3,436 research outputs found
Ultrahard spectra of PeV neutrinos from supernovae in compact star clusters
Starburst regions with multiple powerful winds of young massive stars and
supernova remnants are favorable sites for high-energy cosmic ray acceleration.
A supernova shock colliding with a fast wind from a compact cluster of young
stars allows the acceleration of protons to energies well above the standard
limits of diffusive shock acceleration in an isolated SN. The proton spectrum
in such a wind-supernova PeV accelerator is hard with a large flux in the
high-energy-end of the spectrum producing copious gamma-rays and neutrinos in
inelastic nuclear collisions. We argue that SN shocks in the Westerlund 1
cluster in the Milky Way may accelerate protons to about 40 PeV. Once
accelerated, these CRs will diffuse into surrounding dense clouds and produce
neutrinos with fluxes sufficient to explain a fraction of the events detected
by IceCube Observatory from the inner Galaxy.Comment: 10 pages, 7 figures, MNRAS v.453, p.113-121, 201
Optical spectra of quantum dots: effects of non-adiabaticity
It is shown that in many cases an adequate description of optical spectra of
semiconductor quantum dots requires a treatment beyond the commonly used
adiabatic approximation. We have developed a theory of phonon-assisted optical
transitions in semiconductor quantum dots, which takes into account
non-adiabaticity of the exciton-phonon system. Effects of non-adiabaticity lead
to a mixing of different exciton and phonon states that provides a key to the
understanding of surprisingly high intensities of phonon satellites observed in
photoluminescence spectra of quantum dots. A breakdown of the adiabatic
approximation gives an explanation also for discrepancies between the serial
law, observed in multi-phonon optical spectra of some quantum dots, and the
Franck-Condon progression, prescribed by the adiabatic approach.Comment: 4 pages, 3 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
[email protected]
Magnetic susceptibility of ultra-small superconductor grains
For assemblies of superconductor nanograins, the magnetic response is
analyzed as a function of both temperature and magnetic field. In order to
describe the interaction energy of electron pairs for a huge number of
many-particle states, involved in calculations, we develop a simple
approximation, based on the Richardson solution for the reduced BCS Hamiltonian
and applicable over a wide range of the grain sizes and interaction strengths
at arbitrary distributions of single-electron energy levels in a grain. Our
study is focused upon ultra-small grains, where both the mean value of the
nearest-neighbor spacing of single-electron energy levels in a grain and
variations of this spacing from grain to grain significantly exceed the
superconducting gap in bulk samples of the same material. For these ultra-small
superconductor grains, the overall profiles of the magnetic susceptibility as a
function of magnetic field and temperature are demonstrated to be qualitatively
different from those for normal grains. We show that the analyzed signatures of
pairing correlations are sufficiently stable with respect to variations of the
average value of the grain size and its dispersion over an assembly of
nanograins. The presence of these signatures does not depend on a particular
choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses:
[email protected], [email protected], [email protected]
Paramagnetic Meissner effect in ZrB12 single crystal with non-monotonic vortex-vortex interactions
The magnetic response related to paramagnetic Meissner effect (PME) is
studied in a high quality single crystal ZrB12 with non-monotonic vortex-vortex
interactions. We observe the expulsion and penetration of magnetic flux in the
form of vortex clusters with increasing temperature. A vortex phase diagram is
constructed which shows that the PME can be explained by considering the
interplay among the flux compression, the different temperature dependencies of
the vortex-vortex and the vortex-pin interactions, and thermal fluctuations.
Such a scenario is in good agreement with the results of the magnetic
relaxation measurements.Comment: accepted by New Journal of Physic
Vortices on a superconducting nanoshell: phase diagram and dynamics
In superconductors, the search for special vortex states such as giant
vortices focuses on laterally confined or nanopatterned thin superconducting
films, disks, rings, or polygons. We examine the possibility to realize giant
vortex states and states with non-uniform vorticity on a superconducting
spherical nanoshell, due to the interplay of the topology and the applied
magnetic field. We derive the phase diagram and identify where, as a function
of the applied magnetic field, the shell thickness and the shell radius, these
different vortex phases occur. Moreover, the curved geometry allows these
states (or a vortex lattice) to coexist with a Meissner state, on the same
curved film. We have examined the dynamics of the decay of giant vortices or
states with non-uniform vorticity into a vortex lattice, when the magnetic
field is adapted so that a phase boundary is crossed.Comment: 21 pages, 9 figure
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