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 $\sigma$ 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 $\sigma$ pole, it is accounted by a pole moving with increasing $T$ 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 σ\sigma meson from large N expansion

The spectral function of the scalar-isoscalar channel of the O(N) symmetric linear $\sigma$ 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" $\beta_c = 0.89$. 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