70 research outputs found
Thermal effects on CHNHPbI perovskite from ab-initio molecular dynamics simulations
We present a molecular dynamics simulation study of CHNHPbI based
on forces calculated from density functional theory. The simulation were
performed on model systems having 8 and 27 unit cells, and for a total
simulation time of 40 ps in each case. Analysis of the finite size effects, in
particular the mobility of the organic component, suggests that the smaller
system is over correlated through the long range electrostatic interaction. In
the larger system this finite size artifact is relaxed producing a more
reliable description of the anisotropic rotational behavior of the methyl
ammonium molecules. The thermal effects on the optical properties of the system
were also analyzed. The HOMO-LUMO energy gap fluctuates around its central
value with a standard deviation of approximately 0.1 eV. The projected density
of states consistently place the Fermi level on the orbitals of the I
atoms, and the lowest virtual state on orbitals of the Pb atoms throughout
the whole simulation trajectory.Comment: 16 pages, 11 figure
Quantum Confinement and Negative Heat Capacity
Thermodynamics dictates that the specific heat of a system is strictly
non-negative. However, in finite classical systems there are well known
theoretical and experimental cases where this rule is violated, in particular
finite atomic clusters. Here, we show for the first time that negative heat
capacity can also occur in finite quantum systems. The physical scenario on
which this effect might be experimentally observed is discussed. Observing such
an effect might lead to the design of new light harvesting nano devices, in
particular a solar nano refrigerator.Comment: 8 pages, 5 figure
Molecular kinetics of solid and liquid CHCl
We present a detailed analysis of the molecular kinetics of CHCl in a
range of temperatures covering the solid and liquid phases. Using nuclear
quadrupolar resonance we determine the relaxation times for the molecular
rotations in solid at pre-melting conditions. Molecular dynamics simulations
are used to characterize the rotational dynamics in the solid and liquid phases
and to study the local structure of the liquid in terms of the molecular
relative orientations. We find that in the pre-melting regime the molecules
rotate about the C-H bond, but the rotations are isotropic in the liquid, even
at supercooled conditions.Comment: Chemical Physics Letter (in press). 14 pages, 7 figure
Self-assembly of pseudo-dipolar nanoparticles at low densities and strong coupling
Nanocolloids having directional interactions are highly relevant for designing new self-assembled materials easy to control. In this article we report stochastic dynamics simulations of finite-size pseudo-dipolar colloids immersed in an implicit dielectric solvent using a realistic continuous description of the quasi-hard Coulombic interaction. We investigate structural and dynamical properties near the low-temperature and highly-diluted limits. This system self-assembles in a rich variety of string-like configurations, depicting three clearly distinguishable regimes with decreasing temperature: fluid, composed by isolated colloids; string-fluid, a gas of short string-like clusters; and string-gel, a percolated network. By structural characterization using radial distribution functions and cluster properties, we calculate the state diagram, verifying the presence of string-fluid regime. Regarding the string-gel regime, we show that the antiparallel alignment of the network chains arises as a novel self-assembly mechanism when the characteristic interaction energy exceeds the thermal energy in two orders of magnitude, ud/kBT ≈ 100. This is associated to relevant structural modifications in the network connectivity and porosity. Furthermore, our results give insights about the dynamically-arrested nature of the string-gel regime, where we show that the slow relaxation takes place in minuscule energy steps that reflect local rearrangements of the network.Fil: Brito, Mariano Exequiel. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Carignano, Marcelo A.. Qatar Environment And Energy Research Institute; QatarFil: Marconi, Veronica Iris. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentin
The water supercooled regime as described by four common water models
The temperature scale of simple water models in general does not coincide
with the natural one. Therefore, in order to make a meaningful evaluation of
different water models a temperature rescaling is necessary. In this paper we
introduce a rescaling using the melting temperature and the temperature
corresponding to the maximum of the heat capacity to evaluate four common water
models (TIP4P-Ew, TIP4P-2005, TIP5P-Ew and Six-Sites) in the supercooled
regime. Although all the models show the same general qualitative behavior, the
TIP5P-Ew appears as the best representation of the supercooled regime when the
rescaled temperature is used. We also analyze, using thermodynamic arguments,
the critical nucleus size for ice growth. Finally, we speculate on the possible
reasons why atomistic models do not usually crystalize while the coarse grained
mW model do crystallize.Comment: 8 pages, 8 figure
Contribución de cadenas pendientes al módulo viscoelástico de gomas (en aproximación lineal)
Se estudia la contribución de una cadena pendiente al espectro de relajación de una red perfecta monofuncional, variando la relación entre la longitud de dicha cadena y la de los tramos de la red, y la funcionalidad de ésta. El espectro de relajación se calcula como en la ref. 1 a partir de los autovalores de la matriz de conectividad de la red, esta vez acoplada con la cadena.
En contraste con1, que muestra la contribución a dicho espectro de una “cáscara” suficientemente alejada del borde en el límite de red infinita, en nuestro trabajo encontramos el espectro completo (para una red cuyo tamaño está limitado por la capacidad de la VAX 780) que es cualitativamente diferente del mostrado en1 y permite cálculos del módulo viscoelástico complejo susceptibles de comparación con datos experimentales.
En particular encontramos que los tiempos de relajación más largos están asociados a la presencia de la cadena pendiente y aumentan con la longitud de ésta, siguiendo una ley de escala tipo potencia. El exponente obtenido dista mucho del medido (que es aproximadamente 3,4) indicando la necesidad de introducir en el cálculo la contribución de los entrelazamientos entre cadenas.Facultad de Ciencias Exacta
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