1,219 research outputs found
Transition from accelerated to decelerated regimes in JT and CGHS cosmologies
In this work we discuss the possibility of positive-acceleration regimes, and
their transition to decelerated regimes, in two-dimensional (2D) cosmological
models. We use general relativity and the thermodynamics in a 2D space-time,
where the gas is seen as the sources of the gravitational field. An
early-Universe model is analyzed where the state equation of van der Waals is
used, replacing the usual barotropic equation. We show that this substitution
permits the simulation of a period of inflation, followed by a
negative-acceleration era. The dynamical behavior of the system follows from
the solution of the Jackiw-Teitelboim equations (JT equations) and the
energy-momentum conservation laws. In a second stage we focus the
Callan-Giddings-Harvey-Strominger model (CGHS model); here the transition from
the inflationary period to the decelerated period is also present between the
solutions, although this result depend strongly on the initial conditions used
for the dilaton field. The temporal evolution of the cosmic scale function, its
acceleration, the energy density and the hydrostatic pressure are the physical
quantities obtained in through the analysis.Comment: To appear in Europhysics Letter
Response of a catalytic reaction to periodic variation of the CO pressure: Increased CO_2 production and dynamic phase transition
We present a kinetic Monte Carlo study of the dynamical response of a
Ziff-Gulari-Barshad model for CO oxidation with CO desorption to periodic
variation of the CO presure. We use a square-wave periodic pressure variation
with parameters that can be tuned to enhance the catalytic activity. We produce
evidence that, below a critical value of the desorption rate, the driven system
undergoes a dynamic phase transition between a CO_2 productive phase and a
nonproductive one at a critical value of the period of the pressure
oscillation. At the dynamic phase transition the period-averged CO_2 production
rate is significantly increased and can be used as a dynamic order parameter.
We perform a finite-size scaling analysis that indicates the existence of
power-law singularities for the order parameter and its fluctuations, yielding
estimated critical exponent ratios and . These exponent ratios, together with theoretical symmetry
arguments and numerical data for the fourth-order cumulant associated with the
transition, give reasonable support for the hypothesis that the observed
nonequilibrium dynamic phase transition is in the same universality class as
the two-dimensional equilibrium Ising model.Comment: 18 pages, 10 figures, accepted in Physical Review
Inflationary and dark energy regimes in 2+1 dimensions
In this work we investigate the behavior of three-dimensional (3D)
cosmological models. The simulation of inflationary and dark-energy-dominated
eras are among the possible results in these 3D formulations; taking as
starting point the results obtained by Cornish and Frankel.
Motivated by those results, we investigate, first, the inflationary case
where we consider a two-constituent cosmological fluid: the scalar field
represents the hypothetical inflaton which is in gravitational interaction with
a matter/radiation contribution. For the description of an old universe, it is
possible to simulate its evolution starting with a matter dominated universe
that faces a decelerated/accelerated transition due to the presence of the
additional constituent (simulated by the scalar field or ruled by an exotic
equation of state) that plays the role of dark energy. We obtain, through
numerical analysis, the evolution in time of the scale factor, the
acceleration, the energy densities, and the hydrostatic pressure of the
constituents. The alternative scalar cosmology proposed by Cornish and Frankel
is also under investigation in this work. In this case an inflationary model
can be constructed when another non-polytropic equation of state (the van der
Waals equation) is used to simulate the behavior of an early 3D universe.Comment: Latex file, plus 9 figures. To appear in General Relativity and
Gravitatio
Oxidation of allylic and benzylic alcohols to aldehydes and carboxylic acids
An oxidation of allylic and benzylic alcohols to the corresponding carboxylic
acids is effected by merging a Cu-catalyzed oxidation using O2 as a terminal
oxidant with a subsequent chlorite oxidation (Lindgren oxidation). The
protocol was optimized to obtain pure products without chromatography or
crystallization. Interception at the aldehyde stage allowed for
Z/E-isomerization, thus rendering the oxidation stereoconvergent with respect
to the configuration of the starting material
Decay of metastable phases in a model for the catalytic oxidation of CO
We study by kinetic Monte Carlo simulations the dynamic behavior of a
Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O
CO on a catalytic surface. Finite-size scaling analysis of the fluctuations
and the fourth-order order-parameter cumulant show that below a critical CO
desorption rate, the model exhibits a nonequilibrium first-order phase
transition between low and high CO coverage phases. We calculate several points
on the coexistence curve. We also measure the metastable lifetimes associated
with the transition from the low CO coverage phase to the high CO coverage
phase, and {\it vice versa}. Our results indicate that the transition process
follows a mechanism very similar to the decay of metastable phases associated
with {\it equilibrium} first-order phase transitions and can be described by
the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by
nucleation and growth. In the present case, the desorption parameter plays the
role of temperature, and the distance to the coexistence curve plays the role
of an external field or supersaturation. We identify two distinct regimes,
depending on whether the system is far from or close to the coexistence curve,
in which the statistical properties and the system-size dependence of the
lifetimes are different, corresponding to multidroplet or single-droplet decay,
respectively. The crossover between the two regimes approaches the coexistence
curve logarithmically with system size, analogous to the behavior of the
crossover between multidroplet and single-droplet metastable decay near an
equilibrium first-order phase transition.Comment: 27 pages, 22 figures, accepted by Physical Review
Energetics and Vibrational States for Hydrogen on Pt(111)
We present a combination of theoretical calculations and experiments for the
low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111)
surface. The vibrational band states are calculated based on the full
three-dimensional adiabatic potential energy surface obtained from first
principles calculations. For coverages less than three quarters of a monolayer,
the observed experimental high-resolution electron peaks at 31 and 68meV are in
excellent agreement with the theoretical transitions between selected bands.
Our results convincingly demonstrate the need to go beyond the local harmonic
oscillator picture to understand the dynamics of this system.Comment: In press at Phys. Rev. Lett - to appear in April 200
The Static and Dynamic Lattice Changes Induced by Hydrogen Adsorption on NiAl(110)
Static and dynamic changes induced by adsorption of atomic hydrogen on the
NiAl(110) lattice at 130 K have been examined as a function of adsorbate
coverage. Adsorbed hydrogen exists in three distinct phases. At low coverages
the hydrogen is itinerant because of quantum tunneling between sites and
exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate
mediated interactions produce an ordered superstructure with c(2x2) symmetry,
and at higher coverages, hydrogen exists as a disordered lattice gas. This
picture of how hydrogen interacts with NiAl(110) is developed from our data and
compared to current theoretical predictions.Comment: 36 pages, including 12 figures, 2 tables and 58 reference
Senescence Is the Main Trait Induced by Temozolomide in Glioblastoma Cells.
First-line drug in the treatment of glioblastoma, the most severe brain cancer, is temozolomide (TMZ), a DNA-methylating agent that induces the critical damage O <sup>6</sup> -methylguanine (O <sup>6</sup> MeG). This lesion is cytotoxic through the generation of mismatch repair-mediated DNA double-strand breaks (DSBs), which trigger apoptotic pathways. Previously, we showed that O <sup>6</sup> MeG also induces cellular senescence (CSEN). Here, we show that TMZ-induced CSEN is a late response which has similar kinetics to apoptosis, but at a fourfold higher level. CSEN cells show a high amount of DSBs, which are located outside of telomeres, a high level of ROS and oxidized DNA damage (8-oxo-guanine), and sustained activation of the DNA damage response and histone methylation. Despite the presence of DSBs, CSEN cells are capable of repairing radiation-induced DSBs. Glioblastoma cells that acquired resistance to TMZ became simultaneously resistant to TMZ-induced CSEN. Using a Tet-On glioblastoma cell system, we show that upregulation of MGMT immediately after TMZ completely abrogated apoptosis and CSEN, while induction of MGMT long-term (&gt;72 h) after TMZ did not reduce apoptosis and CSEN. Furthermore, upregulation of MGMT in the senescent cell population had no impact on the survival of senescent cells, indicating that O <sup>6</sup> MeG is required for induction, but not for maintenance of the senescent state. We further show that, in recurrent GBM specimens, a significantly higher level of DSBs and CSEN-associated histone H3K27me3 was observed than in the corresponding primary tumors. Overall, the data indicate that CSEN is a key node induced in GBM following chemotherapy
Interpenetrating Self-Supporting Networks from Anisotropic Semiconductor Nanoparticles and Noble Metal Nanowires
In this work, a new type of multicomponent nanostructures is introduced by forming interpenetrating networks of two different nanomaterials. In detail, gel networks from semiconductor nanorods are interpenetrated by Au nanowires. Two different types of gelling agents, namely S2− and Yb3+, are employed to trigger the network formation. The structural and electrochemical properties of the resulting materials are discussed. (Photo)electrochemical measurements are performed on the structures to compare the materials in terms of their conductivity as well as their efficiency in converting photonic energy to electrical energy. The new type of CdSe/CdS:Au nanostructure gelled with S2− shows one order of magnitude higher photocurrent than the system gelled with Yb3+. Moreover, the introduction of Au nanowires exhibit a photocurrent which is two orders of magnitudes higher than in samples without Au nanowires
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