2,784 research outputs found
“Ineffective” competition: a puzzle?
Conventionally, we think of an increase in competition as weakly decreasing prices, increasing the number of consumers served, thus increasing consumer surplus, decreasing firms profits, etc. Here, we demonstrate that, under some tame circumstances, an increase in competition may lead to a price increase in a horizontally differentiated market. We show this relationship for the petrol market in German cities
Anomaly-induced dynamical refringence in strong-field QED
We investigate the impact of the Adler-Bell-Jackiw anomaly on the
nonequilibrium evolution of strong-field quantum electrodynamics (QED) using
real-time lattice gauge theory techniques. For field strengths exceeding the
Schwinger limit for pair production, we encounter a highly absorptive medium
with anomaly-induced dynamical refractive properties. In contrast to earlier
expectations based on equilibrium properties, where net anomalous effects
vanish because of the trivial vacuum structure, we find that out-of-equilibrium
conditions can have dramatic consequences for the presence of quantum currents
with distinctive macroscopic signatures. We observe an intriguing tracking
behavior, where the system spends longest times near collinear field
configurations with maximum anomalous current. Apart from the potential
relevance of our findings for future laser experiments, similar phenomena
related to the chiral magnetic effect are expected to play an important role
for strong QED fields during initial stages of heavy-ion collision experiments.Comment: 5 pages, 4 figures, references adde
“Ineffective†competition: a puzzle?
Conventionally, we think of an increase in competition as weakly decreasing prices, increasing the number of consumers served, thus increasing consumer surplus, decreasing firms profits, etc. Here, we demonstrate that, under some tame circumstances, an increase in competition may lead to a price increase in a horizontally differentiated market. We show this relationship for the petrol market in German cities.
Mapping atomistic to coarse-grained polymer models using automatic simplex optimization to fit structural properties
We develop coarse-grained force fields for poly (vinyl alcohol) and poly
(acrylic acid) oligomers. In both cases, one monomer is mapped onto a
coarse-grained bead. The new force fields are designed to match structural
properties such as radial distribution functions of various kinds derived by
atomistic simulations of these polymers. The mapping is therefore constructed
in a way to take into account as much atomistic information as possible. On the
technical side, our approach consists of a simplex algorithm which is used to
optimize automatically non-bonded parameters as well as bonded parameters.
Besides their similar conformation (only the functional side group differs),
poly (acrylic acid) was chosen to be in aqueous solution in contrast to a poly
(vinyl alcohol) melt. For poly (vinyl alcohol) a non-optimized bond angle
potential turns out to be sufficient in connection with a special, optimized
non-bonded potential. No torsional potential has to be applied here. For poly
(acrylic acid), we show that each peak of the radial distribution function is
usually dominated by some specific model parameter(s). Optimization of the bond
angle parameters is essential. The coarse-grained forcefield reproduces the
radius of gyration of the atomistic model. As a first application, we use the
force field to simulate longer chains and compare the hydrodynamic radius with
experimental data.Comment: 34 pages, 3 tables, 16 figure
Orientation Correlation in Simplified Models of Polymer Melts
We investigate mutual local chain order in systems of fully flexible polymer
melts in a simple generic bead-spring model. The excluded-volume interaction
together with the connectivity leads to local ordering effects which are
independent of chain length between 25 and 700 monomers, i.e. in the Rouse as
well as in the reptation regime. These ordering phenomena extend to a distance
of about 3 to 4 monomer sizes and decay to zero afterwards.Comment: 5 pages, 3 figure
Mapping atomistic to coarse-grained polymer models using automatic simplex optimization to fit structural properties
We develop coarse-grained force fields for poly (vinyl alcohol) and poly
(acrylic acid) oligomers. In both cases, one monomer is mapped onto a
coarse-grained bead. The new force fields are designed to match structural
properties such as radial distribution functions of various kinds derived by
atomistic simulations of these polymers. The mapping is therefore constructed
in a way to take into account as much atomistic information as possible. On the
technical side, our approach consists of a simplex algorithm which is used to
optimize automatically non-bonded parameters as well as bonded parameters.
Besides their similar conformation (only the functional side group differs),
poly (acrylic acid) was chosen to be in aqueous solution in contrast to a poly
(vinyl alcohol) melt. For poly (vinyl alcohol) a non-optimized bond angle
potential turns out to be sufficient in connection with a special, optimized
non-bonded potential. No torsional potential has to be applied here. For poly
(acrylic acid), we show that each peak of the radial distribution function is
usually dominated by some specific model parameter(s). Optimization of the bond
angle parameters is essential. The coarse-grained forcefield reproduces the
radius of gyration of the atomistic model. As a first application, we use the
force field to simulate longer chains and compare the hydrodynamic radius with
experimental data.Comment: 34 pages, 3 tables, 16 figure
Local Structure and Dynamics of Trans-polyisoprene oligomers
Mono- and poly-disperse melts of oligomers (average length 10 monomers) of
trans-1,4-polyisoprene are simulated in full atomistic detail. The force-field
is developed by means of a mixture of ab initio quantum-chemistry and an
automatic generation of empirical parameters. Comparisons to NMR and scattering
experiments validate the model. The local reorientation dynamics shows that for
CH vectors there is a two-stage process consisting of an initial decay and a
late-stage decorrelation originating from overall reorientation. The atomistic
model can be successfully mapped onto a simple model including only beads for
the monomers with bond springs and bond angle potentials. End-bridging Monte
Carlo as an equilibration stage and molecular dynamics as the subsequent
simulation method together prove to be a useful method for polymer simulations.Comment: 25 pages, 15 figures, accepted by Macromolecule
Photovoltaic effect in an electrically tunable van der Waals heterojunction
Semiconductor heterostructures form the cornerstone of many electronic and
optoelectronic devices and are traditionally fabricated using epitaxial growth
techniques. More recently, heterostructures have also been obtained by vertical
stacking of two-dimensional crystals, such as graphene and related two-
dimensional materials. These layered designer materials are held together by
van der Waals forces and contain atomically sharp interfaces. Here, we report
on a type- II van der Waals heterojunction made of molybdenum disulfide and
tungsten diselenide monolayers. The junction is electrically tunable and under
appropriate gate bias, an atomically thin diode is realized. Upon optical
illumination, charge transfer occurs across the planar interface and the device
exhibits a photovoltaic effect. Advances in large-scale production of
two-dimensional crystals could thus lead to a new photovoltaic solar
technology.Comment: 26 pages, 14 figures, Nano Letters 201
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