42 research outputs found
The Aharonov-Bohm effect for an exciton
We study theoretically the exciton absorption on a ring shreded by a magnetic
flux. For the case when the attraction between electron and hole is
short-ranged we get an exact solution of the problem. We demonstrate that,
despite the electrical neutrality of the exciton, both the spectral position of
the exciton peak in the absorption, and the corresponding oscillator strength
oscillate with magnetic flux with a period ---the universal flux
quantum. The origin of the effect is the finite probability for electron and
hole, created by a photon at the same point, to tunnel in the opposite
directions and meet each other on the opposite side of the ring.Comment: 13 RevTeX 3.0 pages plus 4 EPS-figures, changes include updated
references and an improved chapter on possible experimental realization
The Nature of the Cold Filaments in the California Current System
Data from the Coastal Transition Zone (CTZ) experiment axe used to describe the velocity
fields and water properties associated with cold filaments in the California Current. Combined
with previous field surveys and satellite imagery, these show seasonal variability with maximum
dynamic height ranges and velocities in summer and minimum values in late winter and early
spring. North of Point Arena (between 39 degrees N and 42 degrees N) in spring-summer the flow field on the
outer edge of the cold water has the character of a meandering jet, carrying fresh, nutrient-poor
water from farther north on its offshore side and cold, salty, nutrient-rich water on its inshore
side. At Point Arena in midsummer, the jet often flows offshore and continues south without
meandering back onshore as strongly as it does farther north. The flow field south of Point Arena
in summer takes on more of the character a field of mesoscale eddies, although the meandering
jet from the north continues to be identifiable. The conceptual model for the May-July period
between 36 degrees N and 42 degrees N is thus of a surface jet that meanders through and interacts with a field
of eddies; the eddies are more dominant south of 39 degrees N, where the jet broadens and where multiple
jets and filaments are often present. At the surface, the jet often separates biological communities
and may appear as a barrier to cross-jet transport, especially north of Point Arena early in the
season (March-May). However, phytoplankton pigment and nutrients are carried on the inshore
flank of the jet, and pigment maxima are sometimes found in the core of the jet. The biological
effect of the jet is to define a convoluted, 100 to 400-km-wide region next to the coast, within
which much of the richer water is contained, and also to carry some of that richer water offshore
in meanders along the outer edge of that region.The CTZ program was funded by the Coastal Sciences Program of the Office of Naval Research (Code 1122CS). Support for PTS was provided by ONR grants N00014-87K0009 and N00014-90J1115, with additional support provided by NASA grants NAGW-869 and NAGW-1251
The Effect of Herbicide on the Re-Establishment of Native Grasses in the Blackland Prairie
Falls in people with chronic obstructive pulmonary disease: An observational cohort study
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Modelling oil absorption during post-frying cooling - I: model development
Several studies have highlighted the importance of the cooling period in oil absorption in deep-fat fried products. Specifically, it has been established that the largest proportion of oil which ends up into the food, is sucked into the porous crust region after the fried product is removed from the oil bath, stressing the importance of this time interval. The main objective of this paper was to develop a predictive mechanistic model that can be used to understand the principles behind post-frying cooling oil absorption kinetics, which can also help identifying the key parameters that affect the final oil intake by the fried product. The model was developed for two different geometries, an infinite slab and an infinite cylinder, and was divided into two main sub-models, one describing the immersion frying period itself and the other describing the post-frying cooling period. The immersion frying period was described by a transient moving-front model that considered the movement of the crust/core interface, whereas post-frying cooling oil absorption was considered to be a pressure driven flow mediated by capillary forces. A key element in the model was the hypothesis that oil suction would only begin once a positive pressure driving force had developed. The mechanistic model was based on measurable physical and thermal properties, and process parameters with no need of empirical data fitting, and can be used to study oil absorption in any deep-fat fried product that satisfies the assumptions made