204 research outputs found
A Monte Carlo study of Inverse Symmetry Breaking
We make a Monte Carlo study of the coupled two-scalar
model in four dimensions at finite temperature. We
find no trace of Inverse Symmetry Breaking for values of the renormalized
parameters for which perturbation theory predicts this phenomenon.Comment: 4 pages, revtex, 3 figures include
A lattice Monte Carlo study of Inverse Symmetry Breaking in a two-scalar model in three dimensions
We carry a Monte Carlo study of the coupled two-scalar model in three dimensions. We find no trace of Inverse Symmetry
Breaking in the region of negative 's for which the one-loop effective
potential predicts this phenomenon. Moreover, for 's negative enough,
but still in the stability region for the potential, one of the transitions
turns out to be of first order, both for zero and finite temperature.Comment: 23 pages, 7 figures included, Latex, epsfig. Minor changes in Sec.3
and in the conclusions. In press on Nuc.Phys.
Inverse Symmetry Breaking on the lattice: an accurate MC study
We present here a new MC study of ISB at finite temperature in a model in four dimensions. The results of our simulations,
even if not conclusive, are favourable to ISB. Detection of the effect required
measuring some critical couplings with six-digits precision, a level of
accuracy that could be achieved only by a careful use of FSS techniques. The
gap equations for the Debye masses, resulting from the resummation of the ring
diagrams, seem to provide a qualitatively correct description of the data,
while the simple one-loop formulae appear to be inadequate.Comment: 23 pages, 7 figures, Section 6 revised, in press on Nucl. Phys.
Polarized thermal emission by thin metal wires
We report new measurements of the linear polarization of thermal radiation
emitted by incandescent thin tungsten wires, with thicknesses ranging from five
to hundred microns. Our data show very good agreement with theoretical
predictions, based on Drude-type fits to measured optical properties of
tungsten.Comment: 12 pages, 4 encapsulated figures. This new version matches the one
published in New. J. Phys.. Improved presentation, more references added, and
one new figure include
Johnson-Nyquist noise and the Casimir force between real metals at nonzero temperature
It is well known since a long time that all lossy conductors at finite
temperature display an electronic noise, the Johnson-Nyquist noise, arising
from the thermal agitation of electric charges inside the conductor. The
existence of this noise implies that two nearby discharged conductors at finite
temperature should repel each other, as a result of the electrodynamic
interaction between the Johnson-Nyquist currents in either conductor and the
eddy currents they induce in the other. It is suggested that this force is at
the origin of the recently discovered large repulsive correction to the thermal
Casimir force between two lossy metallic plates. Further support for this
physical picture is obtained by studying a simple system of two linear noisy
antennas. Using elementary concepts from circuit theory, we show that the
repulsive force engendered by the Johnson-Nyquist noise results in the same
kind of thermodynamic inconsistencies found in the Casimir problem. We show
that all inconsistencies are however resolved if account is taken of capacitive
effects associated with the end points of the antennas. Our findings therefore
suggest that capacitive effects resulting from the finite size of the plates,
may be essential for a resolution of the analogous problems met in the thermal
Casimir effect.Comment: 9 pages, 1 encapsulated figure. Contributed to 8th Workshop on
Quantum Field Theory Under the Influence of External Conditions (QFEXT07),
Leipzig, Germany, 17-21 Sep 200
Isoelectronic determination of the thermal Casimir force
Differential force measurements between spheres coated with either nickel or
gold and rotating disks with periodic distributions of nickel and gold are
reported. The rotating samples are covered by a thin layer of titanium and a
layer of gold. While titanium is used for fabrication purposes, the gold layer
(nominal thicknesses of 21, 37, 47 and 87 nm) provides an isoelectronic
environment, and is used to nullify the electrostatic contribution but allow
the passage of long wavelength Casimir photons. A direct comparison between the
experimental results and predictions from Drude and plasma models for the
electrical permittivity is carried out. In the models the magnetic permeability
of nickel is allowed to change to investigate its effects. Possible sources of
errors, both in the experimental and theoretical sides, are taken into account.
It is found that a Drude response with magnetic properties of nickel taken into
account is unequivocally ruled out. The full analysis of the data indicates
that a dielectric plasma response with magnetic properties of Ni included shows
good agreement with the data. Neither a Drude nor a plasma dielectric response
provide a satisfactory description if the magnetic properties of nickel are
disregarded.Comment: 16 pages, 15 figures. Extended version accepted in Physical Review
A generalized Kramers-Kronig transform for Casimir effect computations
Recent advances in experimental techniques now permit to measure the Casimir
force with unprecedented precision. In order to achieve a comparable precision
in the theoretical prediction of the force, it is necessary to accurately
determine the electric permittivity of the materials constituting the plates
along the imaginary frequency axis. The latter quantity is not directly
accessible to experiments, but it can be determined via dispersion relations
from experimental optical data. In the experimentally important case of
conductors, however, a serious drawback of the standard dispersion relations
commonly used for this purpose, is their strong dependence on the chosen
low-frequency extrapolation of the experimental optical data, which introduces
a significant and not easily controllable uncertainty in the result. In this
paper we show that a simple modification of the standard dispersion relations,
involving suitable analytic window functions, resolves this difficulty, making
it possible to reliably determine the electric permittivity at imaginary
frequencies solely using experimental optical data in the frequency interval
where they are available, without any need of uncontrolled data extrapolations.Comment: 10 pages, 6 encapsulated figures. A few typos corrected, some
references added. The new version matches the one accepted for publication on
Phys. Rev.
Material dependence of Casimir forces: gradient expansion beyond proximity
A widely used method for estimating Casimir interactions [H. B. G. Casimir,
Proc. K. Ned. Akad. Wet. 51, 793 (1948)] between gently curved material
surfaces at short distances is the proximity force approximation (PFA). While
this approximation is asymptotically exact at vanishing separations,
quantifying corrections to PFA has been notoriously difficult. Here we use a
derivative expansion to compute the leading curvature correction to PFA for
metals (gold) and insulators (SiO) at room temperature. We derive an
explicit expression for the amplitude of the PFA correction to
the force gradient for axially symmetric surfaces. In the non-retarded limit,
the corrections to the Casimir free energy are found to scale logarithmically
with distance. For gold, has an unusually large temperature
dependence.Comment: 4 pages, 2 figure
Continuum limit of finite temperature from lattice Monte Carlo
The model at finite temperature is simulated on the lattice. For
fixed we compute the transition line for by means of
Finite Size Scaling techniques. The crossings of a Renormalization Group
trajectory with the transition lines of increasing give a well defined
limit for the critical temperature in the continuum. By considering different
RG trajectories, we compute as a function of the renormalized
parameters.Comment: 26 pages, Latex, epsfig, 8 figures include
Making precise predictions of the Casimir force between metallic plates via a weighted Kramers-Kronig transform
The possibility of making precise predictions for the Casimir force is
essential for the theoretical interpretation of current precision experiments
on the thermal Casimir effect with metallic plates, especially for sub-micron
separations. For this purpose it is necessary to estimate very accurately the
dielectric function of a conductor along the imaginary frequency axis. This
task is complicated in the case of ohmic conductors, because optical data do
not usually extend to sufficiently low frequencies to permit an accurate
evaluation of the standard Kramers-Kronig integral used to compute . By making important improvements in the results of a previous paper by
the author, it is shown that this difficulty can be resolved by considering
suitable weighted dispersions relations, which strongly suppress the
contribution of low frequencies. The weighted dispersion formulae presented in
this paper permit to estimate accurately the dielectric function of ohmic
conductors for imaginary frequencies, on the basis of optical data extending
from the IR to the UV, with no need of uncontrolled data extrapolations towards
zero frequency that are instead necessary with standard Kramers-Kronig
relations. Applications to several sets of data for gold films are presented to
demonstrate viability of the new dispersion formulae.Comment: 18 pages, 15 encapsulated figures. In the revised version important
improvements have been made, which affect the main conclusions of the pape
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