4,251 research outputs found

### Vacuum polarization by topological defects in de Sitter spacetime

In this paper we investigate the vacuum polarization effects associated with
a massive quantum scalar field in de Sitter spacetime in the presence of
gravitational topological defects. Specifically we calculate the vacuum
expectation value of the field square, $$. Because this investigation
has been developed in a pure de Sitter space, here we are mainly interested on
the effects induced by the presence of the defects.Comment: Talk presented at the 1st. Mediterranean Conference on Classical and
Quantum Gravity (MCCQG

### Vacuum Polarization for a Massless Spin-1/2 Field in the Global Monopole Spacetime at Nonzero Temperature

In this paper we present the effects produced by the temperature in the
renormalized vacuum expectation value of the zero-zero component of the
energy-momentum tensor associated with massless left-handed spinor field in the
pointlike global monopole spacetime. In order to develop this calculation we
had to obtain the Euclidean thermal Green function in this background. Because
the expression obtained for the thermal energy density cannot be expressed in a
closed form, its explicit dependence on the temperature is not completely
evident. So, in order to obtain concrete information about its thermal
behavior, we develop a numerical analysis of our result in the high-temperature
limit for specific values of the parameter $\alpha$ which codify the presence
of the monopole.Comment: 22 pages, LaTex format, 5 figure

### Nonplanar integrability at two loops

In this article we compute the action of the two loop dilatation operator on
restricted Schur polynomials that belong to the su(2) sector, in the displaced
corners approximation. In this non-planar large N limit, operators that
diagonalize the one loop dilatation operator are not corrected at two loops.
The resulting spectrum of anomalous dimensions is related to a set of decoupled
harmonic oscillators, indicating integrability in this sector of the theory at
two loops. The anomalous dimensions are a non-trivial function of the 't Hooft
coupling, with a spectrum that is continuous and starting at zero at large N,
but discrete at finite N.Comment: version to appear in JHE

### Temperature- and quantum phonon effects on Holstein-Hubbard bipolarons

The one-dimensional Holstein-Hubbard model with two electrons of opposite
spin is studied using an extension of a recently developed quantum Monte Carlo
method, and a very simple yet rewarding variational approach, both based on a
canonically transformed Hamiltonian. The quantum Monte Carlo method yields very
accurate results in the regime of small but finite phonon frequencies,
characteristic of many strongly correlated materials such as, e.g., the
cuprates and the manganites. The influence of electron-electron repulsion,
phonon frequency and temperature on the bipolaron state is investigated.
Thermal dissociation of the intersite bipolaron is observed at high
temperatures, and its relation to an existing theory of the manganites is
discussed.Comment: 12 pages, 7 figures; final version, accepted for publication in Phys.
Rev.

### DeterminaĂ§ĂŁo de metil-xantinas em alimentos por cromatografia lĂquida de alta eficiĂȘncia.

bitstream/item/75983/1/ct55-2002.pd

### Vacuum Polarization by a Magnetic Flux Tube at Finite Temperature in the Cosmic String Spacetime

In this paper we analyse the effect produced by the temperature in the vacuum
polarization associated with charged massless scalar field in the presence of
magnetic flux tube in the cosmic string spacetime. Three different
configurations of magnetic fields are taken into account: $(i)$ a homogeneous
field inside the tube, $(ii)$ a field proportional to $1/r$ and $(iii)$ a
cylindrical shell with $\delta$-function. In these three cases, the axis of the
infinitely long tube of radius $R$ coincides with the cosmic string. Because
the complexity of this analysis in the region inside the tube, we consider the
thermal effect in the region outside. In order to develop this analysis, we
construct the thermal Green function associated with this system for the three
above mentioned situations considering points in the region outside the tube.
We explicitly calculate in the high-temperature limit, the thermal average of
the field square and the energy-momentum tensor.Comment: 16 pages, 1 figur

### Electrostatic self-force in (2+1)-dimensional cosmological gravity

Point sources in (2+1)-dimensional gravity are conical singularities that
modify the global curvature of the space giving rise to self-interaction
effects on classical fields. In this work we study the electrostatic
self-interaction of a point charge in the presence of point masses in
(2+1)-dimensional gravity with a cosmological constant.Comment: 9 pages, Late

### Generalized Lyapunov Exponent and Transmission Statistics in One-dimensional Gaussian Correlated Potentials

Distribution of the transmission coefficient T of a long system with a
correlated Gaussian disorder is studied analytically and numerically in terms
of the generalized Lyapunov exponent (LE) and the cumulants of lnT. The effect
of the disorder correlations on these quantities is considered in weak,
moderate and strong disorder for different models of correlation. Scaling
relations between the cumulants of lnT are obtained. The cumulants are treated
analytically within the semiclassical approximation in strong disorder, and
numerically for an arbitrary strength of the disorder. A small correlation
scale approximation is developed for calculation of the generalized LE in a
general correlated disorder. An essential effect of the disorder correlations
on the transmission statistics is found. In particular, obtained relations
between the cumulants and between them and the generalized LE show that, beyond
weak disorder, transmission fluctuations and deviation of their distribution
from the log-normal form (in a long but finite system) are greatly enhanced due
to the disorder correlations. Parametric dependence of these effects upon the
correlation scale is presented.Comment: 18 pages, 11 figure

### Vacuum Polarization of a Massless Scalar Field in the Background of a Global Monopole with Finite Core

In this paper we analyze the vacuum polarization effects of a massless scalar
field in the background of a global monopole considering a inner structure to
it. Specifically we investigate the effect of its structure on the vacuum
expectation value of the square of the field operator, ,
admitting a non-minimal coupling between the field with the geometry: $\xi
{\cal{R}}\hat{\Phi}^2$. Also we calculate the corrections on the vacuum
expectation value of the energy-momentum tensor, , due to
the inner structure of the monopole. In order to develop these analysis, we
calculate the Euclidean Green function associated with the system for points in
the region outside the core. As we shall see, for specific value of the
coupling parameter $\xi$, the corrections caused by the inner structure of the
monopole can provide relevant contributions on these vacuum polarizations.Comment: Accepted for publication in Classical and Quantum Gravity. Added
references. 22 pages, 1 figur

### Statistical Scattering of Waves in Disordered Waveguides: from Microscopic Potentials to Limiting Macroscopic Statistics

We study the statistical properties of wave scattering in a disordered
waveguide. The statistical properties of a "building block" of length (delta)L
are derived from a potential model and used to find the evolution with length
of the expectation value of physical quantities. In the potential model the
scattering units consist of thin potential slices, idealized as delta slices,
perpendicular to the longitudinal direction of the waveguide; the variation of
the potential in the transverse direction may be arbitrary. The sets of
parameters defining a given slice are taken to be statistically independent
from those of any other slice and identically distributed. In the
dense-weak-scattering limit, in which the potential slices are very weak and
their linear density is very large, so that the resulting mean free paths are
fixed, the corresponding statistical properties of the full waveguide depend
only on the mean free paths and on no other property of the slice distribution.
The universality that arises demonstrates the existence of a generalized
central-limit theorem.
Our final result is a diffusion equation in the space of transfer matrices of
our system, which describes the evolution with the length L of the disordered
waveguide of the transport properties of interest. In contrast to earlier
publications, in the present analysis the energy of the incident particle is
fully taken into account.Comment: 75 pages, 10 figures, submitted to Phys. Rev

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