11 research outputs found
Renormalization Group theory outperforms other approaches in statistical comparison between upscaling techniques for porous media
Determining the pressure differential required to achieve a desired flow rate
in a porous medium requires solving Darcy's law, a Laplace-like equation, with
a spatially varying tensor permeability. In various scenarios, the permeability
coefficient is sampled at high spatial resolution, which makes solving Darcy's
equation numerically prohibitively expensive. As a consequence, much effort has
gone into creating upscaled or low-resolution effective models of the
coefficient while ensuring that the estimated flow rate is well reproduced,
bringing to fore the classic tradeoff between computational cost and numerical
accuracy. Here we perform a statistical study to characterize the relative
success of upscaling methods on a large sample of permeability coefficients
that are above the percolation threshold. We introduce a new technique based on
Mode-Elimination Renormalization-Group theory (MG) to build coarse-scale
permeability coefficients. Comparing the results with coefficients upscaled
using other methods, we find that MG is consistently more accurate,
particularly so due to its ability to address the tensorial nature of the
coefficients. MG places a low computational demand, in the manner that we have
implemented it, and accurate flow-rate estimates are obtained when using
MG-upscaled permeabilities that approach or are beyond the percolation
threshold.Comment: 15 pages, 7 figures, Physical Review
Magnetization Properties of Some Quantum Spin Ladders
The experimental realization of various spin ladder systems has prompted
their detailed theoretical investigations. Here we study the evolution of
ground state magnetization with an external magnetic field for two different
antiferromagnetic systems: a three-legged spin-1/2 ladder, and a two-legged
spin-1/2 ladder with an additional diagonal interaction. The finite system
density-matrix renormalization group method is employed for numerical studies
of the three-chain system, and an effective low-energy Hamiltonian is used in
the limit of strong interchain coupling to study the two- and three-chain
systems. The three-chain system has a magnetization plateau at one-third of the
saturation magnetization. The two-chain system has a plateau at zero
magnetization due to a gap above the singlet ground state. It also has a
plateau at half of the saturation magnetization for a certain range of values
of the couplings. We study the regions of transitions between plateaus
numerically and analytically, and find that they are described, at first order
in a strong-coupling expansion, by an XXZ spin-1/2 chain in a magnetic field;
the second order terms give corrections to the XXZ model. We also study
numerically some low-temperature properties of the three-chain system, such as
the magnetization, magnetic susceptibility and specific heat.Comment: Revtex, 26 pages including 17 epsf figures; a few minor changes; this
is the final published versio
Electron correlation effects in electron-hole recombination in organic light-emitting diodes
We develop a general theory of electron--hole recombination in organic light
emitting diodes that leads to formation of emissive singlet excitons and
nonemissive triplet excitons. We briefly review other existing theories and
show how our approach is substantively different from these theories. Using an
exact time-dependent approach to the interchain/intermolecular charge-transfer
within a long-range interacting model we find that, (i) the relative yield of
the singlet exciton in polymers is considerably larger than the 25% predicted
from statistical considerations, (ii) the singlet exciton yield increases with
chain length in oligomers, and, (iii) in small molecules containing nitrogen
heteroatoms, the relative yield of the singlet exciton is considerably smaller
and may be even close to 25%. The above results are independent of whether or
not the bond-charge repulsion, X_perp, is included in the interchain part of
the Hamiltonian for the two-chain system. The larger (smaller) yield of the
singlet (triplet) exciton in carbon-based long-chain polymers is a consequence
of both its ionic (covalent) nature and smaller (larger) binding energy. In
nitrogen containing monomers, wavefunctions are closer to the noninteracting
limit, and this decreases (increases) the relative yield of the singlet
(triplet) exciton. Our results are in qualitative agreement with
electroluminescence experiments involving both molecular and polymeric light
emitters. The time-dependent approach developed here for describing
intermolecular charge-transfer processes is completely general and may be
applied to many other such processes.Comment: 19 pages, 11 figure
Electron correlation effects in electron–hole recombination and triplet–triplet scattering in organic light emitting diodes
Using a time-dependent model many-body formulation, we follow the electron–hole recombination between a pair of oppositely charged polyene chains and triplet–triplet (T–T) scattering in -conjugated systems. Electron correlations reduce the overall yields and also lead to large differences between the singlet and triplet yields. External electric field has a strong influence on the recombination products. The fraction of singlet yield, η, increases with the chain length. Heteroatoms also change this ratio significantly, as has been observed experimentally. The outcome of the triplet–triplet scattering is also significantly affected by electron correlations
Electron correlation effects in electron–hole recombination and triplet–triplet scattering in organic light emitting diodes
Using a time-dependent model many-body formulation, we follow the electron–hole recombination between a pair of oppositely charged polyene chains and triplet–triplet (T-T) scattering in π-conjugated systems. Electron correlations reduce the overall yields and also lead to large differences between the singlet and triplet yields. External electric field has a strong influence on the recombination products. The fraction of singlet yield, η, increases with the chain length. Heteroatoms also change this ratio significantly, as has been observed experimentally. The outcome of the triplet–triplet scattering is also significantly affected by electron correlations
Symmetrized DMRG studies of low-energy electronic states of poly-para-phenylene (PPP) and poly-para-phenylenevinylene (PPV) systems
Symmetrized DMRG calculations on long oligomers of poly- para-phenylene (PPP) and poly-para-phenylene vinylene (PPV) systems within a `U-V' model have been carried out to obtain the one-photon, two-photon and singlet-triplet gaps in these systems. The extrapolated gaps (in eV) are 2.89, 3.76 and 2.72 in PPP and 3.01, 3.61 and 2.23 in PPV for the one- photon, two-photon and spin gaps respectively. By studying doped systems, we have obtained the exciton binding energies. The larger exciton binding energies, compared to strongly dimerized linear chains emphasizes the role of topology in these polymers. Bond orders, charge and spin correlations in the low-lying states bring out the similarities between the lowest one-photon, the lowest triplet and the lowest bipolaronic states in these systems. The two-photon state bond orders show evidence for strong localization of this excitation in both PPP and PPV systems
Formation cross-sections of singlet and triplet excitons in pi-conjugated polymers
Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought(1) to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, sigma (s), is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, sigma (T); that is, sigma (S)/sigma (T) approximate to 1. However, recent experimental(2) and theoretical(3) work suggests that sigma (S)/sigma (T) may be greater than 1. Here we report direct measurements of sigma (S)/sigma (T) for a large number of pi -conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of sigma (S)/sigma (T) on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated pi -electrons, and quantify this process using exact valence bond calculations. The calculated sigma (S)/sigma (T) reproduces the experimentally observed trend. The calculations also show that the strong dependence of sigma (S)/sigma (T) on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process
Method of Moments for Computational Microemulsion Analysis and Prediction in Tertiary Oil Recovery
We discuss the application of Helfrich’s surface
torque
density concept to microemulsion design and analysis from three different
angles: (i) from the point of view of coarse-grained molecular simulations,
using Dissipative Particle Dynamics, including charge interactions
and added salt, (ii) using an approximate double-film model for the
surface, and (iii) comparison with formulation approaches. The simulations
use that the surface torque can be calculated unambiguously from the
stress profile, provided the surface is tensionless. Very good agreement
is found on predicting optimal salinity (or the absence of that) for
a range of surfactants: dioctyl sodium sulfosuccinate, various twin-tailed
sulfonates and sodium dodecyl sulfate. The simulations are very fast,
on par with times for experiments, thus they could lead to a practical
tool for discovery of more efficient surfactants, although much remains
to be done with respect to other important variables: oil composition,
surfactant mixtures, aggregation in solution, and so on. The microscopic
model (second approach) is highly approximate: it is essentially based
on two opposing swelling tendencies, that are both of osmotic nature.
In accordance with the model, the tails are swollen by the oil and
the charged head groups are confined in a salty layer in Donnan equilibrium
with the salt solution. In this way, the surface interactions are
purely entropic. The comparison of the film model with existing formulation
approaches (third approach) covers the interfacial tension minimum,
Winsor R theory, quantitative structure property relations (QSPR),
hydrophilic–lipophilic deviation (HLD), HLD-net average curvature,
and temperature coefficients. Using the surface torque analysis, we
succeed in deriving in an ab initio way QSPR empirical coefficients
that have been known for decades, but until now, have been obscure
in origin