2,657 research outputs found
Hydrodynamical instability of extragalactic stratified jets
The recent development of tomographic analysis of radio data shows the
presence of sheaths around several extragalactic jets. We have studied the
Kelvin-Helmholtz instability of such stratified jets, describing them as inner
relativistic beams embedded in slower plasma envelopes. We find that the para-
meters of the envelope generally determine the Kelvin-Helmholtz properties of
the jet which thus appears isolated from any ambient external medium.Comment: LaTeX, 5 pages, 1 figure and crckapb.sty, epsf.sty included, To
appear in the proceedings of Girona Conference ``Blazars, Black Holes and
Jets'', 9-12 September 1996, Girona, Spai
Optimal correction of independent and correlated errors
We identify optimal quantum error correction codes for situations that do not
admit perfect correction. We provide analytic n-qubit results for standard
cases with correlated errors on multiple qubits and demonstrate significant
improvements to the fidelity bounds and optimal entanglement decay profiles.Comment: 11 pages, 4 figures. Updated to include fidelity analysi
Linking radio and gamma ray emission in Ap Librae
Ap Lib is one of the rare Low Synchrotron Peaked blazars detected so far at
TeV energies. This type of source is not properly modelled by standard one-zone
leptonic Synchrotron-self-Compton (SSC) emission scenarios. The aim of this
paper is to study the relevance of additional components which should naturally
occur in a SSC scenario for a better understanding of the emission mechanisms,
especially at very high energies (VHE). Methods. We use simultaneous data from
a multi-wavelength campaign of Planck, Swift-UVOT and Swift-XRT telescopes
carried out in February 2010, as well as quasi-simultaneous data of WISE, Fermi
and H.E.S.S. taken in 2010. The multi-lambda emission of Ap Lib is modelled by
a blob-in-jet SSC scenario including the contribution of the base of the VLBI
extended jet, the radiative blob-jet interaction, the accretion disk and its
associated external photon field. We show that signatures of a strong
parsec-scale jet and of an accretion disk emission are present in the SED. We
can link the observationnal VLBI jet features from MOJAVE to parameters
expected for a VHE emitting blob accelerated near the jet base. The VHE
emission appears to be dominated by the inverse-Compton effect of the blob
relativistic electrons interacting with the jet synchrotron radiation. In such
scenario Ap Lib appears as an intermediate source between BL Lac objects and
Flat Spectrum Radio Quasars. Ap Lib could be a bright representative of a
specific class of blazars, in which the parsec-scale jet luminosity is no more
negligible compared to the blob and contributes to the high energy emission via
inverse Compton processes.Comment: 12 pages, 7 figures, Accepted for publication in A&
Extended two-level quantum dissipative system from bosonization of the elliptic spin-1/2 Kondo model
We study the elliptic spin-1/2 Kondo model (spin-1/2 fermions in one
dimension with fully anisotropic contact interactions with a magnetic impurity)
in the light of mappings to bosonic systems using the fermion-boson
correspondence and associated unitary transformations. We show that for fixed
fermion number, the bosonic system describes a two-level quantum dissipative
system with two noninteracting copies of infinitely-degenerate upper and lower
levels. In addition to the standard tunnelling transitions, and the transitions
driven by the dissipative coupling, there are also bath-mediated transitions
between the upper and lower states which simultaneously effect shifts in the
horizontal degeneracy label. We speculate that these systems could provide new
examples of continuous time quantum random walks, which are exactly solvable.Comment: 7 pages, 1 figur
Quasiclassical theory for the superconducting proximity effect in Dirac materials
We derive the quasiclassical non-equilibrium Eilenberger and Usadel equations
to first order in quantities small compared to the Fermi energy, valid for
Dirac edge and surface electrons with spin-momentum locking, as relevant for
topological insulators. We discuss in detail several of the key technical
points and assumptions of the derivation, and provide a Riccati-parametrization
of the equations. Solving first the equilibrium equations for S/N and S/F
bilayers and Josephson junctions, we study the superconducting proximity effect
in Dirac materials. Similarly to related works, we find that the effect of an
exchange field depends strongly on the direction of the field. Only components
normal to the transport direction lead to attenuation of the Cooper pair
wavefunction inside the F. Fields parallel to the transport direction lead to
phase-shifts in the dependence on the superconducting phase difference for both
the charge current and density of states in an S/F/S-junction. Moreover, we
compute the differential conductance in S/N and S/F bilayers with an applied
voltage bias, and determine the dependence on the length of the N and F regions
and the exchange field.Comment: 13 pages, 5 figures. Accepted for publication in Phys. Rev.
Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling
We investigate theoretically how the proximity effect in
superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling
manifests in the density of states and critical temperature. To describe a
general scenario, we allow for both Rashba and Dresselhaus type spin-orbit
coupling. Our results are obtained via the quasiclassical theory of
superconductivity, extended to include spin-orbit coupling in the Usadel
equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works,
we have derived a Riccati parametrization of the Usadel equation with
spin-orbit coupling which allows us to address the full proximity regime.
First, we consider the density of states in both SF bilayers and SFS trilayers,
where the spectroscopic features in the latter case are sensitive to the phase
difference between the two superconductors. We find that the presence of
spin-orbit coupling leaves clear spectroscopic fingerprints in the density of
states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS
structures without spin-orbit coupling, the density of states in the present
case depends strongly on the direction of magnetization. We show that the
spin-orbit coupling can stabilize singlet superconductivity even in the
presence of a strong exchange field . This leads to the
possibility of a magnetically tunable minigap: changing the direction of the
exchange field opens and closes the minigap. We also determine how the critical
temperature of an SF bilayer is affected by spin-orbit coupling and
demonstrate that one can achieve a spin-valve effect with a single ferromagnet.
We find that displays highly non-monotonic behavior both as a function of
the magnetization direction and the type and direction of the spin-orbit
coupling, offering a new way to exert control over the superconductivity of
proximity structures.Comment: 25 pages, 21 figures. Accepted for publication in Phys. Rev.
Observing the sky at extremely high energies with the Cherenkov Telescope Array: Status of the GCT project
The Cherenkov Telescope Array is the main global project of ground-based
gamma-ray astronomy for the coming decades. Performance will be significantly
improved relative to present instruments, allowing a new insight into the
high-energy Universe [1]. The nominal CTA southern array will include a
sub-array of seventy 4 m telescopes spread over a few square kilometers to
study the sky at extremely high energies, with the opening of a new window in
the multi-TeV energy range. The Gamma-ray Cherenkov Telescope (GCT) is one of
the proposed telescope designs for that sub-array. The GCT prototype recorded
its first Cherenkov light on sky in 2015. After an assessment phase in 2016,
new observations have been performed successfully in 2017. The GCT
collaboration plans to install its first telescopes and cameras on the CTA site
in Chile in 2018-2019 and to contribute a number of telescopes to the
subsequent CTA production phase.Comment: 8 pages, 7 figures, ICRC201
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