7,050 research outputs found
Parametric Competition in non-autonomous Hamiltonian Systems
In this work we use the formalism of chord functions (\emph{i.e.}
characteristic functions) to analytically solve quadratic non-autonomous
Hamiltonians coupled to a reservoir composed by an infinity set of oscillators,
with Gaussian initial state. We analytically obtain a solution for the
characteristic function under dissipation, and therefore for the determinant of
the covariance matrix and the von Neumann entropy, where the latter is the
physical quantity of interest. We study in details two examples that are known
to show dynamical squeezing and instability effects: the inverted harmonic
oscillator and an oscillator with time dependent frequency. We show that it
will appear in both cases a clear competition between instability and
dissipation. If the dissipation is small when compared to the instability, the
squeezing generation is dominant and one can see an increasing in the von
Neumann entropy. When the dissipation is large enough, the dynamical squeezing
generation in one of the quadratures is retained, thence the growth in the von
Neumann entropy is contained
Interband polarized absorption in InP polytypic superlattices
Recent advances in growth techniques have allowed the fabrication of
semiconductor nanostructures with mixed wurtzite/zinc-blende crystal phases.
Although the optical characterization of these polytypic structures is well
eported in the literature, a deeper theoretical understanding of how crystal
phase mixing and quantum confinement change the output linear light
polarization is still needed. In this paper, we theoretically investigate the
mixing effects of wurtzite and zinc-blende phases on the interband absorption
and in the degree of light polarization of an InP polytypic superlattice. We
use a single 88 kp Hamiltonian that describes both crystal
phases. Quantum confinement is investigated by changing the size of the
polytypic unit cell. We also include the optical confinement effect due to the
dielectric mismatch between the superlattice and the vaccum and we show it to
be necessary to match experimental results. Our calculations for large wurtzite
concentrations and small quantum confinement explain the optical trends of
recent photoluminescence excitation measurements. Furthermore, we find a high
sensitivity to zinc-blende concentrations in the degree of linear polarization.
This sensitivity can be reduced by increasing quantum confinement. In
conclusion, our theoretical analysis provides an explanation for optical trends
in InP polytypic superlattices, and shows that the interplay of crystal phase
mixing and quantum confinement is an area worth exploring for light
polarization engineering.Comment: 9 pages, 6 figures and 1 tabl
Controlling high-harmonic generation and above-threshold ionization with an attosecond-pulse train
We perform a detailed analysis of how high-order harmonic generation (HHG)
and above-threshold ionization (ATI) can be controlled by a time-delayed
attosecond-pulse train superposed to a strong, near-infrared laser field. In
particular we show that the high-harmonic and photoelectron intensities, the
high-harmonic plateau structure and cutoff energies, and the ATI angular
distributions can be manipulated by changing this delay. This is a direct
consequence of the fact that the attosecond pulse train can be employed as a
tool for constraining the instant an electronic wave packet is ejected in the
continuum. A change in such initial conditions strongly affects its subsequent
motion in the laser field, and thus HHG and ATI. In our studies, we employ the
Strong-Field Approximation and explain the features observed in terms of
interference effects between various electron quantum orbits. Our results are
in agreement with recent experimental findings and theoretical studies
employing purely numerical methods.Comment: 10 pages revtex and 6 figures (eps files
Low redshift constraints on energy-momentum-powered gravity models
There has been recent interest in the cosmological consequences of
energy-momentum-powered gravity models, in which the matter side of Einstein's
equations is modified by the addition of a term proportional to some power,
, of the energy-momentum tensor, in addition to the canonical linear term.
In this work we treat these models as phenomenological extensions of the
standard CDM, containing both matter and a cosmological constant. We
also quantitatively constrain the additional model parameters using low
redshift background cosmology data that are specifically from Type Ia
supernovas and Hubble parameter measurements. We start by studying specific
cases of these models with fixed values of which lead to an analytic
expression for the Friedmann equation; we discuss both their current
constraints and how the models may be further constrained by future
observations of Type Ia supernovas for WFIRST complemented by measurements of
the redshift drift by the ELT. We then consider and constrain a more extended
parameter space, allowing to be a free parameter and considering scenarios
with and without a cosmological constant. These models do not solve the
cosmological constant problem per se. Nonetheless these models can
phenomenologically lead to a recent accelerating universe without a
cosmological constant at the cost of having a preferred matter density of
around instead of the usual . Finally we
also briefly constrain scenarios without a cosmological constant, where the
single component has a constant equation of state which needs not be that of
matter; we provide an illustrative comparison of this model with a more
standard dynamical dark energy model with a constant equation of state.Comment: 13+2 pages, 12+1 figures; A&A (in press
Evaluation of air lime and clayish earth mortars for earthen wall renders
CIAV2013 – International Conference on Vernacular Architecture, 7º ATP, VerSus, 16-20 october 2013An experimental rammed earth wall was traditionally made with local earth and characterized in terms of superficial hardness, compactness, thermal conductivity and water absorption, in exterior environmental conditions. Two mortars were made with an air lime and a mixture of three washed graduated siliceous sands, with volumetric proportions of 1:2 and 1:3 (air lime:sand). A clayish earth was characterized and applied as partial substitutions of air lime on 1:2 mortars and as partial substitutions of the finest sand on 1:3 mortars. Mortars were formulated and characterized in terms of constitution and consistency and samples of mortars applied on ceramic brick were prepared. For each volumetric proportion, mortars without earth and the ones with earth that presented the best workability were applied as renders on panels on the experimental
rammed earth wall; also prismatic samples were made. The mortar samples and the renders on the wall were characterized at 90 days of age. This paper presents and discusses some of the results obtained with the characterization of the rammed earth wall, the fresh mortars and its application on the bricks and on the wall. Differences
between the mortars are highlighted while their compatibility with traditional rammed earth wall is verified
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