22 research outputs found
Excitable media in open and closed chaotic flows
We investigate the response of an excitable medium to a localized
perturbation in the presence of a two-dimensional smooth chaotic flow. Two
distinct types of flows are numerically considered: open and closed. For both
of them three distinct regimes are found, depending on the relative strengths
of the stirring and the rate of the excitable reaction. In order to clarify and
understand the role of the many competing mechanisms present, simplified models
of the process are introduced. They are one-dimensional baker-map models for
the flow and a one-dimensional approximation for the transverse profile of the
filaments.Comment: 14 pages, 16 figure
Finite temperature excitations of a trapped Bose-Fermi mixture
We present a detailed study of the low-lying collective excitations of a
spherically trapped Bose-Fermi mixture at finite temperature in the
collisionless regime. The excitation frequencies of the condensate are
calculated self-consistently using the static Hartree-Fock-Bogoliubov theory
within the Popov approximation. The frequency shifts and damping rates due to
the coupled dynamics of the condensate, noncondensate, and degenerate Fermi gas
are also taken into account by means of the random phase approximation and
linear response theory. In our treatment, the dipole excitation remains close
to the bare trapping frequency for all temperatures considered, and thus is
consistent with the generalized Kohn theorem. We discuss in some detail the
behavior of monopole and quadrupole excitations as a function of the Bose-Fermi
coupling. At nonzero temperatures we find that, as the mixture moves towards
spatial separation with increasing Bose-Fermi coupling, the damping rate of the
monopole (quadrupole) excitation increases (decreases). This provides us a
useful signature to identify the phase transition of spatial separation.Comment: 10 pages, 8 figures embedded; to be published in Phys. Rev.
Emergent global oscillations in heterogeneous excitable media: The example of pancreatic beta cells
Using the standard van der Pol-FitzHugh-Nagumo excitable medium model I
demonstrate a novel generic mechanism, diversity, that provokes the emergence
of global oscillations from individually quiescent elements in heterogeneous
excitable media. This mechanism may be operating in the mammalian pancreas,
where excitable beta cells, quiescent when isolated, are found to oscillate
when coupled despite the absence of a pacemaker region.Comment: See home page http://lec.ugr.es/~julya
Experimental study of the quantum driven pendulum and its classical analogue in atoms optics
We present experimental results for the dynamics of cold atoms in a far detuned amplitude-modulated optical standing wave. Phase-space resonances constitute distinct peaks in the atomic momentum distribution containing up to 65% of all atoms resulting from a mixed quantum chaotic phase space. We characterize the atomic behavior in classical and quantum regimes and we present the applicable quantum and classical theory, which we have developed and refined. We show experimental proof that the size and the position of the resonances in phase space can be controlled by varying several parameters, such as the modulation frequency, the scaled well depth, the modulation amplitude, and the scaled Planck's constant of the system. We have found a surprising stability against amplitude noise. We present methods to accurately control the momentum of an ensemble of atoms using these phase-space resonances which could be used for efficient phase-space state preparation
Resonance Superfluidity: Renormalization of Resonance Scattering Theory
We derive a theory of superfluidity for a dilute Fermi gas that is valid when
scattering resonances are present. The treatment of a resonance in many-body
atomic physics requires a novel mean-field approach starting from an
unconventional microscopic Hamiltonian. The mean-field equations incorporate
the microscopic scattering physics, and the solutions to these equations
reproduce the energy-dependent scattering properties. This theory describes the
high- behavior of the system, and predicts a value of which is a
significant fraction of the Fermi temperature. It is shown that this novel
mean-field approach does not break down for typical experimental circumstances,
even at detunings close to resonance. As an example of the application of our
theory we investigate the feasibility for achieving superfluidity in an
ultracold gas of fermionic Li.Comment: 15 pages, 10 figure
Assessing the feasibility of interrupting the transmission of soil-transmitted helminths through mass drug administration: The DeWorm3 cluster randomized trial protocol
DeWorm3 collectionThe file attached is the Published/publisher’s pdf version of the article.© 2018 Ásbjörnsdóttir et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Viral infection as a regulator of oceanic phytoplankton populations
Viruses are the most abundant organism in seawater across all the world's oceans. Though they are believed to be capable of infecting all phytoplankton species their role in regulating plankton population levels is not well understood. In order to gain an understanding of the potential influence of viruses on phytoplankton population dynamics, particularly ‘blooms’, two plankton ecosystem models with explicit representation of viruses and virally infected phytoplankton are presented, and an initial investigation into their range of behaviours explored. The models are extensions of well-established plankton ecosystem models that now permit the possibility of viral infection and mortality of phytoplankton. Ecological and epidemiological parameters from a number of sources are used to furnish the models. The models are shown to be capable of capturing known features of phytoplankton population dynamics in the presence of viruses: viruses can stably co-exist in the plankton ecosystem without the need to invoke other stabilising processes, and infection can serve to suppress primary production and phytoplankton abundance whilst boosting nutrient levels. Intuitively, viral infection will be most effective when phytoplankton is high. We therefore use the two models to investigate the influence of viral infection on ‘blooms’ in two independent ways: first with a seasonally-forced variability and second with a triggered transient event. It is demonstrated that the impact of viruses can be very noticeable during episodes of enhanced phytoplankton density found during ’blooms’. Viruses serve to attenuate the intensity and duration of these transient events in a manner consistent with observations