Spatio-temporal distribution of nucleation events during crystal growth

We consider irreversible second-layer nucleation that occurs when two adatoms on a terrace meet. We solve the problem analytically in one dimension for zero and infinite step-edge barriers, and numerically for any value of the barriers in one and two dimensions. For large barriers, the spatial distribution of nucleation events strongly differs from $\rho^2$, where $\rho$ is the stationary adatom density in the presence of a constant flux. The probability $Q(t)$ that nucleation occurs at time $t$ after the deposition of the second adatom, decays for short time as a power law [$Q(t)\sim t^{-1/2}$] in $d=1$ and logarithmically [$Q(t)\sim 1/\ln(t/t_0)$] in $d=2$; for long time it decays exponentially. Theories of the nucleation rate $\omega$ based on the assumption that it is proportional to $\rho^2$ are shown to overestimate $\omega$ by a factor proportional to the number of times an adatom diffusing on the terrace visits an already visited lattice site.Comment: 4 pages, 3 figures; accepted for publication on PR

Island nucleation in the presence of step edge barriers: Theory and applications

We develop a theory of nucleation on top of two-dimensional islands bordered by steps with an additional energy barrier $\Delta E_S$ for descending atoms. The theory is based on the concept of the residence time of an adatom on the island,and yields an expression for the nucleation rate which becomes exact in the limit of strong step edge barriers. This expression differs qualitatively and quantitatively from that obtained using the conventional rate equation approach to nucleation [J. Tersoff et al., Phys. Rev. Lett.72, 266 (1994)]. We argue that rate equation theory fails because nucleation is dominated by the rare instances when two atoms are present on the island simultaneously. The theory is applied to two distinct problems: The onset of second layer nucleation in submonolayer growth, and the distribution of the sizes of top terraces of multilayer mounds under conditions of strong step edge barriers. Application to homoepitaxial growth on Pt(111) yields the estimate $\Delta E_S \geq 0.33$ eV for the additional energy barrier at CO-decorated steps.Comment: 13 pages, 3 figure

Glioblastoma cells express functional cell membrane receptors activated by daily used medical drugs

PURPOSE: Calcium ions are highly versatile spacial and temporal intracellular signals of non-excitable cells and have an important impact on nearly every aspect of cellular life controlling cell growth, metabolism, fluid secretion, information processing, transcription, apoptosis, and motility. Neurons and glia respond to stimuli, including neurotransmitters, neuromodulators, and hormones, which increase the intracellular calcium concentration. The function of intracellular calcium in gliomas is unknown. Lots of daily used drugs may act via receptors that can be linked to the intracellular calcium system and therefore could influence glioma biology. METHODS: Glioma cells were loaded with the calcium ion sensitive dye Fura 2-AM. Subsequently, cells were stimulated with 25 different medical drugs for 30 s. The increase of free intracellular calcium ions was measured and calculated by a microscope–camera–computer-unit. RESULTS: Except for the buffer solution HEPES that served as negative control and for the cortisol derivative dexamethasone, all other 24 tested drugs induced a rise of intracellular calcium ions. The cellular calcium responses were classified into seven functional groups. The tested substances activated several types of calcium channels and receptors. CONCLUSIONS: Our study impressively demonstrates that medical drugs are potent inducers of intracellular calcium signals. Totally unexpected, the results show a high amount of functional cellular receptors and channels on glioma cells, which could be responsible for certain biological effects like migration and cell growth. This calcium imaging study proves the usability of the calcium imaging as a screening system for functional receptors on human glioma cells

Inorganic carbon promotes photosynthesis, growth, and maximum biomass of phytoplankton in eutrophic water bodies

1.The traditional perception in limnology has been that phytoplankton biomass in lakes is limited by phosphorus, nitrogen, and light, but not by dissolved inorganic carbon (DIC) because CO2 can be supplied from the atmosphere. We tested the possibility of carbon limitation of photosynthesis, growth, and biomass accumulation of phytoplankton communities across an alkalinity and DIC gradient (0.15–3.26 mM) in nutrient‐rich freshwater. 2.During 47‐day long experiments, we measured phytoplankton biomass, organic carbon, calcium, DIC, pH, and oxygen in indoor, constantly mixed mesocosms with either no removal or a 70% weekly removal of the biomass. Photosynthesis was measured in the morning and in the afternoon at high biomass. 3.Maximum biomass and organic carbon production increased two‐ to four‐fold with DIC, which supported 7% of organic carbon production at low DIC and 53% at high DIC concentration, while atmospheric CO2 uptake supplied the remainder. Weekly biomass removal increased growth rates through improved light conditions leading to enhanced total phytoplankton biomass production at high DIC. Photosynthesis was significantly higher in the morning compared to afternoon due to daily DIC depletion. 4.We conclude that phytoplankton photosynthesis, growth rate, maximum biomass, and organic carbon production can be markedly carbon limited in eutrophic lake waters. Consequently, lakes of high DIC and pH can support a faster primary production by greater DIC use and chemically enhanced atmospheric CO2 uptake.publishedVersio

Adaptive Lévy processes and area-restricted search in human foraging

A considerable amount of research has claimed that animals’ foraging behaviors display movement lengths with power-law distributed tails, characteristic of Lévy flights and Lévy walks. Though these claims have recently come into question, the proposal that many animals forage using Lévy processes nonetheless remains. A Lévy process does not consider when or where resources are encountered, and samples movement lengths independently of past experience. However, Lévy processes too have come into question based on the observation that in patchy resource environments resource-sensitive foraging strategies, like area-restricted search, perform better than Lévy flights yet can still generate heavy-tailed distributions of movement lengths. To investigate these questions further, we tracked humans as they searched for hidden resources in an open-field virtual environment, with either patchy or dispersed resource distributions. Supporting previous research, for both conditions logarithmic binning methods were consistent with Lévy flights and rank-frequency methods–comparing alternative distributions using maximum likelihood methods–showed the strongest support for bounded power-law distributions (truncated Lévy flights). However, goodness-of-fit tests found that even bounded power-law distributions only accurately characterized movement behavior for 4 (out of 32) participants. Moreover, paths in the patchy environment (but not the dispersed environment) showed a transition to intensive search following resource encounters, characteristic of area-restricted search. Transferring paths between environments revealed that paths generated in the patchy environment were adapted to that environment. Our results suggest that though power-law distributions do not accurately reflect human search, Lévy processes may still describe movement in dispersed environments, but not in patchy environments–where search was area-restricted. Furthermore, our results indicate that search strategies cannot be inferred without knowing how organisms respond to resources–as both patched and dispersed conditions led to similar Lévy-like movement distributions

Which executive functioning deficits are associated with AD/HD, ODD/CD and comorbid AD/HD+ODD/CD?

Item does not contain fulltextThis study investigated (1) whether attention deficit/hyperactivity disorder (AD/HD) is associated with executive functioning (EF) deficits while controlling for oppositional defiant disorder/conduct disorder (ODD/CD), (2) whether ODD/CD is associated with EF deficits while controlling for AD/HD, and (3)~whether a combination of AD/HD and ODD/CD is associated with EF deficits (and the possibility that there is no association between EF deficits and AD/HD or ODD/CD in isolation). Subjects were 99~children ages 6–12 years. Three putative domains of EF were investigated using well-validated tests: verbal fluency, working memory, and planning. Independent of ODD/CD, AD/HD was associated with deficits in planning and working memory, but not in verbal fluency. Only teacher rated AD/HD, but not parent rated AD/HD, significantly contributed to the prediction of EF task performance. No EF deficits were associated with ODD/CD. The presence of comorbid AD/HD accounts for the EF deficits in children with comorbid AD/HD+ODD/CD. These results suggest that EF deficits are unique to AD/HD and support the model proposed by R. A. Barkley (1997).17 p

Familial influences on sustained attention and inhibition in preschoolers

Background: In this study several aspects of attention were studied in 237 nearly 6-year-old twin pairs. Specifically, the ability to sustain attention and inhibition were investigated using a computerized test battery (Amsterdam Neuropsychological Tasks). Furthermore, the Teacher's Report Form (TRF) was filled out by the teacher of the child and the attention subscale of this questionnaire was analyzed. Methods: The variance in performance on the different tasks of the test battery and the score on the attention scale of the TRF were decomposed into a contribution of the additive effects of many genes (A), environmental effects that are shared by twins (C) and unique environmental influences not shared by twins (E) by using data from MZ and DZ twins. Results: The genetic model fitting results showed an effect of A and E for the attention scale of the TRF, and for some of the inhibition and sustained attention measures. For most of the attention variables, however, it was not possible to decide between a model with A and E or a model with C and E. Time-on-task effects on reaction time or number of errors and the delay after making an error did not show familial resemblances. A remarkable finding was that the heritability of the attention scale of the TRF was found to be higher than the heritability of indices that can be considered to be more direct measures of attention, such as mean tempo in the sustained attention task and response speed in the Go-NoGo task. Conclusion: In preschoolers, familial resemblances on sustained attention and inhibition were observed. © Association for Child Psychology and Psychiatry, 2004

Assessment of myocardial oxidative metabolism in aortic valve disease using positron emission tomography with C-11 acetate

C-11 acetate has recently been introduced as a tracer of myocardial oxidative metabolism with the use of positron emission tomography. To evaluate this approach in the pressure- or volume-loaded heart, C-11 acetate clearance rate constants were determined in 22 patients with chronic aortic valve disease and in nine normal subjects. Global myocardial C-11 clearance was significantly higher in patients with predominant aortic stenosis (n = 11) or aortic regurgitation (n = 11) than in normal subjects (0.069 +/- 0.017 min-1 and 0.072 +/- 0.010 min-1 compared with 0.050 +/- 0.004 min-1, p r = 0.73, P = 0.0001) for all studies. However, analysis of patient subgroups demonstrated that this correlation held only for aortic stenosis (r = 0.79, p r = 0.89, p < 0.005) but not in patients with aortic regurgitation. Normalization of C-11 acetate clearance rate constants for gradient-corrected rate-pressure product were significantly lower in patients with loaded ventricles, particularly in the presence of a low ejection fraction, compared to normal subjects. Possible mechanisms include myocardial adaptation through hypertrophy or depressed contractility, which would both tend to reduce oxygen consumption under any given load. Serial comparison of C-11 acetate kinetics and noninvasive indexes of oxygen demand may provide assessment of disease progression in pathologic ventricular loading.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30196/1/0000584.pd