59 research outputs found
Brownian motion: a case of temperature fluctuations
A diffusion process of a Brownian particle in a medium of temperature is
re-considered. We assume that temperature of the medium fluctuates around its
mean value. The velocity probability distribution is obtained. It is shown that
the stationary state is not a thermodynamic equilibrium state described by the
Maxwell distribution. Instead a nonequilibrium state is produced by temperature
fluctuations.Comment: accepted for publication in Acta Physica Polonica
Geometric phase as a determinant of a qubit--environment coupling
We investigate the qubit geometric phase and its properties in dependence on
the mechanism for decoherence of a qubit weakly coupled to its environment. We
consider two sources of decoherence: dephasing coupling (without exchange of
energy with environment) and dissipative coupling (with exchange of energy).
Reduced dynamics of the qubit is studied in terms of the rigorous Davies
Markovian quantum master equation, both at zero and non--zero temperature. For
pure dephasing coupling, the geometric phase varies monotonically with respect
to the polar angle (in the Bloch sphere representation) parameterizing an
initial state of the qubit. Moreover, it is antisymmetric about some points on
the geometric phase-polar angle plane. This is in distinct contrast to the case
of dissipative coupling for which the variation of the geometric phase with
respect to the polar angle typically is non-monotonic, displaying local extrema
and is not antisymmetric. Sensitivity of the geometric phase to details of the
decoherence source can make it a tool for testing the nature of the
qubit--environment interaction.Comment: accepted for publication in Quantum Information Processin
Absolute negative mobility induced by white Poissonian noise
We research the transport properties of inertial Brownian particles which
move in a symmetric periodic potential and are subjected to both a symmetric,
unbiased time-periodic external force and biased Poissonian white shot noise
(of non-zero average F) being composed of a random sequence of delta-shaped
pulses with random amplitudes. Upon varying the parameters of white shot-noise
one conveniently can manipulate the transport direction and the overall
nonlinear response behavior. Within tailored parameter regimes, we find that
the response is opposite to the applied average bias F of such white shot
noise. This very transport characteristics thus mimics a nonlinear Absolute
Negative Mobility (ANM) regime. Moreover, such white shot noise driven ANM is
robust with respect to statistics of the shot noise spikes. Our findings can be
checked and corroborated experimentally by use of a setup that consists of a
single resistively and capacitively shunted Josephson junction device.Comment: 14 pages, 12 figures; accepted in J. Stat. Mech.: Theor. Exp. (2013
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
Comment on "White-Noise-Induced Transport in Periodic Structures"
In the paper by J.\L uczka {\em et al.} ({\em Europhys. Lett.}, {\bf 31}
(1995) 431), the authors reported by rigorous calculation that an additive
Poissonian white shot noise can induce a macroscopic current of a dissipative
particle in a periodic potential -- even {\em in the absence} of spatial
asymmetry of the potential. We argue that their main result is an obvious one
caused by the spatially broken symmetry of a probability distribution of the
additive noise, unlike the similar result caused by chaotic noise which has a
symmetric probability distribution ({\em J.Phys.Soc.Jpn.}, {\bf 63} (1994)
2014).Comment: 2 pages (Latex); submitted to Europhys.Let
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
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
Dynamics at the angle of repose: jamming, bistability, and collapse
When a sandpile relaxes under vibration, it is known that its measured angle
of repose is bistable in a range of values bounded by a material-dependent
maximal angle of stability; thus, at the same angle of repose, a sandpile can
be stationary or avalanching, depending on its history. In the nearly jammed
slow dynamical regime, sandpile collapse to a zero angle of repose can also
occur, as a rare event. We claim here that fluctuations of {\it dilatancy} (or
local density) are the key ingredient that can explain such varied phenomena.
In this work, we model the dynamics of the angle of repose and of the density
fluctuations, in the presence of external noise, by means of coupled stochastic
equations. Among other things, we are able to describe sandpile collapse in
terms of an activated process, where an effective temperature (related to the
density as well as to the external vibration intensity) competes against the
configurational barriers created by the density fluctuations.Comment: 15 pages, 1 figure. Minor changes and update
- …