Memory-function approach to the normal-state optical properties of the Bechgaard salt (TMTSF)_2PF_6

The gauge invariant, two-component optical conductivity model, with a correlation gap structure related to the umklapp scattering processes, is applied to the quasi-one-dimensional electronic systems and compared to the recent measurements on the Bechgaard salt (TMTSF)_2PF_6. The optical response of both the insulating and metallic state is found for the half-filled conduction band, depending on the ratio between the correlation energy scale 2 \Delta^0_2 and the transfer integral in the direction perpendicular to the conducting chains, t_{{\rm b}'}. The estimated value 2 \Delta^0_2/t_{{\rm b}'} agrees reasonably well with the previous experimental and theoretical conclusions. Parallel to the chains the thermally activated conduction electrons in the insulating state are found to exhibit an universal behaviour, accounting for the observed single-particle optical conductivity of the ordered ground state of charge-density-wave systems. The band parameters and the related damping energies suitable to the normal metallic state of (TMTSF)_2PF_6 are estimated from the measured spectra. Not only the spectral weights but also the damping energies clearly indicate an opening of the correlation gap in the charge excitation spectrumComment: 11 pages, 7 figure

A radio air shower surface detector as an extension for IceCube and IceTop

The IceCube neutrino detector is built into the Antarctic ice sheet at the South Pole to measure high energy neutrinos. For this, 4800 photomultiplier tubes (PMTs) are being deployed at depths between 1450 and 2450 meters into the ice to measure neutrino induced charged particles like muons. IceTop is a surface air shower detector consisting of 160 Cherenkov ice tanks located on top of IceCube. To extend IceTop, a radio air shower detector could be built to significantly increase the sensitivity at higher shower energies and for inclined showers. As air showers induced by cosmic rays are a major part of the muonic background in IceCube, IceTop is not only an air shower detector, but also a veto to reduce the background in IceCube. Air showers are detectable by radio signals with a radio surface detector. The major emission process is the coherent synchrotron radiation emitted by e+ e- shower particles in the Earths magnetic field (geosynchrotron effect). Simulations of the expected radio signals of air showers are shown. The sensitivity and the energy threshold of different antenna field configurations are estimated.Comment: 4 pages, 6 figures, to be published in Proceedings of the 30th International Cosmic Ray Conferenc

Boundary element methods for acoustic scattering by fractal screens

We study boundary element methods for time-harmonic scattering in R^n (n=2,3) by a fractal planar screen, assumed to be a non-empty bounded subset Gamma of the hyperplane Gamma_\infty=R^{n-1}\times \{0\}. We consider two distinct cases: (i) Gamma is a relatively open subset of Gamma_\infty with fractal boundary (e.g. the interior of the Koch snowflake in the case n=3); (ii) Gamma is a compact fractal subset of Gamma_\infty with empty interior (e.g. the Sierpinski triangle in the case n=3). In both cases our numerical simulation strategy involves approximating the fractal screen Gamma by a sequence of smoother "prefractal" screens, for which we compute the scattered field using boundary element methods that discretise the associated first kind boundary integral equations. We prove sufficient conditions on the mesh sizes guaranteeing convergence to the limiting fractal solution, using the framework of Mosco convergence. We also provide numerical examples illustrating our theoretical results

Radar absorption, basal reflection, thickness and polarization measurements from the Ross Ice Shelf, Antarctica

Radio-glaciological parameters from the Moore’s Bay region of the Ross Ice Shelf, Antarctica, have been measured. The thickness of the ice shelf in Moore’s Bay was measured from reflection times of radio-frequency pulses propagating vertically through the shelf and reflecting from the ocean, and is found to be 576 ± 8 m. Introducing a baseline of 543 ± 7m between radio transmitter and receiver allowed the computation of the basal reflection coefficient, R, separately from englacial loss. The depth-averaged attenuation length of the ice column, 〈L〉 is shown to depend linearly on frequency. The best fit (95% confidence level) is 〈L(ν)〉= (460±20) − (180±40)ν m (20 dB km−1), for the frequencies ν = [0.100–0.850] GHz, assuming no reflection loss. The mean electric-field reflection coefficient is (1.7 dB reflection loss) across [0.100–0.850] GHz, and is used to correct the attenuation length. Finally, the reflected power rotated into the orthogonal antenna polarization i

A Search for MeV to TeV Neutrinos from Fast Radio Bursts with IceCube

We present two searches for IceCube neutrino events coincident with 28 fast radio bursts (FRBs) and 1 repeating FRB. The first improves on a previous IceCube analysis - searching for spatial and temporal correlation of events with FRBs at energies greater than roughly 50 GeV - by increasing the effective area by an order of magnitude. The second is a search for temporal correlation of MeV neutrino events with FRBs. No significant correlation is found in either search; therefore, we set upper limits on the time-integrated neutrino flux emitted by FRBs for a range of emission timescales less than one day. These are the first limits on FRB neutrino emission at the MeV scale, and the limits set at higher energies are an order-of-magnitude improvement over those set by any neutrino telescope