11 research outputs found
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
Localization of interacting electrons in quantum dot arrays driven by an ac-field
We investigate the dynamics of two interacting electrons moving in a
one-dimensional array of quantum dots under the influence of an ac-field. We
show that the system exhibits two distinct regimes of behavior, depending on
the ratio of the strength of the driving field to the inter-electron Coulomb
repulsion. When the ac-field dominates, an effect termed coherent destruction
of tunneling occurs at certain frequencies, in which transport along the array
is suppressed. In the other, weak-driving, regime we find the surprising result
that the two electrons can bind into a single composite particle -- despite the
strong Coulomb repulsion between them -- which can then be controlled by the
ac-field in an analogous way. We show how calculation of the Floquet
quasienergies of the system explains these results, and thus how ac-fields can
be used to control the localization of interacting electron systems.Comment: 7 pages, 6 eps figures V2. Minor changes, this version to be
published in Phys. Rev.
Location of crossings in the Floquet spectrum of a driven two-level system
Calculation of the Floquet quasi-energies of a system driven by a
time-periodic field is an efficient way to understand its dynamics. In
particular, the phenomenon of dynamical localization can be related to the
presence of close approaches between quasi-energies (either crossings or
avoided crossings). We consider here a driven two-level system, and study how
the locations of crossings in the quasi-energy spectrum alter as the field
parameters are changed. A perturbational scheme provides a direct connection
between the form of the driving field and the quasi-energies which is exact in
the limit of high frequencies. We firstly obtain relations for the
quasi-energies for some common types of applied field in the high-frequency
limit. We then show how the locations of the crossings drift as the frequency
is reduced, and find a simple empirical formula which describes this drift
extremely well in general, and appears to be exact for the specific case of
square-wave driving.Comment: 6 pages, 6 figures. Minor changes to text, this version to be
published in Physical Review
Species sensitivity distributions for suspended clays, sediment burial and grain size change in the marine environment
Assessment of the environmental risk of discharges, containing both chemicals and suspended solids (e.g., drilling discharges to the marine environment), requires an evaluation of the effects of both toxic and nontoxic pollutants. To date, a structured evaluation scheme that can be used for prognostic risk assessments for nontoxic stress is lacking. In the present study we challenge this lack of information by the development of marine species sensitivity distributions (SSDs) for three nontoxic stressors: suspended clays, burial by sediment, and change in sediment grain size. Through a literature study, effect levels were obtained for suspended clays, as well as for burial of biota. Information on the species preference range for median grain size was used to assess the sensitivity of marine species to changes in grain size. The 50% hazardous concentrations (HC50) for suspended barite and bentonite based on 50% effect concentrations (EC50s) were 3,010 and 1,830 mg/L, respectively. For burial the 50% hazardous level (HL50) was 5.4 cm. For change in median grain size, two SSDs were constructed; one for reducing and one for increasing the median grain size. The HL50 for reducing the median grain size was 17.8 ¿m. For increasing the median grain size this value was 305 ¿m. The SSDs have been constructed by using information related to offshore oil- and gas-related activities. Nevertheless, the results of the present study may have broader implications. The hypothesis of the present study is that the SSD methodology developed for the evaluation of toxic stress can also be applied to evaluate nontoxic stressors, facilitating the incorporation of nontoxic stressors in prognostic risk assessment tools
An evolutionary approach to practical constraints in scheduling: a case-study of the wine bottling problem
Practical constraints associated with real-world problems are a key differentiator with respect to more artificially formulated problems. They create challenging variations on what might otherwise be considered as straightforward optimization problems from an evolutionary computation perspective. Through solving various commercial and industrial problems using evolutionary algorithms, we have gathered experience in dealing with practical dynamic constraints. Here, we present proven methods for dealing with these issues for scheduling problems. For use in real-world situations, an evolutionary algorithm must be designed to drive a software application that needs to be robust enough to deal with practical constraints in order to meet the demands and expectations of everyday use by domain specialists who are not necessarily optimization experts. In such situations, addressing these issues becomes critical to success. We show how these challenges can be dealt with by making adjustments to genotypic representation, phenotypic decoding, or the evaluation function itself. The ideas presented in this chapter are exemplified by the means of a case study of a real-world commercial problem, namely that of bottling wine in a mass-production environment. The methods described have the benefit of having been proven by a full-fledged implementation into a software application that undergoes continual and vigorous use in a live environment in which time-varying constraints, arising in multiple different combinations, are a routine occurrence.Arvind Mohais, Sven Schellenberg, Maksud Ibrahimov, Neal Wagner, and Zbigniew Michalewic
An accelerated buoyancy adhesion assay combined with 3-D morphometric analysis for assessing osteoblast adhesion on microgrooved substrata
Weak-coupling-like time evolution of driven four-level systems in the strong-coupling regime
Interpreting cancer genomes using systematic host network perturbations by tumour virus proteins
Molecular basis of virus replication, viral pathogenesis and antiviral strategie