8,794 research outputs found
Observability and Synchronization of Neuron Models
Observability is the property that enables to distinguish two different
locations in -dimensional state space from a reduced number of measured
variables, usually just one. In high-dimensional systems it is therefore
important to make sure that the variable recorded to perform the analysis
conveys good observability of the system dynamics. In the case of networks
composed of neuron models, the observability of the network depends
nontrivially on the observability of the node dynamics and on the topology of
the network. The aim of this paper is twofold. First, a study of observability
is conducted using four well-known neuron models by computing three different
observability coefficients. This not only clarifies observability properties of
the models but also shows the limitations of applicability of each type of
coefficients in the context of such models. Second, a multivariate singular
spectrum analysis (M-SSA) is performed to detect phase synchronization in
networks composed by neuron models. This tool, to the best of the authors'
knowledge has not been used in the context of networks of neuron models. It is
shown that it is possible to detect phase synchronization i)~without having to
measure all the state variables, but only one from each node, and ii)~without
having to estimate the phase
Weak localization in multiterminal networks of diffusive wires
We study the quantum transport through networks of diffusive wires connected
to reservoirs in the Landauer-B\"uttiker formalism. The elements of the
conductance matrix are computed by the diagrammatic method. We recover the
combination of classical resistances and obtain the weak localization
corrections. For arbitrary networks, we show how the cooperon must be properly
weighted over the different wires. Its nonlocality is clearly analyzed. We
predict a new geometrical effect that may change the sign of the weak
localization correction in multiterminal geometries.Comment: 4 pages, LaTeX, 4 figures, 8 eps file
Single File Diffusion of particles with long ranged interactions: damping and finite size effects
We study the Single File Diffusion (SFD) of a cyclic chain of particles that
cannot cross each other, in a thermal bath, with long ranged interactions, and
arbitrary damping. We present simulations that exhibit new behaviors
specifically associated to systems of small number of particles and to small
damping. In order to understand those results, we present an original analysis
based on the decomposition of the particles motion in the normal modes of the
chain. Our model explains all dynamic regimes observed in our simulations, and
provides convincing estimates of the crossover times between those regimes.Comment: 30 pages, 9 figure
Current noise through a Kondo quantum dot in a SU(N) Fermi liquid state
The current noise through a mesoscopic quantum dot is calculated and analyzed
in the Fermi liquid regime of the SU(N) Kondo model. Results connect the
Johnson-Nyquist noise to the shot noise for an arbitrary ratio of voltage and
temperature, and show that temperature corrections are sizeable in usual
experiments. For the experimentally relevant SU(4) case, quasiparticle
interactions are shown to increase the shot noise.Comment: 4 pages, 2 figures, to be published in Phys. Rev. Lett. (revised
version
Dephasing due to electron-electron interaction in a diffusive ring
We study the effect of the electron-electron interaction on the weak
localization correction of a ring pierced by a magnetic flux. We compute
exactly the path integral giving the magnetoconductivity for an isolated ring.
The results are interpreted in a time representation. This allows to
characterize the nature of the phase coherence relaxation in the ring. The
nature of the relaxation depends on the time regime (diffusive or ergodic) but
also on the harmonics of the magnetoconductivity. Whereas phase coherence
relaxation is non exponential for the harmonic , it is always exponential
for harmonics . Then we consider the case of a ring connected to
reservoirs and discuss the effect of connecting wires. We recover the behaviour
of the harmonics predicted recently by Ludwig & Mirlin for a large perimeter
(compared to the Nyquist length). We also predict a new behaviour when the
Nyquist length exceeds the perimeter.Comment: 21 pages, RevTeX4, 8 eps figures; version of 10/2006 : eqs.(100-102)
of section V.C correcte
Dark energy from primordial inflationary quantum fluctuations
We show that current cosmic acceleration can be explained by an almost
massless scalar field experiencing quantum fluctuations during primordial
inflation. Provided its mass does not exceed the Hubble parameter today, this
field has been frozen during the cosmological ages to start dominating the
universe only recently. By using supernovae data, completed with baryonic
acoustic oscillations from galaxy surveys and cosmic microwave background
anisotropies, we infer the energy scale of primordial inflation to be around a
few TeV, which implies a negligible tensor-to-scalar ratio of the primordial
fluctuations. Moreover, our model suggests that inflation lasted for an
extremely long period. Dark energy could therefore be a natural consequence of
cosmic inflation close to the electroweak energy scale.Comment: 5 pages, 2 figures, uses RevTeX. Physical discussion extended,
misprints corrected, references added. Matches published versio
An extremal model for amorphous media plasticity
An extremal model for the plasticity of amorphous materials is studied in a
simple two-dimensional anti-plane geometry. The steady-state is analyzed
through numerical simulations. Long-range spatial and temporal correlations in
local slip events are shown to develop, leading to non-trivial and highly
anisotropic scaling laws. In particular, the plastic strain is shown to
statistically concentrate over a region which tends to align perpendicular to
the displacement gradient. By construction, the model can be seen as giving
rise to a depinning transition, the threshold of which (i.e. the macroscopic
yield stress) also reveal scaling properties reflecting the localization of the
activity.Comment: 4 pages, 5 figure
Four-body Efimov effect
We study three same spin state fermions of mass M interacting with a
distinguishable particle of mass m in the unitary limit where the interaction
has a zero range and an infinite s-wave scattering length. We predict an
interval of mass ratio 13.384 < M/m < 13.607 where there exists a purely
four-body Efimov effect, leading to the occurrence of weakly bound tetramers
without Efimov trimers.Comment: 4 pages, 2 figure
Coexisting ordinary elasticity and superfluidity in a model of defect-free supersolid
We present the mechanics of a model of supersolid in the frame of the
Gross-Pitaevskii equation at that do not require defects nor vacancies.
A set of coupled nonlinear partial differential equations plus boundary
conditions is derived. The mechanical equilibrium is studied under external
constrains as steady rotation or external stress. Our model displays a
paradoxical behavior: the existence of a non classical rotational inertia
fraction in the limit of small rotation speed and no superflow under small (but
finite) stress nor external force. The only matter flow for finite stress is
due to plasticity.Comment: 6 pages, 2 figure
Quantitative pretreatment VOI analysis of liver metastases 99mTc-MAA SPECT/CT and FDG PET/CT in relation with treatment response to SIRT
Using quantitive VOI analysis, the percentage Tc-99m-MAA uptake and SUVmax and mean values of liver metastases obtained prior to SIRT were related to treatment response using both a lesion-based and clinical dichotomous approach. Based on the VOI % of Tc-99m-MAA activity, the estimated Y-90-microspheres activity/cc (MBq/cc) was calculated from the effective dose injected. Baseline VOI FDG PET SUVmean and max values and estimated MBq/cc values were related to treatment response using a lesion-based approach (% change in SUVmean >= 50%) and a clinical dichotomous approach. Fifteen treatment sessions were analyzed (13 patients). Using the lesion-based approach (12 treatment sessions) 40 lesions responded and 37 did not. SUVmax and mean values proved significantly different between non-responding and responding lesions; 18:6 (SD 10.8) versus 13.5 (SD 8.4) for SUVmax (p = 0.02) and 11.4 (SD 3.8) versus 6.3 (SD 4.5) for SUVmean (p = 0.002). Using the clinical dichotomous approach (15 treatment sessions / 11 responding), 91 lesions were analyzed; 57 responded. VOI volumes and estimated Y-90-loaded glass microspheres activity (MBq/cc) did not differ between responders and non responders; 24 cc (SD 27) versus 21 cc (SD 21 cc) (p = 0.4) and 1.95 MBq/cc (SD 1.1 MBq/cc) versus 1.90 MB/cc (SD 2.7 MBq/cc) (p = 0.92). On the contrary, SUVmax and mean values proved significantly different between responders and non-responders; 23.7 (SD 9.8) versus 9.4 (SD 3.8) for SUVmax (p = 0.0001) and 13.1 (SD 8.1) versus 4.9 (SD 1.4) for SUVmean. Conclusion: These findings suggest that in patients presenting with high baseline SUVmax and mean values, the administration of higher activities or alternatively, other potentially more useful treatment options might be considered
- …