127 research outputs found
Saddle Points and Stark Ladders: Exact Calculations of Exciton Spectra in Superlattices
A new, exact method for calculating excitonic absorption in superlattices is
described. It is used to obtain high resolution spectra showing the saddle
point exciton feature near the top of the miniband. The evolution of this
feature is followed through a series of structures with increasing miniband
width. The Stark ladder of peaks produced by an axial electric field is
investigated, and it is shown that for weak fields the line shapes are strongly
modified by coupling to continuum states, taking the form of Fano resonances.
The calculated spectra, when suitably broadened, are found to be in good
agreement with experimental results.Comment: 9 pages Revtex v3.0, followed by 4 uuencoded postscript figures,
SISSA-CM-94-00
Electronic structure of intentionally disordered AlAs/GaAs superlattices
We use realistic pseudopotentials and a plane-wave basis to study the
electronic structure of non-periodic, three-dimensional, 2000-atom
(AlAs)_n/(GaAs)_m (001) superlattices, where the individual layer thicknesses
n,m = {1,2,3} are randomly selected. We find that while the band gap of the
equivalent (n = m = 2) ordered superlattice is indirect, random fluctuations in
layer thicknesses lead to a direct gap in the planar Brillouin zone, strong
wavefunction localization along the growth direction, short radiative
lifetimes, and a significant band-gap reduction, in agreement with experiments
on such intentionally grown disordered superlattices.Comment: 10 pages, REVTeX and EPSF macros, 4 figures in postscript. e-mail to
[email protected]
Binding of biexcitons in GaAs/AlxGa1-xAs superlattices
Binding of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices is studied using linear and nonlinear optical techniques. High biexciton binding energies characteristic of quasi two-dimensional biexcitons are observed in superlattices with considerable miniband dispersion
Diseases of the salivary glands in infants and adolescents
<p>Abstract</p> <p>Background</p> <p>Diseases of the salivary glands are rare in infants and children (with the exception of diseases such as parotitis epidemica and cytomegaly) and the therapeutic regimen differs from that in adults. It is therefore all the more important to gain exact and extensive insight into general and special aspects of pathological changes of the salivary glands in these age groups. Etiology and pathogenesis of these entities is still not yet fully known for the age group in question so that general rules for treatment, based on clinical experience, cannot be given, particularly in view of the small number of cases of the different diseases. Swellings of the salivary glands may be caused by acute and chronic inflammatory processes, by autoimmune diseases, by duct translocation due to sialolithiasis, and by tumors of varying dignity. Clinical examination and diagnosis has also to differentiate between salivary gland cysts and inflammation or tumors.</p> <p>Conclusion</p> <p>Salivary gland diseases are rare in childhood and adolescence. Their pattern of incidence differs very much from that of adults. Acute and chronic sialadenitis not responding to conservative treatment requires an appropriate surgical approach. The rareness of salivary gland tumors is particularly true for the malignant parotid tumors which are more frequent in juvenile patients, a fact that has to be considered in diagnosis and therapy.</p
Vibronic coupling at a chromium ion in a trigonal field in GaAs
We present here the preliminary results of our calculations on the Cr 2+-X(0.839 eV lines) system in GaAs. These calculations show that it is necessary, in order to explain the zero field splitting, to introduce a strong trigonal field. The main points that are explained by the presence of a strong trigonal field are the reduction of the Jahn-Teller energy and the occurrence of vibronic lines labelled I and J. A spin Hamiltonian is also presented that explains the zero field fine structure and the line intensities.Nous prĂ©sentons ici les rĂ©sultats prĂ©liminaires des calculs que nous avons effectuĂ©s sur le systĂšme Cr2+-X (raies Ă 0,839 eV) dans GaAs. La structure Ă champ nul de ces raies ne peut ĂȘtre expliquĂ©e qu'Ă la condition d'introduire un champ trigonal fort. Les principaux points qui sont expliquĂ©s uniquement par cette forte valeur sont : la rĂ©duction de l'Ă©nergie Jahn-Teller et l'existence de raies vibroniques notĂ©es I et J. Un Hamiltonien de spin est prĂ©sentĂ© qui permet de dĂ©crire la structure fine Ă champ nul ainsi que les intensitĂ©s de raies
Mucoepidermoid tumors of salivary glands: histoprognostic value of NORs stained with AgNOR technique
Miniband dispersion and excitonic effects on the optical-spectra of GaAs/AlxGa1-xAs superlattices
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