4,257 research outputs found
Pair-production opacity at high and very-high gamma-ray energies
The propagation of high energy (HE, MeV) and very high-energy
gamma-rays (VHE, GeV) in the extra-galactic photon field leads
to pair-production and consequently energy- and distance-dependent attenuation
of the primary intensity. The spectroscopy of an increasing number of
extra-galactic objects at HE and VHE energies has demonstrated indeed the
presence of such an attenuation which in turn has been used to constrain the
photon density in the medium. At large optical depth ()
potential modifications of pair-production due to competing but rare processes
(as, e.g., the presence of sub-neV axion-like particle) may be found.
Indications for a pair-production anomaly have previously been found with
VHE-spectra. Here, we present further indications (at the level of ) for a reduced optical depth at high energies from an analysis of
Fermi-\textit{LAT} data.Comment: 4 pages, 2 figures, to be published in the proceedings
(DESY-PROC-2013-04) of the 9th patras worksho
Quasiclassical theory of disordered Rashba superconductors
We derive the quasiclassical equations that describe two-dimensional
superconductors with a large Rashba spin-orbit coupling and in the presence of
impurities. These equations account for the helical phase induced by an
in-plane magnetic field, with a superconducting order parameter that is
spatially modulated along a direction perpendicular to the field. We also
derive the generalized Ginzburg-Landau functional, which includes a
linear-in-gradient term corresponding to the helical phase. This theory paves
the way for studies of the proximity effect in two-dimensional electron gases
with large spin-orbit coupling.Comment: 6 pages, 1 figur
On detecting oscillations of gamma rays into axion-like particles in turbulent and coherent magnetic fields
Background radiation fields pervade the Universe, and above a certain energy
any -ray flux emitted by an extragalactic source should be attenuated
due to pair production. The opacity could be alleviated if photons
oscillated into hypothetical axion-like particles (ALPs) in ambient magnetic
fields, leading to a -ray excess especially at high optical depths that
could be detected with imaging air Cherenkov telescopes (IACTs).
Here, we introduce a method to search for such a signal in -ray data
and to estimate sensitivities for future observations. Different magnetic
fields close to the -ray source are taken into account in which photons
can convert into ALPs that then propagate unimpeded over cosmological distances
until they re-convert in the magnetic field of the Milky Way. Specifically, we
consider the coherent field at parsec scales in a blazar jet as well as the
turbulent field inside a galaxy cluster. For the latter, we explicitly derive
the transversal components of a magnetic field with gaussian turbulence which
are responsible for the photon-ALP mixing. To illustrate the method, we apply
it to a mock IACT array with characteristics similar to the Cherenkov Telescope
Array and investigate the dependence of the sensitivity to detect a
-ray excess on the magnetic-field parameters.Comment: 31 pages, 9 figures. Published in JCA
Topological Josephson -junctions
We study the effect of a magnetic field on the current-phase relation of a
topological Josephson junction formed by connecting two superconductors through
the helical edge states of a quantum spin-Hall insulator. We predict that the
Zeeman effect along the spin quantization axis of the helical edges results in
an anomalous Josephson relation that allows for a supercurrent to flow in the
absence of superconducting phase bias. We relate the associated field-tunable
phase shift in the Josephson relation of such a -junction to
the existence of a so-called helical superconductivity, which may result from
the interplay of the Zeeman effect and spin-orbit coupling. We analyze the
dependence of the magneto-supercurrent on the junction length and discuss its
observability in suitably designed hybrid structures subject to an in-plane
magnetic field.Comment: 7 pages, 3 figures, Appendix and references adde
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