279 research outputs found

### 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.

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