7,381 research outputs found
Necessary conditions for optimality of decomposable entanglement witnesses
It is well known that the support of an optimal decomposable entanglement
witness is completely entangled. We add two more necessary conditions for the
optimality: The orthogonal complement of the support must have a nonzero
product vector; another one will be given in terms of related faces of a convex
cone. With these necessary conditions, we show that there exist examples of
non-optimal decomposable entanglement witnesses which are the partial
transposes of positive semi-definite matrices supported on completely entangled
spaces, whenever both of the local dimensions are greater than or equal to
three.Comment: Final form to be published in Rep. Math. Phys. The last example is
corrected together with minor text
Sustainability performance measurement : a preliminary classification framework of models and indicators
In this position paper we focus on the diversity of sustainability measurements. Based on existing research on performance measurement, we propose a preliminary classification framework summarizing sustainability models and indicators. By describing illustrative examples, we claim that several models and indicators can be distinguished with their own peculiarities. Having such a framework is interesting for both academia and business to structure the range of models and indicators and to ultimately select the appropriate sustainability measurement approach. The proposed framework should be validated by further research
Variational theory for a single polyelectrolyte chain revisited
We reconsider the electrostatic contribution to the persistence length,
, of a single, infinitely long charged polymer in the presence of
screening. A Gaussian variational method is employed, taking as the
only variational parameter. For weakly charged and flexible chains, crumpling
occurs at small length scales because conformational fluctuations overcome
electrostatic repulsion. The electrostatic persistence length depends on the
square of the screening length, , as first argued by
Khokhlov and Khachaturian by applying the Odijk-Skolnick-Fixman (OSF) theory to
a string of crumpled blobs. We compare our approach to previous theoretical
works (including variational formulations) and show that the result
found by several authors comes from the improper use of
a cutoff at small length scales. For highly charged and stiff chains, crumpling
does not occur; here we recover the OSF result and validate the perturbative
calculation for slightly bent rods.Comment: 11 pages, 6 figure
Vision-Based Autonomous Driving: A Model Learning Approach
We present an integrated approach for perception and control for an
autonomous vehicle and demonstrate this approach in a high-fidelity urban
driving simulator. Our approach first builds a model for the environment, then
trains a policy exploiting the learned model to identify the action to take at
each time-step. To build a model for the environment, we leverage several deep
learning algorithms. To that end, first we train a variational autoencoder to
encode the input image into an abstract latent representation. We then utilize
a recurrent neural network to predict the latent representation of the next
frame and handle temporal information. Finally, we utilize an
evolutionary-based reinforcement learning algorithm to train a controller based
on these latent representations to identify the action to take. We evaluate our
approach in CARLA, a high-fidelity urban driving simulator, and conduct an
extensive generalization study. Our results demonstrate that our approach
outperforms several previously reported approaches in terms of the percentage
of successfully completed episodes for a lane keeping task.Comment:
The Persistence Length of a Strongly Charged, Rod-like, Polyelectrolyte in the Presence of Salt
The persistence length of a single, intrinsically rigid polyelectrolyte
chain, above the Manning condensation threshold is investigated theoretically
in presence of added salt. Using a loop expansion method, the partition
function is consistently calculated, taking into account corrections to
mean-field theory. Within a mean-field approximation, the well-known results of
Odijk, Skolnick and Fixman are reproduced. Beyond mean-field, it is found that
density correlations between counterions and thermal fluctuations reduce the
stiffness of the chain, indicating an effective attraction between monomers for
highly charged chains and multivalent counterions. This attraction results in a
possible mechanical instability (collapse), alluding to the phenomenon of DNA
condensation. In addition, we find that more counterions condense on slightly
bent conformations of the chain than predicted by the Manning model for the
case of an infinite cylinder. Finally, our results are compared with previous
models and experiments.Comment: 13 pages, 2 ps figure
Conformational Instability of Rodlike Polyelectrolytes due to Counterion Fluctuations
The effective elasticity of highly charged stiff polyelectrolytes is studied
in the presence of counterions, with and without added salt. The rigid polymer
conformations may become unstable due to an effective attraction induced by
counterion density fluctuations. Instabilities at the longest, or intermediate
length scales may signal collapse to globule, or necklace states, respectively.
In the presence of added-salt, a generalized electrostatic persistence length
is obtained, which has a nontrivial dependence on the Debye screening length.
It is also found that the onset of conformational instability is a re-entrant
phenomenon as a function of polyelectrolyte length for the unscreened case, and
the Debye length or salt concentration for the screened case. This may be
relevant in understanding the experimentally observed re-entrant condensation
of DNA.Comment: 8 pages, 4 figure
RNA–protein binding kinetics in an automated microfluidic reactor
Microfluidic chips can automate biochemical assays on the nanoliter scale, which is of considerable utility for RNA–protein binding reactions that would otherwise require large quantities of proteins. Unfortunately, complex reactions involving multiple reactants cannot be prepared in current microfluidic mixer designs, nor is investigation of long-time scale reactions possible. Here, a microfluidic ‘Riboreactor’ has been designed and constructed to facilitate the study of kinetics of RNA–protein complex formation over long time scales. With computer automation, the reactor can prepare binding reactions from any combination of eight reagents, and is optimized to monitor long reaction times. By integrating a two-photon microscope into the microfluidic platform, 5-nl reactions can be observed for longer than 1000 s with single-molecule sensitivity and negligible photobleaching. Using the Riboreactor, RNA–protein binding reactions with a fragment of the bacterial 30S ribosome were prepared in a fully automated fashion and binding rates were consistent with rates obtained from conventional assays. The microfluidic chip successfully combines automation, low sample consumption, ultra-sensitive fluorescence detection and a high degree of reproducibility. The chip should be able to probe complex reaction networks describing the assembly of large multicomponent RNPs such as the ribosome
Atomically dispersed Pt-N-4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction
Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt-N-4 sites doped on a carbon nanotube (Pt-1/CNT) can catalyse the CER with excellent activity and selectivity. The Pt-1/CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt-1/CNT exhibits near 100% CER selectivity even in acidic media, with low Cl- concentrations (0.1M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl- on Pt-N-4 sites during the CER. Density functional theory calculations suggest the PtN4C12 site as the most plausible active site structure for the CER
Binding of molecules to DNA and other semiflexible polymers
A theory is presented for the binding of small molecules such as surfactants
to semiflexible polymers. The persistence length is assumed to be large
compared to the monomer size but much smaller than the total chain length. Such
polymers (e.g. DNA) represent an intermediate case between flexible polymers
and stiff, rod-like ones, whose association with small molecules was previously
studied. The chains are not flexible enough to actively participate in the
self-assembly, yet their fluctuations induce long-range attractive interactions
between bound molecules. In cases where the binding significantly affects the
local chain stiffness, those interactions lead to a very sharp, cooperative
association. This scenario is of relevance to the association of DNA with
surfactants and compact proteins such as RecA. External tension exerted on the
chain is found to significantly modify the binding by suppressing the
fluctuation-induced interaction.Comment: 15 pages, 7 figures, RevTex, the published versio
Phase behaviour of a model of colloidal particles with a fluctuating internal state
Colloidal particles are not simple rigid particles, in general an isolated
particle is a system with many degrees of freedom in its own right, e.g., the
counterions around a charged colloidal particle.The behaviour of model
colloidal particles, with a simple phenomenological model to account for these
degrees of freedom, is studied. It is found that the interaction between the
particles is not pairwise additive. It is even possible that the interaction
between a triplet of particles is attractive while the pair interaction is
repulsive. When this is so the liquid phase is either stable only in a small
region of the phase diagram or absent altogether.Comment: 12 pages including 4 figure
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