1,703 research outputs found
Vanishing Cosmological Constant in Modified Gauss-Bonnet Gravity with Conformal Anomaly
We consider dark energy cosmology in a de Sitter universe filled with quantum
conformal matter. Our model represents a Gauss-Bonnet model of gravity with
contributions from quantum effects. To the General Relativity action an
arbitrary function of the GB invariant, f(G), is added, and taking into account
quantum effects from matter the cosmological constant is studied. For the
considered model the conditions for a vanishing cosmological constant are
considered. Creation of a de Sitter universe by quantum effects in a GB
modified gravity is discussed.Comment: 8 pages latex, 1 figure. To appear in Int. J. Mod. Phys.
Ultrafast non-linear optical signal from a single quantum dot: exciton and biexciton effects
We present results on both the intensity and phase-dynamics of the transient
non-linear optical response of a single quantum dot (SQD).
The time evolution of the Four Wave Mixing (FWM) signal on a subpicosecond
time scale is dominated by biexciton effects. In particular, for the
cross-polarized excitation case a biexciton bound state is found. In this
latter case, mean-field results are shown to give a poor description of the
non-linear optical signal at small times. By properly treating exciton-exciton
effects in a SQD, coherent oscillations in the FWM signal are clearly
demonstrated. These oscillations, with a period corresponding to the inverse of
the biexciton binding energy, are correlated with the phase dynamics of the
system's polarization giving clear signatures of non-Markovian effects in the
ultrafast regime.Comment: 10 pages, 3 figure
Interplay between excitation kinetics and reaction-center dynamics in purple bacteria
Photosynthesis is arguably the fundamental process of Life, since it enables
energy from the Sun to enter the food-chain on Earth. It is a remarkable
non-equilibrium process in which photons are converted to many-body excitations
which traverse a complex biomolecular membrane, getting captured and fueling
chemical reactions within a reaction-center in order to produce nutrients. The
precise nature of these dynamical processes -- which lie at the interface
between quantum and classical behaviour, and involve both noise and
coordination -- are still being explored. Here we focus on a striking recent
empirical finding concerning an illumination-driven transition in the
biomolecular membrane architecture of {\it Rsp. Photometricum} purple bacteria.
Using stochastic realisations to describe a hopping rate model for excitation
transfer, we show numerically and analytically that this surprising shift in
preferred architectures can be traced to the interplay between the excitation
kinetics and the reaction center dynamics. The net effect is that the bacteria
profit from efficient metabolism at low illumination intensities while using
dissipation to avoid an oversupply of energy at high illumination intensities.Comment: 21 pages, 13 figures, accepted for publication in New Journal of
Physic
An Experimental Comparison of Stand Management Approaches to Sudden Oak Death: Prevention vs. Restoration
Many coastal forests stretching from central California to southwest Oregon are threatened or have been impacted by the invasive forest pathogen Phytophthora ramorum, the cause of sudden oak death. We analyzed a set of stand-level forest treatments aimed at preventing or mitigating disease impacts on stand composition, biomass, and fuels using a before–after-control-intervention experiment with a re-evaluation after 5 years. We compared the effects of restorative management for invaded stands and preventative treatments for uninvaded forests with two stand-level experiments. The restorative treatments contrasted two approaches to mastication, hand-crew thinning, and thinning with pile burning with untreated controls replicated at three distinct sites (N = 30), while the preventative treatments were limited to hand-crew thinning (N = 10) conducted at a single site. Half of the restoration treatments had basal sprouts removed 2 and 4 years after treatment. All treatments significantly reduced stand density and increased average tree size without significantly decreasing total basal area, both immediately and 5 years after treatments. Preventative treatments did not reduce the basal area or density of timber species not susceptible to P. ramorum, suggesting the relative dominance of these species increased in accordance with host removal. Follow-up basal sprout removal in the restoration experiment appears to maintain treatment benefits for average tree size and may be associated with small decreases in stand density 5 years after initial treatment. Our study demonstrates that for at least 5 years, a range of common stand management practices can improve forest conditions threatened or impacted by sudden oak death
Spin and Center of Mass in Axially Symmetric Einstein-Maxwell Spacetimes
We give a definition and derive the equations of motion for the center of
mass and angular momentum of an axially symmetric, isolated system that emits
gravitational and electromagnetic radiation. A central feature of this
formulation is the use of Newman-Unti cuts at null infinity that are generated
by worldlines of the spacetime. We analyze some consequences of the results and
comment on the generalization of this work to general asymptotically flat
spacetimes.Comment: 20 page
Analytic results for particles with interaction in two dimensions and an external magnetic field
The -dimensional quantum problem of particles (e.g. electrons) with
interaction in a two-dimensional parabolic potential
(e.g. quantum dot) and magnetic field , reduces exactly to solving a
-dimensional problem which is independent of and . An
exact, infinite set of relative mode excitations are obtained for any . The
problem reduces to that of a ficticious particle in a two-dimensional,
non-linear potential of strength , subject to a ficticious magnetic
field , the relative angular momentum.Comment: To appear in Physical Review Letters (in press). RevTeX file. Two
figures available from [email protected] or
[email protected]
Bundle Theory of Improper Spin Transformations
{\it We first give a geometrical description of the action of the parity
operator () on non relativistic spin Pauli spinors in
terms of bundle theory. The relevant bundle, , is a
non trivial extension of the universal covering group .
is the non relativistic limit of the corresponding Dirac matrix
operator and obeys . Then, from the direct
product of O(3) by , naturally induced by the structure of the galilean
group, we identify, in its double cover, the time reversal operator ()
acting on spinors, and its product with . Both, and
, generate the group . As in the case of parity,
is the non relativistic limit of the corresponding Dirac matrix
operator , and obeys .}Comment: 8 pages, Plaintex; titled changed, minor text modifications, one
reference complete
Microscopic analytical theory of a correlated, two-dimensional N-electron gas in a magnetic field
We present a microscopic, analytical theory describing a confined N-electron
gas in two dimensions subject to an external magnetic field. The number of
electrons N and strength of the electron-electron interaction can be
arbitrarily large, and all Landau levels are included implicitly. A possible
connection with the Integer and Fractional Quantum Hall Effects is proposed.Comment: The revised version contains minor changes to text. To be published
in J. Phys: Condens. Mat
Adiabatic Compression of Soliton Matter Waves
The evolution of atomic solitary waves in Bose-Einstein condensate (BEC)
under adiabatic changes of the atomic scattering length is investigated. The
variations of amplitude, width, and velocity of soliton are found for both
spatial and time adiabatic variations. The possibility to use these variations
to compress solitons up to very high local matter densities is shown both in
absence and in presence of a parabolic confining potential.Comment: to appear in J.Phys.
Synthetic Molecular Clouds from Supersonic MHD and Non-LTE Radiative Transfer Calculations
The dynamics of molecular clouds is characterized by supersonic random
motions in the presence of a magnetic field. We study this situation using
numerical solutions of the three-dimensional compressible magneto-hydrodynamic
(MHD) equations in a regime of highly supersonic random motions. The non-LTE
radiative transfer calculations are performed through the complex density and
velocity fields obtained as solutions of the MHD equations, and more than
5x10^5 synthetic molecular spectra are obtained. We use a numerical flow
without gravity or external forcing. The flow is super-Alfvenic and corresponds
to model A of Padoan and Nordlund (1997). Synthetic data consist of sets of
90x90 synthetic spectra with 60 velocity channels, in five molecular
transitions: J=1-0 and J=2-1 for 12CO and 13CO, and J=1-0 for CS. Though we do
not consider the effects of stellar radiation, gravity, or mechanical energy
input from discrete sources, our models do contain the basic physics of
magneto-fluid dynamics and non-LTE radiation transfer and are therefore more
realistic than previous calculations. As a result, these synthetic maps and
spectra bear a remarkable resemblance to the corresponding observations of real
clouds.Comment: 33 pages, 12 figures included, 5 jpeg figures not included (fig1a,
fig1b, fig3, fig4 fig5), submitted to Ap
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