255 research outputs found
Engineering transport by concatenated maps
We present a generalized kick rotor model in which the phase of the kick can
vary from kick to kick. This additional freedom allows one to control the
transport in phase space. For a specific choice of kick-to-kick phases, we
predict novel forms of accelerator modes which are potentially of high
relevance for future experimental studies
Controlling the Momentum Current of an Off-resonant Ratchet
We experimentally investigate the phenomenon of a quantum ratchet created by
exposing a Bose-Einstein Condensate to short pulses of a potential which is
periodic in both space and time. Such a ratchet is manifested by a directed
current of particles, even though there is an absence of a net bias force. We
confirm a recent theoretical prediction [M. Sadgrove and S. Wimberger, New J.
Phys. \textbf{11}, 083027 (2009)] that the current direction can be controlled
by experimental parameters which leave the underlying symmetries of the system
unchanged. We demonstrate that this behavior can be understood using a single
variable containing many of the experimental parameters and thus the ratchet
current is describable using a single universal scaling law.Comment: arXiv admin note: substantial text overlap with arXiv:1210.565
Reliance on exotic plants by two groups of threatened samango monkeys, Cercopithecus albogularis labiatus, at their southern range limit
Understanding how threatened species adapt their behavior to landscapes shaped by humans is increasingly important to ensuring they persist in a changing world. Matrix habitats can be shared spaces where human and nonhuman primates coexist. We set out to determine how an endemic, threatened forest specialist, the frugivorous, arboreal samango monkey (Cercopithecus albogularis labiatus), has responded to a matrix habitat made up of residential gardens and commercial plantations in Eastern Cape province, South Africa. We followed two groups from dawn to dusk for a mean of 3 days/mo for 12 mo (February 1, 2011 to January 31, 2012) using scan sampling to collect data on their diet, activity, and ranging patterns. We used resource abundance transects to describe the groups’ home ranges and monitored tree phenology to calculate fruit and seed availability indices. Monkeys from both groups consumed large quantities of exotic plant species, accounting for >50% of their overall annual diet, with seeds of the invasive black wattle (Acacia mearnsii) the most commonly consumed exotic species followed by acorns of two oak species (Quercus robur and Q. palustris.). However, monkeys responded to the availability of indigenous rather than exotic fruits and seeds and increased their consumption of exotics when indigenous fruits were less available. Although monkeys spent less time moving when feeding on exotic species compared to indigenous species, eating exotics did not free up monkeys’ time to rest or socialize, as additional time was required to process exotic foods. To offset the possible negative consequences of the monkeys’ reliance on exotic seeds, including escalating conflict between monkeys and people in gardens, we suggest gradual removal of exotic plant species in the habitat and replacement with indigenous species as one mitigation strategy
Can quantum fractal fluctuations be observed in an atom-optics kicked rotor experiment?
We investigate the parametric fluctuations in the quantum survival
probability of an open version of the delta-kicked rotor model in the deep
quantum regime. Spectral arguments [Guarneri I and Terraneo M 2001 Phys. Rev. E
vol. 65 015203(R)] predict the existence of parametric fractal fluctuations
owing to the strong dynamical localisation of the eigenstates of the kicked
rotor. We discuss the possibility of observing such dynamically-induced
fractality in the quantum survival probability as a function of the kicking
period for the atom-optics realisation of the kicked rotor. The influence of
the atoms' initial momentum distribution is studied as well as the dependence
of the expected fractal dimension on finite-size effects of the experiment,
such as finite detection windows and short measurement times. Our results show
that clear signatures of fractality could be observed in experiments with cold
atoms subjected to periodically flashed optical lattices, which offer an
excellent control on interaction times and the initial atomic ensemble.Comment: 18 pp, 7 figs., 1 tabl
Nonlinear resonant tunneling of Bose-Einstein condensates in tilted optical lattices
We study the tunneling decay of a Bose-Einstein condensate out of tilted
optical lattices within the mean-field approximation. We introduce a novel
method to calculate also excited resonance eigenstates of the Gross-Pitaevskii
equation, based on a grid relaxation procedure with complex absorbing
potentials. This algorithm works efficiently in a wide range of parameters
where established methods fail. It allows us to study the effects of the
nonlinearity in detail in the regime of resonant tunneling, where the decay
rate is enhanced by resonant coupling to excited unstable states.Comment: Revised and enlarged version, including 1 additional figur
Steering random walks with kicked ultracold atoms
A kicking sequence of the atom optics kicked rotor at quantum resonance can
be interpreted as a quantum random walk in momentum space. We show how to steer
such a random walk by applying a random sequence of intensities and phases of
the kicking lattice chosen according to a probability distribution. This
distribution converts on average into the final momentum distribution of the
kicked atoms. In particular, it is shown that a power-law distribution for the
kicking strengths results in a L\'evy walk in momentum space and in a power-law
with the same exponent in the averaged momentum distribution. Furthermore, we
investigate the stability of our predictions in the context of a realistic
experiment with Bose-Einstein condensates.Comment: detailed study of random walks and their implementation with a Bose
condensate, 12 pages, 7 figure
Europa. Central. Maniobras de campaña. 18 (1796). 1:862998
Relieve sombreadoOrientado con flech
Dynamical tunneling of a Bose-Einstein condensate in periodically driven systems
We report measurements of dynamical tunneling rates of a Bose-Einstein
condensate across a barrier in classical phase space. The atoms are initially
prepared in quantum states that extend over a classically regular island
region. We focus on the specific system of quantum accelerator modes of the
kicked rotor in the presence of gravity. Our experimental data is supported by
numerical simulations taking into account imperfections mainly from spontaneous
emission. Furthermore, we predict experimentally accessible parameter ranges
over which direct tunneling could be readily observed if spontaneous emission
was further suppressed. Altogether, we provide a proof-of-principle for the
experimental accessibility of dynamical tunneling rates in periodically driven
systems.Comment: Improved versio
Allometric trajectories of body and head morphology in three sympatric Arctic charr (Salvelinus alpinus (L.)) morphs
A study of body and head development in three sympatric reproductively isolated Arctic charr (Salvelinus alpinus (L.)) morphs from a subarctic lake (Skogsfjordvatn, northern Norway) revealed allometric trajectories that resulted in morphological differences. The three morphs were ecologically assigned to a littoral omnivore, a profundal benthivore and a profundal piscivore, and this was confirmed by genetic analyses (microsatellites). Principal component analysis was used to identify the variables responsible for most of the morphological variation of the body and head shape. The littoral omnivore and the profundal piscivore morph had convergent allometric trajectories for the most important head shape variables, developing bigger mouths and relatively smaller eyes with increasing head size. The two profundal morphs shared common trajectories for the variables explaining most of the body and head shape variation, namely head size relative to body size, placement of the dorsal and pelvic fins, eye size and mouth size. In contrast, the littoral omnivore and the profundal benthivore morphs were not on common allometric trajectories for any of the examined variables. The findings suggest that different selective pressures could have been working on traits related to their trophic niche such as habitat and diet utilization of the three morphs, with the two profundal morphs experiencing almost identical environmental conditions
Resonant forcing of select degrees of freedom of multidimensional chaotic map dynamics
We study resonances of multidimensional chaotic map dynamics. We use the
calculus of variations to determine the additive forcing function that induces
the largest response, that is, the greatest deviation from the unperturbed
dynamics. We include the additional constraint that only select degrees of
freedom be forced, corresponding to a very general class of problems in which
not all of the degrees of freedom in an experimental system are accessible to
forcing. We find that certain Lagrange multipliers take on a fundamental
physical role as the efficiency of the forcing function and the effective
forcing experienced by the degrees of freedom which are not forced directly.
Furthermore, we find that the product of the displacement of nearby
trajectories and the effective total forcing function is a conserved quantity.
We demonstrate the efficacy of this methodology with several examples.Comment: 11 pages, 3 figure
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