10,890 research outputs found
Coupled Electron-Phonon System with Coulomb Interaction
In this paper quasiparticle spectra of an electron-phonon-Coulomb
system have been discussed Explicit expressions for renormalized
phonon frequency, phonon attenuation and electron self energy have been
derived. Spectral weight function for such system has been graphically
evaluated. It has also been shown that on the basis of the quasiparticle
picture, the phonon self-energy is much greater than the electron self-energy.
Finally, starting from the Bardeen-Pines Hamiltonian the
results have been extended to include the effects of ionic motion
On the third order structure function for rotating 3D homogeneous turbulent flow
A form for the two-point third order structure function has been calculated
for three dimensional homogeneous incompressible slowly rotating turbulent
fluid. It has been argued that it may possibly hint at the initiation of the
phenomenon of two-dimensionalisation of the 3D incompressible turbulence owing
to rotation.Comment: This revised version corrects some serious flaws in the discussions
after the equation (2) and the equation (13) of the earlier version. Some
typos are also correcte
Exact mean field inference in asymmetric kinetic Ising systems
We develop an elementary mean field approach for fully asymmetric kinetic
Ising models, which can be applied to a single instance of the problem. In the
case of the asymmetric SK model this method gives the exact values of the local
magnetizations and the exact relation between equal-time and time-delayed
correlations. It can also be used to solve efficiently the inverse problem,
i.e. determine the couplings and local fields from a set of patterns, also in
cases where the fields and couplings are time-dependent. This approach
generalizes some recent attempts to solve this dynamical inference problem,
which were valid in the limit of weak coupling. It provides the exact solution
to the problem also in strongly coupled problems. This mean field inference can
also be used as an efficient approximate method to infer the couplings and
fields in problems which are not infinite range, for instance in diluted
asymmetric spin glasses.Comment: 10 pages, 7 figure
Non-linear vibration of a travelling beam having an intermediate guide
The free and forced responses of a non-linear travelling beam having an intermediate guide have been analysed. The guide is modelled as a purely elastic constraint with no inertia. While a suitably located guide increases the natural frequencies, the friction present in the guide-beam interface tends to destabilize the system. The presence of the guide reduces the vibration level by avoiding resonance conditions. The effect of the non-linear term is very sensitive to the location of the guide if the guide stiffness is small. It is suggested that the guide is placed near the antinode of the predominantly excited mode
Parametrically excited Non-linear traveling beams with and without external forcing
The effects of parametric excitation on a traveling beam, both with and without an external harmonic excitation, have been studied including the non-linear terms. Non-linear, complex normal modes have been used for the response analysis. Detailed numerical results are presented to show the effects of non-linearity on the stability of the parametrically excited system. In the presence of both parametric and external harmonic excitations, the response characteristics are found to be similar to that of a Duffing oscillator. The results are sensitive to the relative strengths of and the phase difference between the two forms of excitations
A new deterministic model of strange stars
The observed evidence for the existence of strange stars and the concomitant
observed masses and radii are used to derive an interpolation formula for the
mass as a function of the radial coordinate. The resulting general mass
function becomes an effective model for a strange star. The analysis is based
on the MIT bag model and yields the energy density, as well as the radial and
transverse pressures. Using the interpolation function for the mass, it is
shown that a mass-radius relation due to Buchdahl is satisfied in our model. We
find the surface redshift () corresponding to the compactness of the stars.
Finally, from our results, we predict some characteristics of a strange star of
radius 9.9 km.Comment: one new figures and minor revisions have been done. To appear in
Eur.Phys.J.
Positive Feedback Regulation Results in Spatial Clustering and Fast Spreading of Active Signaling Molecules on a Cell Membrane
Positive feedback regulation is ubiquitous in cell signaling networks, often
leading to binary outcomes in response to graded stimuli. However, the role of
such feedbacks in clustering, and in spatial spreading of activated molecules,
has come to be appreciated only recently. We focus on the latter, using a
simple model developed in the context of Ras activation with competing negative
and positive feedback mechanisms. We find that positive feedback, in the
presence of slow diffusion, results in clustering of activated molecules on the
plasma membrane, and rapid spatial spreading as the front of the cluster
propagates with a constant velocity (dependent on the feedback strength). The
advancing fronts of the clusters of the activated species are rough, with
scaling consistent with the Kardar-Parisi-Zhang (KPZ) equation in one
dimension. Our minimal model is general enough to describe signal transduction
in a wide variety of biological networks where activity in the
membrane-proximal region is subject to feedback regulation.Comment: 37 pages, 8 figures. Journal of Chemical Physics (in press
Composite Fermions in Quantum Dots
We demonstrate the formation of composite fermions in two-dimensional quantum
dots under high magnetic fields. The composite fermion interpretation provides
a simple way to understand several qualitative and quantitative features of the
numerical results obtained earlier in exact diagonalization studies. In
particular, the ground states are recognized as compactly filled quasi-Landau
levels of composite fermions.Comment: Revtex. Postscript files of figures are appended the tex
Theory of the Half-Polarized Quantum Hall States
We report a theoretical analysis of the half-polarized quantum Hall states
observed in a recent experiment. Our numerical results indicate that the ground
state energy of the quantum Hall and states versus spin
polarization has a downward cusp at half the maximal spin polarization. We map
the two-component fermion system onto a system of excitons and describe the
ground state as a liquid state of excitons with non-zero values of exciton
angular momentum.Comment: 4 pages (RevTeX), 3 figures (PostScript), added reference
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