804 research outputs found
A new interpretation of dielectric data in molecular glass formers
Literature dielectric data of glycerol, propylene carbonate and
ortho-terphenyl (OTP) show that the measured dielectric relaxation is a decade
faster than the Debye expectation, but still a decade slower than the breakdown
of the shear modulus. From a comparison of time scales, the dielectric
relaxation seems to be due to a process which relaxes not only the molecular
orientation, but the entropy, the short-range order and the density as well. On
the basis of this finding, we propose an alternative to the
Gemant-DiMarzio-Bishop extension of the Debye picture.Comment: 7 pages, 4 figures, 68 references; 3. version extended following
referee advic
Scaling the Temperature-dependent Boson Peak of Vitreous Silica with the high-frequency Bulk Modulus derived from Brillouin Scattering Data
The position and strength of the boson peak in silica glass vary considerably
with temperature . Such variations cannot be explained solely with changes
in the Debye energy. New Brillouin scattering measurements are presented which
allow determining the -dependence of unrelaxed acoustic velocities. Using a
velocity based on the bulk modulus, scaling exponents are found which agree
with the soft-potential model. The unrelaxed bulk modulus thus appears to be a
good measure for the structural evolution of silica with and to set the
energy scale for the soft potentials.Comment: Accepted for publication in Physical Review Letter
Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
refereein
Dynamic rotor mode in antiferromagnetic nanoparticles
We present experimental, numerical, and theoretical evidence for a new mode
of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering
experiments on 8 nm particles of hematite display a loss of diffraction
intensity with temperature, the intensity vanishing around 150 K. However, the
signal from inelastic neutron scattering remains above that temperature,
indicating a magnetic system in constant motion. In addition, the precession
frequency of the inelastic magnetic signal shows an increase above 100 K.
Numerical Langevin simulations of spin dynamics reproduce all measured neutron
data and reveal that thermally activated spin canting gives rise to a new type
of coherent magnetic precession mode. This "rotor" mode can be seen as a
high-temperature version of superparamagnetism and is driven by exchange
interactions between the two magnetic sublattices. The frequency of the rotor
mode behaves in fair agreement with a simple analytical model, based on a high
temperature approximation of the generally accepted Hamiltonian of the system.
The extracted model parameters, as the magnetic interaction and the axial
anisotropy, are in excellent agreement with results from Mossbauer
spectroscopy
Shadowing of Ultrahigh Energy Neutrinos
The rise with energy of the neutrino--nucleon cross section implies that at
energies above few TeV the Earth is becoming opaque to cosmic neutrinos. The
neutrinos interact with the nucleons through the weak charged current,
resulting into absorption, and the weak neutral current, which provides a
redistribution of the neutrino energy. We Mellin transform the neutrino
transport equation and find its exact solution in the moment space. A simple
analytical formula is provided, which describes accurately the neutrino
spectrum, after the neutrinos have traversed the Earth. The effect of the weak
neutral current is most prominent for an initial flat neutrino spectrum and we
find that at low energies (around 1 TeV) the neutrino intensity is even
enhanced.Comment: gziped, tar file of LaTeX paper plus 2 postscript figures, 13 page
Detection of the Crab Nebula using a Random Forest Analysis of the first TAIGA IACT Data
The Tunka Advanced Instrument for Gamma- and cosmic-ray Astronomy (TAIGA) is
a multicomponent experiment for the measurement of TeV to PeV gamma- and cosmic
rays. Our goal is to establish a novel hybrid direct air shower technique,
sufficient to access the energy domain of the long-sought Pevatrons. The hybrid
air Cherenkov light detection technique combines the strengths of the HiSCORE
shower front sampling array, and two 4 m class, 9.6 deg field
of view Imaging Air Cherenkov Telescopes (IACTs). The HiSCORE array provides
good angular and shower core position resolution, while the IACTs provide the
image shape and orientation for gamma-hadron separation. In future, an
additional muon detector will be used for hadron tagging at 100 TeV
energies. Here, only data from the first IACT of the TAIGA experiment are used.
A random forest algorithm was trained using Monte Carlo (MC) simulations and
real data, and applied to 85 h of selected observational data tracking the Crab
Nebula at a mean zenith angle of 33.5 deg, resulting in a threshold energy of 6
TeV for this dataset. The analysis was performed using the gammapy package. A
total of 163.5 excess events were detected, with a statistical significance of
8.5 sigma. The observed spectrum of the Crab Nebula is best fit with a power
law above 6 TeV with a flux normalisation of at a reference energy of 13 TeV and a spectral index
of .Comment: 8 pages, 9 figures, accepted by MNRA
The Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems
New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient and, in particular, its low frequency
limit. This study indicates that in the limit
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.Comment: Revtex, 4 ps figure
Molecular observation of contour-length fluctuations limiting topological confinement in polymer melts
In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time
Tunka-Rex: the Cost-Effective Radio Extension of the Tunka Air-Shower Observatory
Tunka-Rex is the radio extension of the Tunka cosmic-ray observatory in
Siberia close to Lake Baikal. Since October 2012 Tunka-Rex measures the radio
signal of air-showers in coincidence with the non-imaging air-Cherenkov array
Tunka-133. Furthermore, this year additional antennas will go into operation
triggered by the new scintillator array Tunka-Grande measuring the secondary
electrons and muons of air showers. Tunka-Rex is a demonstrator for how
economic an antenna array can be without losing significant performance: we
have decided for simple and robust SALLA antennas, and we share the existing
DAQ running in slave mode with the PMT detectors and the scintillators,
respectively. This means that Tunka-Rex is triggered externally, and does not
need its own infrastructure and DAQ for hybrid measurements. By this, the
performance and the added value of the supplementary radio measurements can be
studied, in particular, the precision for the reconstructed energy and the
shower maximum in the energy range of approximately eV. Here
we show first results on the energy reconstruction indicating that radio
measurements can compete with air-Cherenkov measurements in precision.
Moreover, we discuss future plans for Tunka-Rex.Comment: Proceeding of UHECR 2014, Springdale, Utah, USA, accepted by JPS
Conference Proceeding
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