378,448 research outputs found
Symmetry constraints on phonon dispersion in graphene
Taking into account the constraints imposed by the lattice symmetry, we
calculate the phonon dispersion for graphene with interactions between the
first, second, and third nearest neighbors in the framework of the Born--von
Karman model. Analytical expressions obtained for the dispersion of the
out-of-plane (bending) modes give the nonzero sound velocity. The dispersion of
four in-plane modes is determined by coupled equations. Values of the force
constants are found in fitting with frequencies at critical points and with
elastic constants measured on graphite.Comment: 5 pages, 2 figure
Jet-induced gauge field instabilities in the quark-gluon plasma
We discuss the properties of the collective modes of a system composed by a
thermalized quark-gluon plasma traversed by a relativistic jet of partons. The
transport equations obeyed by the components of the plasma and of the jet are
studied in the Vlasov approximation. Assuming that the partons in the jet can
be described with a tsunami-like distribution function we derive the
expressions of the dispersion law of the collective modes. Then the behavior of
the unstable gauge modes of the system is analyzed for various values of the
velocity of the jet, of the momentum of the collective modes and of the angle
between these two quantities. We find that the most unstable modes are those
with momentum orthogonal to the velocity of the jet, and the effect is stronger
for ultrarelativistic jet velocities. Our results suggest a new possible
collective mechanism for the description of the jet quenching phenomena in
heavy ion collisions.Comment: 7 pages; invited talk at the YITP symposium on "Fundamental Problems
in Hot and/or dense QCD", Kyoto, Japan, 3-6 March 200
The Nuclear Scissors Mode in a Solvable Model
The coupled dynamics of the scissors mode and the isovector giant quadrupole
resonance is studied in a model with separable quadrupole-quadrupole residual
interactions. The method of Wigner function moments is applied to derive the
dynamical equations for angular momentum and quadrupole moment. Analytical
expressions for energies, B(M1)- and B(E2)-values, sum rules and flow-patterns
of both modes are found for arbitrary values of the deformation parameter. Some
predictions for the case of superdeformation are given. The subtle nature of
the phenomenon and its peculiarities are clarified.Comment: 49 pages, 3 figures. We corrected the force constant which influenced
mostly the results of the superdeformed region. Flow patterns are left
without any change
Jet-induced gauge field instabilities in the quark-gluon plasma: A kinetic theory approach
We discuss the properties of the collective modes of a system composed by a
thermalized quark-gluon plasma traversed by a relativistic jet of partons. The
transport equations obeyed by the components of the plasma and of the jet are
studied in the Vlasov approximation. Assuming that the partons in the jet can
be described with a tsunami-like distribution function we derive the
expressions of the dispersion law of the collective modes. Then the behavior of
the unstable gauge modes of the system is analyzed for various values of the
velocity of the jet, of the momentum of the collective modes and of the angle
between these two quantities. We find that the most unstable modes are those
with momentum orthogonal to the velocity of the jet and that these
instabilities appear when the velocity of the jet is higher than a threshold
value, which depends on the plasma and jet frequencies. The results obtained
within the Vlasov approximation are compared with the corresponding results
obtained using a chromohydrodynamical approach.The effect we discuss here
suggests a possible collective mechanism for the description of the jet
quenching phenomena in heavy ion collisions.Comment: 13 pages, 6 figure
Frequency shift and mode coupling in the nonlinear dynamics of a Bose condensed gas
We investigate the behavior of large amplitude oscillations of a trapped
Bose-condensed gas of alkali atoms at zero temperature, by solving the
equations of hydrodynamics for collective modes. Due to the atom-atom
interaction, the equations of motion are nonlinear and give rise to significant
frequency shift and mode coupling. We provide analytic expressions for the
frequency shift, pointing out the crucial role played by the anisotropy of the
confining potential. For special values of the anisotropy parameter the mode
coupling is particularly strong and the frequency shift becomes large,
revealing a peculiar behavior of the Bose-condensed gas. Consequences on the
theory of collapse and revival of collective excitations are also discussed.Comment: 10 pages, RevTeX, 9 figures, more info at
http://www-phys.science.unitn.it/bec/BEC.htm
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