Small Island Size Research Articles

Nonlinear control for stabilization of small neoclassical tearing modes in ITER

In this paper, the feasibility of feedback stabilization of neoclassical tearing modes at small island sizes, corresponding to otherwise unstable island sizes in ITER scenario 2, is demonstrated. The islands are stabilized by application of electron cyclotron resonance heating and current drive in a regime where the application of current drive in open loop normally results in a complete suppression of the island. By applying current drive in closed loop with feedback of real-time measurements of the island width, complete suppression is avoided and the island is stabilized at a specific reduced size. In contrast to complete suppression, control of islands at a specific size will allow the manipulation of a plasma's current density profile in hybrid scenarios. Three conceptual (non-)linear feedback controllers with varying complexity, performance, robustness and required model knowledge are introduced. Simulations show the theoretical feasibility of small island stabilization at a specific reduced width. The controllers are applied to the generalized Rutherford equation, which governs the island evolution subject to electron cyclotron current drive. A strategy for the gradual implementation of the controllers is suggested. Stabilization of small islands by feedback control will allow the use of system identification to extend the model knowledge on the evolution of small islands, and in addition will extend the operational regime.

Ostwald ripening of three-dimensional clusters on a surface studied with an ultrafast kinetic Monte Carlo algorithm

We have studied the Ostwald ripening of three-dimensional islands on a homogeneous surface with an original off-lattice kinetic Monte Carlo algorithm. In this algorithm, adatom trajectories are highly simplified, while still ensuring that the adatom fluxes between islands are exactly described. From the simulations, we obtained the evolution of the island size distribution over a large time range. The simulations obtained are compared with the results of numerical integration of rate equations derived from a mean-field approximation. Both results indicate that the equilibrium radius of the islands follows a power-law behavior in the limit of a very dilute phase, with an exponent close to 1/4. A general, excellent agreement is obtained, showing the validity of our approach, whereas the validity of the mean-field approximation is discussed for a very small mean island size, or for a large fraction of the surface covered by the islands.

New Records of Butterflies from Yap Outer Islands, Micronesia: Fais Island and Ngulu, Ulithi, and Woleai Atolls

Eight species of butterflies are recorded from among four different island groups in Yap Outer Islands, Federated States of Micronesia (FSM). Five species (63%) belong to the family Lycaenidae; the three others to Nymphalidae. Hypolimnas bolina is the most ubiquitous species, being the only one recorded on all the islands. Ngulu Atoll, which has the smallest land area, also has one of the most depauperate butterfly faunas, with only two species recorded, but it is located between Palau and Yap proper, which host the richest butterfly faunas in southwestern Micronesia. Ulithi Atoll, which is nearest to potential source populations on Yap, has the largest number of species. Small island size, limited habitat diversity, and lack of sufficient host plants combined with distance from potential source populations are likely to be the main factors contributing to the small number of species on these low-lying coralline islands.

Early colonisation population structure of a Norway rat island invasion

Colonists undergo non-equilibrium processes such as founder effects, inbreeding and changing population size which influence the mating system and demography of a population. Understanding these processes in colonising populations informs management and helps prevent further invasions. We sampled and genotyped most individuals of a Norway rat (Rattus norvegicus) reinvasion on Moturemu island (5 ha) in New Zealand. Population size was most likely between 30 and 33 rats. Genetic methods detected a clear bottleneck signal from the founding population. Parentage assignment revealed promiscuous mating dominated by a few individuals with increasing inbreeding, both putatively a result of small island size. Combining ecological and genetic data from a single sample allowed inferences on population structure and functioning. Invading Norway rats rapidly achieve population structure similar to established island populations despite a small number of colonists and associated inbreeding. Overcoming these initial obstacles to population establishment contributes to the global success of invasive rats.

Strain relaxation dependent island nucleation rates during the Stranski–Krastanow growth of GaN on AlN by molecular beam epitaxy

This study reports on the correlation between strain relaxation and nucleation kinetics during the Stranski–Krastanow growth of GaN on (0001)AlN by plasma-assisted molecular beam epitaxy. Using reflection high-energy electron diffraction and real-time desorption mass spectrometry, the strain-related Ga adatom detachment and desorption rates were determined, giving information about the average GaN island nucleation rate. Two different regimes were found: one at low-temperature growth (690<TS<720 °C), where strain relaxation occurred slowly, yielding impeded island nucleation rates and small island sizes (diameter ∼8–12 nm and height ∼2.3–2.7 nm). In the other, i.e., high-temperature growth regime (TS>720 °C), islands showed an abrupt relaxation mode, accompanied by a fast nucleation rate toward island sizes twice as large.

Realization of high-resistance GaN by controlling the annealing pressure of the nucleation layer in metal-organic chemical vapor deposition

The influence of the annealing pressure of the nucleation layer (NL) on the resistance of GaN films grown on sapphire by means of metal-organic chemical vapor deposition has been investigated. It is found that the sheet resistance of GaN increases significantly with decreasing the annealing pressure. When the annealing pressure is 10 kPa, GaN with sheet resistance higher than 10 11 Ω/sq is achieved. It is determined that the edge-component threading dislocations (ETDs) rather than the screw-component ones are the dominant structural feature responsible for the resistance variation. Based on the analysis of in-situ reflectance and atomic force microscopy, it is believed that the island density and island size in the NL are the key factors accounting for the resistance variation in GaN. The high island density and small island size in the NL not only lead to more ETDs in GaN, which introduce more acceptors to compensate the background electrons, but also enhance the rapid coalescence in GaN and thus effectively limit the diffusion of oxygen impurities into GaN epilayers from GaN/sapphire interfaces.

Avoiding collapse: Pre-European sustainability on Pacific Islands

Both geography and cultural practices were important in guiding the ecological histories of Pacific Islands. This paper focuses on cases where cultural adaptation was the key to sustainability on small islands prone to deforestation. Environmental conditions including small island size and low rainfall required Polynesians to adapt their cultures and in some cases abandon the traditional reliance on shifting cultivation seen in the western Pacific. Successful adaptations involved avoiding agricultural strategies associated with fire while shifting to a reliance on irrigated pondfields or arboriculture. Easter Island, which suffered severe deforestation, is different from most other Pacific Islands because its environment is particularly fragile and there was no option for either irrigated agriculture or arboriculture.

Large-area and high-density gold nanoparticle arrays with sub-10 nm gaps

Electrochemical techniques are widely used for the fabrication of nanostructured materials, yet a desired high-density nanoparticle arrays remains a challenge. Here large-area and high-density gold nanoparticle arrays with sub-10 nm gaps have been, for the first time, synthesized on Si(1 0 0) substrate within an electrochemical deposition system via the application of an unusually high over-potential. The extremely high over-potential contributes to the relatively small critical island size and high nucleation rate. It is believed that this method can be extended to the electrochemical fabrication nanoparticle arrays of other materials.

Local-strain effects in Si∕SiGe∕Si islands on oxide

The tensile-strained Si, based on the misfit between Si and SiGe gives high speed and high drive current to the metal-oxide-silicon field effect transistors. In order to achieve the total system minimum energy, the island edge, within some characteristic length, bends upwards giving rise to a distorted lattice, as simulated by the finite element method. The finding indicates that the conventional strain partition rule of Si∕SiGe∕Si layers used for a large island size (>10μm) is not adequate for a small island size (<200nm) due to the significant local-strain effect of the edge lattice distortion. For a small island size, the bending from the edge can significantly affect the strain on the surface of the top Si layer, and a compressive strain or reduced tensile strain occurs at the center of the top Si layer, while the conventional strain partition rule predicts a uniform tensile strain on the top Si layer for any size of Si∕SiGe∕Si islands.

Limitations of the thermodynamic Gibbs-Thompson analysis of nanoisland decay

The interpretation of island-within-island decay experiments is commonly based on a quasiequilibrium analysis with the island evaporation rate related to the inverse of its curvature, i.e., the Gibbs-Thompson chemical potential $\ensuremath{\mu}(r)$, where $r$ is the island size. However, it has been suggested that the quasiequilibrium analysis fails for sufficiently small island sizes because the distribution of atoms, with different coordination and detachment barriers at the island perimeter controls the evolution. With realistic Monte Carlo simulations that use calculated barriers for Ag(111), published in the literature and consistent with measured equilibrium island shapes, we have examined the island decay law. Deviations of the decay law of an adatom island from the expected quasiequilibrium analysis for the case of diffusion-limited kinetics are observed $N(t)={({N}_{0}\ensuremath{-}ct)}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\approx}1$ (instead of the expected $\ensuremath{\alpha}\ensuremath{\approx}2∕3$). In addition, the decay of a corresponding vacancy island for the same island-within-island geometry (without a step edge barrier), which is expected to be the same as the adatom island decay (in the quasiequilibrium analysis) is found to be faster. This also signals independently the failure of the quasiequilibrium analysis.

Growth mechanism of InAs quantum dots on GaAs by metal-organic chemical-vapor deposition

The growth parameters affecting the deposition of InAs quantum dots (QDs) by metal-organic chemical-vapor deposition are reported. Experiments with arsine pause, gas switching, and hydrogen shroud flow show that a low V∕III ratio is the key to obtaining three-dimensional InAs island formation with high density and uniformity. Based on atomic force microscopy images of InAs QDs deposited under different growth conditions, a physical model for the epitaxial growth of three-dimensional islands is proposed. In this model, the InAs QD growth is governed by two types of arsenic sources at the growth surface: free arsenic atoms arriving at the boundary layer and dangling arsenic bonds available at the GaAs wafer surface. At high V∕III ratio, free arsenic atoms arriving at the boundary layer are the dominant hydride species and produce a low density of InAs islands with irregular shape and polycrystalline defects. At low V∕III ratio arsenic bonds on the GaAs surface are the main sites for indium atoms to attach to, thus producing high island densities and small coherent island sizes.

Scaling of submonolayer island sizes in surfactant-mediated epitaxy of semiconductors

We study submonolayer island size distributions in the epitaxy of Si and Ge on the Si(111) surface with Bi as a surfactant. We show that sizes of Si islands at different growth temperatures scale to a standard scaling function that peaks at the mean island size. Size distribution of Ge islands demonstrates qualitatively different behavior: With the decreasing temperature the most probable size in the population of Ge islands shifts towards small island sizes so that the peaked scaling function degenerates to a decreasing one. The observed scaling phenomena are found to be inherent to the mechanism of growth which involves exchange and de-exchange processes of deposited atoms with the surfactant and the strong passivation of step edges in the presence of the surfactant.

Onset of neoclassical tearing modes on JET

A detailed study has been made of the onset and behaviour of (m = 3, n = 2) neoclassical tearing modes (NTMs) on JET, validating many of the underlying concepts in NTM physics. In particular, fitting onset thresholds in terms of the correctly motivated physics parameters produces scalings closer to theoretical expectations. The use of such locally measured parameters also removes previous inconsistencies in the data between different divertor configurations. The evolution of island size with β confirms the predicted levels of bootstrap current, and highlights the role of small island size stabilization terms, which give rise to a seeding requirement for NTMs from some other instability. This seeding requirement is confirmed, and is generally provided by sawteeth in the cases studied here. Although there are a variety of possible seeding mechanisms, for the particular cases studied here, it appears doubtful that the seeds are induced by magnetic coupling. In particular the NTM always appears well below the n = 2 sawtooth precursor harmonic frequency—although they often lock together once the NTM has grown close to saturation. Also, while plasma rotation profiles are favourable for a three wave interaction involving (4, 3) and n = 1 modes, with irregular matches to the appropriate mode frequency condition, there is no clear correlation between such matches and (3, 2) NTM growth rates. Thus, other seeding models should be considered—we have discussed two possibilities: a temporary change to classical tearing stability due to the sawtooth, and flow variation leading to a change in ion polarization current polarity, but neither of these are yet confirmed by the data. A complete resolution of this issue is essential if a predictive theory of NTMs is to be obtained, but it requires further, more detailed experimental measurements, and development of theoretical models. Finally, the nature of the (4, 3) NTM has also been explored. This mode also requires a seed perturbation to be triggered (in these cases supplied by a sawtooth), and occurs at similar local parameter values as the (3, 2) NTM, but with lower heating powers and βN. A cascade process emerges in which successively lower mode number NTMs occur as β is raised.

Energy distribution in Ge islands on Si(001): A spectral and site-resolved analysis versus size and morphology

By a spectral and site-resolved analysis of the energy distribution (both stress and surface originated) at the atomistic scale, calculated by classical molecular dynamics for Ge islands with different morphologies on Si(001), we show how domes actually provide the largest strain release. Moreover, we point out that the usual partition of the total energy into a volume plus a surface contribution also corresponds to two separate spectral energy regions, which are the same in any morphology. However, it turns out that the volume-scaling contribution is quite complex and that the real strain relaxation term corresponds to the lower part of the energy spectrum, not scaling as the volume for small island size.

Effects of ion beams on the early stages of MgO growth

Molecular dynamics simulations are performed to examine the effects of ion beams in selecting MgO island orientations during ion beam assisted deposition. Sputter yields are determined as a function of ion beam orientation for MgO islands of different sizes. While channeling does occur in small islands, the difference in sputter yields between channeling and nonchanneling orientations is much smaller in small islands than in larger islands or a continuous film. The ion beam is capable of disrupting the entire island structure at small island sizes, while channeling dependent sputtering occurs at larger island size. Since less sputtering occurs for channeling orientations, the ion beam produces an orientation-dependent island growth rate. This leads to island orientation-dependent nucleation rates. A new analytical model parameterized using the molecular dynamics sputter yield data is developed which predicts how growth conditions affect these island-orientation nucleation rates. Based upon these simulations and model results, we suggest a strategy to efficiently grow thin, highly textured MgO films.

Smoluchowski ripening and random percolation in epitaxialSi1−xGex/Si(001)islands

Island size distributions of ripened ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(001)$ $(x=0.3\ensuremath{-}0.7)$ islands exhibit a slow decay at small island sizes and an asymmetric bell-shaped distribution peaked at a large size. We explain the ripening process in ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(001)$ islands by proposing a model based on random percolation and Smoluchowski ripening. Drawing an analogy with percolation theory, we use a shifted average height to represent the time variable, which is also applicable in the Smoluchowski ripening model; this shifted average height is used to analyze size distributions and correlation functions. Critical exponents derived from the site percolation model at $d=3$ agree with our measurements. Island diffusion $D\ensuremath{\propto}{s}^{\ensuremath{-}\ensuremath{\alpha}}$ leading to coalescence events, shows a weak dependence on size s: \ensuremath{\alpha}=0.28\ifmmode\pm\else\textpm\fi{}0.20, attributable to the inverse process of sequential incorporation of atoms at step edges due to the high stress concentration at island terrace perimeters. While random percolation dominates for small sizes regime, Smoluchowski ripening controls the distribution peak at large island sizes. Good agreement is obtained between the model and the measured island size distributions.

Gibbs-Thomson formula for small island sizes: Corrections for high vapor densities.

In this paper we report simulation studies of equilibrium features, namely circular islands on model surfaces, using Monte-Carlo methods. In particular, we are interested in studying the relationship between the density of vapour around a curved island and its curvature-the Gibbs-Thomson formula. Numerical simulations of a lattice gas model, performed for various sizes of islands, don't fit very well to the Gibbs-Thomson formula. We show how corrections to this form arise at high vapour densities, wherein a knowledge of the exact equation of state (as opposed to the ideal gas approximation) is necessary to predict this relationship. Exploiting a mapping of the lattice gas to the Ising model one can compute the corrections to the Gibbs-Thomson formula using high field series expansions. We also investigate finite size effects on the stability of the islands both theoretically and through simulations. Finally the simulations are used to study the microscopic origins of the Gibbs-Thomson formula. A heuristic argument is suggested in which it is partially attributed to geometric constraints on the island edge.

A vibrational characterisation of the O/A1(111) system: a reassignment of HREELS data

In light of recent STM measurements of the O/A1(111) system, we reassign the dipole active modes observed at low coverage to resolve discrepancies between the interpretation of Strong, Firey, deWette and Erskine [Phys. Rev. B 26 (1982) 3483], invoking subsurface oxygen, and a variety of other studies which find no evidence for surface oxygen. The STM results, which show that very small island sizes are stabilized over an exposure range up to ∼ 200 L with a total coverage ≤ 0.2 ML, are incompatible with the assumption of long range periodicity required for lattice dynamical modeling. The consequence is that vibrational modes polarized parallel to the surface may become dipole active. Within an Al 3O cluster model appropriate to exposures ≤ 3 L where most oxygen atoms are isolated species in three-fold hollow sites, the strong feature at 584 cm −1 (72 meV) is still attributed to top-layer oxygen motion perpecdicular to the surface (the symmetric Al 3O stretch) but the second intense feature at 480 cm −1 (60 meV) is assigned to the umbrella mode involving predominantly Al motion parallel to the surface rather than the motion of two AlO layers moving perpendicular to the surface out of phase with each other. The lowest frequency mode near 224 cm −1 (28 meV) derives from the frustrated translation of the cluster perpendicular to the surface. At higher exposures (> 10 L) where multiple oxygen islands begin to appear, totally symmetric combinations of the E-derived asymmetric Al 3O stretching motion polarised nominally parallel to the surface become dipole allowed and can be assigned to the loss at 850 cm −1 (105 meV), which was previously attributed to subsurface oxygen.

Biodiversity on Oceanic Islands: Its Origin and Extinction1

SYNOPSIS. The isolation and small size of oceanic islands make them attractive models for studies of diversification; the sensitivity of their biota makes them important subjects for studies of extinction. I explore the origin of island biotas through dispersal and in situ diversification, and examine the fate of these biotas since human contact. Island biotas start out depauperate and disharmonic, facilitating the survival of relict taxa and stimulating adaptive radiations. The often highly restricted range and small population size of insular species, together with their limited diversity of defenses, make island biotas particularly vulnerable to extinction, largely through habitat loss or interactions with introduced species.

Molt of Bristle-Thighed Curlews in the Northwestern Hawailan Islands

-I studied molt of Bristle-thighed Curlews (Numenius tahitiensis) on Laysan Island in the Northwestern Hawaiian Islands from 1988-1991. Adult curlews underwent a complete prebasic molt between August and December. Duration of primary molt was about 92 days, which is rapid compared with other shorebirds that molt in tropical and southern latitudes. Adults replaced large numbers of primaries and secondaries simultaneously, and about 50% of the birds became flightless during molt. The prealternate molt began in winter and ended in early spring; it involved the body feathers and variable numbers of rectrices but no remiges. Juveniles molted body feathers and some rectrices during their first autumn and winter but did not replace their juvenal primaries until the spring and summer of their second calendar year. Second-year curlews began replacing their new first basic primaries in late summer, in some cases before the outer juvenal primaries had been dropped. The delayed first prebasic primary molt is probably an adaptation allowing inexperienced birds to devote energy expenditure in their first winter to obtaining food rather than to molting remiges. Because second-year birds remain on the wintering grounds throughout the year and do not prepare for migration, there is no selection against replacing new primaries. Unlike most shorebirds, adult Bristle-thighed Curlews gained mass steadily throughout the autumn and winter. Their rapid prebasic molt in autumn may be an adaptation to allow the birds ample time to build up fat reserves during winter. I suggest that the absence of rich intertidal feeding areas and the frequency of winter storms make it difficult for curlews to take on large fat stores in short time periods as do species that winter on continental coasts. The lack of predators and the small size of remote oceanic islands obviate the need for curlews to maintain peak flight efficiency, allowing birds to become flightless during molt in autumn and to carry increasingly large fat stores throughout the winter. Received 8 October 1992, accepted 15 December 1992. SEVERAL GENERAL PATTERNS in the timing and duration of the prebasic molt of shorebirds have been identified. For example, most shorebirds do not overlap molt of flight feathers with either breeding or migration, although molt of body feathers may begin on the breeding grounds (Ferns 1978, Johnson and Johnson 1983). Notable exceptions include some populations of Purple Sandpipers (Calidris maritima; Sutton and Parmelee 1955, Bengston 1975) and Dunlins (C. alpina; Holmes 1966) that replace their primaries while breeding, and Common Redshanks (Tringa totanus; Pienkowski et al. 1976) that molt while migrating. Also, shorebirds wintering in tropical and southern latitudes tend to initiate molt later, and to molt more slowly, than do species that winter farther north (Prater 1981). Despite substantial infor' Present address: Cooperative Wildlife Research Unit, University of Montana, Missoula, Montana 59812, USA. mation on the molt of shorebirds (e.g. Boere 1976, Morrison 1976, Pienkowski et al. 1976, Prater 1981), the ecological factors responsible for these general patterns have been difficult to elucidate. Consequently, there remains a need to document molt in additional species. Bristle-thighed Curlews (Numenius tahitiensis) are one of the rarest and least-studied shorebirds in the world. They breed in remote areas of western Alaska and winter on small islands in the tropical and subtropical Pacific Ocean. The only published information on their molt comes from a small sample of subadults collected in the Marshall Islands during summer (Johnson 1977). Bristle-thighed Curlews exhibit several unusual traits that make a study of their molt particularly interesting: (1) they are the only migratory shorebird whose winter range is restricted to oceanic islands, which they reach after nonstop flights of at least 4,000 km (Marks et al. 1990); (2) they have high annual survivorship and extreme longevity (Marks 1992); (3) most subadults remain on the wintering