Island size distributions in submonolayer growth: successful prediction by mean field theory with coverage dependent capture numbers

We show that mean-field rate equations for submonolayer growth can successfully predict island size distributions in the pre-coalescence regime if the full dependence of capture numbers on both the island size and the coverage is taken into account. This is demonstrated by extensive Kinetic Monte Carlo simulations for a growth kinetics with hit and stick aggregation. A detailed analysis of the capture numbers reveals a nonlinear dependence on the island size for small islands. This nonlinearity turns out to be crucial for the successful prediction of the island size distribution and renders an analytical treatment based on a continuum limit of the mean-field rate equations difficult.Comment: 4 pages, 4 figue

Crossover in the scaling of island size and capture zone distributions

Simulations of irreversible growth of extended (fractal and square) islands with critical island sizes i=1 and 2 are performed in broad ranges of coverage \theta and diffusion-to-deposition ratios R in order to investigate scaling of island size and capture zone area distributions (ISD, CZD). Large \theta and small R lead to a crossover from the CZD predicted by the theory of Pimpinelli and Einstein (PE), with Gaussian right tail, to CZD with simple exponential decays. The corresponding ISD also cross over from Gaussian or faster decays to simple exponential ones. For fractal islands, these features are explained by changes in the island growth kinetics, from a competition for capture of diffusing adatoms (PE scaling) to aggregation of adatoms with effectively irrelevant diffusion, which is characteristic of random sequential adsorption (RSA) without surface diffusion. This interpretation is confirmed by studying the crossover with similar CZ areas (of order 100 sites) in a model with freezing of diffusing adatoms that corresponds to i=0. For square islands, deviations from PE predictions appear for coverages near \theta=0.2 and are mainly related to island coalescence. Our results show that the range of applicability of the PE theory is narrow, thus observing the predicted Gaussian tail of CZD may be difficult in real systems.Comment: 9 pages, 7 figure

Capture-zone scaling in island nucleation: phenomenological theory of an example of universal fluctuation behavior

In studies of island nucleation and growth, the distribution of capture zones, essentially proximity cells, can give more insight than island-size distributions. In contrast to the complicated expressions, ad hoc or derived from rate equations, usually used, we find the capture-zone distribution can be described by a simple expression generalizing the Wigner surmise from random matrix theory that accounts for the distribution of spacings in a host of fluctuation phenomena. Furthermore, its single adjustable parameter can be simply related to the critical nucleus of growth models and the substrate dimensionality. We compare with extensive published kinetic Monte Carlo data and limited experimental data. A phenomenological theory sheds light on the result.Comment: 5 pages, 4 figures, originally submitted to Phys. Rev. Lett. on Dec. 15, 2006; revised version v2 tightens and focuses the presentation, emphasizes the importance of universal features of fluctuations, corrects an error for d=1, replaces 2 of the figure

Acceptability of intrapartum ultrasound by mothers in an African population

Intrapartum ultrasound is gaining high acceptance by many women as another method for assessing labour progression. Despite growing evidence of the effectiveness of ultrasound in labour, the acceptance of intrapartum ultrasound has not been previously investigated in black Africans. This study aimed to determine women’s acceptance of intrapartum ultrasound and their preference for transperineal ultrasound or digital vaginal examination (digital VE) in Ghana. An analytical cross-sectional study was conducted among mothers who had had both digital VE and transperineal ultrasound during labour in a tertiary hospital. Information about their sociodemographic characteristics, experience with, and preference for ultrasound or digital VE in labour using a pretested structured questionnaire was obtained. Their experiences were categorised as ‘tolerable, ‘quite uncomfortable’ or ‘very uncomfortable’. Categorical variables were compared using Fisher’s exact test. A p value < 0.05 was considered statistically significant. Altogether, 196 women were recruited into the study. The mean age of the women was 26.7 years (standard deviation, 4.6 years). Nearly half (47%) of the women had never delivered before. Significantly more women considered transperineal ultrasound to be more tolerable than digital VE (66% vs. 40%; p < 0.001). Almost all the women (97.5%) described their experience with transperineal ultrasound to be better than digital VE, and would choose transperineal ultrasound over digital VE in the future (98.5% vs. 1.5%; p < 0.001). The findings of this study are comparable to those of other related studies reported recently. This research confirms high acceptance of ultrasound in labour by mothers from different countries and across continents, implying that cultural differences do not influence women’s responses to and interest in intrapartum ultrasound. Most women found ultrasound in labour to be more tolerable than digital VE. Whenever possible, transperineal ultrasound should be provided as an alternative to digital VE during labour.N/

Continuous and correlated nucleation during nonstandard island growth at Ag/Si(111)-7x7 heteroepitaxy

We present a combined experimental and theoretical study of submonolayer heteroepitaxial growth of Ag on Si(111)-7x7 at temperatures from 420 K to 550 K when Ag atoms can easily diffuse on the surface and the reconstruction 7x7 remains stable. STM measurements for coverages from 0.05 ML to 0.6 ML show that there is an excess of smallest islands (each of them fills up just one half-unit cell - HUC) in all stages of growth. Formation of 2D wetting layer proceeds by continuous nucleation of the smallest islands in the proximity of larger 2D islands (extended over several HUCs) and following coalescence with them. Such a growth scenario is verified by kinetic Monte Carlo simulation which uses a coarse-grained model based on a limited capacity of HUC and a mechanism which increases nucleation probability in a neighbourhood of already saturated HUCs (correlated nucleation). The model provides a good fit for experimental dependences of the relative number of Ag-occupied HUCs and the preference in occupation of faulted HUCs on temperature and amount of deposited Ag. Parameters obtained for the hopping of Ag adatoms between HUCs agree with those reported earlier for initial stages of growth. The model provides two new parameters - maximum number of Ag atoms inside HUC, and on HUC boundary.Comment: LaTeX2e, BibTeX, 9 pages, 7 images, accepted to Phys. Rev.

The seasonal cycle of ocean-atmosphere CO2 Flux in Ryder Bay, West Antarctic Peninsula

Approximately 15 million km2 of the Southern Ocean is seasonally ice covered, yet the processes affecting carbon cycling and gas exchange in this climatically important region remain inadequately understood. Here, 3 years of dissolved inorganic carbon (DIC) measurements and carbon dioxide (CO2) fluxes from Ryder Bay on the west Antarctic Peninsula (WAP) are presented. During spring and summer, primary production in the surface ocean promotes atmospheric CO2 uptake. In winter, higher DIC, caused by net heterotrophy and vertical mixing with Circumpolar Deep Water, results in outgassing of CO2 from the ocean. Ryder Bay is found to be a net sink of atmospheric CO2 of 0.59–0.94 mol C m−2 yr−1 (average of 3 years). Seasonal sea ice cover increases the net annual CO2 uptake, but its effect on gas exchange remains poorly constrained. A reduction in sea ice on the WAP shelf may reduce the strength of the oceanic CO2 sink in this region

Nonequilibrium phase transitions in models of adsorption and desorption

The nonequilibrium phase transition in a system of diffusing, coagulating particles in the presence of a steady input and evaporation of particles is studied. The system undergoes a transition from a phase in which the average number of particles is finite to one in which it grows linearly in time. The exponents characterizing the mass distribution near the critical point are calculated in all dimensions.Comment: 10 pages, 2 figures (To appear in Phys. Rev. E

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

Intrapartum ultrasound assessment of cervical dilatation and its value in detecting active labor.

Introduction We aimed to examine the agreement between ultrasound and digital vaginal examination in assessing cervical dilatation in an African population and to assess the value of ultrasound in detecting active labor. Method A cross-sectional study was conducted in a teaching hospital in Ghana between April and September of 2016. Anterior–posterior and transverse diameters of cervical dilatation were measured with ultrasound and the mean value was compared with digital vaginal examination in 195 women in labor. Agreement between methods was examined with correlation coefficients and with Bland–Altman plots. Active labor was defined when cervix was dilated ≥ 4 cm with vaginal examinations. ROC curve analysis was conducted on the diagnostic performance of ultrasound in detecting active labor. Results Data were analyzed in 175 out of 195 (90%) cases where ultrasound could clearly visualize the cervix. The remaining 20 cases were all determined by digital vaginal examination as advanced cervical dilatation (≥ 8 cm), advanced head station (≥ + 2), and with ruptured membranes. The Pearson correlation coefficient (r) was 0.78 (95% CI 0.72–0.83) and the intra-class correlation coefficient was 0.76 (95% CI 0.69–0.81). Bland–Altman analysis obtained a mean difference of − 0.03 cm (95% CI − 0.18 to 0.12) with zero included in the CI intervals, indicating no significant difference between methods. Limits of agreement were from − 2.01 to 1.95 cm. Ultrasound predicted active labor with 0.87 (95% CI 0.75–0.99) as the area under the ROC curve. Conclusion Ultrasound measurements showed good agreement with digital vaginal examinations in assessing cervical dilatation during labor and ultrasound may be used to detect active labor.N/

The effectiveness of intrapartum ultrasonography in assessing cervical dilatation, head station and position: A systematic review and meta-analysis

The objective of this review was to assess the effectiveness of intrapartum ultrasonography in measuring cervical dilatation, head station and position. Electronic literature searches were carried out of MEDLINE, CINAHL, and Web of Knowledge, plus manual reference list checks of all relevant articles. All published prospective studies comparing intrapartum ultrasonography with digital VE in the determination of cervical dilatation, head station and position were then evaluated for the success rate and level of agreement between ultrasonography and digital VE. Ultrasonography had higher success rate than digital VE in the determination of fetal head position, with a statistically significant difference in the first stage of labour. Second, although the successful determination of cervical dilatation was in favour of digital VE, the difference was not statistically significant. In addition, there was high agreement between ultrasound and digital VE findings on cervical dilatation. Lastly, a significant but moderate correlation between digital VE and ultrasound methods was found in the assessment of fetal head station. However, no meta-analysis could be done for the fetal head station due to the methodological differences between ultrasound anatomical landmarks and that of digital VE. The findings suggest that ultrasonography is superior to digital VE in the assessment of fetal head position, but has moderate correlation with digital VE in the assessment of head station. It also showed high agreement with digital VE in the assessment of cervical dilatation with no statistically significant difference in terms of success rate