Condensate fraction in metallic superconductors and ultracold atomic vapors

We investigate the condensate density and the condensate fraction of conduction electrons in weak-coupling superconductors by using the BCS theory and the concept of off-diagonal-long-range-order. We discuss the analytical formula of the zero-temperature condensate density of Cooper pairs as a function of Debye frequency and energy gap, and calculate the condensate fraction for some metals. We study the density of Cooper pairs also at finite temperature showing its connection with the gap order parameter and the effects of the electron-phonon coupling. Finally, we analyze similarities and differences between superconductors and ultracold Fermi atoms in the determination of their condensate density by using the BCS theory.Comment: 14 pages, 1 figure, 1 table, to be published in 'Fermions: Flavors, Properties, and Types' (Nova Science Publishers, New York)

Pulsed Quantum Tunneling with Matter Waves

In this report we investigate the macroscopic quantum tunneling of a Bose condensate falling under gravity and scattering on a Gaussian barrier that could model a mirror of far-detuned sheet of light. We analyze the effect of the inter-atomic interaction and that of a transverse confining potential. We show that the quantum tunneling can be quasi-periodic and in this way one could generate coherent Bose condensed atomic pulses. In the second part of the report, we discuss an effective 1D time-dependent non-polynomial nonlinear Schrodinger equation (NPSE), which describes cigar-shaped condensates. NPSE is obtained from the 3D Gross-Pitaevskii equation by using a variational approach. We find that NPSE gives much more accurate results than all other effective 1D equations recently proposed.Comment: 9 pages, 5 figures, report for the X International Laser Physics Workshop, Seminar on Bose-Einstein Condensation of Trapped Atoms, Moscow, July 3-7, 200

Classical and Quantum Perturbation Theory for two Non--Resonant Oscillators with Quartic Interaction

We study the classical and quantum perturbation theory for two non--resonant oscillators coupled by a nonlinear quartic interaction. In particular we analyze the question of quantum corrections to the torus quantization of the classical perturbation theory (semiclassical mechanics). We obtain up to the second order of perturbation theory an explicit analytical formula for the quantum energy levels, which is the semiclassical one plus quantum corrections. We compare the "exact" quantum levels obtained numerically to the semiclassical levels studying also the effects of quantum corrections.Comment: 11 pages, Latex, no figures, to be published in Meccanic

Instabilities, Point Attractors and Limit Cycles in a Inflationary Universe

We study the stability of a scalar inflaton field and analyze its point attractors in the phase space. We show that the value of the inflaton field in the vacuum is a bifurcation parameter and prove the possible existence of a limit cycle by using analytical and numerical arguments.Comment: Latex, 11 pages, 3 figures (available upon request), to be published in Modern Physics Letters

Reply to a Comment on "the Role of Dimensionality in the Stability of a Confined Condensed Bose Gas"

As pointed out by the authors of the comment quant-ph/9712046, in our paper quant-ph/9712030 we studied in detail the metastability of a Bose-Einstein Condensate (BEC) confined in an harmonic trap with zero-range interaction. As well known, the BEC with attractive zero-range interaction is not stable but can be metastable. In our paper we analyzed the role of dimensionality for the metastability of the BEC with attractive and repulsive interaction.Comment: 4 pages, Latex, no figure

Particles and Anti-Particles in a Relativistic Bose Condensate

We study the Bose-Einstein condensation (BEC) for a relativistic ideal gas of bosons. In the framework of canonical thermal field theory, we analyze the role of particles and anti-particles in the determination of BEC transition temperature. At the BEC transition point we obtain two universal curves, i.e. valid for any mass value: the scaled critical temperature as a function of the scaled charge density of the Bose system, and the density ratio of anti-particles versus the scaled critical temperature. Moreover, we numerically investigate charge densities and condensed fraction ranging from the non-relativistic to the ultra-relativistic temperature, where analytical results are obtained.Comment: RevTex, 16 pages, 4 figures, to be published in Nuovo Cimento

On the Limit Cycle of an Inflationary Universe

We study the dynamics of a scalar inflaton field with a symmetric double--well potential and prove rigorously the existence of a limit cycle in its phase space. By using analytical and numerical arguments we show that the limit cycle is stable and give an analytical formula for its period.Comment: Latex, 11 pages, 3 figures (available upon request), to be published in Nuovo Cimento

On the Torus Quantization of Two Anyons with Coulomb Interaction in a Magnetic Field

We study two anyons with Coulomb interaction in a uniform magnetic field $B$. By using the torus quantization we obtain the modified Landau and Zeeman formulas for the two anyons. Then we derive a simple algebraic equation for the full spectral problem up to the second order in $B$.Comment: latex, 10 pages, to be published in Modern Physics Letters