437 research outputs found
Computer simulation of pulsed field gel runs allows the quantitation of radiation-induced double-strand breaks in yeast
A procedure for the quantification of double-strand breaks in yeast is presented that utilizes pulsed field gel electrophoresis (PFGE) and a comparison of the observed DNA mass distribution in the gel lanes with calculated distributions. Calculation of profiles is performed as follows. If double-strand breaks are produced by sparsely ionizing radiation, one can assume that they are distributed randomly in the genome, and the resulting DNA mass distribution in molecular length can be predicted by means of a random breakage model. The input data for the computation of molecular length profiles are the breakage frequency per unit length, , as adjustable parameter, and the molecular lengths of the intact chromosomes. The obtained DNA mass distributions in molecular length must then be transformed into distributions of DNA mass in migration distance. This requires a calibration of molecular length vs. migration distance that is specific for the gel lane in question. The computed profiles are then folded with a Lorentz distribution with adjusted spread parameter to account for and broadening. The DNA profiles are calculated for different breakage frequencies and for different values of , and the parameters resulting in the best fit of the calculated to the observed profile are determined
Charge density dependent nongeminate recombination in organic bulk heterojunction solar cells
Apparent recombination orders exceeding the value of two expected for
bimolecular recombination have been reported for organic solar cells in various
publications. Two prominent explanations are bimolecular losses with a carrier
concentration dependent prefactor due to a trapping limited mobility, and
protection of trapped charge carriers from recombination by a donor--acceptor
phase separation until reemission from these deep states. In order to clarify
which mechanism is dominant we performed temperature and illumination dependent
charge extraction measurements under open circuit as well as short circuit
conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-Cbutyric acid
methyl ester (P3HT:PCBM) and PTB7:PCBM
(Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]])
solar cells in combination with current--voltage characteristics. We show that
the charge carrier density dependence of the mobility and the
recombination prefactor are different for PCBM at temperatures below
300K and PTB7:PCBM at room temperature. Therefore, in addition to
a detrapping limited recombination in systems with at least partial
donor--acceptor phase separation is required to explain the high recombination
orders.Comment: 7 pages, 4 figure
Interfaces in partly compatible polymer mixtures: A Monte Carlo simulation approach
The structure of polymer coils near interfaces between coexisting phases of
symmetrical polymer mixtures (AB) is discussed, as well as the structure of
symmetric diblock copolymers of the same chain length N adsorbed at the
interface. The problem is studied by Monte Carlo simulations of the bond
fluctuation model on the simple cubic lattice, using massively parallel
computers (CRAY T3D). While homopolymer coils in the strong segregation limit
are oriented parallel to the interface, the diblocks form ``dumbbells''
oriented perpendicular to the interface. However, in the dilute case
(``mushroom regime'' rather than ``brush regime''), the diblocks are only
weakly stretched. Distribution functions for monomers at the chain ends and in
the center of the polymer are obtained, and a comparison to the self consistent
field theory is made.Comment: to appear in Physica
Synchronous vs. asynchronous dynamics of diffusion-controlled reactions
An analytical method based on the classical ruin problem is developed to
compute the mean reaction time between two walkers undergoing a generalized
random walk on a 1d lattice. At each time step, either both walkers diffuse
simultaneously with probability (synchronous event) or one of them diffuses
while the other remains immobile with complementary probability (asynchronous
event). Reaction takes place through same site occupation or position exchange.
We study the influence of the degree of synchronicity of the walkers and
the lattice size on the global reaction's efficiency. For odd , the
purely synchronous case () is always the most effective one, while for
even , the encounter time is minimized by a combination of synchronous and
asynchronous events. This new parity effect is fully confirmed by Monte Carlo
simulations on 1d lattices as well as for 2d and 3d lattices. In contrast, the
1d continuum approximation valid for sufficiently large lattices predicts a
monotonic increase of the efficiency as a function of . The relevance of the
model for several research areas is briefly discussed.Comment: 21 pages (including 12 figures and 4 tables), uses revtex4.cls,
accepted for publication in Physica
Bulk Mediated Surface Diffusion: Non Markovian Desorption with Finite First Moment
Here we address a fundamental issue in surface physics: the dynamics of
adsorbed molecules. We study this problem when the particle's desorption is
characterized by a non Markovian process, while the particle's adsorption and
its motion in the bulk are governed by a Markovian dynamics. We study the
diffusion of particles in a semi-infinite cubic lattice, and focus on the
effective diffusion process at the interface . We calculate analytically
the conditional probability to find the particle on the plane as well as
the surface dispersion as functions of time. The comparison of these results
with Monte Carlo simulations show an excellent agreement.Comment: 16 pages, 7 figs. European Physical Journal B (in press
Charge photogeneration in few-layer MoS2
The two-dimensional semiconductor MoS2 in its mono- and few-layer form is
expected to have a significant exciton binding energy of several 100 meV,
leading to the consensus that excitons are the primary photoexcited species.
Nevertheless, even single layers show a strong photovoltaic effect and work as
the active material in high sensitivity photodetectors, thus indicating
efficient charge carrier photogeneration (CPG). Here we use continuous wave
photomodulation spectroscopy to identify the optical signature of long-lived
charge carriers and femtosecond pump-probe spectroscopy to follow the CPG
dynamics. We find that intitial photoexcitation yields a branching between
excitons and charge carriers, followed by excitation energy dependent hot
exciton dissociation as an additional CPG mechanism. Based on these findings,
we make simple suggestions for the design of more efficient MoS2 photovoltaic
and photodetector devices
Localization transition of random copolymers at interfaces
We consider adsorption of random copolymer chains onto an interface within
the model of Garel et al. Europhysics Letters 8, 9 (1989). By using the replica
method the adsorption of the copolymer at the interface is mapped onto the
problem of finding the ground state of a quantum mechanical Hamiltonian. To
study this ground state we introduce a novel variational principle for the
Green's function, which generalizes the well-known Rayleigh-Ritz method of
Quantum Mechanics to nonstationary states. Minimization with an appropriate
trial Green's function enables us to find the phase diagram for the
localization-delocalization transition for an ideal random copolymer at the
interface.Comment: 5 page
Stochastic processes with finite correlation time: modeling and application to the generalized Langevin equation
The kangaroo process (KP) is characterized by various forms of the covariance
and can serve as a useful model of random noises. We discuss properties of that
process for the exponential, stretched exponential and algebraic (power-law)
covariances. Then we apply the KP as a model of noise in the generalized
Langevin equation and simulate solutions by a Monte Carlo method. Some results
appear to be incompatible with requirements of the fluctuation-dissipation
theorem because probability distributions change when the process is inserted
into the equation. We demonstrate how one can construct a model of noise free
of that difficulty. This form of the KP is especially suitable for physical
applications.Comment: 22 pages (RevTeX) and 4 figure
Diblock copolymers at a homopolymer-homopolymer-interface: a Monte Carlo simulation
The properties of diluted symmetric A-B diblock copolymers at the interface
between A and B homopolymer phases are studied by means of Monte Carlo (MC)
simulations of the bond fluctuation model. We calculate segment density
profiles as well as orientational properties of segments, of A and B blocks,
and of the whole chain. Our data support the picture of oriented ``dumbbells'',
which consist of mildly perturbed A and B Gaussian coils. The results are
compared to a self consistent field theory (SCFT) for single copolymer chains
at a homopolymer interface. We also discuss the number of interaction contacts
between monomers, which provide a measure for the ``active surface'' of
copolymers or homopolymers close to the interface
A symmetric polymer blend confined into a film with antisymmetric surfaces: interplay between wetting behavior and phase diagram
We study the phase behavior of a symmetric binary polymer blend which is
confined into a thin film. The film surfaces interact with the monomers via
short range potentials. We calculate the phase behavior within the
self-consistent field theory of Gaussian chains. Over a wide range of
parameters we find strong first order wetting transitions for the semi-infinite
system, and the interplay between the wetting/prewetting behavior and the phase
diagram in confined geometry is investigated. Antisymmetric boundaries, where
one surface attracts the A component with the same strength than the opposite
surface attracts the B component, are applied. The phase transition does not
occur close to the bulk critical temperature but in the vicinity of the wetting
transition. For very thin films or weak surface fields one finds a single
critical point at . For thicker films or stronger surface fields
the phase diagram exhibits two critical points and two concomitant coexistence
regions. Only below a triple point there is a single two phase coexistence
region. When we increase the film thickness the two coexistence regions become
the prewetting lines of the semi-infinite system, while the triple temperature
converges towards the wetting transition temperature from above. The behavior
close to the tricritical point, which separates phase diagrams with one and two
critical points, is studied in the framework of a Ginzburg-Landau ansatz.
Two-dimensional profiles of the interface between the laterally coexisting
phases are calculated, and the interfacial and line tensions analyzed. The
effect of fluctuations and corrections to the self-consistent field theory are
discussed.Comment: Phys.Rev.E in prin
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