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

Enhancement of four reflection shifts by a three-layer surface plasmon resonance

We investigate the effect of a surface plasmon resonance on Goos-Hanchen and Imbert-Fedorov spatial and angular shifts in the reflection of a light beam by considering a three-layer system made of glass, gold and air. We calculate these spatial and angular shifts as a function of the incidence angle showing that they are strongly enhanced in correspondence of the resonant angle. In particular, we find giant spatial and angular Goos-Hanchen shits for the p-wave light close to the plasmon resonance. We also predict a similar, but less pronounced, resonant effect on spatial and angular Imbert-Fedorov shifts for both s-wave and p-wave light.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.

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

Quantum Signature of the Chaos-Order Transition in a Homogeneous SU(2) Yang-Mills-Higgs System

We analyze a spatially homogeneous SU(2) Yang-Mills-Higgs system both in classical and quantum mechanics. By using the Toda criterion of the Gaussian curvature we find a classical chaos-order transition as a function of the Higgs vacuum, the Yang-Mills coupling constant and the energy of the system. Then, we study the nearest-neighbour spacing distribution of the energy levels, which shows a Wigner-Poisson transition by increasing the value of the Higgs field in the vacuum. This transition is a clear quantum signature of the classical chaos-order transition of the system.Comment: Latex, 10 pages, 1 table, talk to the VIII International Conference on Symmetry Methods in Physics, 27 July -- 2 August 1997, Joint Institute for Nuclear Physics, Dubna (Russia

Critical Temperature of an Interacting Bose Gas in a Generic Power-Law Potential

We investigate the critical temperature of an interacting Bose gas confined in a trap described by a generic isotropic power-law potential. We compare the results with respect to the non-interacting case. In particular, we derive an analytical formula for the shift of the critical temperature holding to first order in the scattering length. We show that this shift scales as $N^{n\over 3(n+2)}$, where $N$ is the number of Bosons and $n$ is the exponent of the power-law potential. Moreover, the sign of the shift critically depends on the power-law exponent $n$. Finally, we find that the shift of the critical temperature due to finite-size effects vanishes as $N^{-{2n\over 3(n+2)}}$.Comment: 9 pages, 1 figure, 1 table, to be published in Int. J. Mod. Phys. B, related papers can be found at http://www.mi.infm.it/salasnich/tdqg.htm

Chaos Suppression in the SU(2) Yang--Mills--Higgs System

We study the classical chaos--order transition in the spatially homogenous SU(2) Yang--Mills--Higgs system by using a quantal analog of Chirikov's resonance overlap criterion. We obtain an analytical estimation of the range of parameters for which there is chaos suppression.Comment: LaTex, 10 pages, to be published in Phys. Rev.