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 $T$. Such variations cannot be explained solely with changes in the Debye energy. New Brillouin scattering measurements are presented which allow determining the $T$-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 $T$ 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 $\thicksim$4 m class, $\sim$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 $\ge$ 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 $(3.20\pm0.42)\cdot10^{-10} TeV^{-1} cm^{-2} s^{-1})$ at a reference energy of 13 TeV and a spectral index of $-2.74\pm0.16$.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 $C(\omega)$ and, in particular, its low frequency limit. This study indicates that in the $\omega \to 0$ limit $C(\omega)$ 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 $10^{17}-10^{18}\,$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