69 research outputs found
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
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
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
Phonon `notches' in a-b -plane optical conductivity of high-Tc superconductors
It is shown that a correlation between the positions of the -axis
longitudinal optic () phonons and ``notch''-like structures in the
- plane conductivity of high- superconductors results from
phonon-mediated interaction between electrons in different layers. It is found
that the relative size of the notches depends on
, where ,
and are the effective coupling strength, the frequency and the
width of the optical phonon which is responsible for the notch. Even for
the effect can be large if the phonon is very sharp.Comment: 5 pages, REVTeX, 4 uuencoded figure
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
Negative Mobility induced by Colored Thermal Fluctuations
Anomalous transport of non-Markovian, thermal Brownian particle dynamics in
spatially-periodic symmetric systems that is driven by time-periodic symmetric
driving and constant bias is investigated numerically. The Brownian dynamics is
modeled by a Generalized Langevin equation with exponentially correlated
Gaussian thermal noise, obeying the fluctuation-dissipation theorem. We study
the role of non-zero correlation time of thermal fluctuations for the
occurrence of absolute negative (linear) mobility (ANM) near zero bias,
negative-valued, nonlinear mobility (NNM) and negative differential mobility
(NDM) at finite bias away from equilibrium. We detect that a non-zero thermal
correlation time can either enhance or also diminish the value of ANM.
Moreover, finite thermal noise correlation can induce NDM and NNM in regions of
parameter space for which such ANM- and NNM-behavior is distinctly absent for
limiting white thermal noise. In parts of the parameter space, we find a
complex structure of regions of linear and nonlinear negative mobility: islands
and tongues which emerge and vanish under parameters manipulation. While
certain such anomalous transport regimes fade away with increasing temperature
some specific regions interestingly remain rather robust. Outside those regimes
with anomalous mobility, the ac/dc driven transport is either normal or the
driven Brownian particles are not transported at all
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
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
Arrival time distribution for a driven system containing quenched dichotomous disorder
We study the arrival time distribution of overdamped particles driven by a
constant force in a piecewise linear random potential which generates the
dichotomous random force. Our approach is based on the path integral
representation of the probability density of the arrival time. We explicitly
calculate the path integral for a special case of dichotomous disorder and use
the corresponding characteristic function to derive prominent properties of the
arrival time probability density. Specifically, we establish the scaling
properties of the central moments, analyze the behavior of the probability
density for short, long, and intermediate distances. In order to quantify the
deviation of the arrival time distribution from a Gaussian shape, we evaluate
the skewness and the kurtosis.Comment: 18 pages, 5 figure
Analytically solvable model of a driven system with quenched dichotomous disorder
We perform a time-dependent study of the driven dynamics of overdamped
particles which are placed in a one-dimensional, piecewise linear random
potential. This set-up of spatially quenched disorder then exerts a dichotomous
varying random force on the particles. We derive the path integral
representation of the resulting probability density function for the position
of the particles and transform this quantity of interest into the form of a
Fourier integral. In doing so, the evolution of the probability density can be
investigated analytically for finite times. It is demonstrated that the
probability density contains both a -singular contribution and a
regular part. While the former part plays a dominant role at short times, the
latter rules the behavior at large evolution times. The slow approach of the
probability density to a limiting Gaussian form as time tends to infinity is
elucidated in detail.Comment: 18 pages, 5 figure
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