a-4695-b5e7-f4642f4211cc | 6579bc7b-693a-4695-b5e7-f4642f4211cc | 6579bc7b-693a-4695-b5e7-f4642f4211cc | human | null | null | none | abstracts | Fischler-Susskind holographic cosmology revisited | null | When Fischler and Susskind proposed a holographic prescription based on the
Particle Horizon, they found that spatially closed cosmological models do not
verify it due to the apparently unavoidable recontraction of the Particle
Horizon area. In this article, after a short review of their original work, we
expose graphically and analytically that spatially closed cosmological models
can avoid this problem if they expand fast enough. It has been also shown that
the Holographic Principle is saturated for a codimension one brane dominated
Universe. The Fischler-Susskind prescription is used to obtain the maximum
number of degrees of freedom per Planck volume at the Planck era compatible
with the Holographic Principle.
|
e749d360-a960-4e5f-b658-1c773efdddce | e749d360-a960-4e5f-b658-1c773efdddce | e749d360-a960-4e5f-b658-1c773efdddce | human | null | null | none | abstracts | Parity doubling in particle physics | null | Parity doubling in excited hadrons is reviewed. Parity degeneracy in hadrons
was first experimentally observed 40 years ago. Recently new experimental data
on light mesons caused much excitement and renewed interest to the phenomenon,
which still remains to be enigmatic. The present retrospective review is an
attempt to trace the history of parity doubling phenomenon, thus providing a
kind of introduction to the subject. We begin with early approaches of 1960s
(Regge theory and dynamical symmetries) and end up with the latest trends
(manifestations of broader degeneracies and AdS/QCD). We show the evolution of
various ideas about parity doubling. The experimental evidence for this
phenomenon is scrutinized in the non-strange sector. Some experiments of 1960s
devoted to the search for missing non-strange bosons are re-examined and it is
argued that results of these experiments are encouraging from the modern
perspective.
|
5f6f5a70-02a1-4d89-b484-bc0297b64865 | 5f6f5a70-02a1-4d89-b484-bc0297b64865 | 5f6f5a70-02a1-4d89-b484-bc0297b64865 | human | null | null | none | abstracts | Bergman kernels and equilibrium measures for ample line bundles | null | Let L be an ample holomorphic line bundle over a compact complex Hermitian
manifold X. Any fixed smooth Hermitian metric on L induces a Hilbert space
structure on the space of global holomorphic sections with values in the k:th
tensor power of L. In this paper various convergence results are obtained for
the corresponding Bergman kernels. The convergence is studied in the large k
limit and is expressed in terms of the equilibrium metric associated to the
fixed metric, as well as in terms of the Monge-Ampere measure of the fixed
metric itself on a certain support set. It is also shown that the equilibrium
metric has Lipschitz continuous first derivatives. These results can be seen as
generalizations of well-known results concerning the case when the curvature of
the fixed metric is positive (the corresponding equilibrium metric is then
simply the fixed metric itself).
|
5704752a-a0ad-4504-9116-8ad3af510221 | 5704752a-a0ad-4504-9116-8ad3af510221 | 5704752a-a0ad-4504-9116-8ad3af510221 | human | null | null | none | abstracts | Collective excitations of hard-core Bosons at half filling on square and
triangular lattices: Development of roton minima and collapse of roton gap | null | We study ground state properties and excitation spectra for hard-core Bosons
on square and triangular lattices, at half filling, using series expansion
methods. Nearest-neighbor repulsion between the Bosons leads to the development
of short-range density order at the antiferromagnetic wavevector, and
simultaneously a roton minima in the density excitation spectra. On the
square-lattice, the model maps on to the well studied XXZ model, and the roton
gap collapses to zero precisely at the Heisenberg symmetry point, leading to
the well known spectra for the Heisenberg antiferromagnet. On the
triangular-lattice, the collapse of the roton gap signals the onset of the
supersolid phase. Our results suggest that the transition from the superfluid
to the supersolid phase maybe weakly first order. We also find several features
in the density of states, including two-peaks and a sharp discontinuity, which
maybe observable in experimental realization of such systems.
|
6f3633e6-a5cf-4ddb-bedd-89f78d5c96a4 | 6f3633e6-a5cf-4ddb-bedd-89f78d5c96a4 | 6f3633e6-a5cf-4ddb-bedd-89f78d5c96a4 | human | null | null | none | abstracts | Non-Relativistic Propagators via Schwinger's Method | null | In order to popularize the so called Schwinger's method we reconsider the
Feynman propagator of two non-relativistic systems: a charged particle in a
uniform magnetic field and a charged harmonic oscillator in a uniform magnetic
field. Instead of solving the Heisenberg equations for the position and the
canonical momentum operators, ${\bf R}$ and ${\bf P}$, we apply this method by
solving the Heisenberg equations for the gauge invariant operators ${\bf R}$
and $\mathversion{bold}${\pi}$ = {\bf P}-e{\bf A}$, the latter being the
mechanical momentum operator. In our procedure we avoid fixing the gauge from
the beginning and the result thus obtained shows explicitly the gauge
dependence of the Feynman propagator.
|
4083fdcb-4bd7-4c1e-9fb2-14d9ce1083eb | 4083fdcb-4bd7-4c1e-9fb2-14d9ce1083eb | 4083fdcb-4bd7-4c1e-9fb2-14d9ce1083eb | human | null | null | none | abstracts | A linear RFQ ion trap for the Enriched Xenon Observatory | null | The design, construction, and performance of a linear radio-frequency ion
trap (RFQ) intended for use in the Enriched Xenon Observatory (EXO) are
described. EXO aims to detect the neutrinoless double-beta decay of $^{136}$Xe
to $^{136}$Ba. To suppress possible backgrounds EXO will complement the
measurement of decay energy and, to some extent, topology of candidate events
in a Xe filled detector with the identification of the daughter nucleus
($^{136}$Ba). The ion trap described here is capable of accepting, cooling, and
confining individual Ba ions extracted from the site of the candidate
double-beta decay event. A single trapped ion can then be identified, with a
large signal-to-noise ratio, via laser spectroscopy.
|
6a348c0f-f4c9-4701-9562-0766ee9670d2 | 6a348c0f-f4c9-4701-9562-0766ee9670d2 | 6a348c0f-f4c9-4701-9562-0766ee9670d2 | human | null | null | none | abstracts | Spectral Analysis of the Chandra Comet Survey | null | We present results of the analysis of cometary X-ray spectra with an extended
version of our charge exchange emission model (Bodewits et al. 2006). We have
applied this model to the sample of 8 comets thus far observed with the Chandra
X-ray observatory and ACIS spectrometer in the 300-1000 eV range. The surveyed
comets are C/1999 S4 (LINEAR), C/1999 T1 (McNaught-Hartley), C/2000 WM1
(LINEAR), 153P/2002 (Ikeya-Zhang), 2P/2003 (Encke), C/2001 Q4 (NEAT), 9P/2005
(Tempel 1) and 73P/2006-B (Schwassmann-Wachmann 3) and the observations include
a broad variety of comets, solar wind environments and observational
conditions. The interaction model is based on state selective, velocity
dependent charge exchange cross sections and is used to explore how cometary
X-ray emission depend on cometary, observational and solar wind
characteristics. It is further demonstrated that cometary X-ray spectra mainly
reflect the state of the local solar wind. The current sample of Chandra
observations was fit using the constrains of the charge exchange model, and
relative solar wind abundances were derived from the X-ray spectra. Our
analysis showed that spectral differences can be ascribed to different solar
wind states, as such identifying comets interacting with (I) fast, cold wind,
(II), slow, warm wind and (III) disturbed, fast, hot winds associated with
interplanetary coronal mass ejections. We furthermore predict the existence of
a fourth spectral class, associated with the cool, fast high latitude wind.
|
5db5e19f-07da-4670-89ba-b359f8167988 | 5db5e19f-07da-4670-89ba-b359f8167988 | 5db5e19f-07da-4670-89ba-b359f8167988 | human | null | null | none | abstracts | Correlations, fluctuations and stability of a finite-size network of
coupled oscillators | null | The incoherent state of the Kuramoto model of coupled oscillators exhibits
marginal modes in mean field theory. We demonstrate that corrections due to
finite size effects render these modes stable in the subcritical case, i.e.
when the population is not synchronous. This demonstration is facilitated by
the construction of a non-equilibrium statistical field theoretic formulation
of a generic model of coupled oscillators. This theory is consistent with
previous results. In the all-to-all case, the fluctuations in this theory are
due completely to finite size corrections, which can be calculated in an
expansion in 1/N, where N is the number of oscillators. The N -> infinity limit
of this theory is what is traditionally called mean field theory for the
Kuramoto model.
|
a04b7f8c-0e56-4f30-974e-fb7fe2b1ec2c | a04b7f8c-0e56-4f30-974e-fb7fe2b1ec2c | a04b7f8c-0e56-4f30-974e-fb7fe2b1ec2c | human | null | null | none | abstracts | Route to Lambda in conformally coupled phantom cosmology | null | In this letter we investigate acceleration in the flat cosmological model
with a conformally coupled phantom field and we show that acceleration is its
generic feature. We reduce the dynamics of the model to a 3-dimensional
dynamical system and analyze it on a invariant 2-dimensional submanifold. Then
the concordance FRW model with the cosmological constant $\Lambda$ is a global
attractor situated on a 2-dimensional invariant space. We also study the
behaviour near this attractor, which can be approximated by the dynamics of the
linearized part of the system. We demonstrate that trajectories of the
conformally coupled phantom scalar field with a simple quadratic potential
crosses the cosmological constant barrier infinitely many times in the phase
space. The universal behaviour of the scalar field and its potential is also
calculated. We conclude that the phantom scalar field conformally coupled to
gravity gives a natural dynamical mechanism of concentration of the equation of
state coefficient around the magical value $w_{\text{eff}}=-1$. We demonstrate
route to Lambda through the infinite times crossing the $w_{\text{eff}}=-1$
phantom divide.
|
7c278368-5836-4cfe-bbcc-d5ee36ffbed7 | 7c278368-5836-4cfe-bbcc-d5ee36ffbed7 | 7c278368-5836-4cfe-bbcc-d5ee36ffbed7 | human | null | null | none | abstracts | Scaling cosmologies, geodesic motion and pseudo-susy | null | One-parameter solutions in supergravity carried by scalars and a metric trace
out curves on the scalar manifold. In ungauged supergravity these curves
describe a geodesic motion. It is known that a geodesic motion sometimes occurs
in the presence of a scalar potential and for time-dependent solutions this can
happen for scaling cosmologies. This note contains a further study of such
solutions in the context of pseudo-supersymmetry for multi-field systems whose
first-order equations we derive using a Bogomol'nyi-like method. In particular
we show that scaling solutions that are pseudo-BPS must describe geodesic
curves. Furthermore, we clarify how to solve the geodesic equations of motion
when the scalar manifold is a maximally non-compact coset such as occurs in
maximal supergravity. This relies upon a parametrization of the coset in the
Borel gauge. We then illustrate this with the cosmological solutions of
higher-dimensional gravity compactified on a $n$-torus.
|
3fa11bec-09c2-4a6e-b5ce-9746caeef01c | 3fa11bec-09c2-4a6e-b5ce-9746caeef01c | 3fa11bec-09c2-4a6e-b5ce-9746caeef01c | human | null | null | none | abstracts | The Peculiar Velocities of Local Type Ia Supernovae and their Impact on
Cosmology | null | We quantify the effect of supernova Type Ia peculiar velocities on the
derivation of cosmological parameters. The published distant and local Ia SNe
used for the Supernova Legacy Survey first-year cosmology report form the
sample for this study. While previous work has assumed that the local SNe are
at rest in the CMB frame (the No Flow assumption), we test this assumption by
applying peculiar velocity corrections to the local SNe using three different
flow models. The models are based on the IRAS PSCz galaxy redshift survey, have
varying beta = Omega_m^0.6/b, and reproduce the Local Group motion in the CMB
frame. These datasets are then fit for w, Omega_m, and Omega_Lambda using
flatness or LambdaCDM and a BAO prior. The chi^2 statistic is used to examine
the effect of the velocity corrections on the quality of the fits. The most
favored model is the beta=0.5 model, which produces a fit significantly better
than the No Flow assumption, consistent with previous peculiar velocity
studies. By comparing the No Flow assumption with the favored models we derive
the largest potential systematic error in w caused by ignoring peculiar
velocities to be Delta w = +0.04. For Omega_Lambda, the potential error is
Delta Omega_Lambda = -0.04 and for Omega_m, the potential error is Delta
Omega_m < +0.01. The favored flow model (beta=0.5) produces the following
cosmological parameters: w = -1.08 (+0.09,-0.08), Omega_m = 0.27 (+0.02,-0.02)
assuming a flat cosmology, and Omega_Lambda = 0.80 (+0.08,-0.07) and Omega_m =
0.27 (+0.02,-0.02) for a w = -1 (LambdaCDM) cosmology.
|
1ba239b9-7a95-4bfb-a670-0d30c7cfba5b | 1ba239b9-7a95-4bfb-a670-0d30c7cfba5b | 1ba239b9-7a95-4bfb-a670-0d30c7cfba5b | human | null | null | none | abstracts | Temperature-driven transition from the Wigner Crystal to the
Bond-Charge-Density Wave in the Quasi-One-Dimensional Quarter-Filled band | null | It is known that within the interacting electron model Hamiltonian for the
one-dimensional 1/4-filled band, the singlet ground state is a Wigner crystal
only if the nearest neighbor electron-electron repulsion is larger than a
critical value. We show that this critical nearest neighbor Coulomb interaction
is different for each spin subspace, with the critical value decreasing with
increasing spin. As a consequence, with the lowering of temperature, there can
occur a transition from a Wigner crystal charge-ordered state to a spin-Peierls
state that is a Bond-Charge-Density Wave with charge occupancies different from
the Wigner crystal. This transition is possible because spin excitations from
the spin-Peierls state in the 1/4-filled band are necessarily accompanied by
changes in site charge densities. We apply our theory to the 1/4-filled band
quasi-one-dimensional organic charge-transfer solids in general and to 2:1
tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene
(TMTSF) cationic salts in particular. We believe that many recent experiments
strongly indicate the Wigner crystal to Bond-Charge-Density Wave transition in
several members of the TMTTF family. We explain the occurrence of two different
antiferromagnetic phases but a single spin-Peierls state in the generic phase
diagram for the 2:1 cationic solids. The antiferromagnetic phases can have
either the Wigner crystal or the Bond-Charge-Spin-Density Wave charge
occupancies. The spin-Peierls state is always a Bond-Charge-Density Wave.
|
66fb6040-a46f-4fd4-86d7-db0cd426b11f | 66fb6040-a46f-4fd4-86d7-db0cd426b11f | 66fb6040-a46f-4fd4-86d7-db0cd426b11f | human | null | null | none | abstracts | Bubbling Surface Operators And S-Duality | null | We construct smooth asymptotically AdS_5xS^5 solutions of Type IIB
supergravity corresponding to all the half-BPS surface operators in N=4 SYM.
All the parameters labeling a half-BPS surface operator are identified in the
corresponding bubbling geometry. We use the supergravity description of surface
operators to study the action of the SL(2,Z) duality group of N=4 SYM on the
parameters of the surface operator, and find that it coincides with the recent
proposal by Gukov and Witten in the framework of the gauge theory approach to
the geometrical Langlands with ramification. We also show that whenever a
bubbling geometry becomes singular that the path integral description of the
corresponding surface operator also becomes singular.
|
9cd3ff38-52f6-4b5c-b300-4b153d9dc95c | 9cd3ff38-52f6-4b5c-b300-4b153d9dc95c | 9cd3ff38-52f6-4b5c-b300-4b153d9dc95c | human | null | null | none | abstracts | Neutrino-cooled accretion and GRB variability | null | For accretion rates Mdot~0.1 Msun/s to a few solar mass black hole the inner
part of the disk is expected to make a transition from advection dominance to
neutrino cooling. This transition is characterized by sharp changes of the disk
properties. I argue here that during this transition, a modest increase of the
accretion rate leads to powerful enhancement of the Poynting luminosity of the
GRB flow and decrease of its baryon loading. These changes of the
characteristics of the GRB flow translate into changing gamma-ray spectra from
the photosphere of the flow. The photospheric interpretation of the GRB
emission explains the observed narrowing of GRB pulses with increasing photon
energy and the luminosity-spectral peak relation within and among bursts.
|
04e6f90a-c0cd-4ba5-a183-859e987fcd11 | 04e6f90a-c0cd-4ba5-a183-859e987fcd11 | 04e6f90a-c0cd-4ba5-a183-859e987fcd11 | human | null | null | none | abstracts | Can a Black Hole Collapse to a Space-time Singularity? | null | A critique of the singularity theorems of Penrose, Hawking, and Geroch is
given. It is pointed out that a gravitationally collapsing black hole acts as
an ultrahigh energy particle accelerator that can accelerate particles to
energies inconceivable in any terrestrial particle accelerator, and that when
the energy $E$ of the particles comprising matter in a black hole is $\sim
10^{2} GeV$ or more, or equivalently, the temperature $T$ is $\sim 10^{15} K$
or more, the entire matter in the black hole is converted into quark-gluon
plasma permeated by leptons. As quarks and leptons are fermions, it is
emphasized that the collapse of a black-hole to a space-time singularity is
inhibited by Pauli's exclusion principle. It is also suggested that ultimately
a black hole may end up either as a stable quark star, or as a pulsating quark
star which may be a source of gravitational radiation, or it may simply explode
with a mini bang of a sort.
|
8ed42b4f-0592-423f-b07e-944bade00238 | 8ed42b4f-0592-423f-b07e-944bade00238 | 8ed42b4f-0592-423f-b07e-944bade00238 | human | null | null | none | abstracts | Dynamics of single polymers under extreme confinement | null | We study the dynamics of a single chain polymer confined to a two dimensional
cell. We introduce a kinetically constrained lattice gas model that preserves
the connectivity of the chain, and we use this kinetically constrained model to
study the dynamics of the polymer at varying densities through Monte Carlo
simulations. Even at densities close to the fully-packed configuration, we find
that the monomers comprising the chain manage to diffuse around the box with a
root mean square displacement of the order of the box dimensions over time
scales for which the overall geometry of the polymer is, nevertheless, largely
preserved. To capture this shape persistence, we define the local tangent field
and study the two-time tangent-tangent correlation function, which exhibits a
glass-like behavior. In both closed and open chains, we observe reptational
motion and reshaping through local fingering events which entail global monomer
displacement.
|
6bd9ac4b-8607-4d7f-9231-56b92b8f3e3b | 6bd9ac4b-8607-4d7f-9231-56b92b8f3e3b | 6bd9ac4b-8607-4d7f-9231-56b92b8f3e3b | human | null | null | none | abstracts | Stochastic fluctuations in metabolic pathways | null | Fluctuations in the abundance of molecules in the living cell may affect its
growth and well being. For regulatory molecules (e.g., signaling proteins or
transcription factors), fluctuations in their expression can affect the levels
of downstream targets in a network. Here, we develop an analytic framework to
investigate the phenomenon of noise correlation in molecular networks.
Specifically, we focus on the metabolic network, which is highly inter-linked,
and noise properties may constrain its structure and function. Motivated by the
analogy between the dynamics of a linear metabolic pathway and that of the
exactly soluable linear queueing network or, alternatively, a mass transfer
system, we derive a plethora of results concerning fluctuations in the
abundance of intermediate metabolites in various common motifs of the metabolic
network. For all but one case examined, we find the steady-state fluctuation in
different nodes of the pathways to be effectively uncorrelated. Consequently,
fluctuations in enzyme levels only affect local properties and do not propagate
elsewhere into metabolic networks, and intermediate metabolites can be freely
shared by different reactions. Our approach may be applicable to study
metabolic networks with more complex topologies, or protein signaling networks
which are governed by similar biochemical reactions. Possible implications for
bioinformatic analysis of metabolimic data are discussed.
|
eae974bf-fb5e-4694-9a9b-da8852420c5b | eae974bf-fb5e-4694-9a9b-da8852420c5b | eae974bf-fb5e-4694-9a9b-da8852420c5b | human | null | null | none | abstracts | The azimuth structure of nuclear collisions -- I | null | We describe azimuth structure commonly associated with elliptic and directed
flow in the context of 2D angular autocorrelations for the purpose of precise
separation of so-called nonflow (mainly minijets) from flow. We extend the
Fourier-transform description of azimuth structure to include power spectra and
autocorrelations related by the Wiener-Khintchine theorem. We analyze several
examples of conventional flow analysis in that context and question the
relevance of reaction plane estimation to flow analysis. We introduce the 2D
angular autocorrelation with examples from data analysis and describe a
simulation exercise which demonstrates precise separation of flow and nonflow
using the 2D autocorrelation method. We show that an alternative correlation
measure based on Pearson's normalized covariance provides a more intuitive
measure of azimuth structure.
|
d7c28993-fb95-4ec9-903b-8f5e1a2a7176 | d7c28993-fb95-4ec9-903b-8f5e1a2a7176 | d7c28993-fb95-4ec9-903b-8f5e1a2a7176 | human | null | null | none | abstracts | Exploiting Social Annotation for Automatic Resource Discovery | null | Information integration applications, such as mediators or mashups, that
require access to information resources currently rely on users manually
discovering and integrating them in the application. Manual resource discovery
is a slow process, requiring the user to sift through results obtained via
keyword-based search. Although search methods have advanced to include evidence
from document contents, its metadata and the contents and link structure of the
referring pages, they still do not adequately cover information sources --
often called ``the hidden Web''-- that dynamically generate documents in
response to a query. The recently popular social bookmarking sites, which allow
users to annotate and share metadata about various information sources, provide
rich evidence for resource discovery. In this paper, we describe a
probabilistic model of the user annotation process in a social bookmarking
system del.icio.us. We then use the model to automatically find resources
relevant to a particular information domain. Our experimental results on data
obtained from \emph{del.icio.us} show this approach as a promising method for
helping automate the resource discovery task.
|
c54b5b1d-8cd0-4f75-a01f-75afdfe85228 | c54b5b1d-8cd0-4f75-a01f-75afdfe85228 | c54b5b1d-8cd0-4f75-a01f-75afdfe85228 | human | null | null | none | abstracts | Personalizing Image Search Results on Flickr | null | The social media site Flickr allows users to upload their photos, annotate
them with tags, submit them to groups, and also to form social networks by
adding other users as contacts. Flickr offers multiple ways of browsing or
searching it. One option is tag search, which returns all images tagged with a
specific keyword. If the keyword is ambiguous, e.g., ``beetle'' could mean an
insect or a car, tag search results will include many images that are not
relevant to the sense the user had in mind when executing the query. We claim
that users express their photography interests through the metadata they add in
the form of contacts and image annotations. We show how to exploit this
metadata to personalize search results for the user, thereby improving search
performance. First, we show that we can significantly improve search precision
by filtering tag search results by user's contacts or a larger social network
that includes those contact's contacts. Secondly, we describe a probabilistic
model that takes advantage of tag information to discover latent topics
contained in the search results. The users' interests can similarly be
described by the tags they used for annotating their images. The latent topics
found by the model are then used to personalize search results by finding
images on topics that are of interest to the user.
|
90e3a1d6-f6e7-4ddc-9fd0-da637a88a266 | 90e3a1d6-f6e7-4ddc-9fd0-da637a88a266 | 90e3a1d6-f6e7-4ddc-9fd0-da637a88a266 | human | null | null | none | abstracts | Resummed Cross Section for Jet Production at Hadron Colliders | null | We study the resummation of large logarithmic perturbative corrections to the
single-inclusive jet cross section at hadron colliders. The corrections we
address arise near the threshold for the partonic reaction, when the incoming
partons have just enough energy to produce the high-transverse-momentum final
state. The structure of the resulting logarithmic corrections is known to
depend crucially on the treatment of the invariant mass of the produced jet at
threshold. We allow the jet to have a non-vanishing mass at threshold, which
most closely corresponds to the situation in experiment. Matching our results
to available semi-analytical next-to-leading-order calculations, we derive
resummed results valid to next-to-leading logarithmic accuracy. We present
numerical results for the resummation effects at Tevatron and RHIC energies.
|
4813f797-36c7-4624-a472-3360cccd1db5 | 4813f797-36c7-4624-a472-3360cccd1db5 | 4813f797-36c7-4624-a472-3360cccd1db5 | human | null | null | none | abstracts | Settling the Complexity of Computing Two-Player Nash Equilibria | null | We settle a long-standing open question in algorithmic game theory. We prove
that Bimatrix, the problem of finding a Nash equilibrium in a two-player game,
is complete for the complexity class PPAD Polynomial Parity Argument, Directed
version) introduced by Papadimitriou in 1991.
This is the first of a series of results concerning the complexity of Nash
equilibria. In particular, we prove the following theorems:
Bimatrix does not have a fully polynomial-time approximation scheme unless
every problem in PPAD is solvable in polynomial time. The smoothed complexity
of the classic Lemke-Howson algorithm and, in fact, of any algorithm for
Bimatrix is not polynomial unless every problem in PPAD is solvable in
randomized polynomial time. Our results demonstrate that, even in the simplest
form of non-cooperative games, equilibrium computation and approximation are
polynomial-time equivalent to fixed point computation. Our results also have
two broad complexity implications in mathematical economics and operations
research: Arrow-Debreu market equilibria are PPAD-hard to compute. The P-Matrix
Linear Complementary Problem is computationally harder than convex programming
unless every problem in PPAD is solvable in polynomial time.
|
a30577e6-4b3e-448d-9738-2e0723100ef9 | a30577e6-4b3e-448d-9738-2e0723100ef9 | a30577e6-4b3e-448d-9738-2e0723100ef9 | human | null | null | none | abstracts | Equation of State in Relativistic Magnetohydrodynamics: variable versus
constant adiabatic index | null | The role of the equation of state for a perfectly conducting, relativistic
magnetized fluid is the main subject of this work. The ideal constant
$\Gamma$-law equation of state, commonly adopted in a wide range of
astrophysical applications, is compared with a more realistic equation of state
that better approximates the single-specie relativistic gas. The paper focus on
three different topics. First, the influence of a more realistic equation of
state on the propagation of fast magneto-sonic shocks is investigated. This
calls into question the validity of the constant $\Gamma$-law equation of state
in problems where the temperature of the gas substantially changes across
hydromagnetic waves. Second, we present a new inversion scheme to recover
primitive variables (such as rest-mass density and pressure) from conservative
ones that allows for a general equation of state and avoids catastrophic
numerical cancellations in the non-relativistic and ultrarelativistic limits.
Finally, selected numerical tests of astrophysical relevance (including
magnetized accretion flows around Kerr black holes) are compared using
different equations of state. Our main conclusion is that the choice of a
realistic equation of state can considerably bear upon the solution when
transitions from cold to hot gas (or viceversa) are present. Under these
circumstances, a polytropic equation of state can significantly endanger the
solution.
|
8d16f433-5b1c-4360-a3f7-e9648ceb7f62 | 8d16f433-5b1c-4360-a3f7-e9648ceb7f62 | 8d16f433-5b1c-4360-a3f7-e9648ceb7f62 | human | null | null | none | abstracts | Carbon Nanotube Thin Film Field Emitting Diode: Understanding the System
Response Based on Multiphysics Modeling | null | In this paper, we model the evolution and self-assembly of randomly oriented
carbon nanotubes (CNTs), grown on a metallic substrate in the form of a thin
film for field emission under diode configuration. Despite high output, the
current in such a thin film device often decays drastically. The present paper
is focused on understanding this problem. A systematic, multiphysics based
modelling approach is proposed. First, a nucleation coupled model for
degradation of the CNT thin film is derived, where the CNTs are assumed to
decay by fragmentation and formation of clusters. The random orientation of the
CNTs and the electromechanical interaction are then modeled to explain the
self-assembly. The degraded state of the CNTs and the electromechanical force
are employed to update the orientation of the CNTs. Field emission current at
the device scale is finally obtained by using the Fowler-Nordheim equation and
integration over the computational cell surfaces on the anode side. The
simulated results are in close agreement with the experimental results. Based
on the developed model, numerical simulations aimed at understanding the
effects of various geometric parameters and their statistical features on the
device current history are reported.
|
24b62220-00b8-43c2-8ef4-7692ac3e377b | 24b62220-00b8-43c2-8ef4-7692ac3e377b | 24b62220-00b8-43c2-8ef4-7692ac3e377b | human | null | null | none | abstracts | Demographics of Transition Objects | null | The unusual properties of transition objects (young stars with an optically
thin inner disc surrounded by an optically thick outer disc) suggest that
significant disc evolution has occured in these systems. We explore the nature
of these systems by examining their demographics, specifically their stellar
accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T
Tauri stars of comparable age. We find that transition objects in Taurus occupy
a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition
single stars in Taurus, they have stellar accretion rates that are typically
~10 times lower at the same disc mass and median disc masses ~4 times larger.
These properties are anticipated by several proposed planet formation theories
and suggest that the formation of Jovian mass planets may play a significant
role in explaining the origin of at least some transition objects. Considering
transition objects as a distinct demographic group among accreting T Tauri
stars leads to a tighter relationship between disc masses and stellar accretion
rates, with a slope between the two quantities that is close to the value of
unity expected in simple theories of disc accretion.
|
65804473-f4ce-498a-a23e-ee72e731e82e | 65804473-f4ce-498a-a23e-ee72e731e82e | 65804473-f4ce-498a-a23e-ee72e731e82e | human | null | null | none | abstracts | Modeling the Field Emission Current Fluctuation in Carbon Nanotube Thin
Films | null | Owing to their distinct properties, carbon nanotubes (CNTs) have emerged as
promising candidate for field emission devices. It has been found
experimentally that the results related to the field emission performance show
variability. The design of an efficient field emitting device requires the
analysis of the variabilities with a systematic and multiphysics based modeling
approach. In this paper, we develop a model of randomly oriented CNTs in a thin
film by coupling the field emission phenomena, the electron-phonon transport
and the mechanics of single isolated CNT. A computational scheme is developed
by which the states of CNTs are updated in time incremental manner. The device
current is calculated by using Fowler-Nordheim equation for field emission to
study the performance at the device scale.
|
e5d85ae7-46e4-464e-9d4c-1082dd7e42e6 | e5d85ae7-46e4-464e-9d4c-1082dd7e42e6 | e5d85ae7-46e4-464e-9d4c-1082dd7e42e6 | human | null | null | none | abstracts | The molecular environment of massive star forming cores associated with
Class II methanol maser emission | null | Methanol maser emission has proven to be an excellent signpost of regions
undergoing massive star formation (MSF). To investigate their role as an
evolutionary tracer, we have recently completed a large observing program with
the ATCA to derive the dynamical and physical properties of molecular/ionised
gas towards a sample of MSF regions traced by 6.7 GHz methanol maser emission.
We find that the molecular gas in many of these regions breaks up into multiple
sub-clumps which we separate into groups based on their association
with/without methanol maser and cm continuum emission. The temperature and
dynamic state of the molecular gas is markedly different between the groups.
Based on these differences, we attempt to assess the evolutionary state of the
cores in the groups and thus investigate the role of class II methanol masers
as a tracer of MSF.
|
ba674f74-eccb-41bb-ba5e-5df43c3277fa | ba674f74-eccb-41bb-ba5e-5df43c3277fa | ba674f74-eccb-41bb-ba5e-5df43c3277fa | human | null | null | none | abstracts | Two Results on Homogeneous Hessian Nilpotent Polynomials | null | Let $z=(z_1, ..., z_n)$ and $\Delta=\sum_{i=1}^n \frac {\partial^2}{\partial
z^2_i}$ the Laplace operator. A formal power series $P(z)$ is said to be {\it
Hessian Nilpotent}(HN) if its Hessian matrix $\Hes P(z)=(\frac {\partial^2
P}{\partial z_i\partial z_j})$ is nilpotent. In recent developments in [BE1],
[M] and [Z], the Jacobian conjecture has been reduced to the following
so-called {\it vanishing conjecture}(VC) of HN polynomials: {\it for any
homogeneous HN polynomial $P(z)$ $($of degree $d=4$$)$, we have $\Delta^m
P^{m+1}(z)=0$ for any $m>>0$.} In this paper, we first show that, the VC holds
for any homogeneous HN polynomial $P(z)$ provided that the projective
subvarieties ${\mathcal Z}_P$ and ${\mathcal Z}_{\sigma_2}$ of $\mathbb C
P^{n-1}$ determined by the principal ideals generated by $P(z)$ and
$\sigma_2(z):=\sum_{i=1}^n z_i^2$, respectively, intersect only at regular
points of ${\mathcal Z}_P$. Consequently, the Jacobian conjecture holds for the
symmetric polynomial maps $F=z-\nabla P$ with $P(z)$ HN if $F$ has no non-zero
fixed point $w\in \mathbb C^n$ with $\sum_{i=1}^n w_i^2=0$. Secondly, we show
that the VC holds for a HN formal power series $P(z)$ if and only if, for any
polynomial $f(z)$, $\Delta^m (f(z)P(z)^m)=0$ when $m>>0$.
|
e27cc405-b78c-4a9f-8796-bcc878e3dfc2 | e27cc405-b78c-4a9f-8796-bcc878e3dfc2 | e27cc405-b78c-4a9f-8796-bcc878e3dfc2 | human | null | null | none | abstracts | A Vanishing Conjecture on Differential Operators with Constant
Coefficients | null | In the recent progress [BE1], [Me] and [Z2], the well-known JC (Jacobian
conjecture) ([BCW], [E]) has been reduced to a VC (vanishing conjecture) on the
Laplace operators and HN (Hessian nilpotent) polynomials (the polynomials whose
Hessian matrix are nilpotent). In this paper, we first show that the vanishing
conjecture above, hence also the JC, is equivalent to a vanishing conjecture
for all 2nd order homogeneous differential operators $\Lambda$ and
$\Lambda$-nilpotent polynomials $P$ (the polynomials $P(z)$ satisfying
$\Lambda^m P^m=0$ for all $m\ge 1$). We then transform some results in the
literature on the JC, HN polynomials and the VC of the Laplace operators to
certain results on $\Lambda$-nilpotent polynomials and the associated VC for
2nd order homogeneous differential operators $\Lambda$. This part of the paper
can also be read as a short survey on HN polynomials and the associated VC in
the more general setting. Finally, we discuss a still-to-be-understood
connection of $\Lambda$-nilpotent polynomials in general with the classical
orthogonal polynomials in one or more variables. This connection provides a
conceptual understanding for the isotropic properties of homogeneous
$\Lambda$-nilpotent polynomials for the 2nd order homogeneous full rank
differential operators $\Lambda$ with constant coefficients.
|
96bb0d31-224d-4ed2-a3eb-61f8c7a2e4ab | 96bb0d31-224d-4ed2-a3eb-61f8c7a2e4ab | 96bb0d31-224d-4ed2-a3eb-61f8c7a2e4ab | human | null | null | none | abstracts | Effects of accelerating growth on the evolution of weighted complex
networks | null | Many real systems possess accelerating statistics where the total number of
edges grows faster than the network size. In this paper, we propose a simple
weighted network model with accelerating growth. We derive analytical
expressions for the evolutions and distributions for strength, degree, and
weight, which are relevant to accelerating growth. We also find that
accelerating growth determines the clustering coefficient of the networks.
Interestingly, the distributions for strength, degree, and weight display a
transition from scale-free to exponential form when the parameter with respect
to accelerating growth increases from a small to large value. All the
theoretical predictions are successfully contrasted with extensive numerical
simulations.
|
ce11f36a-b566-49f8-8ae5-46bb884bdb40 | ce11f36a-b566-49f8-8ae5-46bb884bdb40 | ce11f36a-b566-49f8-8ae5-46bb884bdb40 | human | null | null | none | abstracts | Locally Decodable Codes From Nice Subsets of Finite Fields and Prime
Factors of Mersenne Numbers | null | A k-query Locally Decodable Code (LDC) encodes an n-bit message x as an N-bit
codeword C(x), such that one can probabilistically recover any bit x_i of the
message by querying only k bits of the codeword C(x), even after some constant
fraction of codeword bits has been corrupted. The major goal of LDC related
research is to establish the optimal trade-off between length and query
complexity of such codes.
Recently [Y] introduced a novel technique for constructing locally decodable
codes and vastly improved the upper bounds for code length. The technique is
based on Mersenne primes. In this paper we extend the work of [Y] and argue
that further progress via these methods is tied to progress on an old number
theory question regarding the size of the largest prime factors of Mersenne
numbers.
Specifically, we show that every Mersenne number m=2^t-1 that has a prime
factor p>m^\gamma yields a family of k(\gamma)-query locally decodable codes of
length Exp(n^{1/t}). Conversely, if for some fixed k and all \epsilon > 0 one
can use the technique of [Y] to obtain a family of k-query LDCs of length
Exp(n^\epsilon); then infinitely many Mersenne numbers have prime factors arger
than known currently.
|
39739e17-f11a-4a6e-a07c-9c46e94dd864 | 39739e17-f11a-4a6e-a07c-9c46e94dd864 | 39739e17-f11a-4a6e-a07c-9c46e94dd864 | human | null | null | none | abstracts | Effect of the Spatial Dispersion on the Shape of a Light Pulse in a
Quantum Well | null | Reflectance, transmittance and absorbance of a symmetric light pulse, the
carrying frequency of which is close to the frequency of interband transitions
in a quantum well, are calculated. Energy levels of the quantum well are
assumed discrete, and two closely located excited levels are taken into
account. A wide quantum well (the width of which is comparable to the length of
the light wave, corresponding to the pulse carrying frequency) is considered,
and the dependance of the interband matrix element of the momentum operator on
the light wave vector is taken into account. Refractive indices of barriers and
quantum well are assumed equal each other. The problem is solved for an
arbitrary ratio of radiative and nonradiative lifetimes of electronic
excitations. It is shown that the spatial dispersion essentially affects the
shapes of reflected and transmitted pulses. The largest changes occur when the
radiative broadening is close to the difference of frequencies of interband
transitions taken into account.
|
d3e3eb1f-61a2-4993-9197-907faf9ba92f | d3e3eb1f-61a2-4993-9197-907faf9ba92f | d3e3eb1f-61a2-4993-9197-907faf9ba92f | human | null | null | none | abstracts | Very High Energy Gamma Rays from Supernova Remnants and Constraints on
the Galactic Interstellar Radiation Field | null | The large-scale Galactic interstellar radiation field (ISRF) is the result of
stellar emission and dust re-processing of starlight. Where the energy density
of the ISRF is high (e.g., the Galactic Centre), the dominant gamma-ray
emission in individual supernova remnants (SNRs), such as G0.9+0.1, may come
from inverse Compton (IC) scattering of the ISRF. Several models of the ISRF
exist. The most recent one, which has been calculated by us, predicts a
significantly higher ISRF than the well-used model of Mathis, Mezger, and
Panagia. However,comparison with data is limited to local observations. Based
on our current estimate of the ISRF we predict the gamma-ray emission in the
SNRs G0.9+0.1 and RXJ1713, and pair-production absorption features above 20 TeV
in the spectra of G0.9+0.1, J1713-381, and J1634-472. We discuss how GLAST,
along with current and future very high energy instruments, may be able to
provide upper bounds on the large-scale ISRF.
|
00b37f1d-e697-4c6a-910a-c5f8b40a3547 | 00b37f1d-e697-4c6a-910a-c5f8b40a3547 | 00b37f1d-e697-4c6a-910a-c5f8b40a3547 | human | null | null | none | abstracts | Magneto-oscillations due to electron-electron interactions in the ac
conductivity of a 2D electron gas | null | Electron-electron interactions give rise to the correction,
\delta\sigma^{int}(\omega), to the ac magnetoconductivity, \sigma(\omega), of a
clean 2D electron gas that is periodic in \omega_c^{-1}, where \omega_c is the
cyclotron frequency. Unlike conventional harmonics of the cyclotron resonance,
which are periodic with \omega, this correction is periodic with \omega^{3/2}.
Oscillations in \delta\sigma^{int}(\omega) develop at low magnetic fields,
\omega_c\ll\omega, when the conventional harmonics are suppressed by the
disorder. Their origin is a {\em double} backscattering of an electron from the
impurity-induced Friedel oscillations. During the time \sim\omega^{-1} between
the two backscattering events the electron travels only a {\em small portion}
of the Larmour circle.
|
7f5787de-d23e-437a-af6c-72c4a6031b4c | 7f5787de-d23e-437a-af6c-72c4a6031b4c | 7f5787de-d23e-437a-af6c-72c4a6031b4c | human | null | null | none | abstracts | Implementation of a Gauss convoluted Pandel PDF for track reconstruction
in Neutrino Telescopes | null | A probability distribution function is presented which provides a realistic
description of the detection of scattered photons. The resulting probabilities
can be described analytically by means of a superposition of several special
functions. These exact expressions can be evaluated numerically only for small
distances and limited time residuals, due to computer accuracy limitations. In
this report we provide approximations for the exact expressions in different
regions of the distance-time residual space, defined by the detector geometry
and the space-time scale of an event. These approximations can be evaluated
numerically with a relative error with respect to the exact expression at the
boundaries of less than 0.001.
|
da634a9a-70c8-4faf-bc54-2cb8a407f251 | da634a9a-70c8-4faf-bc54-2cb8a407f251 | da634a9a-70c8-4faf-bc54-2cb8a407f251 | human | null | null | none | abstracts | A Cut-free Sequent Calculus for Bi-Intuitionistic Logic: Extended
Version | null | Bi-intuitionistic logic is the extension of intuitionistic logic with a
connective dual to implication. Bi-intuitionistic logic was introduced by
Rauszer as a Hilbert calculus with algebraic and Kripke semantics. But her
subsequent ``cut-free'' sequent calculus for BiInt has recently been shown by
Uustalu to fail cut-elimination. We present a new cut-free sequent calculus for
BiInt, and prove it sound and complete with respect to its Kripke semantics.
Ensuring completeness is complicated by the interaction between implication and
its dual, similarly to future and past modalities in tense logic. Our calculus
handles this interaction using extended sequents which pass information from
premises to conclusions using variables instantiated at the leaves of failed
derivation trees. Our simple termination argument allows our calculus to be
used for automated deduction, although this is not its main purpose.
|
4f711ff8-b4ba-417b-923d-01478a03c328 | 4f711ff8-b4ba-417b-923d-01478a03c328 | 4f711ff8-b4ba-417b-923d-01478a03c328 | human | null | null | none | abstracts | Further Analysis of Excitations of Quarks at Finite Temperature -- Mass
Effect and Pole Structure -- | null | We calculate the spectral function of the massive quark at finite temperature
(T) using a Yukawa model and show that the peak in the negative energy region
among the three-peaks found in a previous work for the massless quark is
largely suppressed. To explore the underlying mechanism of this behavior, we
also investigate the pole structure of the retarded Green function of the
quark. We will show the result only for the massless quark. We find the
residues of the poles corresponding the three-peaks are all comparable at
$T\sim m_b$. We also show that the multi-peak structure of the quark spectra is
well described in the pole approximation which indicates that the quasi-paricle
picture is valid in this $T$ region.
|
7e213769-69ad-43e0-bde7-dcbf7f077309 | 7e213769-69ad-43e0-bde7-dcbf7f077309 | 7e213769-69ad-43e0-bde7-dcbf7f077309 | human | null | null | none | abstracts | An SiO Maser Search off the Galactic Plane | null | We have searched for the SiO J=1--0 v= 1 and 2 maser lines at ~ 43 GHz in 277
2MASS/MSX/IRAS sources off the Galactic plane (|b|> 3 deg), which resulted in
119 (112 new) detections. Among the new detections, are two very faint objects
with MSX 12 micron flux densities below 2 Jy. These are likely to be O-rich
AGB-stars associated with dwarf-galaxy tidal tails. The sample also includes
medium bright MSX objects at moderately high Galactic latitudes (3 deg<|b|<5
deg) and in the IRAS gap at higher latitudes. A signature of a warp of the
inner Galactic disk is found for a disk subsample. This warp appears relatively
strongly in the area of 0<l<45 deg and 3<|b|<5 deg. We also found a group of
stars that does not follow to the Galactic rotation. This feature appears in
the Galactic disk at l~ 27 deg, and extends more than 15 deg in Galactic
latitude, like a stream of tidal debris from a dwarf galaxy.
|
a81e81a2-dc40-4a5c-a35c-9599df099763 | a81e81a2-dc40-4a5c-a35c-9599df099763 | a81e81a2-dc40-4a5c-a35c-9599df099763 | human | null | null | none | abstracts | Dislocation Dynamics in a Crystal Lattice (Peierls-Nabarro) Relief | null | The theory of the dislocation motion in the periodic potential relief of the
crystal lattice (the Peierls-Nabarro barriers) is reviewed. On the basis of the
kink mechanism the temperature dependence of the flow stress is described for a
wide class of materials. The theory of quantum mechanical dislocation
tunnelling through the Peierls-Nabarro barriers is extended and compared with
experimental data on the plasticity of alkali halides, BCC and HCP metals at
low temperatures. The behavior of the flow stress at the range of athermic
anomalies is modeled by changing the mechanism of the dislocation motion from
the thermally activated hopping over the barriers to the quantum tunnelling
through them. Some results of previous calculations are represented in a more
explicit convenient for applications form. The pronounced effect of the
switching between the normal and the superconducting states on the flow stress
of metals is explained on the basis of the change in the dissipative properties
of the electron subsystem affecting the dislocation motion.
|
3a020859-4a9f-4937-9f7a-2d44ecb3ee11 | 3a020859-4a9f-4937-9f7a-2d44ecb3ee11 | 3a020859-4a9f-4937-9f7a-2d44ecb3ee11 | human | null | null | none | abstracts | Spectrum of cosmic rays, produced in supernova remnants | null | Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova
remnants is employed to calculate CR spectra. The magnetic field in SNRs is
assumed to be significantly amplified by the efficiently accelerating nuclear
CR component. It is shown that the calculated CR spectra agree in a
satisfactory way with the existing measurements up to the energy $10^{17}$ eV.
The power law spectrum of protons extends up to the energy $3\times 10^{15}$ eV
with a subsequent exponential cutoff. It gives a natural explanation for the
observed knee in the Galactic CR spectrum. The maximum energy of the
accelerated nuclei is proportional to their charge number $Z$. Therefore the
break in the Galactic CR spectrum is the result of the contribution of
progressively heavier species in the overall CR spectrum so that at $10^{17}$
eV the CR spectrum is dominated by iron group nuclei. It is shown that this
component plus a suitably chosen extragalactic CR component can give a
consistent description for the entire Galactic CR spectrum.
|
56612136-66ac-4a9a-ac1c-25befca70f08 | 56612136-66ac-4a9a-ac1c-25befca70f08 | 56612136-66ac-4a9a-ac1c-25befca70f08 | human | null | null | none | abstracts | A resummed perturbative estimate for the quarkonium spectral function in
hot QCD | null | By making use of the finite-temperature real-time static potential that was
introduced and computed to leading non-trivial order in Hard Thermal Loop
resummed perturbation theory in recent work, and solving numerically a
Schr\"odinger-type equation, we estimate the quarkonium (in practice,
bottomonium) contribution to the spectral function of the electromagnetic
current in hot QCD. The spectral function shows a single resonance peak which
becomes wider and then disappears as the temperature is increased beyond 450
MeV or so. This behaviour can be compared with recently attempted lattice
reconstructions of the same quantity, based on the ``maximum entropy method'',
which generically show several peaks. We also specify the dependence of our
results on the spatial momentum of the electromagnetic current, as well as on
the baryon chemical potential characterising the hot QCD plasma.
|
16718ca7-8cd4-4540-8d2d-d802976f0de2 | 16718ca7-8cd4-4540-8d2d-d802976f0de2 | 16718ca7-8cd4-4540-8d2d-d802976f0de2 | human | null | null | none | abstracts | A User-Friendly Code to Diagnose Chromospheric Plasmas | null | The physical interpretation of spectropolarimetric observations of lines of
neutral helium, such as those of the 10830 A multiplet, represents an excellent
opportunity for investigating the magnetism of plasma structures in the solar
chromosphere. Here we present a powerful forward modeling and inversion code
that permits either to calculate the emergent intensity and polarization for
any given magnetic field vector or to infer the dynamical and magnetic
properties from the observed Stokes profiles. This diagnostic tool is based on
the quantum theory of spectral line polarization, which self-consistently
accounts for the Hanle and Zeeman effects in the most general case of the
incomplete Paschen-Back effect regime. We also take into account radiative
transfer effects. An efficient numerical scheme based on global optimization
methods has been applied. Our Stokes inversion code permits a fast and reliable
determination of the global minimum.
|
e15a71cd-b9f3-4eb3-81c8-19448337e5ae | e15a71cd-b9f3-4eb3-81c8-19448337e5ae | e15a71cd-b9f3-4eb3-81c8-19448337e5ae | human | null | null | none | abstracts | Latin bitrades derived from groups | null | A latin bitrade is a pair of partial latin squares which are disjoint, occupy
the same set of non-empty cells, and whose corresponding rows and columns
contain the same set of entries. Dr\'apal (\cite{Dr9}) showed that a latin
bitrade is equivalent to three derangements whose product is the identity and
whose cycles pairwise have at most one point in common. By letting a group act
on itself by right translation, we show how some latin bitrades may be derived
from groups without specifying an independent group action. Properties of latin
trades such as homogeneousness, minimality (via thinness) and orthogonality may
also be encoded succinctly within the group structure. We apply the
construction to some well-known groups, constructing previously unknown latin
bitrades. In particular, we show the existence of minimal, $k$-homogeneous
latin trades for each odd $k\geq 3$. In some cases these are the smallest known
such examples.
|