407 research outputs found
Low-temperature anomalous specific heat without tunneling modes: a simulation for a-Si with voids
Using empirical potential molecular dynamics we compute dynamical matrix
eigenvalues and eigenvectors for a 4096 atom model of amorphous silicon and a
set of models with voids of different size based on it. This information is
then employed to study the localization properties of the low-energy
vibrational states, calculate the specific heat C(T) and examine the
low-temperature properties of our models usually attributed to the presence of
tunneling states in amorphous silicon. The results of our calculations for C(T)
and "excess specific heat bulge" in the C(T)/T^3 vs. T graph for voidless a-Si
appear to be in good agreement with experiment; moreover our investigation
shows that the presence of localized low-energy excitations in the vibrational
spectrum of our models with voids strongly manifests itself as a sharp peak in
C(T)/T^3 dependence at T < 3K. To our knowledge this is the first numerical
simulation that provides adequate agreement with experiment for the very
low-temperature properties of specific heat in disordered systems within the
limits of harmonic approximation.Comment: 5 pages with 2 ps figures, submitted to PR
Vector-meson contributions do not explain the rate and spectrum in K_L -> pi0 gamma gamma
We analyze the recent NA48 data for the reaction K_L -> pi0 gamma gamma with
and without the assumption of vector meson dominance (VMD). We find that the
data is well described by a three-parameter expression inspired by O(p^6)
chiral perturbation theory. We also find that it is impossible to fit the shape
of the decay distribution and the overall rate simultaneously if one imposes
the VMD constraints on the three parameters. We comment on the different fits
and their implications for the CP-conserving component of the decay K_L -> pi0
e+ e-.Comment: Version accepted for publication on Phys. Rev. D. 19 pages, LaTeX, 8
figures, uses epsf.st
Exact renormalization group flow equations for non-relativistic fermions: scaling towards the Fermi surface
We construct exact functional renormalization group (RG) flow equations for
non-relativistic fermions in arbitrary dimensions, taking into account not only
mode elimination but also the rescaling of the momenta, frequencies and the
fermionic fields. The complete RG flow of all relevant, marginal and irrelevant
couplings can be described by a system of coupled flow equations for the
irreducible n-point vertices. Introducing suitable dimensionless variables, we
obtain flow equations for generalized scaling functions which are continuous
functions of the flow parameter, even if we consider quantities which are
dominated by momenta close to the Fermi surface, such as the density-density
correlation function at long wavelengths. We also show how the problem of
constructing the renormalized Fermi surface can be reduced to the problem of
finding the RG fixed point of the irreducible two-point vertex at vanishing
momentum and frequency. We argue that only if the degrees of freedom are
properly rescaled it is possible to reach scale-invariant non-Fermi liquid
fixed points within a truncation of the exact RG flow equations.Comment: 20 Revtex pages, with 4 figures; final version to appear in Phys.
Rev. B; references and some explanations adde
Biological Rhythms Workshop I: Introduction to Chronobiology
In this chapter, we present a series of four articles derived from a Introductory Workshop on Biological Rhythms presented at the 72nd Annual Cold Spring Harbor Symposium on Quantitative Biology: Clocks and Rhythms. A diverse range of species, from cyanobacteria to humans, evolved endogenous biological clocks that allow for the anticipation of daily variations in light and temperature. The ability to anticipate environmental variation promotes optimal performance and survival. In the first article, Introduction to Chronobiology, we present a brief historical timeline of how circadian concepts and terminology have emerged since the early observation of daily leaf movement in plants made by an astronomer in the 1700s. Workshop Part IA provides an overview of the molecular basis for rhythms generation in several key model organisms, Workshop Part IB focuses on how biology built a brain clock capable of coordinating the daily timing of essential brain and physiological processes, and Workshop Part IC gives key insight into how researchers study sleep and rhythms in humans
The Structure and Dynamics of the Upper Chromosphere and Lower Transition Region as Revealed by the Subarcsecond VAULT Observations
The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond (0.5") images of this region with high
sensitivity and cadence. Detailed analyses of those observations have
contributed significantly to new ideas about the nature of the transition
region. Here, we present a broad overview of the Ly-a atmosphere as revealed by
the VAULT observations, and bring together past results and new analyses from
the second VAULT flight to create a synthesis of our current knowledge of the
high-resolution Ly-a Sun. We hope that this work will serve as a good reference
for the design of upcoming Ly-a telescopes and observing plans.Comment: 28 pages, 11 figure
Minimal Noncanonical Cosmologies
We demonstrate how much it is possible to deviate from the standard
cosmological paradigm of inflation-assisted LambdaCDM, keeping within current
observational constraints, and without adding to or modifying any theoretical
assumptions. We show that within a minimal framework there are many new
possibilities, some of them wildly different from the standard picture. We
present three illustrative examples of new models, described phenomenologically
by a noncanonical scalar field coupled to radiation and matter. These models
have interesting implications for inflation, quintessence, reheating,
electroweak baryogenesis, and the relic densities of WIMPs and other exotics.Comment: 20 pages, 5 figures, 3 table
Gamma-ray and radio tests of the e+e- excess from DM annihilations
PAMELA and ATIC recently reported an excess in e+e- cosmic rays. We show that
if it is due to Dark Matter annihilations, the associated gamma-ray flux and
the synchrotron emission produced by e+e- in the galactic magnetic field
violate HESS and radio observations of the galactic center and HESS
observations of dwarf Spheroidals, unless the DM density profile is
significantly less steep than the benchmark NFW and Einasto profiles.Comment: 16 pages, 4 figures; v2: normalizations fixed in Table 2 and typos
corrected (no changes in the analysis nor the results), some references and
comments added; v3: minor additions, matches published versio
B physics and extra dimensions
We compute the dominant new physics contributions to the processes Z -> b b and B - B in the context of two representative models with extra dimensions. The main thrust of the calculations focuses on how to control the effects of the infinite tower of Kaluza-Klein modes inside the relevant one-loop diagrams. By comparing the results with the existing experimental data, most importantly those for Rb, we show that one may derive interesting lower bounds on the size of the compactification scale Mc
Probing mSUGRA via the Extreme Universe Space Observatory
An analysis is carried out within mSUGRA of the estimated number of events
originating from upward moving ultra-high energy neutralinos that could be
detected by the Extreme Universe Space Observatory (EUSO). The analysis
exploits a recently proposed technique that differentiates ultra-high energy
neutralinos from ultra-high energy neutrinos using their different absorption
lengths in the Earth's crust. It is shown that for a significant part of the
parameter space, where the neutralino is mostly a Bino and with squark mass
TeV, EUSO could see ultra-high energy neutralino events with
essentially no background. In the energy range 10^9 GeV < E < 10^11 GeV, the
unprecedented aperture of EUSO makes the telescope sensitive to neutralino
fluxes as low as 1.1 \times 10^{-6} (E/GeV)^{-1.3} GeV^{-1} cm^{-2} yr^{-1}
sr^{-1}, at the 95% CL. Such a hard spectrum is characteristic of supermassive
particles' -body hadronic decay. The case in which the flux of ultra-high
energy neutralinos is produced via decay of metastable heavy particles with
uniform distribution throughout the universe is analyzed in detail. The
normalization of the ratio of the relics' density to their lifetime has been
fixed so that the baryon flux produced in the supermassive particle decays
contributes to about 1/3 of the events reported by the AGASA Collaboration
below 10^{11} GeV, and hence the associated GeV gamma-ray flux is in complete
agreement with EGRET data. For this particular case, EUSO will collect between
4 and 5 neutralino events (with 0.3 of background) in ~ 3 yr of running. NASA's
planned mission, the Orbiting Wide-angle Light-collectors (OWL), is also
briefly discussed in this context.Comment: Some discussion added, final version to be published in Physical
Review
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
- âŠ