4,516 research outputs found
Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields
We present a calculation of exciton states in semiconductor coupled quantum
wells (CQWs) in the presence of electric and magnetic fields applied
perpendicular to the QW plane. The exciton Schr\"odinger equation is solved in
real space in three dimensions to obtain the Landau levels of both direct and
indirect excitons. Calculation of the exciton energy levels and oscillator
strengths enables mapping of the electric and magnetic field dependence of the
exciton absorption spectrum. For the ground state of the system, we evaluate
the Bohr radius, optical lifetime, binding energy and dipole moment. The
exciton mass renormalization due to the magnetic field is calculated using a
perturbative approach. We predict a non-monotonous dependence of the exciton
ground state effective mass on magnetic field. Such a trend is explained in a
classical picture, in terms of the ground state tending from an indirect to a
direct exciton with increasing magnetic field.Comment: 20 pages, 7 figure
Apollo experience report: A use of network simulation techniques in the design of the Apollo lunar surface experiments package support system
A case study of data-communications network modeling and simulation is presented. The applicability of simulation techniques in early system design phases is demonstrated, and the ease with which model parameters can be changed and comprehensive statistics gathered is shown. The discussion of the model design and application also yields an insight into the design and implementation of the Apollo lunar surface experiments package ground-support system
Terahertz emitters based on microcavity dipolaritons
We propose the use of dipolaritons -- quantum well excitons with large dipole
moment, coupled to a planar microcavity -- for generating terahertz (THz)
radiation. This is achieved by exciting the system with two THz detuned lasers
that leads to dipole moment oscillations of the exciton polariton at the
detuning frequency, thus generating a THz emission. We have optimized the
structural parameters of a system with microcavity embedded AlGaAs double
quantum wells and shown that the THz emission intensity is maximized if the
laser frequencies both match different dipolariton states. The influence of the
electronic tunnel coupling between the wells on the frequency and intensity of
the THz radiation is also investigated, demonstrating a trade-off between the
polariton dipole moment and the Rabi splitting.Comment: 4 pages, 4 figures. This article has been submitted to Applied
Physics Letter
Drift-diffusion model of the fragmentation of the external ring structure in the photoluminescence pattern of indirect excitons in coupled quantum wells
Under optical excitation, coupled quantum wells are known to reveal
fascinating features in the photoluminescence pattern originating from dipole
orientated indirect excitons. The appearance of an external ring has been
attributed to macroscopic charge separation in the quantum well plane. We
present a classical model of non-linear diffusion to account for the observed
fragmentation of the external ring into a periodic array of islands. The model
incorporates the Coulomb interactions between electrons, holes and indirect
excitons. At low temperatures, these interactions lead to pattern formation
similar to the experimentally observed ring fragmentation. The fragmentation is
found to persist to temperatures above the quantum degeneracy temperature of
indirect excitons.Comment: 5 pages, 3 figure
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Evolution of a transdisciplinary "One Medicine-One Health" approach to global health education at the University of California, Davis.
In today's world health events in one nation or geographic area often have repercussions for the health and well-being of populations beyond that region; sometimes even globally. In recent years many factors, most notably concern about emerging infectious diseases, have contributed to an increasing appreciation of the interdependency of human, animal and ecosystem health worldwide. Integrated global approaches to improve the health of humans, animals and their shared environments are proving to be in the best interest of many countries. A special symposium and award were established in memoriam to the internationally renowned epidemiologist, Dr. Calvin W. Schwabe, who (while at the University of California, Davis) was a significant advocate of the "One Medicine" approach to public health, calling upon all health professionals, including veterinarians, to work collaboratively and comparatively to improve human health. This paper discusses the evolution of the "One Medicine" concept into a global "One Health" approach to research, training capacity and service infrastructure, focused not only on disease, but also on health at the individual, population, and ecosystem levels. Projects involving UCD faculty which attempt to integrate a One Health approach include the Health for Animals and Livelihood Improvement (HALI) Project in Tanzania, Envirovet Summer Institute, Avian Flu School and Newcastle Immunization Program in Africa, a web-based virtual global health training program, and the Calvin Schwabe One Health Project
A methodological analysis of national models of integration: time to think without the models?
National models of integration are widely used to understand the relationship between nationalism and integration-immigration policies. In this methodological article, we highlight two key concerns. First, national models of integration emerged out of inductive and normative case studies. The analytical value of models which are based on inductive and normative reasoning is not directly generalisable beyond very similar cases. Second, and despite their inductive limitations, the generalisation and application of national models beyond these analytical limits have produced tautological and essentialist outcomes – fitting empirical data into the models and overlooking other possible correlations between nationalism and integration policies. Amid recent scholarly attempts at amending the national models, the question remains whether national models are truly useful tools for the comparative studies of the relationships between nationalism and integration-immigration policies. We argue that comparative examinations require a more robust theoretical and methodological approach that can be used across periods and contexts without becoming tautological and essentialist
Kinetics of the inner ring in the exciton emission pattern in GaAs coupled quantum wells
We report on the kinetics of the inner ring in the exciton emission pattern.
The formation time of the inner ring following the onset of the laser
excitation is found to be about 30 ns. The inner ring was also found to
disappear within 4 ns after the laser termination. The latter process is
accompanied by a jump in the photoluminescence (PL) intensity. The spatial
dependence of the PL-jump indicates that the excitons outside of the region of
laser excitation, including the inner ring region, are efficiently cooled to
the lattice temperature even during the laser excitation. The ring formation
and disappearance are explained in terms of exciton transport and cooling.Comment: 19 pages, 6 figure
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