404 research outputs found
Field Measurements of Terrestrial and Martian Dust Devils
Surface-based measurements of terrestrial and martian dust devils/convective vortices provided from mobile and stationary platforms are discussed. Imaging of terrestrial dust devils has quantified their rotational and vertical wind speeds, translation speeds, dimensions, dust load, and frequency of occurrence. Imaging of martian dust devils has provided translation speeds and constraints on dimensions, but only limited constraints on vertical motion within a vortex. The longer mission durations on Mars afforded by long operating robotic landers and rovers have provided statistical quantification of vortex occurrence (time-of-sol, and recently seasonal) that has until recently not been a primary outcome of more temporally limited terrestrial dust devil measurement campaigns. Terrestrial measurement campaigns have included a more extensive range of measured vortex parameters (pressure, wind, morphology, etc.) than have martian opportunities, with electric field and direct measure of dust abundance not yet obtained on Mars. No martian robotic mission has yet provided contemporaneous high frequency wind and pressure measurements. Comparison of measured terrestrial and martian dust devil characteristics suggests that martian dust devils are larger and possess faster maximum rotational wind speeds, that the absolute magnitude of the pressure deficit within a terrestrial dust devil is an order of magnitude greater than a martian dust devil, and that the time-of-day variation in vortex frequency is similar. Recent terrestrial investigations have demonstrated the presence of diagnostic dust devil signals within seismic and infrasound measurements; an upcoming Mars robotic mission will obtain similar measurement types
Very high energy observations of the BL Lac objects 3C 66A and OJ 287
Using the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), we
have observed the BL Lac objects 3C 66A and OJ 287. These are members of the
class of low-frequency-peaked BL Lac objects (LBLs) and are two of the three
LBLs predicted by Costamante and Ghisellini to be potential sources of very
high energy (>100 GeV) gamma-ray emission. The third candidate, BL Lacertae,
has recently been detected by the MAGIC collaboration. Our observations have
not produced detections; we calculate a 99% CL upper limit of flux from 3C 66A
of 0.15 Crab flux units and from OJ 287 our limit is 0.52 Crab. These limits
assume a Crab-like energy spectrum with an effective energy threshold of 185
GeV.Comment: 24 pages, 15 figures, Accepted for publication in Astroparticle
Physic
Effects of Al doping on the structural and electronic properties of Mg(1-x)Al(x)B2
We have studied the structural and electronic properties of Mg(1-x)Al(x)B2
within the Virtual Crystal Approximation (VCA) by means of first-principles
total-energy calculations. Results for the lattice parameters, the electronic
band structure, and the Fermi surface as a function of Al doping for 0<x<0.6
are presented. The ab initio VCA calculations are in excellent agreement with
the experimentally observed change in the lattice parameters of Al doped MgB2.
The calculations show that the Fermi surface associated with holes a the boron
planes collapses gradually with aluminum doping and vanishes for x=0.56. In
addition, an abrupt topological change in the sigma-band Fermi surface was
found for x=0.3. The calculated hole density correlates closely with existing
experimental data for Tc(x), indicating that the observed loss of
superconductivity in Mg(1-x)Al(x)B2 is a result of hole bands filling.Comment: 4 pages (revtex) and 4 figures (postscript
Crystal and magnetic structure of LaTiO3 : evidence for non-degenerate -orbitals
The crystal and magnetic structure of LaTiO3 ~ has been studied by x-ray and
neutron diffraction techniques using nearly stoichiometric samples. We find a
strong structural anomaly near the antiferromagnetic ordering, T=146 K. In
addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies
a splitting of the t2g-levels. Our results indicate that LaTiO3 should be
considered as a Jahn-Teller system where the structural distortion and the
resulting level splitting are enhanced by the magnetic ordering.Comment: 4 pages 5 figure
Observational signatures of a non-singular bouncing cosmology
We study a cosmological scenario in which inflation is preceded by a bounce.
In this scenario, the primordial singularity, one of the major shortcomings of
inflation, is replaced by a non-singular bounce, prior to which the universe
undergoes a phase of contraction. Our starting point is the bouncing cosmology
investigated in Falciano et al. (2008), which we complete by a detailed study
of the transfer of cosmological perturbations through the bounce and a
discussion of possible observational effects of bouncing cosmologies. We focus
on a symmetric bounce and compute the evolution of cosmological perturbations
during the contracting, bouncing and inflationary phases. We derive an
expression for the Mukhanov-Sasaki perturbation variable at the onset of the
inflationary phase that follows the bounce. Rather than being in the
Bunch-Davies vacuum, it is found to be in an excited state that depends on the
time scale of the bounce. We then show that this induces oscillations
superimposed on the nearly scale-invariant primordial spectra for scalar and
tensor perturbations. We discuss the effects of these oscillations in the
cosmic microwave background and in the matter power spectrum. We propose a new
way to indirectly measure the spatial curvature energy density parameter in the
context of this model.Comment: 40 pages, 5 figures, typos corrected and reference adde
Domain growth and aging scaling in coarsening disordered systems
Using extensive Monte Carlo simulations we study aging properties of two
disordered systems quenched below their critical point, namely the
two-dimensional random-bond Ising model and the three-dimensional
Edwards-Anderson Ising spin glass with a bimodal distribution of the coupling
constants. We study the two-times autocorrelation and space-time correlation
functions and show that in both systems a simple aging scenario prevails in
terms of the scaling variable , where is the time-dependent
correlation length, whereas is the waiting time and is the observation
time. The investigation of the space-time correlation function for the
random-bond Ising model allows us to address some issues related to
superuniversality.Comment: 8 pages, 9 figures, to appear in European Physical Journal
The First VERITAS Telescope
The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic
Radiation Imaging Telescope Array System) has been in operation since February
2005. We present here a technical description of the instrument and a summary
of its performance. The calibration methods are described, along with the
results of Monte Carlo simulations of the telescope and comparisons between
real and simulated data. The analysis of TeV -ray observations of the
Crab Nebula, including the reconstructed energy spectrum, is shown to give
results consistent with earlier measurements. The telescope is operating as
expected and has met or exceeded all design specifications.Comment: Accepted by Astroparticle Physic
A pleurocidin analogue with greater conformational flexibility, enhanced antimicrobial potency and in vivo therapeutic efficacy.
Antimicrobial peptides (AMPs) are a potential alternative to classical antibiotics that are yet to achieve a therapeutic breakthrough for treatment of systemic infections. The antibacterial potency of pleurocidin, an AMP from Winter Flounder, is linked to its ability to cross bacterial plasma membranes and seek intracellular targets while also causing membrane damage. Here we describe modification strategies that generate pleurocidin analogues with substantially improved, broad spectrum, antibacterial properties, which are effective in murine models of bacterial lung infection. Increasing peptide-lipid intermolecular hydrogen bonding capabilities enhances conformational flexibility, associated with membrane translocation, but also membrane damage and potency, most notably against Gram-positive bacteria. This negates their ability to metabolically adapt to the AMP threat. An analogue comprising D-amino acids was well tolerated at an intravenous dose of 15 mg/kg and similarly effective as vancomycin in reducing EMRSA-15 lung CFU. This highlights the therapeutic potential of systemically delivered, bactericidal AMPs
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation
We study a cosmological scenario in which the DBI action governing the motion
of a D3-brane in a higher-dimensional spacetime is supplemented with an induced
gravity term. The latter reduces to the quartic Galileon Lagrangian when the
motion of the brane is non-relativistic and we show that it tends to violate
the null energy condition and to render cosmological fluctuations ghosts. There
nonetheless exists an interesting parameter space in which a stable phase of
quasi-exponential expansion can be achieved while the induced gravity leaves
non trivial imprints. We derive the exact second-order action governing the
dynamics of linear perturbations and we show that it can be simply understood
through a bimetric perspective. In the relativistic regime, we also calculate
the dominant contribution to the primordial bispectrum and demonstrate that
large non-Gaussianities of orthogonal shape can be generated, for the first
time in a concrete model. More generally, we find that the sign and the shape
of the bispectrum offer powerful diagnostics of the precise strength of the
induced gravity.Comment: 34 pages including 9 figures, plus appendices and bibliography.
Wordings changed and references added; matches version published in JCA
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