30,187 research outputs found
TEC enhancement due to energetic electrons above Taiwan and the West Pacific
The energetic electrons of the inner radiation belt during a geomagnetic
disturbance can penetrate in the forbidden range of drift shells located at the
heights of the topside equatorial ionosphere (<1000 km). A good correlation was
previously revealed between positive ionospheric storms and intense fluxes of
quasi-trapped 30-keV electrons at ~900 km height in the forbidden zone. In the
present work, we use statistics to validate an assumption that the intense
electron fluxes in the topside equatorial ionosphere can be an important source
of the ionization in the low-latitude ionosphere. The data on the energetic
electrons were obtained from polar orbiting satellites over the periods of the
62 strong geomagnetic storms from 1999 to 2006. Ionospheric response to the
selected storms was determined using global ionospheric maps of vertical total
electron content (VTEC). A case-event study of a major storm on 9 November 2004
provided experimental evidence in support to the substantial ionization effect
of energetic electrons during positive ionospheric storms at the low latitudes.
Statistical analysis of nine magnetic storms indicated that the VTEC increases
coincided with and coexisted with intense 30-keV electron fluxes irrespective
of local time and phase of geomagnetic storm. We concluded that extremely
intense fluxes of the 30-keV electrons in the topside low-latitude ionosphere
can contribute ~ 10 - 30 TECU to the localized positive ionospheric storms.Comment: 15 pages, 4 figures, 1 table accepted for publication in Terrestrial,
Atmospheric and Oceanic Sciences (TAO), Dec. 2012 A special issue on
"Connection of solar and heliospheric activities with near-Earth space
weather: Sun-Earth connection
Influence of substrate temperature on lattice strain field and phase transition in MeV oxygen ion implanted GaAs crystals
A detailed study of the influence of substrate temperature on the radiation-induced lattice strain field and crystalline-to-amorphous (c-a) phase transition in MeV oxygen ion implanted GaAs crystals has been made using channeling Rutherford backscattering spectroscopy, secondary ion mass spectrometry, and the x-ray rocking curve technique. A comparison has been made between the cases of room temperature (RT) and low temperature (LT) (about 100 K) implantation. A strong in situ dynamic annealing process is found in RT implantation at a moderate beam current, resulting in a uniform positive strain field in the implanted layer. LT implantation introduces a freeze-in effect which impedes the recombination and diffusion of initial radiation-created lattice damage and defects, and in turn drives more efficiently the c-a transition as well as strain saturation and relaxation. The results are interpreted with a spike damage model in which the defect production process is described in terms of the competition between defect generation by nuclear spikes and defects diffusion and recombination stimulated by electronic spikes. It is also suggested that the excess population of vacancies and their complexes is responsible for lattice spacing expansion in ion-implanted GaAs crystals
Fluxon Dynamics of a Long Josephson Junction with Two-gap Superconductors
We investigate the phase dynamics of a long Josephson junction (LJJ) with
two-gap superconductors. In this junction, two channels for tunneling between
the adjacent superconductor (S) layers as well as one interband channel within
each S layer are available for a Cooper pair. Due to the interplay between the
conventional and interband Josephson effects, the LJJ can exhibit unusual phase
dynamics. Accounting for excitation of a stable 2-phase texture arising
from the interband Josephson effect, we find that the critical current between
the S layers may become both spatially and temporally modulated. The spatial
critical current modulation behaves as either a potential well or barrier,
depending on the symmetry of superconducting order parameter, and modifies the
Josephson vortex trajectories. We find that these changes in phase dynamics
result in emission of electromagnetic waves as the Josephson vortex passes
through the region of the 2-phase texture. We discuss the effects of this
radiation emission on the current-voltage characteristics of the junction.Comment: 14 pages, 6 figure
The Amplitude in an External Homogeneous Electromagnetic Field
Neutrino-photon interactions in the presence of an external homogeneous
constant electromagnetic field are studied. The amplitude is
calculated in an electromagnetic field of the general type, when the two field
invariants are nonzero.Comment: 7 pages, 1 figur
Connection Between Wave Functions in the Dirac and Foldy-Wouthuysen Representations
The connection between wave functions in the Dirac and Foldy-Wouthuysen
representations is found. When the Foldy-Wouthuysen transformation is exact,
upper spinors in two representations differ only by constant factors, and lower
spinors in the Foldy-Wouthuysen representation are equal to zero.Comment: 7 page
Damping in high-frequency metallic nanomechanical resonators
We have studied damping in polycrystalline Al nanomechanical resonators by
measuring the temperature dependence of their resonance frequency and quality
factor over a temperature range of 0.1 - 4 K. Two regimes are clearly
distinguished with a crossover temperature of 1 K. Below 1 K we observe a
logarithmic temperature dependence of the frequency and linear dependence of
damping that cannot be explained by the existing standard models. We attribute
these phenomena to the effect of the two-level systems characterized by the
unexpectedly long (at least two orders of magnitude longer) relaxation times
and discuss possible microscopic models for such systems. We conclude that the
dynamics of the two-level systems is dominated by their interaction with
one-dimensional phonon modes of the resonators.Comment: 5 pages, 3 figure
Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular Lattices
We study the phase diagram of two-dimensional Bose-Fermi mixtures of
ultracold atoms on a triangular optical lattice, in the limit when the velocity
of bosonic condensate fluctuations is much larger than the Fermi velocity.
We contrast this work with our previous results for a square lattice system
in Phys. Rev. Lett. {\bf 97}, 030601 (2006).
Using functional renormalization group techniques we show that the phase
diagrams for a triangular lattice contain exotic superconducting phases. For
spin-1/2 fermions on an isotropic lattice we find a competition of -, -,
extended -, and -wave symmetry, as well as antiferromagnetic order. For
an anisotropic lattice, we further find an extended p-wave phase. A Bose-Fermi
mixture with spinless fermions on an isotropic lattice shows a competition
between - and -wave symmetry.
These phases can be traced back to the geometric shapes of the Fermi surfaces
in various regimes, as well as the intrinsic frustration of a triangular
lattice.Comment: 6 pages, 4 figures, extended version, slight modification
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