11,191 research outputs found
Short-Time Critical Dynamics of Damage Spreading in the Two-Dimensional Ising Model
The short-time critical dynamics of propagation of damage in the Ising
ferromagnet in two dimensions is studied by means of Monte Carlo simulations.
Starting with equilibrium configurations at and magnetization
, an initial damage is created by flipping a small amount of spins in one
of the two replicas studied. In this way, the initial damage is proportional to
the initial magnetization in one of the configurations upon quenching the
system at , the Onsager critical temperature of the
ferromagnetic-paramagnetic transition. It is found that, at short times, the
damage increases with an exponent , which is much larger
than the exponent characteristic of the initial increase of the
magnetization . Also, an epidemic study was performed. It is found that
the average distance from the origin of the epidemic ()
grows with an exponent , which is the same,
within error bars, as the exponent . However, the survival
probability of the epidemics reaches a plateau so that . On the other
hand, by quenching the system to lower temperatures one observes the critical
spreading of the damage at , where all the measured
observables exhibit power laws with exponents , , and .Comment: 11 pages, 9 figures (included). Phys. Rev. E (2010), in press
Orphan penumbrae: Submerging horizontal fields
We investigate the properties of orphan penumbrae, which are photospheric
filamentary structures observed in active regions near polarity inversion lines
that resemble the penumbra of regular sunspots but are not connected to any
umbra. We use Hinode data from the Solar Optical Telescope to determine the
properties of orphan penumbrae. Spectropolarimetric data are employed to obtain
the vector magnetic field and line-of-sight velocities in the photosphere.
Magnetograms are used to study the overall evolution of these structures, and
G-band and Ca II H filtergrams are to investigate their brightness and apparent
horizontal motions. Orphan penumbrae form between regions of opposite polarity
in places with horizontal magnetic fields. Their magnetic configuration is that
of -shaped flux ropes. In the two cases studied here, the
opposite-polarity regions approach each other with time and the whole structure
submerges as the penumbral filaments disappear. Orphan penumbrae are very
similar to regular penumbrae, including the existence of strong gas flows.
Therefore, they could have a similar origin. The main difference between them
is the absence of a "background" magnetic field in orphan penumbrae. This could
explain most of the observed differences. The fast flows we detect in orphan
penumbrae may be caused by the siphon flow mechanism. Based on the similarities
between orphan and regular penumbrae, we propose that the Evershed flow is also
a manifestation of siphon flows.Comment: 15 pages, 15 figure
Time and energy-resolved two photon-photoemission of the Cu(100) and Cu(111) metal surfaces
We present calculations on energy- and time-resolved two-photon photoemission
spectra of images states in Cu(100) and Cu(111) surfaces. The surface is
modeled by a 1D effective potential and the states are propagated within a
real-space, real-time method. To obtain the energy resolved spectra we employ a
geometrical approach based on a subdivision of space into two regions. We treat
electronic inelastic effects by taking into account the scattering rates
calculated within a GW scheme. To get further insight into the decaying
mechanism we have also studied the effect of the variation of the classical
Hartree potential during the excitation. This effect turns out to be small.Comment: 11 pages, 7 figure
Phenolics, depsides and triterpenes from the chilean lichen pseudocyphellaria nudata (zahlbr.) D.J. Galloway
Indexación: ScieloThe lichen Pseudocyphellaria nudata is a species endemic to southern South América. From the lichen tallus, methyl orsellinate, 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde, methyl-evernate, tenuiorin, hopan-6ß,22-diol and hopan-6α,76,22-triol were isolated and identified as the main lichen constituents. This is the first report of the occurrence of 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde in lichens.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=s0717-97072008000300017&nrm=is
A linear acoustic model for multi-cylinder IC engine intake manifolds including the effects of the intake throttle
This paper presents a linear acoustic model of a multi-cylinder intake manifold that can be used as part of a hybrid time/frequency domain method to calculate the intake wave dynamics of practical naturally aspirated engines.
The method allows the user to construct a model of almost any manifold of complex geometry. The model is constructed as an assemblage of sub-models:
(i) A model for a straight pipe with both ends open and through-flow.
(ii) A model for an expansion chamber consisting of three lengths of pipe laid end-to-end: a narrow bore pipe expanding into a wide bore pipe contracting into a narrower bore pipe once more.
(iii) A model of a side-branch, which includes a model for a straight pipe with one end closed and a model for the three way junction that joins the side-branch to a length of flow pipe.
(iv) A model for an expansion with two (or more) side-branches, which combines the sub-models (i, ii, iii) into a multi-way (n-way) junction model.
(v) A model for an intake throttle.
Good agreement with measurement has been found for each sub-model when bench-tested in isolation and encouraging agreement has been found when many sub-models are used together to model a complex intake manifold on a running engine
Enhancements to the GW space-time method
We describe the following new features which significantly enhance the power
of the recently developed real-space imaginary-time GW scheme (Rieger et al.,
Comp. Phys. Commun. 117, 211 (1999)) for the calculation of self-energies and
related quantities of solids: (i) to fit the smoothly decaying time/energy
tails of the dynamically screened Coulomb interaction and other quantities to
model functions, treating only the remaining time/energy region close to zero
numerically and performing the Fourier transformation from time to energy and
vice versa by a combination of analytic integration of the tails and
Gauss-Legendre quadrature of the remaining part and (ii) to accelerate the
convergence of the band sum in the calculation of the Green's function by
replacing higher unoccupied eigenstates by free electron states (plane waves).
These improvements make the calculation of larger systems (surfaces, clusters,
defects etc.) accessible.Comment: 10 pages, 6 figure
A TDDFT study of the excited states of DNA bases and their assemblies
We present a detailed study of the optical absorption spectra of DNA bases
and base pairs, carried out by means of time dependent density functional
theory. The spectra for the isolated bases are compared to available
theoretical and experimental data and used to assess the accuracy of the method
and the quality of the exchange-correlation functional: Our approach turns out
to be a reliable tool to describe the response of the nucleobases. Furthermore,
we analyze in detail the impact of hydrogen bonding and -stacking in the
calculated spectra for both Watson-Crick base pairs and Watson-Crick stacked
assemblies. We show that the reduction of the UV absorption intensity
(hypochromicity) for light polarized along the base-pair plane depends strongly
on the type of interaction. For light polarized perpendicular to the basal
plane, the hypochromicity effect is reduced, but another characteristic is
found, namely a blue shift of the optical spectrum of the base-assembly
compared to that of the isolated bases. The use of optical tools as
fingerprints for the characterization of the structure (and type of
interaction) is extensively discussed.Comment: 31 pages, 8 figure
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
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