284 research outputs found
Vibrational ratchets
Transport in a one-dimensional symmetric device can be activated by the
combination of thermal noise and a bi-harmonic drive. For the study case of an
overdamped Brownian particle diffusing on a periodic one-dimensional substrate,
we distinguish two apparently different bi-harmonic regimes: (i) Harmonic
mixing, where the two drive frequencies are commensurate and of the order of
some intrinsic dynamical relaxation rate. A comparison of new simulation
results with earlier theoretical predictions shows that the analytical
understanding of this frequency mixing mechanism is not satisfactory, yet; (ii)
Vibrational mixing, where one harmonic drive component is characterized by a
high frequency but finite amplitude-to-frequency ratio. Its effect on the
device response to either a static or a low-frequency additional input signal
is accurately reproduced by rescaling each spatial Fourier component of the
substrate potential, separately. Contrary to common wisdom based on the linear
response theory, we show that extremely high-frequency modulations can indeed
influence the response of slowly (or dc) operated devices, with potential
applications in sensor technology and cellular physiology. Finally, the mixing
of two high-frequency beating signal is also investigated both numerically and
analytically.Comment: 8 pages, 9 figure
Recycled Noise Rectification: A Dumb Maxwell's Daemon
The one dimensional motion of a massless Brownian particle on a symmetric
periodic substrate can be rectified by re-injecting its driving noise through a
realistic recycling procedure. If the recycled noise is multiplicatively
coupled to the substrate, the ensuing feed-back system works like a passive
Maxwell's daemon, capable of inducing a net current that depends on both the
delay and the autocorrelation times of the noise signals. Extensive numerical
simulations show that the underlying rectification mechanism is a resonant
nonlinear effect: The observed currents can be optimized for an appropriate
choice of the recycling parameters with immediate application to the design of
nanodevices for particle transport.Comment: 7 pages, 6 figure
Two coupled Josephson junctions: dc voltage controlled by biharmonic current
We study transport properties of two Josephson junctions coupled by an
external shunt resistance. One of the junction (say, the first) is driven by an
unbiased ac current consisting of two harmonics. The device can rectify the ac
current yielding a dc voltage across the first junction. For some values of
coupling strength, controlled by an external shunt resistance, a dc voltage
across the second junction can be generated. By variation of system parameters
like the relative phase or frequency of two harmonics, one can conveniently
manipulate both voltages with high efficiency, e.g., changing the dc voltages
across the first and second junctions from positive to negative values and vice
versa.Comment: 15 pages, 7 figures, to appear in J. Phys. Condens. Matter (2012
AC Driven Jumps Distribution on a Periodic Substrate
A driven Brownian particle (e.g. an adatom on a surface) diffusing on a
low-viscosity, periodic substrate may execute multiple jumps. In the presence
of an additional periodic drive, the jump lengths and time durations become
statistically modulated according to a syncronyzation mechanism reminiscent of
asymmetric stochastic resonance. Here, too, bistability plays a key role, but
in a dynamical sense, inasmuch as a particle switches between locked and
running states.Comment: 4 pages, 4 figures, RevTeX, to be published in Surface Science
Letter
Stochastic resonance with weak monochromatic driving: gains above unity induced by high-frequency signals
We study the effects of a high-frequency (HF) signal on the response of a
noisy bistable system to a low-frequency subthreshold sinusoidal signal. We
show that, by conveniently choosing the ratio of the amplitude of the HF signal
to its frequency, stochastic resonance gains greater than unity can be measured
at the low-frequency value. Thus, the addition of the HF signal can entail an
improvement in the detection of weak monochromatic signals. The results are
explained in terms of an effective model and illustrated by means of numerical
simulations.Comment: 5 pages, 2 figure
String ratchets: ac driven asymmetric kinks.
We simulated numerically the time evolution of a one-kink bearing, damped elastic string sitting on noiseless periodic substrates of two types: (I) asymmetric, time independent, (II) symmetric, periodically deformable. An asymmetric kink subjected to an ac drive is shown to drift steadily with finite average speed independent of its initial kinetic conditions. In the overdamped regime the resulting net kink transport can be attributed to the rectification of the Brownian motion of a pointlike particle with oscillating mass. For intermediate to low damping completely different features show up, due to the finite size of the objects being transported; in particular, the kink current hits a maximum for an optimal value of the damping constant, resonates at the kink internal-mode frequency and, finally, reverses sign within a certain range of the drive parameters
Transport by bi-harmonic drives: from harmonic to vibrational mixing
Transport in a one-dimensional symmetric device can be activated by the combination of thermal noise and a bi-harmonic drive. The results of extensive simulations allow us to distinguish between two apparently different bi-harmonic regimes: (i) harmonic mixing, where the two drive frequencies are commensurate but not too high; (ii) vibrational mixing, where one harmonic drive component possesses a high frequency but finite amplitude-to-frequency ratio. A comparison with the earlier theoretical predictions shows that at present the analytical understanding of nonlinear frequency mixing is still not satisfactory
Stokes' Drift of linear Defects
A linear defect, viz. an elastic string, diffusing on a planar substrate
traversed by a travelling wave experiences a drag known as Stokes' drift. In
the limit of an infinitely long string, such a mechanism is shown to be
characterized by a sharp threshold that depends on the wave parameters, the
string damping constant and the substrate temperature. Moreover, the onset of
the Stokes' drift is signaled by an excess diffusion of the string center of
mass, while the dispersion of the drifting string around its center of mass may
grow anomalous.Comment: 14 pages, no figures, to be published in Phys.Rev.
Deterministic ratchets: route to diffusive transport
The rectification efficiency of an underdamped ratchet operated in the
adiabatic regime increases according to a scaling current-amplitude curve as
the damping constant approaches a critical threshold; below threshold the
rectified signal becomes extremely irregular and eventually its time average
drops to zero. Periodic (locked) and diffusive (fully chaotic) trajectories
coexist on fine tuning the amplitude of the input signal. The transition from
regular to chaotic transport in noiseless ratchets is studied numerically.Comment: 9 pages, 5 figures, to be published in Phys. Rev.
Arthrobacter sp. Inoculation Improves Cactus Pear Growth, Quality of Fruits, and Nutraceutical Properties of Cladodes
A study was undertaken to determine the effects of a strain of Arthrobacter sp., a Plant Growth-Promoting Bacteria (PGPB), on plant phenology and qualitative composition of Opuntia ficus-indica (L.) Mill. fruits and cladodes. The strain was inoculated in soil, and its effects on cactus pear plants were detected and compared to nontreated plants. Compared to the latter, the treatment with bacteria promoted an earlier plant sprouting (2 months before the control) and fruitification, ameliorating fruit quality (i.e., improved fresh and dry weight: + 24% and + 26%, respectively, increased total solid content by 30% and polyphenols concentrations by 22%). The quality and quantity of monosaccharides of cladodes were also increased by Arthrobacter sp. with a positive effect on their nutraceutical value. In summer, the mean values of xylose, arabinose, and mannose were significantly higher in treated compared to not treated plants (+ 3.54; + 7.04; + 4.76 mg/kg d.w. respectively). A similar trend was observed in autumn, when the cladodes of inoculated plants had higher contents, i.e., 33% xylose, 65% arabinose, and 40% mannose, respect to the controls. In conclusion, Arthrobacter sp. plays a role in the improvement of nutritional and nutraceutical properties of cactus pear plants due to its capabilities to promote plant growth. Therefore, these results open new perspectives in PGPB application in the agro-farming system as alternative strategy to improve cactus pear growth, yield, and cladodes quality, being the latter the main by-product to be utilized for additional industrial uses
- …