2,246 research outputs found
Effect of time delay on feedback control of a flashing ratchet
It was recently shown that the use of feedback control can improve the
performance of a flashing ratchet. We investigate the effect of a time delay in
the implementation of feedback control in a closed-loop collective flashing
ratchet, using Langevin dynamics simulations. Surprisingly, for a large
ensemble, a well-chosen delay time improves the ratchet performance by allowing
the system to synchronize into a quasi-periodic stable mode of oscillation that
reproduces the optimal average velocity for a periodically flashing ratchet.
For a small ensemble, on the other hand, finite delay times significantly
reduce the benefit of feedback control for the time-averaged velocity, because
the relevance of information decays on a time scale set by the diffusion time
of the particles. Based on these results, we establish that experimental use of
feedback control is realistic.Comment: 6 pages, 6 figures, to appear in Europhysics Letter
Phase Ordering Dynamics of Theory with Hamiltonian Equations of Motion
Phase ordering dynamics of the (2+1)- and (3+1)-dimensional theory
with Hamiltonian equations of motion is investigated numerically. Dynamic
scaling is confirmed. The dynamic exponent is different from that of the
Ising model with dynamics of model A, while the exponent is the same.Comment: to appear in Int. J. Mod. Phys.
Symmetry of two terminal, non-linear electric conduction
The well-established symmetry relations for linear transport phenomena can
not, in general, be applied in the non-linear regime. Here we propose a set of
symmetry relations with respect to bias voltage and magnetic field for the
non-linear conductance of two-terminal electric conductors. We experimentally
confirm these relations using phase-coherent, semiconductor quantum dots.Comment: 4 pages, 4 figure
Araracuara, un nuevo género de Rhamnaceae de la AmazonÃa colombiana
Se describe e ilustra Araracuara Fern. Alonso, un nuevo género de
la familia Rhamnaceae conocido tan sólo de las mesetas de arenisca
de la AmazonÃa colombiana. Se discuten sus posibles afinidades en
la familia y se sugiere que estarÃamos ante un género relÃctico, relacionado
con el pantropical Colubrina y en menor medida con el
amazónico Ampelozizyphus. Se incluye una clave para los géneros
de Rhamnaceae de Colombia y un apéndice con una muestra representativa
de especies de los géneros estudiados, algunos de ellos
pobremente documentados de territorio colombiano.Peer reviewe
A microfabricated ion trap with integrated microwave circuitry
We describe the design, fabrication and testing of a surface-electrode ion
trap, which incorporates microwave waveguides, resonators and coupling elements
for the manipulation of trapped ion qubits using near-field microwaves. The
trap is optimised to give a large microwave field gradient to allow
state-dependent manipulation of the ions' motional degrees of freedom, the key
to multiqubit entanglement. The microwave field near the centre of the trap is
characterised by driving hyperfine transitions in a single laser-cooled 43Ca+
ion.Comment: 4 pages, 5 figure
Voltage rectification effects in mesoscopic superconducting triangles: experiment and modelling
The interaction of externally applied currents with persistent currents
induced by magnetic field in a mesoscopic triangle is investigated. As a
consequence of the superposition of these currents, clear voltage rectification
effects are observed. We demonstrate that the amplitude of the rectified signal
strongly depends on the configurations of the current leads with the lowest
signal obtained when the contacts are aligned along a median of the triangle.
When the contacts are aligned off-centered compared to the geometrical center,
the voltage response shows oscillations as a function of the applied field,
whose sign can be controlled by shifting the contacts. These results are in
full agreement with theoretical predictions for an analogous system consisting
of a closed loop with a finite number of identical Josephson junctions.Comment: 5 pages, 4 figures, published in Phys. Rev.
Solid-state NMR study of the YadA membrane-anchor domain in the bacterial outer membrane.
MAS-NMR was used to study the structure and dynamics at ambient temperatures of the membrane-anchor domain of YadA (YadA-M) in a pellet of the outer membrane of E. coli in which it was expressed. YadA is an adhesin from the pathogen Yersinia enterocolitica that is involved in interactions with the host cell, and it is a model protein for studying the autotransport process. Existing assignments were sucessfully transferred to a large part of the YadA-M protein in the E. coli lipid environment by using 13C-13C DARR and PDSD spectra at different mixing times. The chemical shifts in most regions of YadA-M are unchanged relative to those in microcrystalline YadA-M preparations from which a structure has previously been solved, including the ASSA region that is proposed to be involved in transition-state hairpin formation for transport of the soluble domain. Comparisons of the dynamics between the microcrystalline and membrane-embedded samples indicate greater flexibility of the ASSA region in the outer-membrane preparation at physiological temperatures. This study will pave the way towards MAS-NMR structure determination of membrane proteins, and a better understanding of functionally important dynamic residues in native membrane environments
Analytical model of brittle destruction based on hypothesis of scale similarity
The size distribution of dust particles in nuclear fusion devices is close to
the power function. A function of this kind can be the result of brittle
destruction. From the similarity assumption it follows that the size
distribution obeys the power law with the exponent between -4 and -1. The model
of destruction has much in common with the fractal theory. The power exponent
can be expressed in terms of the fractal dimension. Reasonable assumptions on
the shape of fragments concretize the power exponent, and vice versa possible
destruction laws can be inferred on the basis of measured size distributions.Comment: 10 pages, 3 figure
Is there a Landau Pole Problem in QED?
We investigate a lattice version of QED by numerical simulations. For the
renormalized charge and mass we find results which are consistent with the
renormalized charge vanishing in the continuum limit. A detailed study of the
relation between bare and renormalized quantities reveals that the Landau pole
lies in a region of parameter space which is made inaccessible by spontaneous
chiral symmetry breaking
Higher-Twist Contribution to Pion Structure Function: 4-Fermi Operators
We present quenched lattice QCD results for the contribution of higher-twist
operators to the lowest non-trivial moment of the pion structure function. To
be specific, we consider the combination which has and receives contributions from 4-Fermi
operators only. We introduce the basis of lattice operators. The
renormalization of the operators is done perturbatively in the
scheme using the 't Hooft-Veltman prescription for , taking
particular care of mixing effects. The contribution is found to be of
, relative to the leading contribution to the moment of
.Comment: Version to appear in Nucl. Phys.
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