1,934 research outputs found
Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn_{1-x}Mn_xSe/ZnSe quantum disc samples of different radii
Quantum disc structures (with diameters of 200 nm and 100 nm) were prepared
from a Zn_{0.72}Mn_{0.28}Se/ZnSe single quantum well structure by electron beam
lithography followed by an etching procedure which combined dry and wet etching
techniques. The quantum disc structures and the parent structure were studied
by photoluminescence and photoluminescence excitation spectroscopy. For the
light-hole excitons in the quantum well region, shifts of the energy positions
are observed following fabrication of the discs, confirming that strain
relaxation occurs in the pillars. The light-hole exciton lines also sharpen
following disc fabrication: this is due to an interplay between strain effects
(related to dislocations) and the lateral size of the discs. A further
consequence of the small lateral sizes of the discs is that the intensity of
the donor-bound exciton emission from the disc is found to decrease with the
disc radius. These size-related effects occur before the disc radius is reduced
to dimensions necessary for lateral quantum confinement to occur but will
remain important when the discs are made small enough to be considered as
quantum dots.Comment: LaTeX2e, 13 pages, 6 figures (epsfig
Activation of mating type genes by transposition in Saccharomyces cerevisiae
Yeast Saccharomyces cerevisiae may express an a or alpha mating type. These cells types may be interconverted as a consequence of heritable genetic alteractions at the mating type locus (MAT). According to the more general controlling element model [Oshima, U. & Takano, I. (1971) Genetics 67, 327--335] and the specific cassette model [Hicks, J., Strathern, J. & Herskowitz, I. (1977) in DNA Insertion Elements, Plasmids and Episomes, eds. Bukhari, A. I., Shapiro, J.A. & Adhya, S. L.(Cold Spring Harbor Laboratory, Cold Spring Harbor, NY), pp. 457--462], the regulatory information required for switching the MAT locus exists at two other loosely linked loci, HMa and HMalpha. Specifically, the HMa and HMalpha loci are proposed to carry silent alpha and silent a genes, respectively. According to these models, switching occurs when a replica of a silent gene replaces the resident information at the mating type locus and is thereby expressed. These models predict that mutations at the silent ("storage") loci would generate defective MAT loci subsequent to the switching process. Therefore, the behavior of HMalpha mutants during the mating type interconversion was investigated. The results demonstrate that defective MATa alleles are generated by switching the MATalpha locus in HMalpha mutants. Thus, the genetic information from HMalpha is transposed to the mating type locus. These results provide genetic evidence in support of these models
Measurement of focusing properties for high numerical aperture optics using an automated submicron beamprofiler
The focusing properties of three aspheric lenses with numerical aperture (NA)
between 0.53 and 0.68 were directly measured using an interferometrically
referenced scanning knife-edge beam profiler with sub-micron resolution. The
results obtained for two of the three lenses tested were in agreement with
paraxial gaussian beam theory. It was also found that the highest NA aspheric
lens which was designed for 830nm was not diffraction limited at 633nm. This
process was automated using motorized translation stages and provides a direct
method for testing the design specifications of high numerical aperture optics.Comment: 6 pages 4 figure
Comparison of Zn_{1-x}Mn_xTe/ZnTe multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity
Large-area high density patterns of quantum dots with a diameter of 200 nm
have been prepared from a series of four Zn_{0.93}Mn_{0.07}Te/ZnTe multiple
quantum well structures of different well width (4 nm, 6 nm, 8 nm and 10 nm) by
electron beam lithography followed by Ar+ ion beam etching. Below-bandgap
photomodulated reflectivity spectra of the quantum dot samples and the parent
heterostructures were then recorded at 10 K and the spectra were fitted to
extract the linewidths and the energy positions of the excitonic transitions in
each sample. The fitted results are compared to calculations of the transition
energies in which the different strain states in the samples are taken into
account. We show that the main effect of the nanofabrication process is a
change in the strain state of the quantum dot samples compared to the parent
heterostructures. The quantum dot pillars turn out to be freestanding, whereas
the heterostructures are in a good approximation strained to the ZnTe lattice
constant. The lateral size of the dots is such that extra confinement effects
are not expected or observed.Comment: 23 pages, LaTeX2e (amsmath, epsfig), 7 EPS figure
FATP4 missense and nonsense mutations cause similar features in Ichthyosis Prematurity Syndrome
<p>Abstract</p> <p>Background</p> <p>Ichthyosis Prematurity Syndrome (IPS) is an autosomal recessive disorder characterized by premature birth, non-scaly ichthyosis and atopic manifestations. The disease was recently shown to be caused by mutations in the gene encoding the fatty acid transport protein 4 (FATP4) and a specific reduction in the incorporation of very long chain fatty acids (VLCFA) into cellular lipids.</p> <p>Findings</p> <p>We screened probands from five families segregating IPS for mutations in the <it>FATP4 </it>gene. Four probands were compound heterozygous for four different mutations of which three are novel. Four patients were heterozygous and one patient homozygous for the previously reported non-sense mutation p.C168X (c.504c > a). All patients had clinical characteristics of IPS and a similar clinical course.</p> <p>Conclusions</p> <p>Missense mutations and non-sense mutations in <it>FATP4 </it>are associated with similar clinical features suggesting that missense mutations have a severe impact on FATP4 function. The results broaden the mutational spectrum in <it>FATP4 </it>associated with IPS for molecular diagnosis of and further functional analysis of FATP4.</p
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