5,246 research outputs found
Renormalized parameters and perturbation theory for an n-channel Anderson model with Hund's rule coupling: Asymmetric case
We explore the predictions of the renormalized perturbation theory for an
n-channel Anderson model, both with and without Hund's rule coupling, in the
regime away from particle-hole symmetry. For the model with n=2 we deduce the
renormalized parameters from numerical renormalization group calculations, and
plot them as a function of the occupation at the impurity site, nd. From these
we deduce the spin, orbital and charge susceptibilities, Wilson ratios and
quasiparticle density of states at T=0, in the different parameter regimes,
which gives a comprehensive overview of the low energy behavior of the model.
We compare the difference in Kondo behaviors at the points where nd=1 and nd=2.
One unexpected feature of the results is the suppression of the charge
susceptibility in the strong correlation regime over the occupation number
range 1 <nd <3.Comment: 9 pages, 17 figure
The emission of energetic electrons from the complex streamer corona adjacent to leader stepping
We here propose a model to capture the complexity of the streamer corona
adjacent to leader stepping and relate it to the production of energetic
electrons serving as a source of X-rays and -rays, manifesting in
terrestrial gamma-ray flashes (TGFs). During its stepping, the leader tip is
accompanied by a corona consisting of multitudinous streamers perturbing the
air in its vicinity and leaving residual charge behind. We explore the relative
importance of air perturbations and preionization on the production of
energetic run-away electrons by 2.5D cylindrical Monte Carlo particle
simulations of streamers in ambient fields of 16 kV cm and 50 kV
cm at ground pressure. We explore preionization levels between
m and m, channel widths between 0.5 and 1.5 times the
original streamer widths and air perturbation levels between 0\% and 50\% of
ambient air. We observe that streamers in preionized and perturbed air
accelerate more efficiently than in non-ionized and uniform air with air
perturbation dominating the streamer acceleration. We find that in unperturbed
air preionization levels of m are sufficient to explain
run-away electron rates measured in conjunction with terrestrial gamma-ray
flashes. In perturbed air, the production rate of runaway electrons varies from
s to s with maximum electron energies from
some hundreds of eV up to some hundreds of keV in fields above and below the
breakdown strength. In the presented simulations the number of runaway
electrons matches with the number of energetic electrons measured in alignment
with the observations of terrestrial gamma-ray flashes. Conclusively, the
complexity of the streamer zone ahead of leader tips allows explaining the
emission of energetic electrons and photons from streamer discharges.Comment: 29 pages, 11 figures, 2 table
Full counting statistics for orbital-degenerate impurity Anderson model with Hund's rule exchange coupling
We study non-equilibrium current fluctuations through a quantum dot, which
includes a ferromagnetic Hund's rule coupling , in the low-energy Fermi
liquid regime using the renormalized perturbation theory. The resulting
cumulant for the current distribution in the particle-hole symmetric case,
shows that spin-triplet and spin-singlet pairs of quasiparticles are formed in
the current due to the Hund's rule coupling and these pairs enhance the current
fluctuations. In the fully screened higher-spin Kondo limit, the Fano factor
takes a value determined by the orbital degeneracy .
We also investigate crossover between the small and large limits in the
two-orbital case M=2, using the numerical renormalization group approach.Comment: 5 pages, 3 figure
Acceleration Mechanics in Relativistic Shocks by the Weibel Instability
Plasma instabilities (e.g., Buneman, Weibel and other two-stream
instabilities) created in collisionless shocks may be responsible for particle
(electron, positron, and ion) acceleration. Using a 3-D relativistic
electromagnetic particle (REMP) code, we have investigated long-term particle
acceleration associated with relativistic electron-ion or electron-positron jet
fronts propagating into an unmagnetized ambient electron-ion or
electron-positron plasma. These simulations have been performed with a longer
simulation system than our previous simulations in order to investigate the
nonlinear stage of the Weibel instability and its particle acceleration
mechanism. The current channels generated by the Weibel instability are
surrounded by toroidal magnetic fields and radial electric fields. This radial
electric field is quasi stationary and accelerates particles which are then
deflected by the magnetic field.Comment: 17 pages, 5 figures, accepted for publication in ApJ, A full
resolution ot the paper can be found at
http://gammaray.nsstc.nasa.gov/~nishikawa/accmec.pd
Fermi Liquids and the Luttinger Integral
The Luttinger Theorem, which relates the electron density to the volume of
the Fermi surface in an itinerant electron system, is taken to be one of the
essential features of a Fermi liquid. The microscopic derivation of this result
depends on the vanishing of a certain integral, the Luttinger integral , which is also the basis of the Friedel sum rule for impurity models,
relating the impurity occupation number to the scattering phase shift of the
conduction electrons. It is known that non-zero values of with
, occur in impurity models in phases with non-analytic low
energy scattering, classified as singular Fermi liquids. Here we show the same
values, , occur in an impurity model in phases with regular
low energy Fermi liquid behavior. Consequently the Luttinger integral can be
taken to characterize these phases, and the quantum critical points separating
them interpreted as topological.Comment: 5 pages 7 figure
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