2,743 research outputs found
Duality in Perturbation Theory and the Quantum Adiabatic Approximation
Duality is considered for the perturbation theory by deriving, given a series
solution in a small parameter, its dual series with the development parameter
being the inverse of the other. A dual symmetry in perturbation theory is
identified. It is then shown that the dual to the Dyson series in quantum
mechanics is given by a recent devised series having the adiabatic
approximation as leading order. A simple application of this result is given by
rederiving a theorem for strongly perturbed quantum systems.Comment: 9 pages, revtex. Improved english and presentation. Final version
accepted for publication by Physical Review
Exact solutions of classical scalar field equations
We give a class of exact solutions of quartic scalar field theories. These
solutions prove to be interesting as are characterized by the production of
mass contributions arising from the nonlinear terms while maintaining a
wave-like behavior. So, a quartic massless equation has a nonlinear wave
solution with a dispersion relation of a massive wave and a quartic scalar
theory gets its mass term renormalized in the dispersion relation through a
term depending on the coupling and an integration constant. When spontaneous
breaking of symmetry is considered, such wave-like solutions show how a mass
term with the wrong sign and the nonlinearity give rise to a proper dispersion
relation. These latter solutions do not change the sign maintaining the
property of the selected value of the equilibrium state. Then, we use these
solutions to obtain a quantum field theory for the case of a quartic massless
field. We get the propagator from a first order correction showing that is
consistent in the limit of a very large coupling. The spectrum of a massless
quartic scalar field theory is then provided. From this we can conclude that,
for an infinite countable number of exact classical solutions, there exist an
infinite number of equivalent quantum field theories that are trivial in the
limit of the coupling going to infinity.Comment: 7 pages, no figures. Added proof of existence of a zero mode and two
more references. Accepted for publication in Journal of Nonlinear
Mathematical Physic
Green functions and nonlinear systems: Short time expansion
We show that Green function methods can be straightforwardly applied to
nonlinear equations appearing as the leading order of a short time expansion.
Higher order corrections can be then computed giving a satisfactory agreement
with numerical results. The relevance of these results relies on the
possibility of fully exploiting a gradient expansion in both classical and
quantum field theory granting the existence of a strong coupling expansion.
Having a Green function in this regime in quantum field theory amounts to
obtain the corresponding spectrum of the theory.Comment: 7 pages, 3 figures. Version accepted for publication in International
Journal of Modern Physics
Photospheric and chromospheric activity in four young solar-type stars
We present a photometric and spectroscopic study of four G-K dwarfs, namely
HD 166, epsilon Eri, chi1 Ori and kappa1 Cet. In three cases, we find a clear
spatial association between photospheric and chromospheric active regions. For
chi1 Ori we do not find appreciable variations of photospheric temperature, and
chromospheric Halpha emission. We applied a spot/plage model to the observed
rotational modulation of temperature and flux to derive spot/plage parameters
and to reconstruct a rough three-dimensional map of the outer atmosphere of
kappa1 Cet, HD 166 and epsilon Eri.Comment: 12 pages, 3 tables, 9 figures. Submitted to Ap
Contemporaneous broad-band photometry and H observations of T Tauri stars
The study of contemporaneous variations of the continuum flux and emission
lines is of great importance to understand the different astrophysical
processes at work in T Tauri stars. In this paper we present the results of a
simultaneous and H photometric monitoring, contemporaneous to
medium-resolution spectroscopy of six T Tauri stars in the Taurus-Auriga star
forming region. We have characterized the H photometric system using
synthetic templates and the contemporaneous spectra of the targets. We show
that we can achieve a precision corresponding to 23 \AA\ in the H
equivalent width, in typical observing conditions. The spectral analysis has
allowed us to determine the basic stellar parameters and the values of
quantities related to the accretion. In particular, we have measured a
significant veiling only for the three targets with the strongest H
emission (T Tau, FM Tau, and DG Tau). The broad-band photometric variations are
found to be in the range 0.050.70 mag and are often paired to variations in
the H intensity, which becomes stronger when the stellar continuum is
weaker. In addition, we have mostly observed a redder and a bluer
color as the stars become fainter. For most of the targets, the timescales of
these variations seem to be longer than the rotation period. One exception is T
Tau, for which the broad-band photometry varies with the rotation period. The
most plausible interpretation of these photometric and H variations is
that they are due to non-stationary mass accretion onto the stars, but
rotational modulation can play a major role in some cases.Comment: 21 pages, 11 figures, accepted for publication in Acta Astronomic
A Quantum Many-Body Instability in the Thermodynamic Limit
Intrinsic decoherence in the thermodynamic limit is shown for a large class
of many-body quantum systems in the unitary evolution in NMR and cavity QED.
The effect largely depends on the inability of the system to recover the
phases. Gaussian decaying in time of the fidelity is proved for spin systems
and radiation-matter interaction.Comment: 11 pages, 1 figure. Final version accepted for publication in Modern
Physics Letters
Dynamical decoherence in a cavity with a large number of two-level atoms
We consider a large number of two-level atoms interacting with the mode of a
cavity in the rotating-wave approximation (Tavis-Cummings model). We apply the
Holstein-Primakoff transformation to study the model in the limit of the number
of two-level atoms, all in their ground state, becoming very large. The unitary
evolution that we obtain in this approximation is applied to a macroscopic
superposition state showing that, when the coherent states forming the
superposition are enough distant, then the state collapses on a single coherent
state describing a classical radiation mode. This appear as a true dynamical
effect that could be observed in experiments with cavities.Comment: 9 pages, no figures. This submission substitutes paper
quant-ph/0212148 that was withdrawn. Version accepted for publication in
Journal of Physics B: Atomic, Molecular & Optical Physic
REM near-IR and optical photometric monitoring of Pre-Main Sequence Stars in Orion
We performed an intensive photometric monitoring of the PMS stars falling in
a field of about 10x10 arc-minutes in the vicinity of the Orion Nebula Cluster
(ONC). Photometric data were collected between November 2006 and January 2007
with the REM telescope in the VRIJHK' bands. The largest number of observations
is in the I band (about 2700 images) and in J and H bands (about 500 images in
each filter). From the observed rotational modulation, induced by the presence
of surface inhomogeneities, we derived the rotation periods for 16 stars and
improved previous determinations for the other 13. The analysis of the spectral
energy distributions and, for some stars, of high-resolution spectra provided
us with the main stellar parameters (luminosity, effective temperature, mass,
age, and vsini). We also report the serendipitous detection of two strong
flares in two of these objects. In most cases, the light-curve amplitudes
decrease progressively from the R to H band as expected for cool starspots,
while in a few cases, they can only be modelled by the presence of hot spots,
presumably ascribable to magnetospheric accretion. The application of our own
spot model to the simultaneous light curves in different bands allowed us to
deduce the spot parameters and particularly to disentangle the spot temperature
and size effects on the observed light curves.Comment: 29 pages, 24 figure
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