27 research outputs found
Condensation of quasiparticles and density modulation beyond the superfluid critical velocity
We investigate the effect of a constant external velocity field on the ground
state of a bosonic quasiparticle Hamiltonian. Below a critical velocity the
ground state is a quasiparticle vacuum, corresponding to a pure superfluid
phase at zero temperature. Beyond the critical velocity energy minimization
leads to a macroscopic condensation of quasiparticles at a nonzero wave vector
k_v parallel to the velocity v. Simultaneously, physical particles also undergo
a condensation at k_v and, to a smaller extent, at -k_v. Together with the BEC
at k=0, the three entangled condensates give rise to density modulations of
wave vectors k_v and 2k_v. For larger |v| our model predicts a bifurcation of
k_v with corresponding two pure condensates and no density modulation.Comment: Proof of quasiparticle condensation complete
The scalar-isoscalar spectral function of strong matter in a large N approximation
The enhancement of the scalar-isoscalar spectral function near the two-pion
threshold is studied in the framework of an effective linear model,
using a large N approximation in the number of the Goldstone bosons. The effect
is rather insensitive to the detailed T=0 characteristics of the pole,
it is accounted by a pole moving with increasing along the real axis of the
second Riemann sheet towards the threshold location from below.Comment: 5 pages, poster presented at SEWM2002, Heidelberg, October 200
Finite temperature spectral function of the meson from large N expansion
The spectral function of the scalar-isoscalar channel of the O(N) symmetric
linear model is studied in the broken symmetry phase. The
investigation is based on the leading order evaluation of the self-energy in
the limit of large number of Goldstone bosons. We describe its temperature
dependent variation in the whole low temperature phase. This variation closely
reflects the trajectory of the scalar-isoscalar quasiparticle pole. In the
model with no explicit chiral symmetry breaking we have studied near the
critical point also the corresponding dynamical exponent.Comment: 9 pages, 3 figures. To be published in Proc. of Budapest'02 Workshop
on Quark and Hadron Dynamics, Budapest, Hungary, March 3--7, 200
Analytic determination of the T-\mu phase diagram of the chiral quark model
Using a gap equation for the pion mass a nonperturbative method is given for
solving the chiral quark-meson model in the chiral limit at the lowest order in
the fermion contributions encountered in a large N_f approximation. The
location of the tricritical point is analytically determined. A mean field
potential is constructed from which critical exponents can be obtained.Comment: 8 pages, 2 figures. To be published in Proc.of Budapest'04 Workshop
on Hot and Dense Matter in Relativistic Heavy Ion Physics, Budapest, Hungary,
March 24-27, 200
Energies And Damping Rates of Elementary Excitations in Spin-1 Bose-einstein-condensed Gases
The finite temperature Green's function technique is used to calculate the energies and damping rates of the elementary excitations of homogeneous, dilute, spin-1 Bose gases below the Bose-Einstein condensation temperature in both the density and spin channels. For this purpose a self-consistent dynamical Hartree-Fock model is formulated, which takes into account the direct and exchange processes on equal footing by summing up certain classes of Feynman diagrams. The model is shown to satisfy the Goldstone theorem and to exhibit the hybridization of one-particle and collective excitations correctly. The results are applied to gases of Na-23 and Rb-87 atoms
Landau damping in dilute Bose gases
Landau damping in weakly interacting Bose gases is investigated by means of
perturbation theory. Our approach points out the crucial role played by
Bose-Einstein condensation and yields an explicit expression for the decay rate
of elementary excitations in both uniform and non uniform gases. Systematic
results are derived for the phonon width in homogeneous gases interacting with
repulsive forces. Special attention is given to the low and high temperature
regimes.Comment: 11 pages, latex, 1 figure available upon request. The paper accepted
for publication in Phys. Lett.
Chaotic Repellers in Antiferromagnetic Ising Model
For the first time we present the consideration of the antiferromagnetic
Ising model in case of fully developed chaos and obtain the exact connection
between this model and chaotic repellers. We describe the chaotic properties of
this statistical mechanical system via the invariants characterizing a fractal
set and show that in chaotic region it displays phase transition at {\it
positive} "temperature" . We obtain the density of the
invariant measure on the chaotic repeller.Comment: LaTeX file, 10 pages, 4 PS figurs upon reques
Ultracold atom superfluidity induced by the Feshbach resonance
We discuss the possible signatures of superfluidity induced by the Feshbach
resonance in ultracold gas of fermion atoms. Approaching the phase transition
from above there appear various manifestations of the gradually emerging order
parameter, but yet the long range coherence is not established due to strong
quantum fluctuations. The single particle excitation spectrum becomes gapped
while at the same time the pair excitations are characterized by the narrow
quasiparticle peak surrounded by the incoherent background. This quasiparticle
shows up certain collective features such as the remnant of the "first sound"
which at Tc spreads down to low momenta. Presence of this Goldstone mode is the
most unambiguous proof for appearance of the superfluid state. We discuss how
such mode can be detected experimentally.Comment: 5 pages, 3 figure