159 research outputs found
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
Frequency Windows of Absolute Negative Conductance in Josephson Junctions
We report on anomalous conductance in a resistively and capacitively shunted
Josephson junction which is simultaneously driven by ac and dc currents. The
dependence of the voltage across the junction on the frequency of the ac
current shows windows of absolute negative conductance regimes, i.e. for a
positive (negative) dc current, the voltage is negative (positive).Comment: 4 pages, 1 figur
Negative conductances of Josephson junctions: Voltage fluctuations and energetics
We study a resistively and capacitively shunted Josephson junction, which is
driven by a combination of time-periodic and constant currents. Our
investigations concern three main problems: (A) The voltage fluctuations across
the junction; (B) The quality of transport expressed in terms of the P\'eclet
number; (C) The efficiency of energy transduction from external currents. These
issues are discussed in different parameter regimes that lead to: (i) absolute
negative conductance; (ii) negative differential conductance, and (iii) normal,
Ohmic-like conductance. Conditions for optimal operation of the system are
studied.Comment: 7 pages, 4 figures, Presented at the "Frontiers of Quantum and
Mesoscopic Thermodynamics", 28 July - 2 August 2008, Prague, Czech Republi
Absolute negative mobility induced by thermal equilibrium fluctuations
A novel transport phenomenon is identified that is induced by inertial
Brownian particles which move in simple one-dimensional, symmetric periodic
potentials under the influence of both a time periodic and a constant, biasing
driving force. Within tailored parameter regimes, thermal equilibrium
fluctuations induce the phenomenon of absolute negative mobility (ANM), which
means that the particle noisily moves {\it backwards} against a small constant
bias. When no thermal fluctuations act, the transport vanishes identically in
these tailored regimes. There also exist parameter regimes, where ANM can occur
in absence of fluctuations on grounds which are rooted solely in the complex,
inertial deterministic dynamics. The experimental verification of this new
transport scheme is elucidated for the archetype symmetric physical system: a
convenient setup consisting of a resistively and capacitively shunted Josephson
junction device.Comment: 4 pages, 3 figures. Phys. Rev. Lett. (in press
Anomalous transport in biased ac-driven Josephson junctions: Negative conductances
We investigate classical anomalous electrical transport in a driven,
resistively and capacitively shunted Josephson junction device. Novel transport
phenomena are identified in chaotic regimes when the junction is subjected to
both, a time periodic (ac) and a constant, biasing (dc) current. The dependence
of the voltage across the junction on the dc-current exhibits a rich diversity
of anomalous transport characteristics: In particular, depending on the chosen
parameter regime we can identify so termed absolute negative conductance around
zero dc-bias, the occurrence of negative differential conductance and, after
crossing a zero conductance, the emergence of a negative nonlinear conductance
in the non-equilibrium response regime remote from zero dc-bias.Comment: 7 pages, 5 figure
Brownian motors: current fluctuations and rectification efficiency
With this work we investigate an often neglected aspect of Brownian motor
transport: The r\^{o}le of fluctuations of the noise-induced current and its
consequences for the efficiency of rectifying noise. In doing so, we consider a
Brownian inertial motor that is driven by an unbiased monochromatic,
time-periodic force and thermal noise. Typically, we find that the asymptotic,
time- and noise-averaged transport velocities are small, possessing rather
broad velocity fluctuations. This implies a corresponding poor performance for
the rectification power. However, for tailored profiles of the ratchet
potential and appropriate drive parameters, we can identify a drastic
enhancement of the rectification efficiency. This regime is marked by
persistent, uni-directional motion of the Brownian motor with few back-turns,
only. The corresponding asymmetric velocity distribution is then rather narrow,
with a support that predominantly favors only one sign for the velocity.Comment: 9 pages, 4 figure
Demon-free quantum Brownian motors
A quantum Smoluchowski equation is put forward that consistently describes
thermal quantum states. In particular, it notably does not induce a violation
of the second law of thermodynamics. This so modified kinetic equation is
applied to study {\it analytically} directed quantum transport at strong
friction in arbitrarily shaped ratchet potentials that are driven by nonthermal
two-state noise. Depending on the mutual interplay of quantum tunneling and
quantum reflection these quantum corrections can induce both, either a sizable
enhancement or a suppression of transport. Moreover, the threshold for current
reversals becomes markedly shifted due to such quantum fluctuations.Comment: 4 pages 3 figure
Interleukin 1-β, Interleukin-1 Receptor Antagonist, and Interleukin 18 in Children with Acute Spontaneous Urticaria
Very little is known about the role of interleukin-1β (IL-1β) and interleukin-18 (IL-18) in urticaria. Material and Methods. Serum levels of IL-1β, IL-1 receptor antagonist (IL-1RA), and IL-18 were measured in 56 children with urticaria and in 41 healthy subjects. Results. Serum IL-1β did not differ between children with acute urticaria and controls. Children with single episode of urticaria had higher levels of IL-1RA and IL-18 than healthy subjects. In children with single episode of urticaria, level of IL-1RA correlated with C-reactive protein (CRP), D-dimer, and IL-1β levels. In subjects with recurrence of urticaria IL-1RA was positively correlated with WBC and D-dimer levels. No correlation of cytokine levels and urticaria severity scores (UAS) in all children with urticaria was observed. In children with single episode of urticaria UAS correlated with CRP level. In the group with single episode of urticaria and in children with symptoms of upper respiratory infection, IL-1RA and IL-18 levels were higher than in controls. The former was higher than in noninfected children with urticaria. In conclusion, this preliminary study documents that serum IL-1RA and IL-18 levels are increased in some children with acute urticaria. However further studies are necessary to define a pathogenic role of IL-1β, IL-1RA, and IL-18 in urticaria
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