548 research outputs found
Analytical models for growth by metal organic vapor phase epitaxy. I.Isothermal models
For a proper description of growth by metal organic vapour phase epitaxy the three-dimensional Navier-Stokes partial differential equations need to be solved which govern the following series of processes: (i) transport by diffusion and flow through the gas phase, (ii) reactions which take place in this gas phase, (iii) reactions which take place at the surface. The authors are at first interested in the medium- and higher-temperature regions, which cover the growth determined by diffusion through the gas phase (medium temperature) and the growth that is determined by the desorption of growth species (higher temperature). Using a number of well justified assumptions one can reduce the problem to a two-dimensional one. For the diffusion-limited region (i.e. medium-temperature region) the effect of different flow profiles (plug flow, parabolic flow, linear increasing velocity and combination of plug and linear profile) on the growth rate has been studied under isothermal conditions. It was found that all profiles yield the same growth rate within a few per cent, so that it suffices to use the simple plug flow profile in growth rate calculations. It is also shown that axial diffusion is an important effect only at the end of long reactors. Finally a model is derived in which surface reaction kinetics is combined with the diffusion-limited model for the isothermal case
Diffusion enhancement in on/off ratchets
We show a diffusion enhancement of suspended polystyrene particles in an electrical on/off ratchet. The enhancement can be described by a simple master equation model. Furthermore, we find that the diffusion enhancement can be described by a general curve whose shape is only determined by the asymmetry of the ratchet repeat unit. The scaling of this curve can be explained from an analytical expression valid for small off-times. Finally, we demonstrate how the master equation model can be used to find the driving parameters for optimal particle separation. (C) 2013 American Institute of Physics
Ion channeling for strain analysis in buried nanofilms (
The title should have been: \"Ion channeling for strain anal. in buried nanofilms
Type 0 T-Duality and the Tachyon Coupling
We consider the T-duality relations between Type 0A and 0B theories, and show
that this constraints the possible couplings of the tachyon to the RR-fields.
Due to the `doubling' of the RR sector in Type 0 theories, we are able to
introduce a democratic formulation for the Type 0 effective actions, in which
there is no Chern-Simons term in the effective action. Finally we discuss how
to embed Type II solutions into Type 0 theories.Comment: some misprints corrected and a reference adde
Partitioning Schemes and Non-Integer Box Sizes for the Box-Counting Algorithm in Multifractal Analysis
We compare different partitioning schemes for the box-counting algorithm in
the multifractal analysis by computing the singularity spectrum and the
distribution of the box probabilities. As model system we use the Anderson
model of localization in two and three dimensions. We show that a partitioning
scheme which includes unrestricted values of the box size and an average over
all box origins leads to smaller error bounds than the standard method using
only integer ratios of the linear system size and the box size which was found
by Rodriguez et al. (Eur. Phys. J. B 67, 77-82 (2009)) to yield the most
reliable results.Comment: 10 pages, 13 figure
Acute effect of pegvisomant on cardiovascular risk markers in healthy men: implications for the pathogenesis of atherosclerosis in GH deficiency
Cardiovascular risk is increased in GH deficiency (GHD). GHD adults are
frequently abdominally obese and display features of the metabolic
syndrome. Otherwise healthy abdominally obese subjects have low GH levels
and show features of the metabolic syndrome as well. We investigated in
healthy nonobese males the effect of the GH receptor antagonist
pegvisomant in different metabolic conditions. This is a model for acute
GHD without the alterations in body composition associated with GHD. We
compared the effect of pegvisomant with that of placebo before and after 3
d of fasting. In addition, we investigated the effect of pegvisomant under
normal, i.e. fed, conditions. Three days of fasting as well as pegvisomant
alone decreased serum free IGF-I levels (1.0 +/- 0.15 vs. 0.31 +/- 0.05
ng/ml and 0.86 +/- 0.23 vs. 0.46 +/- 0.23 ng/ml, respectively). Fasting in
combination with pegvisomant also decreased serum free IGF-I levels (1.0
+/- 0.15 vs. 0.31 +/- 0.07 ng/ml). Treatment with pegvisomant had no
additional influence on the decline of free IGF-I induced by fasting.
Pegvisomant alone had no influence on insulin sensitivity. The increase in
insulin sensitivity induced by fasting was comparable to the increase in
insulin sensitivity induced by fasting combined with pegvisomant. Among
serum lipid concentrations, only serum triglycerides increased
significantly as a result of pegvisomant alone (1.0 +/- 0.2 vs. 1.6 +/-
0.4 mmol/liter). The changes in lipid concentrations induced by fasting
alone or pegvisomant were not different from those induced by pegvisomant
alone. von Willebrand factor antigen levels declined significantly under
the influence of pegvisomant alone (1.1 +/- 0.07 vs. 0.8 +/- 0.06 U/ml).
In conclusion, in different metabolic conditions the GH receptor
antagonist pegvisomant induces no significant acute changes in the major
risk markers for cardiovascular disease. These data suggest that the
secondary metabolic changes, e.g. abdominal obesity or inflammatory
factors, that develop as a result of long-standing GHD are of primary
importance in the pathogenesis of atherosclerosis in patients with GHD
Temperature-dependent aluminum incorporation in aluminum gallium arsenide layers grown by metalorganic vapor phase epitaxy
Influence of a partially oxidized calcium cathode on the performance of polymeric light emitting diodes
We investigated the influence of the presence of oxygen during the deposition of the calcium cathode on the structure and on the performance of polymeric light emitting diodes (pLEDs). The oxygen background pressure during deposition of the calcium cathode of polymeric LEDs was varied. Subsequently, the oxygen depth distribution was measured and correlated with the performance of the pLEDs. The devices have been fabricated in a recently built ultraclean setup. The polymer layers of the pLEDs have been spincoated in a dry nitrogen atmosphere and transported directly into an ultrahigh vacuum chamber where the metal electrodes have been deposited by evaporation. We used indium–tin–oxide as anode, OC1C10 PPV as electroluminescent polymer, calcium as cathode, and aluminum as protecting layer. We achieved reproducibility of about 15% in current and brightness for devices fabricated in an oxygen atmosphere o
Blockade of the growth hormone (GH) receptor unmasks rapid GH-releasing peptide-6-mediated tissue-specific insulin resistance
The roles of GH and its receptor (GHR) in metabolic control are not yet
fully understood. We studied the roles of GH and the GHR using the GHR
antagonist pegvisomant for metabolic control of healthy nonobese men in
fasting and nonfasting conditions. Ten healthy subjects were enrolled in a
double blind, placebo-controlled study on the effects of pegvisomant on
GHRH and GH-releasing peptide-6 (GHRP-6)-induced GH secretion before and
after 3 days of fasting and under nonfasting conditions (n = 5). Under the
condition of GHR blockade by pegvisomant in the nonfasting state, GHRP-6
(1 microg/kg) caused a increase in serum insulin (10.3 +/- 2.1 vs. 81.3
+/- 25.4 mU/L; P < 0.001) and glucose (4.2 +/- 0.3 vs. 6.0 +/- 0.6 mmol/L;
P < 0.05) concentrations. In this group, a rapid decrease in serum free
fatty acids levels was also observed. These changes were not observed
under GHR blockade during fasting or in the absence of pegvisomant. We
conclude that although these results were obtained from an acute study,
and long-term administration of pegvisomant could render different
results, blockade of the GHR in the nonfasting state induces
tissue-specific changes in insulin sensitivity, resulting in an increase
in glucose and insulin levels (indicating insulin resistance of
liver/muscle), but probably also in an increase in lipogenesis (indicating
normal insulin sensitivity of adipose tissue). These GHRP-6-mediated
changes indicate that low GH bioactivity on the tissue level can induce
changes in metabolic control, which are characterized by an increase in
fat mass and a decrease in lean body mass. As a mechanism of these
GHRP-6-mediated metabolic changes in the nonfasting state, direct
nonpituitary-mediated GHRP-6 effects on the gastroentero-hepatic axis seem
probable
Somatostatin Receptor Expression in GH-Secreting Pituitary Adenomas Treated with Long-Acting Somatostatin Analogues in Combination with Pegvisomant
Background: Growth hormone secreting pituitary adenomas (somatotroph adenoma) predominantly express somatostatin receptors (SSTRs) subtypes 2 and 5. Higher SSTR2 expression on somatotroph adenomas results in a better response to somatostatin analogues (SSAs), which preferentially bind, but also down regulate, SSTR2. The effect of the combined treatment with SSAs and the GH receptor antagonist pegvisomant (PEGV) on SSTR expression in somatotroph adenomas is currently unknown. Aim of the Study: To assess SSTR2 and SSTR5 expression in three groups of somatotroph adenomas: drug-naive, treated with long-acting (LA) SSA monotherapy, or LA-SSA/PEGV combination therapy before surgery. Additionally, we evaluated the required PEGV dose to achieve IGF-I normalization in relation to the SSTR expression. Materials and Methods: At our Pituitary Center Rotterdam, we selected acromegalic patients who underwent transsphenoidal neurosurgery. All patients were eventually treated with LA-SSA/PEGV combination therapy during their medical history. SSTR2 and SSTR5 expression in somatotroph adenomas tissues was determined using immunohistochemistry. Results: Out of 39 somatotroph adenoma tissue samples, 23 were drug-naive, 9 received pre-treatment with LA-SSA and 7 LA-SSA/PEGV combined treatment. SSTR2 expression was significantly higher in treatment-naive compared to combined treatment somatotroph adenomas (p = 0.048), while SSTR5 expression did not differ. Noteworthy, SSTR2 expression in naive somatotroph adenoma tissues was inversely correlated to the required PEGV dose to achieve IGF-I normalization during post-surgical medical treatment (ρ = -0.538, p = 0.024). Conclusion: In our specific cohort, the SSTR2 expression is lower in patients pre-treated with LA-SSA/PEGV compared to the drug-naive acromegalic patients. Additionally, the SSTR2 expression in treatment naive somatotroph adenoma tissues was inversely correlated with the required PEGV dose to achieve IGF-I normalization
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