392 research outputs found
Monitoring dispersion and re-agglomeration phenomena during the manufacture of polymer nanocomposites
Correlated Hybrid Fluctuations from Inflation with Thermal Dissipation
We investigate the primordial scalar perturbations in the thermal dissipative
inflation where the radiation component (thermal bath) persists and the density
fluctuations are thermally originated. The perturbation generated in this model
is hybrid, i.e. it consists of both adiabatic and isocurvature components. We
calculate the fractional power ratio () and the correlation coefficient
() between the adiabatic and the isocurvature perturbations at the
commencing of the radiation regime. Since the adiabatic/isocurvature
decomposition of hybrid perturbations generally is gauge-dependent at
super-horizon scales when there is substantial energy exchange between the
inflaton and the thermal bath, we carefully perform a proper decomposition of
the perturbations. We find that the adiabatic and the isocurvature
perturbations are correlated, even though the fluctuations of the radiation
component is considered uncorrelated with that of the inflaton. We also show
that both and depend mainly on the ratio between the
dissipation coefficient and the Hubble parameter during inflation.
The correlation is positive () for strong dissipation cases
where , and is negative for weak dissipation instances where
. Moreover, and in this model are not
independent of each other. The predicted relation between and
is consistent with the WMAP observation. Other testable predictions are also
discussed.Comment: 18 pages using revtex4, accepted for publication in PR
The Sachs-Wolfe Effect: Gauge Independence and a General Expression
In this paper we address two points concerning the Sachs-Wolfe effect: (i)
the gauge independence of the observable temperature anisotropy, and (ii) a
gauge-invariant expression of the effect considering the most general situation
of hydrodynamic perturbations. The first result follows because the gauge
transformation of the temperature fluctuation at the observation event only
contributes to the isotropic temperature change which, in practice, is absorbed
into the definition of the background temperature. Thus, we proceed without
fixing the gauge condition, and express the Sachs-Wolfe effect using the
gauge-invariant variables.Comment: 5 pages, closer to published versio
The effects of macroscopic inhomogeneities on the magneto transport properties of the electron gas in two dimensions
In experiments on electron transport the macroscopic inhomogeneities in the
sample play a fundamental role. In this paper and a subsequent one we introduce
and develop a general formalism that captures the principal features of sample
inhomogeneities (density gradients, contact misalignments) in the magneto
resistance data taken from low mobility heterostructures. We present detailed
assessments and experimental investigations of the different regimes of
physical interest, notably the regime of semiclassical transport at weak
magnetic fields, the plateau-plateau transitions as well as the
plateau-insulator transition that generally occurs at much stronger values of
the external field only.
It is shown that the semiclassical regime at weak fields plays an integral
role in the general understanding of the experiments on the quantum Hall
regime. The results of this paper clearly indicate that the plateau-plateau
transitions, unlike the the plateau-insulator transition, are fundamentally
affected by the presence of sample inhomogeneities. We propose a universal
scaling result for the magneto resistance parameters. This result facilitates,
amongst many other things, a detailed understanding of the difficulties
associated with the experimental methodology of H.P. Wei et.al in extracting
the quantum critical behavior of the electron gas from the transport
measurements conducted on the plateau-plateau transitions.Comment: 20 pages, 9 figure
Passing through the bounce in the ekpyrotic models
By considering a simplified but exact model for realizing the ekpyrotic
scenario, we clarify various assumptions that have been used in the literature.
In particular, we discuss the new ekpyrotic prescription for passing the
perturbations through the singularity which we show to provide a spectrum
depending on a non physical normalization function. We also show that this
prescription does not reproduce the exact result for a sharp transition. Then,
more generally, we demonstrate that, in the only case where a bounce can be
obtained in Einstein General Relativity without facing singularities and/or
violation of the standard energy conditions, the bounce cannot be made
arbitrarily short. This contrasts with the standard (inflationary) situation
where the transition between two eras with different values of the equation of
state can be considered as instantaneous. We then argue that the usually
conserved quantities are not constant on a typical bounce time scale. Finally,
we also examine the case of a test scalar field (or gravitational waves) where
similar results are obtained. We conclude that the full dynamical equations of
the underlying theory should be solved in a non singular case before any
conclusion can be drawn.Comment: 17 pages, ReVTeX 4, 13 figures, minor corrections, conclusions
unchange
Optically Driven Qubits in Artificial Molecules
We present novel models of quantum gates based on coupled quantum dots in
which a qubit is regarded as the superposition of ground states in each dot.
Coherent control on the qubit is performed by both a frequency and a
polarization of a monochromatic light pulse illuminated on the quantum dots. We
also show that a simple combination of two single qubit gates functions as a
controlled NOT gate resulting from an electron-electron interaction. To examine
the decoherence of quantum states, we discuss electronic relaxation contributed
mainly by LA phonon processes.Comment: 11 pages, 4 figures, submitted to Physical Review
Inflationary models inducing non-Gaussian metric fluctuations
We construct explicit models of multi-field inflation in which the primordial
metric fluctuations do not necessarily obey Gaussian statistics. These models
are realizations of mechanisms in which non-Gaussianity is first generated by a
light scalar field and then transferred into curvature fluctuations. The
probability distribution functions of the metric perturbation at the end of
inflation are computed. This provides a guideline for designing strategies to
search for non-Gaussian signals in future CMB and large scale structure
surveys.Comment: 4 pages, 7 figure
Oscillations During Inflation and the Cosmological Density Perturbations
Adiabatic (curvature) perturbations are produced during a period of
cosmological inflation that is driven by a single scalar field, the inflaton.
On particle physics grounds -- though -- it is natural to expect that this
scalar field is coupled to other scalar degrees of freedom. This gives rise to
oscillations between the perturbation of the inflaton field and the
perturbations of the other scalar degrees of freedom, similar to the phenomenon
of neutrino oscillations. Since the degree of the mixing is governed by the
squared mass matrix of the scalar fields, the oscillations can occur even if
the energy density of the extra scalar fields is much smaller than the energy
density of the inflaton field. The probability of oscillation is resonantly
amplified when perturbations cross the horizon and the perturbations in the
inflaton field may disappear at horizon crossing giving rise to perturbations
in scalar fields other than the inflaton. Adiabatic and isocurvature
perturbations are inevitably correlated at the end of inflation and we provide
a simple expression for the cross-correlation in terms of the slow-roll
parameters.Comment: 23 pages, uses LaTeX, added few reference
Dark energy and dark matter from an inhomogeneous dilaton
A cosmological scenario is proposed where the dark matter (DM) and dark
energy (DE) of the universe are two simultaneous manifestations of an
inhomogenous dilaton. The equation of state of the field is scale-dependent and
pressureless at galactic and larger scales and it has negative pressure as a DE
at very large scales. The dilaton drives an inflationary phase followed by a
kinetic energy-dominated one, as in the "quintessential inflation" model
introduced by Peebles & Vilenkin, and soon after the end of inflation particle
production seeds the first inhomogeneities that lead to galaxy formation. The
dilaton is trapped near the minimum of the potential where it oscillates like a
massive field, and the excess of kinetic energy is dissipated via the mechanism
of "gravitational cooling" first introduced by Seidel & Suen. The
inhomogeneities therefore behave like solitonic oscillations around the minimum
of the potential, known as "oscillatons", that we propose account for most DM
in galaxies. Those regions where the dilaton does not transform enough kinetic
energy into reheating or carry an excess of it from regions that have cooled,
evolve to the tail of the potential as DE, driving the acceleration of the
universe.Comment: 9 pages, 8 figures, uses revtex, submitted PR
Deuteron Electroweak Disintegration
We study the deuteron electrodisintegration with inclusion of the neutral
currents focusing on the helicity asymmetry of the exclusive cross section in
coplanar geometry. We stress that a measurement of this asymmetry in the quasi
elastic region is of interest for an experimental determination of the weak
form factors of the nucleon, allowing one to obtain the parity violating
electron neutron asymmetry. Numerically, we consider the reaction at low
momentum transfer and discuss the sensitivity of the helicity asymmetry to the
strangeness radius and magnetic moment. The problems coming from the finite
angular acceptance of the spectrometers are also considered.Comment: 30 pages, Latex, 7 eps figures, submitted to Phys.Rev.C e-mail:
[email protected] , [email protected]
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