813 research outputs found
Dynamical localization and eigenstate localization in trap models
The one-dimensional random trap model with a power-law distribution of mean
sojourn times exhibits a phenomenon of dynamical localization in the case where
diffusion is anomalous: The probability to find two independent walkers at the
same site, as given by the participation ratio, stays constant and high in a
broad domain of intermediate times. This phenomenon is absent in dimensions two
and higher. In finite lattices of all dimensions the participation ratio
finally equilibrates to a different final value. We numerically investigate
two-particle properties in a random trap model in one and in three dimensions,
using a method based on spectral decomposition of the transition rate matrix.
The method delivers a very effective computational scheme producing numerically
exact results for the averages over thermal histories and initial conditions in
a given landscape realization. Only a single averaging procedure over disorder
realizations is necessary. The behavior of the participation ratio is compared
to other measures of localization, as for example to the states' gyration
radius, according to which the dynamically localized states are extended. This
means that although the particles are found at the same site with a high
probability, the typical distance between them grows. Moreover the final
equilibrium state is extended both with respect to its gyration radius and to
its Lyapunov exponent. In addition, we show that the phenomenon of dynamical
localization is only marginally connected with the spectrum of the transition
rate matrix, and is dominated by the properties of its eigenfunctions which
differ significantly in dimensions one and three.Comment: 10 pages, 10 figures, submitted to EPJ
Rescattering effects in laser-assisted electron-atom bremsstrahlung
Rescattering effects in nonresonant spontaneous laser-assisted electron-atom
bremsstrahlung (LABrS) are analyzed within the framework of time-dependent
effective-range (TDER) theory. It is shown that high energy LABrS spectra
exhibit rescattering plateau structures that are similar to those that are
well-known in strong field laser-induced processes as well as those that have
been predicted theoretically in laser-assisted collision processes. In the
limit of a low-frequency laser field, an analytic description of LABrS is
obtained from a rigorous quantum analysis of the exact TDER results for the
LABrS amplitude. This amplitude is represented as a sum of factorized terms
involving three factors, each having a clear physical meaning. The first two
factors are the exact field-free amplitudes for electron-atom bremsstrahlung
and for electron-atom scattering, and the third factor describes free electron
motion in the laser field along a closed trajectory between the first
(scattering) and second (rescattering) collision events. Finally, a
generalization of these TDER results to the case of LABrS in a Coulomb field is
discussed
Mud crab susceptibility to disease from white spot syndrome virus is species-dependent
<p>Abstract</p> <p>Background</p> <p>Based on a report for one species (<it>Scylla serrata</it>), it is widely believed that mud crabs are relatively resistant to disease caused by white spot syndrome virus (WSSV). We tested this hypothesis by determining the degree of susceptibility in two species of mud crabs, <it>Scylla olivacea </it>and <it>Scylla paramamosain</it>, both of which were identified by mitochondrial 16 S ribosomal gene analysis. We compared single-dose and serial-dose WSSV challenges on <it>S. olivacea </it>and <it>S. paramamosain</it>.</p> <p>Findings</p> <p>In a preliminary test using <it>S. olivacea </it>alone, a dose of 1 × 10<sup>6 </sup>WSSV copies/g gave 100% mortality within 7 days. In a subsequent test, 17 <it>S. olivacea </it>and 13 <it>S. paramamosain </it>were divided into test and control groups for challenge with WSSV at 5 incremental, biweekly doses starting from 1 × 10<sup>4 </sup>and ending at 5 × 10<sup>6 </sup>copies/g. For 11 <it>S. olivacea </it>challenged, 3 specimens died at doses between 1 × 10<sup>5 </sup>and 5 × 10<sup>5 </sup>copies/g and none died for 2 weeks after the subsequent dose (1 × 10<sup>6 </sup>copies/g) that was lethal within 7 days in the preliminary test. However, after the final challenge on day 56 (5 × 10<sup>6 </sup>copies/g), the remaining 7 of 11 <it>S. olivacea </it>(63.64%) died within 2 weeks. There was no mortality in the buffer-injected control crabs. For 9 <it>S. paramamosain </it>challenged in the same way, 5 (55.56%) died after challenge doses between 1 × 10<sup>4 </sup>and 5 × 10<sup>5 </sup>copies/g, and none died for 2 weeks after the challenge dose of 1 × 10<sup>6 </sup>copies/g. After the final challenge (5 × 10<sup>6 </sup>copies/g) on day 56, no <it>S. paramamosain </it>died during 2 weeks after the challenge, and 2 of 9 WSSV-infected <it>S. paramamosain </it>(22.22%) remained alive together with the control crabs until the end of the test on day 106. Viral loads in these survivors were low when compared to those in the moribund crabs.</p> <p>Conclusions</p> <p><it>S. olivacea </it>and <it>S. paramamosain </it>show wide variation in response to challenge with WSSV. <it>S. olivacea </it>and <it>S. paramamosain </it>are susceptible to white spot disease, and <it>S. olivacea </it>is more susceptible than <it>S. paramamosain</it>. Based on our single-challenge and serial challenge results, and on previous published work showing that <it>S. serrata </it>is relatively unaffected by WSSV infection, we propose that susceptibility to white spot disease in the genus <it>Scylla </it>is species-dependent and may also be dose-history dependent. In practical terms for shrimp farmers, it means that <it>S. olivacea </it>and <it>S. paramamosain </it>may pose less threat as WSSV carriers than <it>S. serrata</it>. For crab farmers, our results suggest that rearing of <it>S. serrata </it>would be a better choice than <it>S. paramamosain </it>or <it>S. olivacea </it>in terms of avoiding losses from seasonal outbreaks of white spot disease.</p
Plateau Structure in Resonant Laser-Assisted Electron-Atom Scattering
Orders of magnitude increases are predicted in the cross sections for electron-atom scattering accompanied by absorption or emission of n laser photons for incident electron energies at which the electron, by emitting μ laser photons, can be captured by the atom to form a negative ion. Enhancements are most significant in the plateau region (n \u3e\u3e μ) of the scattered electron spectrum, whose shape is predicted to replicate that of the ion’s (n + μ)-photon detachment spectrum
Resonant electron-atom bremsstrahlung in an intense laser field
We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for recombination or attachment to the intermediate state, and for ionization or detachment of this state. The high accuracy of our general analytic parametrization of the resonant BrS cross section is shown by comparison with exact numerical results for laser-assisted BrS spectra obtained within time-dependent effective range theory. Numerical estimates of resonant BrS in electron scattering from a Coulomb potential are also presented
Analytic description of elastic electron-atom scattering in an elliptically polarized laser field
An analytic description of laser-assisted electron-atom scattering (LAES) in an elliptically polarized field is presented using time-dependent effective range (TDER) theory to treat both electron-laser and electron-atom interactions nonperturbatively. Closed-form formulas describing plateau features in LAES spectra are derived quantum mechanically in the low-frequency limit. These formulas provide an analytic explanation for key features of the LAES differential cross section. For the low-energy region of the LAES spectrum, our result generalizes the Kroll-Watson formula to the case of elliptic polarization. For the high-energy (rescattering) plateau in the LAES spectrum, our result generalizes prior results for a linearly polarized field valid for the high-energy end of the rescattering plateau [Flegel et al., J. Phys. B 42, 241002 (2009)] and confirms the factorization of the LAES cross section into three factors: two field-free elastic electron-atom scattering cross sections (with laser-modified momenta) and a laser field-dependent factor (insensitive to the scattering potential) describing the laser-driven motion of the electron in the elliptically polarized field. We present also approximate analytic expressions for the exact TDER LAES amplitude that are valid over the entire rescattering plateau and reduce to the three-factor form in the plateau cutoff region. The theory is illustrated for the cases of e-H scattering in a CO2-laser field and e-F scattering in a midinfrared laser field of wavelength λ = 3.5 μm, for which the analytic results are shown to be in good agreement with exact numerical TDER results
Resonant phenomena in laser-assisted radiative attachment or recombination
Resonant enhancements are predicted in cross sections σn for laser-assisted radiative attachment or electron–ion recombination accompanied by absorption of n laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting μ laser photons followed by emission of a spontaneous photon upon absorbing n + μ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for σn and on numerical results for e−H attachment
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