26,776 research outputs found
Dynamic Structure Factor of Normal Fermi Gas from Collisionless to Hydrodynamic Regime
The dynamic structure factor of a normal Fermi gas is investigated by using
the moment method for the Boltzmann equation. We determine the spectral
function at finite temperatures over the full range of crossover from the
collisionless regime to the hydrodynamic regime. We find that the Brillouin
peak in the dynamic structure factor exhibits a smooth crossover from zero to
first sound as functions of temperature and interaction strength. The dynamic
structure factor obtained using the moment method also exhibits a definite
Rayleigh peak (), which is a characteristic of the hydrodynamic
regime. We compare the dynamic structure factor obtained by the moment method
with that obtained from the hydrodynamic equations.Comment: 19 pages, 9 figure
Tunable pinning of a superconducting vortex a by a magnetic vortex
The interaction between a straight vortex line in a superconducting film and
a soft magnetic nanodisk in the magnetic vortex state in the presence of a
magnetic field applied parallel to the film surfaces is studied theoretically.
The superconductor is described by London theory and the nanodisk by the
Landau-Lifshitz continuum theory of magnetism, using the approximation known as
the rigid vortex model. Pinning of the vortex line by the nanodisk is found to
result, predominantly, from the interaction between the vortex line and the
changes in the nanodisk magnetization induced by the magnetic field of the
vortex line and applied field. In the context of the rigid vortex model, these
changes result from the displacement of the magnetic vortex. This displacement
is calculated analytically by minimizing the energy, and the pinning potential
is obtained. The applied field can tune the pinning potential by controlling
the displacement of the magnetic vortex. The nanodisk magnetization curve is
predicted to change in the presence of the vortex lineComment: 9 pages, 8 figures. Submitted to Phys. Rev.
Wounded quarks and diquarks in high energy collisions
Particle production in Au-Au, Cu-Cu, d-Au and p-p collisions at 200 GeV c.m.
energy are analyzed in the wounded quark-diquark model. Existing data are well
reproduced. Emission functions of wounded and unwounded constituents are
determined. Implications for the collective evolution of the system are
discussed.Comment: version to be published in Phys. Rev. C, minor changes, discussion
extende
Surprises in nonperturbative dynamics in sigma-model at finite density
The linear sigma-model occupies a unique place in
elementary particle physics and quantum field theory. It has been recently
realized that when a chemical potential for hypercharge is added, it becomes a
toy model for the description of the dynamics of the kaon condensate in high
density QCD. We review recent results in nonperturbative dynamics obtained in
the ungauged and gauged versions of this model.Comment: Brief review. 16 pages, 5 figure
Relaxation Mechanism for Ordered Magnetic Materials
We have formulated a relaxation mechanism for ferrites and ferromagnetic
metals whereby the coupling between the magnetic motion and lattice is based
purely on continuum arguments concerning magnetostriction. This theoretical
approach contrasts with previous mechanisms based on microscopic formulations
of spin-phonon interactions employing a discrete lattice. Our model explains
for the first time the scaling of the intrinsic FMR linewidth with frequency,
and 1/M temperature dependence and the anisotropic nature of magnetic
relaxation in ordered magnetic materials, where M is the magnetization. Without
introducing adjustable parameters our model is in reasonable quantitative
agreement with experimental measurements of the intrinsic magnetic resonance
linewidths of important class of ordered magnetic materials, insulator or
metals
On-Shell Description of Unsteady Flames
The problem of non-perturbative description of unsteady premixed flames with
arbitrary gas expansion is solved in the two-dimensional case. Considering the
flame as a surface of discontinuity with arbitrary local burning rate and gas
velocity jumps given on it, we show that the front dynamics can be determined
without having to solve the flow equations in the bulk. On the basis of the
Thomson circulation theorem, an implicit integral representation of the gas
velocity downstream is constructed. It is then simplified by a successive
stripping of the potential contributions to obtain an explicit expression for
the vortex component near the flame front. We prove that the unknown potential
component is left bounded and divergence-free by this procedure, and hence can
be eliminated using the dispersion relation for its on-shell value (i.e., the
value along the flame front). The resulting system of integro-differential
equations relates the on-shell fuel velocity and the front position. As
limiting cases, these equations contain all theoretical results on flame
dynamics established so far, including the linear equation describing the
Darrieus-Landau instability of planar flames, and the nonlinear
Sivashinsky-Clavin equation for flames with weak gas expansion.Comment: 21 pages, 3 figures; extended discussion of causality, new references
adde
Vector order parameter in general relativity. Covariant equations
Phase transitions with spontaneous symmetry breaking and vector order
parameter are considered in multidimensional theory of general relativity.
Covariant equations, describing the gravitational properties of topological
defects, are derived. The topological defects are classified in accordance with
the symmetry of the covariant derivative of the vector order parameter. The
abilities of the derived equations are demonstrated in application to the brane
world concept. New solutions of the Einstein equations with a transverse vector
order parameter are presented. In the vicinity of phase transition the
solutions are found analytically
Microwave Response and Spin Waves in Superconducting Ferromagnets
Excitation of spin waves is considered in a superconducting ferromagnetic
slab with the equilibrium magnetization both perpendicular and parallel to the
surface. The surface impedance is calculated and its behavior near propagation
thresholds is analyzed. Influence of non-zero magnetic induction at the surface
is considered in various cases. The results provide a basis for investigation
of materials with coexisting superconductivity and magnetism by microwave
response measurements.Comment: 10 pages, 7 figure
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