62,227 research outputs found
Vortex Nucleation Induced Phonon Radiation from a Moving Electron Bubble in Superfluid 4He
We construct an efficient zero-temperature semi-local density functional to
dynamically simulate an electron bubble passing through superfluid 4He under
various pressures and electric fields up to nanosecond timescale. Our simulated
drift velocity can be quantitatively compared to experiments particularly when
pressure approaches zero. We find that the high-speed bubble experiences
remarkable expansion and deformation before vortex nucleation occurs.
Accompanied by vortex-ring shedding, drastic surface vibration is generated
leading to intense phonon radiation into the liquid. The amount of energy
dissipated by these phonons is found to be greater than the amount carried away
solely by the vortex rings. These results may enrich our understanding about
the vortex nucleation induced energy dissipation in this fascinating system.Comment: 7 pages, 5 figure
Radio light curves during the passage of cloud G2 near Sgr A*
We calculate radio light curves produced by the bow shock that is likely to
form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*.
The shock acceleration of the radio-emitting electrons is captured
self-consistently by means of first-principles particle-in-cell simulations. We
show that the radio luminosity is expected to reach maximum in early 2013,
roughly a month after the bow shock crosses the orbit pericenter. We estimate
the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit
orientation and parameters. We show that the most promising frequencies for
radio observations are in the 0.1<nu<1 GHz range, for which the bow shock
emission will be much stronger than the intrinsic radio flux for all the models
considered.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Entanglement detection beyond the CCNR criterion for infinite-dimensions
In this paper, in terms of the relation between the state and the reduced
states of it, we obtain two inequalities which are valid for all separable
states in infinite-dimensional bipartite quantum systems. One of them provides
an entanglement criterion which is strictly stronger than the computable
cross-norm or realignment (CCNR) criterion.Comment: 11 page
A multiplexing architecture for mixed-signal CMOS fuzzy controllers
Limits to precision impose limits to the complexity of analog circuits, hence fuzzy analog
controllers are usually oriented to fast low-power systems with low-medium complexity. This
paper presents a strategy to preserve most of the advantages of an analog implementation, while
allowing a marked increment in system complexity.The works in this papaer has been partially funded by the spanish
C.I.C.Y.T. under contract TIC96-1392-C02-02 (SIVA
Particle acceleration by collisionless shocks containing large-scale magnetic-field variations
Diffusive shock acceleration at collisionless shocks is thought to be the
source of many of the energetic particles observed in space. Large-scale
spatial variations of the magnetic field has been shown to be important in
understanding observations. The effects are complex, so here we consider a
simple, illustrative model. Here, we solve numerically the Parker transport
equation for a shock in the presence of large-scale sinusoidal magnetic-field
variations. We demonstrate that the familiar planar-shock results can be
significantly altered as a consequence of large-scale, meandering magnetic
lines of force. Because perpendicular diffusion coefficient is
generally much smaller than parallel diffusion coefficient ,
the energetic charged particles are trapped and preferentially accelerated
along the shock front in the regions where the connection points of magnetic
field lines intersecting the shock surface converge, and thus create the "hot
spots" of the accelerated particles. For the regions where the connection
points separate from each other, the acceleration to high energies will be
suppressed. Further, the particles diffuse away from the "hot spot" regions and
modify the spectra of downstream particle distribution. These features are
qualitatively similar to the recent Voyager's observation in the Heliosheath.
These results are potentially important for particle acceleration at shocks
propagating in turbulent magnetized plasmas as well as those which contain
large-scale nonplanar structures. Examples include anomalous cosmic rays
accelerated by the solar wind termination shock, energetic particles observed
in propagating heliospheric shocks, and galactic cosmic rays accelerated by
supernova blast waves, etc.Comment: accepted to Ap
Comparison of Recoil-Induced Resonances (RIR) and Collective Atomic Recoil Laser (CARL)
The theories of recoil-induced resonances (RIR) [J. Guo, P. R. Berman, B.
Dubetsky and G. Grynberg, Phys. Rev. A {\bf 46}, 1426 (1992)] and the
collective atomic recoil laser (CARL) [ R. Bonifacio and L. De Salvo, Nucl.
Instrum. Methods A {\bf 341}, 360 (1994)] are compared. Both theories can be
used to derive expressions for the gain experienced by a probe field
interacting with an ensemble of two-level atoms that are simultaneously driven
by a pump field. It is shown that the RIR and CARL formalisms are equivalent.
Differences between the RIR and CARL arise because the theories are typically
applied for different ranges of the parameters appearing in the theory. The RIR
limit considered in this paper is , while the CARL
limit is , where is the magnitude of the
difference of the wave vectors of the pump and probe fields, is the
width of the atomic momentum distribution and is a recoil
frequency. The probe gain for a probe-pump detuning equal to zero is analyzed
in some detail, in order to understand how the gain arises in a system which,
at first glance, might appear to have vanishing gain. Moreover, it is shown
that the calculations, carried out in perturbation theory have a range of
applicability beyond the recoil problem. Experimental possibilities for
observing CARL are discussed.Comment: 16 pages, 1 figure. Submitted to Physical Review
Ground state magnetic structure of MnGe
We have used spherical neutron polarimetry to investigate the magnetic
structure of the Mn spins in the hexagonal semimetal MnGe, which exhibits a
large intrinsic anomalous Hall effect. Our analysis of the polarimetric data
finds a strong preference for a spin structure with symmetry relative
to the point group. We show that weak ferromagnetism is an inevitable
consequence of the symmetry of the observed magnetic structure, and that sixth
order anisotropy is needed to select a unique ground state
Microturbulence studies in RFX-mod
Present-days Reversed Field Pinches (RFPs) are characterized by quasi-laminar
magnetic configurations in their core, whose boundaries feature sharp internal
transport barriers, in analogy with tokamaks and stellarators. The abatement of
magnetic chaos leads to the reduction of associated particle and heat transport
along wandering field lines. At the same time, the growth of steep temperature
gradients may trigger drift microinstabilities. In this work we summarize the
work recently done in the RFP RFX-mod in order to assess the existence and the
impact upon transport of such electrostatic and electromagnetic
microinstabilities as Ion Temperature Gradient (ITG), Trapped Electron Modes
(TEM) and microtearing modes.Comment: Work presented at the 2010 Varenna workshop "Theory of Fusion
Plasmas". To appear in Journal of Physics Conference Serie
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