16,273 research outputs found
Asymptotic stability at infinity for differentiable vector fields of the plane
Let X:R2\Dr->R2 be a differentiable (but not necessarily C1) vector field,
where r>0 and Dr={z\in R2:|z|\le r}. If for some e>0 and for all p\in R2\Dr, no
eigenvalue of D_p X belongs to (-e,0]\cup {z\in\C:\mathcal{R}(z)\ge 0}, then
(a)For all p\in R2\Dr, there is a unique positive semi--trajectory of X
starting at p; (b)\mathcal{I}(X), the index of X at infinity, is a well defined
number of the extended real line [-\infty,\infty); (c) There exists a constant
vector v\in R2 such that if \mathcal{I}(X) is less than zero (resp. greater or
equal to zero), then the point at infinity \infty of the Riemann sphere
R2\cup\set{\infty} is a repellor (resp. an attractor) of the vector field X+v.Comment: 16 pages, 7 figure
Observation of the Quantum Zeno and Anti-Zeno effects in an unstable system
We report the first observation of the Quantum Zeno and Anti-Zeno effects in
an unstable system. Cold sodium atoms are trapped in a far-detuned standing
wave of light that is accelerated for a controlled duration. For a large
acceleration the atoms can escape the trapping potential via tunneling.
Initially the number of trapped atoms shows strong non-exponential decay
features, evolving into the characteristic exponential decay behavior. We
repeatedly measure the number of atoms remaining trapped during the initial
period of non-exponential decay. Depending on the frequency of measurements we
observe a decay that is suppressed or enhanced as compared to the unperturbed
system.Comment: 4 pages, 5 figures, submitted to PR
Affine interval exchange transformations with flips and wandering intervals
There exist uniquely ergodic affine interval exchange transformations of
[0,1] with flips having wandering intervals and such that the support of the
invariant measure is a Cantor set.Comment: 7 page
Experimental Study of the Role of Atomic Interactions on Quantum Transport
We report an experimental study of quantum transport for atoms confined in a
periodic potential and compare between thermal and BEC initial conditions. We
observe ballistic transport for all values of well depth and initial
conditions, and the measured expansion velocity for thermal atoms is in
excellent agreement with a single-particle model. For weak wells, the expansion
of the BEC is also in excellent agreement with single-particle theory, using an
effective temperature. We observe a crossover to a new regime for the BEC case
as the well depth is increased, indicating the importance of interactions on
quantum transport.Comment: 4 pages, 3 figure
Unique ergodicity of circle and interval exchange transformations with flips
We study the existence of transitive exchange maps with flips defined on the
unit circle. We provide a complete answer to the question of whether there
exists a transitive exchange map of the unit circle defined on n subintervals
and having f flips.Comment: 13 pages, 6 figures; notational changes, smaller figure
Local mapping of dissipative vortex motion
We explore, with unprecedented single vortex resolution, the dissipation and
motion of vortices in a superconducting ribbon under the influence of an
external alternating magnetic field. This is achieved by combing the phase
sensitive character of ac-susceptibility, allowing to distinguish between the
inductive-and dissipative response, with the local power of scanning Hall probe
microscopy. Whereas the induced reversible screening currents contribute only
inductively, the vortices do leave a fingerprint in the out-of-phase component.
The observed large phase-lag demonstrates the dissipation of vortices at
timescales comparable to the period of the driving force (i.e. 13 ms). These
results indicate the presence of slow microscopic loss mechanisms mediated by
thermally activated hopping transport of vortices between metastable states.Comment: 5 pages, 2 figure
Behavioral Risks for West Nile Virus Disease, Northern Colorado, 2003
Protective practices may affect the level of illness in a community
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