13,225 research outputs found
Fungos micorrízicos arbusculares em diferentes agroecossistemas na região do Baixo Amazonas.
bitstream/item/57631/1/CPATU-PA195.pd
Equilibrium Shape and Size of Supported Heteroepitaxial Nanoislands
We study the equilibrium shape, shape transitions and optimal size of
strained heteroepitaxial nanoislands with a two-dimensional atomistic model
using simply adjustable interatomic pair potentials. We map out the global
phase diagram as a function of substrate-adsorbate misfit and interaction. This
phase diagram reveals all the phases corresponding to different well-known
growth modes. In particular, for large enough misfits and attractive substrate
there is a Stranski-Krastanow regime, where nano-sized islands grow on top of
wetting films. We analyze the various terms contributing to the total island
energy in detail, and show how the competition between them leads to the
optimal shape and size of the islands. Finally, we also develop an analytic
interpolation formula for the various contributions to the total energy of
strained nanoislands.Comment: 9 pages, 7 figure
X-ray Lags in PDS 456 Revealed by Suzaku Observations
X-ray reverberation lags from the vicinity of supermassive black holes have
been detected in almost 30 AGN. The soft lag, which is the time delay between
the hard and soft X-ray light curves, is usually interpreted as the time
difference between the direct and reflected emission, but is alternatively
suggested to arise from the direct and scattering emission from distant clouds.
By analysing the archival Suzaku observations totalling an exposure time of ~
770 ks, we discover a soft lag of ks at Hz in
the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier
frequency reported to date. In this study, we use the maximum likelihood method
to deal with non-continuous nature of the Suzaku light curves. The result
follows the mass-scaling relation for soft lags, which further supports that
soft lags originate from the innermost areas of AGN and hence are best
interpreted by the reflection scenario. Spectral analysis has been performed in
this work and we find no evidence of clumpy partial-covering absorbers. The
spectrum can be explained by a self-consistent relativistic reflection model
with warm absorbers, and spectral variations over epochs can be accounted for
by the change of the continuum, and of column density and ionization states of
the warm absorbers.Comment: accepted for publication in MNRA
Dynamical transitions and sliding friction of the phase-field-crystal model with pinning
We study the nonlinear driven response and sliding friction behavior of the
phase-field-crystal (PFC) model with pinning including both thermal
fluctuations and inertial effects. The model provides a continuous description
of adsorbed layers on a substrate under the action of an external driving force
at finite temperatures, allowing for both elastic and plastic deformations. We
derive general stochastic dynamical equations for the particle and momentum
densities including both thermal fluctuations and inertial effects. The
resulting coupled equations for the PFC model are studied numerically. At
sufficiently low temperatures we find that the velocity response of an
initially pinned commensurate layer shows hysteresis with dynamical melting and
freezing transitions for increasing and decreasing applied forces at different
critical values. The main features of the nonlinear response in the PFC model
are similar to the results obtained previously with molecular dynamics
simulations of particle models for adsorbed layers.Comment: 7 pages, 8 figures, to appear in Physcial Review
Hydrodynamical Simulations of the Barred Spiral Galaxy NGC 1097
NGC 1097 is a nearby barred spiral galaxy believed to be interacting with the
elliptical galaxy NGC 1097A located to its northwest. It hosts a Seyfert 1
nucleus surrounded by a circumnuclear starburst ring. Two straight dust lanes
connected to the ring extend almost continuously out to the bar. The other ends
of the dust lanes attach to two main spiral arms. To provide a physical
understanding of its structural and kinematical properties, two-dimensional
hydrodynamical simulations have been carried out. Numerical calculations reveal
that many features of the gas morphology and kinematics can be reproduced
provided that the gas flow is governed by a gravitational potential associated
with a slowly rotating strong bar. By including the self-gravity of the gas
disk in our calculation, we have found the starburst ring to be gravitationally
unstable which is consistent with the observation in \citet{hsieh11}. Our
simulations show that the gas inflow rate is 0.17 M_\sun yr into the
region within the starburst ring even after its formation, leading to the
coexistence of both a nuclear ring and a circumnuclear disk.Comment: 32 pages, 14 figures, 1 table, accepted for publication in the Ap
Propagação de tomate a partir da ponteira enraizada.
bitstream/item/64482/1/Oriental-RecBas4.pd
Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands
We study numerically the equilibrium shapes, shape transitions and
dislocation nucleation of small strained epitaxial islands with a
two-dimensional atomistic model, using simple interatomic pair potentials. We
first map out the phase diagram for the equilibrium island shapes as a function
of island size (up to N = 105 atoms) and lattice misfit with the substrate and
show that nanoscopic islands have four generic equilibrium shapes, in contrast
with predictions from the continuum theory of elasticity. For increasing
substrate-adsorbate attraction, we find islands that form on top of a finite
wetting layer as observed in Stranski-Krastanow growth. We also investigate
energy barriers and transition paths for transitions between different shapes
of the islands and for dislocation nucleation in initially coherent islands. In
particular, we find that dislocations nucleate spontaneously at the edges of
the adsorbate-substrate interface above a critical size or lattice misfit.Comment: 4 pages, 3 figures, uses wrapfig.sty and epsfig.st
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