515 research outputs found
Resonant Activation Phenomenon for Non-Markovian Potential-Fluctuation Processes
We consider a generalization of the model by Doering and Gadoua to
non-Markovian potential-switching generated by arbitrary renewal processes. For
the Markovian switching process, we extend the original results by Doering and
Gadoua by giving a complete description of the absorption process. For all
non-Markovian processes having the first moment of the waiting time
distributions, we get qualitatively the same results as in the Markovian case.
However, for distributions without the first moment, the mean first passage
time curves do not exhibit the resonant activation minimum. We thus come to the
conjecture that the generic mechanism of the resonant activation fails for
fluctuating processes widely deviating from Markovian.Comment: RevTeX 4, 5 pages, 4 figures; considerably shortened version accepted
as a brief report to Phys. Rev.
On Orbits of the Ring Zmn under Action of the Group SL(m, Zn)
We consider the action of the finite matrix group SL(m,Zn ) on the ring Zmn. We determine orbits of this action for n arbitrary natural number. It is a generalization of the task which was studied by A. A. Kirillov for m = 2 and n prime number
Critical Current 0- Transition in Designed Josephson Quantum Dot Junctions
We report on quantum dot based Josephson junctions designed specifically for
measuring the supercurrent. From high-accuracy fitting of the current-voltage
characteristics we determine the full magnitude of the supercurrent (critical
current). Strong gate modulation of the critical current is observed through
several consecutive Coulomb blockade oscillations. The critical current crosses
zero close to, but not at, resonance due to the so-called 0- transition in
agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at
http://www.fys.ku.dk/~hij/public/nl_supp.pdf
Simple models suffice for the single dot quantum shuttle
A quantum shuttle is an archetypical nanoelectromechanical device, where the
mechanical degree of freedom is quantized. Using a full-scale numerical
solution of the generalized master equation describing the shuttle, we have
recently shown [Novotn\'{y} {\it et al.}, Phys. Rev. Lett. {\bf 92}, 248302
(2004)] that for certain limits of the shuttle parameters one can distinguish
three distinct charge transport mechanisms: (i) an incoherent tunneling regime,
(ii) a shuttling regime, where the charge transport is synchronous with the
mechanical motion, and (iii) a coexistence regime, where the device switches
between the tunneling and shuttling regimes. While a study of the cross-over
between these three regimes requires the full numerics, we show here that by
identifying the appropriate time-scales it is possible to derive vastly simpler
equations for each of the three regimes. The simplified equations allow a clear
physical interpretation, are easily solved, and are in good agreement with the
full numerics in their respective domains of validity.Comment: 23 pages, 14 figures, invited paper for the Focus issue of the New
Journal of Physics on Nano-electromechanical system
Support for Expert Estimations in Transportation Projects
This paper deals with risk analysis as a part of the financial assessment of transportation projects. Two approaches to risk assessment are discussed. A risk can be evaluated either directly in terms of the probabilistic distribution of the assessment criterion; or an indirect determination of the risk can be applied without constructing the probability distribution, but by determining the characteristic features of the project.
Full counting statistics of nano-electromechanical systems
We develop a theory for the full counting statistics (FCS) for a class of
nanoelectromechanical systems (NEMS), describable by a Markovian generalized
master equation. The theory is applied to two specific examples of current
interest: vibrating C60 molecules and quantum shuttles. We report a numerical
evaluation of the first three cumulants for the C60-setup; for the quantum
shuttle we use the third cumulant to substantiate that the giant enhancement in
noise observed at the shuttling transition is due to a slow switching between
two competing conduction channels. Especially the last example illustrates the
power of the FCS.Comment: 7 pages, 3 figures; minor changes - final version as published in
Europhys. Let
Finding the optimum activation energy in DNA breathing dynamics: A Simulated Annealing approach
We demonstrate how the stochastic global optimization scheme of Simulated
Annealing can be used to evaluate optimum parameters in the problem of DNA
breathing dynamics. The breathing dynamics is followed in accordance with the
stochastic Gillespie scheme with the denaturation zones in double stranded DNA
studied as a single molecule time series. Simulated Annealing is used to find
the optimum value of the activation energy for which the equilibrium bubble
size distribution matches with a given value. It is demonstrated that the
method overcomes even large noise in the input surrogate data.Comment: 9 pages, 4 figures, iop article package include
On apparent breaking the second law of thermodynamics in quantum transport studies
We consider a model for stationary electronic transport through a
one-dimensional chain of two leads attached to a perturbed central region
(quantum dot) in the regime where the theory proposed recently by Capek for a
similar model of phonon transport predicts the striking phenomenon of a
permanent current between the leads. This result based on a rigorous but
asymptotic Davies theory is at variance with the zero current yielded by direct
transport calculations which can be carried out in the present model. We find
the permanent current to be within the error of the asymptotic expansion for
finite couplings, and identify cancelling terms of the same order.Comment: 5 pages, 3 figure
Analytical calculation of the excess current in the OTBK theory
We present an analytical derivation of the excess current in Josephson
junctions within the Octavio-Tinkham-Blonder-Klapwijk theory for both symmetric
and asymmetric barrier strengths. We confirm the result found numerically by
Flensberg et al. for equal barriers [Phys. Rev. B 38, 8707 (1988)], including
the prediction of negative excess current for low transparencies, and we
generalize it for differing barriers. Our analytical formulae provide for
convenient fitting of experimental data, also in the less studied, but
practically relevant case of the barrier asymmetry.Comment: 13 pages, 3 figures, submitted to Superconductor Science and
Technolog
Electron transport in single wall carbon nanotube weak links in the Fabry-Perot regime
We fabricated reproducible high transparency superconducting contacts
consisting of superconducting Ti/Al/Ti trilayers to gated single-walled carbon
nanotubes (SWCNTs). The reported semiconducting SWCNT have normal state
differential conductance up to and exhibit clear Fabry-Perot
interference patterns in the bias spectroscopy plot. We observed subharmonic
gap structure in the differential conductance and a distinct peak in the
conductance at zero bias which is interpreted as a manifestation of a
supercurrent. The gate dependence of this supercurrent as well as the excess
current are examined and compared to a coherent theory of superconducting point
contacts with good agreement.Comment: 10 pages, 4 figure
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