491 research outputs found
Metachronal wave and hydrodynamic interaction for deterministic switching rowers
We employ a model system, called rowers, as a generic physical framework to
define the problem of the coordinated motion of cilia (the metachronal wave) as
a far from equilibrium process. Rowers are active (two-state) oscillators
interacting solely through forces of hydrodynamic origin. In this work, we
consider the case of fully deterministic dynamics, find analytical solutions of
the equation of motion in the long wavelength (continuum) limit, and
investigate numerically the short wavelength limit. We prove the existence of
metachronal waves below a characteristic wavelength. Such waves are unstable
and become stable only if the sign of the coupling is reversed. We also find
that with normal hydrodynamic interaction the metachronal pattern has the form
of stable trains of traveling wave packets sustained by the onset of
anti-coordinated beating of consecutive rowers.Comment: 11 pages, 7 figure
Chaos in Pseudo-Newtonian Black Holes with Halos
The Newtonian as well as the special relativistic dynamics are used to study
the stability of orbits of a test particle moving around a black hole plus a
dipolar halo. The black hole is modeled by either the usual monopole potential
or the Paczynki-Wiita pseudo-Newtonian potential. The full general relativistic
similar case is also considered. The Poincare section method and the Lyapunov
characteristic exponents show that the orbits for the pseudo-Newtonian
potential models are more unstable than the corresponding general relativistic
geodesics.Comment: RevTEX, 7 eps figures, to appear in Astron Astrophy
Very low shot noise in carbon nanotubes
We have performed noise measurements on suspended ropes of single wall carbon
nanotubes (SWNT) between 1 and 300 K for different values of dc current through
the ropes. We find that the shot noise is suppressed by more than a factor 100
compared to the full shot noise 2eI. We have also measured an individual SWNT
and found a level of noise which is smaller than the minimum expected. Another
finding is the very low level of 1/f noise, which is significantly lower than
previous observations. We propose two possible interpretations for this strong
shot noise reduction: i) Transport within a rope takes place through a few
nearly ballistic tubes within a rope and possibly involves non integer
effective charges. ii) A substantial fraction of the tubes conduct with a
strong reduction of effective charge (by more than a factor 50).Comment: Submitted to Eur. Phys. J. B (Jan. 2002) Higher resolution pictures
are posted on http://www.lps.u-psud.fr/Collectif/gr_07/publications.htm
Fast Witness Extraction Using a Decision Oracle
The gist of many (NP-)hard combinatorial problems is to decide whether a
universe of elements contains a witness consisting of elements that
match some prescribed pattern. For some of these problems there are known
advanced algebra-based FPT algorithms which solve the decision problem but do
not return the witness. We investigate techniques for turning such a
YES/NO-decision oracle into an algorithm for extracting a single witness, with
an objective to obtain practical scalability for large values of . By
relying on techniques from combinatorial group testing, we demonstrate that a
witness may be extracted with queries to either a deterministic or
a randomized set inclusion oracle with one-sided probability of error.
Furthermore, we demonstrate through implementation and experiments that the
algebra-based FPT algorithms are practical, in particular in the setting of the
-path problem. Also discussed are engineering issues such as optimizing
finite field arithmetic.Comment: Journal version, 16 pages. Extended abstract presented at ESA'1
Superconductivity in ropes of carbon nanotubes
Recent experimental and theoretical results on intrinsic superconductivity in
ropes of single-wall carbon nanotubes are reviewed and compared. We find strong
experimental evidence for superconductivity when the distance between the
normal electrodes is large enough. This indicates the presence of attractive
phonon-mediated interactions in carbon nanotubes, which can even overcome the
repulsive Coulomb interactions. The effective low-energy theory of rope
superconductivity explains the experimental results on the
temperature-dependent resistance below the transition temperature in terms of
quantum phase slips. Quantitative agreement with only one fit parameter can be
obtained. Nanotube ropes thus represent superconductors in an extreme 1D limit
never explored before.Comment: 19 pages, 9 figures, to appear in special issue of Sol. State Com
Hydrodynamic flow patterns and synchronization of beating cilia
We calculate the hydrodynamic flow field generated far from a cilium which is
attached to a surface and beats periodically. In the case of two beating cilia,
hydrodynamic interactions can lead to synchronization of the cilia, which are
nonlinear oscillators. We present a state diagram where synchronized states
occur as a function of distance of cilia and the relative orientation of their
beat. Synchronized states occur with different relative phases. In addition,
asynchronous solutions exist. Our work could be relevant for the synchronized
motion of cilia generating hydrodynamic flows on the surface of cells.Comment: 5 pages, 4 figures, v2: minor correction
Superconductivity in Ropes of Single-Walled Carbon Nanotubes
We report measurements on ropes of Single Walled Carbon Nanotubes (SWNT) in
low-resistance contact to non-superconducting (normal) metallic pads, at low
voltage and at temperatures down to 70 mK. In one sample, we find a two order
of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic
field of the order of 1T, or by a d.c. current greater than 2.5 microA. These
features strongly suggest the existence of superconductivity in ropes of SWNT.Comment: Accepted for publication in Phys. Rev. Let
- …