1,796 research outputs found
Non-innocent side-chains with dipole moments in organic solar cells improve charge separation
Providing sustainable energy is one of the biggest challenges nowadays. An attractive answer is the use of organic solar cells to capture solar energy. Recently a promising route to increase their efficiency has been suggested: developing new organic materials with a high dielectric constant. This solution focuses on lowering the coulomb attraction between electrons and holes, thereby increasing the yield of free charges. In here, we demonstrate from a theoretical point of view that incorporation of dipole moments in organic materials indeed lowers the coulomb attraction. A combination of molecular dynamics simulations for modelling the blend and ab initio quantum chemical calculations to study specific regions was performed. This approach gives predictive insight in the suitability of new materials for application in organic solar cells. In addition to all requirements that make conjugated polymers suitable for application in organic solar cells, this study demonstrates the importance of large dipole moments in polymer side-chains
Raise and Peel Models of fluctuating interfaces and combinatorics of Pascal's hexagon
The raise and peel model of a one-dimensional fluctuating interface (model A)
is extended by considering one source (model B) or two sources (model C) at the
boundaries. The Hamiltonians describing the three processes have, in the
thermodynamic limit, spectra given by conformal field theory. The probability
of the different configurations in the stationary states of the three models
are not only related but have interesting combinatorial properties. We show
that by extending Pascal's triangle (which gives solutions to linear relations
in terms of integer numbers), to an hexagon, one obtains integer solutions of
bilinear relations. These solutions give not only the weights of the various
configurations in the three models but also give an insight to the connections
between the probability distributions in the stationary states of the three
models. Interestingly enough, Pascal's hexagon also gives solutions to a
Hirota's difference equation.Comment: 33 pages, an abstract and an introduction are rewritten, few
references are adde
A refined Razumov-Stroganov conjecture II
We extend a previous conjecture [cond-mat/0407477] relating the
Perron-Frobenius eigenvector of the monodromy matrix of the O(1) loop model to
refined numbers of alternating sign matrices. By considering the O(1) loop
model on a semi-infinite cylinder with dislocations, we obtain the generating
function for alternating sign matrices with prescribed positions of 1's on
their top and bottom rows. This seems to indicate a deep correspondence between
observables in both models.Comment: 21 pages, 10 figures (3 in text), uses lanlmac, hyperbasics and epsf
macro
The Multiplicity of Eigenvalues of the Hodge Laplacian on 5-Dimensional Compact Manifolds
We study multiplicity of the eigenvalues of the Hodge Laplacian on smooth,
compact Riemannian manifolds of dimension five for generic families of metrics.
We prove that generically the Hodge Laplacian, restricted to the subspace of
co-exact two-forms, has nonzero eigenvalues of multiplicity two. The proof is
based on the fact that Hodge Laplacian restricted to the subspace of co-exact
two-forms is minus the square of the Beltrami operator, a first-order operator.
We prove that for generic metrics the spectrum of the Beltrami operator is
simple. Because the Beltrami operator in this setting is a skew-adjoint
operator, this implies the main result for the Hodge Laplacian
Finite-size left-passage probability in percolation
We obtain an exact finite-size expression for the probability that a
percolation hull will touch the boundary, on a strip of finite width. Our
calculation is based on the q-deformed Knizhnik--Zamolodchikov approach, and
the results are expressed in terms of symplectic characters. In the large size
limit, we recover the scaling behaviour predicted by Schramm's left-passage
formula. We also derive a general relation between the left-passage probability
in the Fortuin--Kasteleyn cluster model and the magnetisation profile in the
open XXZ chain with diagonal, complex boundary terms.Comment: 21 pages, 8 figure
Bethe Ansatz Solution of the Asymmetric Exclusion Process with Open Boundaries
We derive the Bethe ansatz equations describing the complete spectrum of the
transition matrix of the partially asymmetric exclusion process with the most
general open boundary conditions. For totally asymmetric diffusion we calculate
the spectral gap, which characterizes the approach to stationarity at large
times. We observe boundary induced crossovers in and between massive, diffusive
and KPZ scaling regimes.Comment: 4 pages, 2 figures, published versio
Conformal invariance and its breaking in a stochastic model of a fluctuating interface
Using Monte-Carlo simulations on large lattices, we study the effects of
changing the parameter (the ratio of the adsorption and desorption rates)
of the raise and peel model. This is a nonlocal stochastic model of a
fluctuating interface. We show that for the system is massive, for
it is massless and conformal invariant. For the conformal
invariance is broken. The system is in a scale invariant but not conformal
invariant phase. As far as we know it is the first example of a system which
shows such a behavior. Moreover in the broken phase, the critical exponents
vary continuously with the parameter . This stays true also for the critical
exponent which characterizes the probability distribution function of
avalanches (the critical exponent staying unchanged).Comment: 22 pages and 20 figure
Relaxation rate of the reverse biased asymmetric exclusion process
We compute the exact relaxation rate of the partially asymmetric exclusion
process with open boundaries, with boundary rates opposing the preferred
direction of flow in the bulk. This reverse bias introduces a length scale in
the system, at which we find a crossover between exponential and algebraic
relaxation on the coexistence line. Our results follow from a careful analysis
of the Bethe ansatz root structure.Comment: 22 pages, 12 figure
Autocorrelations in the totally asymmetric simple exclusion process and Nagel-Schreckenberg model
We study via Monte Carlo simulation the dynamics of the Nagel-Schreckenberg
model on a finite system of length L with open boundary conditions and parallel
updates. We find numerically that in both the high and low density regimes the
autocorrelation function of the system density behaves like 1-|t|/tau with a
finite support [-tau,tau]. This is in contrast to the usual exponential decay
typical of equilibrium systems. Furthermore, our results suggest that in fact
tau=L/c, and in the special case of maximum velocity 1 (corresponding to the
totally asymmetric simple exclusion process) we can identify the exact
dependence of c on the input, output and hopping rates. We also emphasize that
the parameter tau corresponds to the integrated autocorrelation time, which
plays a fundamental role in quantifying the statistical errors in Monte Carlo
simulations of these models.Comment: 7 pages, 6 figure
On the relation between local and charge-transfer exciton binding energies in organic photovoltaic materials
In organic photovoltaic devices two types of excitons can be generated for which different binding energies can be defined: the binding energy of the local exciton generated immediately after light absorption on the polymer and the binding energy of the charge-transfer exciton generated through the electron transfer from polymer to PCBM. Lowering these two binding energies is expected to improve the efficiency of the devices. Using (time-dependent) density functional theory, we studied whether a relation exists between the two different binding energies. For a series of related co-monomers, we found that the local exciton binding energy on a monomer is not directly related to that of the charge-transfer exciton on a monomer-PCBM complex because the variation in exciton binding energy depends mainly on the variation in electron affinity, which does not affect in a direct way the charge-transfer exciton binding energy. Furthermore, for the studied co-monomers and their corresponding trimers, we provide detailed information on the amount of charge transfer upon excitation and on the charge transfer excitation length. This detailed study of the excitation process reveals that the thiophene unit that links the donor and acceptor fragments of the co-monomer actively participates in the charge transfer process
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