83,774 research outputs found
Modulated rotating waves in the magnetized spherical Couette system
We present a study devoted to a detailed description of modulated rotating
waves (MRW) in the magnetized spherical Couette system. The set-up consists of
a liquid metal confined between two differentially rotating spheres and
subjected to an axially applied magnetic field. When the magnetic field
strength is varied, several branches of MRW are obtained by means of three
dimensional direct numerical simulations (DNS). The MRW originate from parent
branches of rotating waves (RW) and are classified according to Rand's (Arch.
Ration. Mech. Anal 79:1-37, 182) and Coughling & Marcus (J. Fluid Mech.
234:1-18,1992) theoretical description. We have found relatively large
intervals of multistability of MRW at low magnetic field, corresponding to the
radial jet instability known from previous studies. However, at larger magnetic
field, corresponding to the return flow regime, the stability intervals of MRW
are very narrow and thus they are unlikely to be found without detailed
knowledge of their bifurcation point. A careful analysis of the spatio-temporal
symmetries of the most energetic modes involved in the different classes of MRW
will allow in the future a comparison with the HEDGEHOG experiment, a
magnetized spherical Couette device hosted at the Helmholtz-Zentrum
Dresden-Rossendorf.Comment: Contains 3 tables and 8 figures. Published in the Journal of
Nonlinear Scienc
Electron energy loss in carbon nanostructures
The response of fullerenes and carbon nanotubes is investigated by
representing each carbon atom by its atomic polarizability. The polarization of
each carbon atom produces an induced dipole that is the result of the
interaction with a given external field plus the mutual interaction among
carbon atoms. The polarizability is obtained from the dielectric function of
graphite after invoking the Clausius-Mossotti relation. This formalism is
applied to the simulation of electron energy loss spectra both in fullerenes
and in carbon nanotubes. The case of broad electron beams is considered and the
loss probability is analyzed in detail as a function of the electron deflection
angle within a fully quantum-mechanical description of the electrons. A general
good agreement with available experiments is obtained in a wide range of probe
energies between 1 keV and 60 keV.Comment: 8 pages, 6 figures, submitted to PR
Collapse of the ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor
Experiments on the Electron Spin Resonance (ESR) in the filled
skutterudite (), at temperatures
where the host resistivity manifests a smooth insulator-metal crossover,
provides evidence of the underlying Kondo physics associated with this system.
At low temperatures (below ), behaves
as a Kondo-insulator with a relatively large hybridization gap, and the
ESR spectra displays a fine structure with lorentzian line shape,
typical of insulating media. The electronic gap is attributed to the large
hybridization present in the coherent regime of a Kondo lattice, when Ce
4f-electrons cooperate with band properties at half-filling. Mean-field
calculations suggest that the electron-phonon interaction is fundamental at
explaining the strong 4f-electron hybridization in this filled skutterudite.
The resulting electronic structure is strongly temperature dependent, and at
about the system undergoes an insulator-to-metal
transition induced by the withdrawal of 4f-electrons from the Fermi volume, the
system becoming metallic and non-magnetic. The ESR fine structure
coalesces into a single dysonian resonance, as in metals. Still, our
simulations suggest that exchange-narrowing via the usual Korringa mechanism,
alone, is not capable of describing the thermal behavior of the ESR spectra in
the entire temperature region ( - K). We propose that temperature
activated fluctuating-valence of the Ce ions is the missing ingredient that,
added to the usual exchange-narrowing mechanism, fully describes this unique
temperature dependence of the ESR fine structure observed in
.Comment: 19 pages, 6 figure
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