Nearest-neighbour Separation Research Articles

Optical and imaging properties of a novel multi-segment spectacle lens designed to slow myopia progression.

High sampling density optical metrology combined with pupil- and image-plane numerical analyses were applied to evaluate a novel spectacle lens containing multiple small zones designed to slow myopia progression. High-resolution aberrometry (ClearWave, www.lumetrics.com) was used to sample wavefront slopes of a novel spectacle lens, Defocus Incorporated Multiple Segments (DIMS) (www.hoya.com), incorporating many small, positive-powered lenslets in its periphery. Using wavefront slope and error maps, custom MATLAB software ('Indiana Wavefront Analyzer') was used to compute image-plane point-spread functions (PSF), modulation transfer functions (MTF), simulated images and power distributions created by the dual-focus optic for different pupil sizes and target vergences. Outside of a central 10mm zone containing single distance optical power, a hexagonal array of small 1mm lenslets with nearest-neighbour separations of 0.5mm were distributed over the lens periphery. Sagittal and curvature-based measures of optical power imperfectly captured the consistent +3.50D add produced by the lenslets. Image plane simulations revealed multiple PSFs and poor image quality at the lenslet focal plane. Blur at the distance optic focal plane was consistent with a combination of diffraction blur from the distance optic and the approximately +3.50D of defocus from the 1mm diameter near optic zones. Converging the defocused beams generated by the multiple small (1mm diameter) lenslets to a blurred image at the distance focal plane produced a blur magnitude determined by the small lenslet diameter and not the overall pupil diameter. The distance optic located in between the near-add lenslets determines the limits of the optical quality achievable by the lens. When compared to the optics of a traditional concentric-zone dual-focus contact lens, the optics of the DIMS lens generates higher-contrast images at low spatial frequencies (<7 cycles per degree), but lower-contrast at high spatial frequencies.

A novel approach to text clustering using genetic algorithm based on the nearest neighbour heuristic

In this paper, we propose a novel clustering algorithm which uses a weighted combination of several criteria as its fitness function. We demonstrate the suitability of the new method in the case of clustering text documents. The proposed algorithm leverages the concept of nearest neighbour separation (NNS) to enhance the separation of the clusters and also outlines a heuristic to compute the NNS. A new parameterized fitness function has been proposed which can be tuned to provide more weightage to the traditional metrics based on inter- and intra-cluster distances of clusters or on the NNS. Genetic Algorithm has been used to perform the actual clustering and the results obtained has been compared with the traditional K-Means algorithm. The performance of the algorithm has been tested on different standard datasets, and the results have been presented.

Determining the presence of characteristic fragmentation length-scales in filaments

Theories suggest that filament fragmentation should occur on a characteristic fragmentation length-scale. This fragmentation length-scale can be related to filament properties, such as the width and the dynamical state of the filament. Here we present a study of a number of fragmentation analysis techniques applied to filaments, and their sensitivity to characteristic fragmentation length-scales. We test the sensitivity to both single-tier and two-tier fragmentation, i.e. when the fragmentation can be characterised with one or two fragmentation length-scales respectively. The nearest neighbour separation, minimum spanning tree separation and two-point correlation function are all able to robustly detect characteristic fragmentation length-scales. The Fourier power spectrum and the Nth nearest neighbour technique are both poor techniques, and require very little scatter in the core spacings for the characteristic length-scale to be successfully determined. We develop a null hypothesis test to compare the results of the nearest neighbour and minimum spanning tree separation distribution with randomly placed cores. We show that a larger number of cores is necessary to successfully reject the null hypothesis if the underlying fragmentation is two-tier, N>20. Once the null is rejected we show how one may decide if the observed fragmentation is best described by single-tier or two-tier fragmentation, using either Akaike's information criterion or the Bayes factor. The analysis techniques, null hypothesis tests, and model selection approaches are all included in a new open-source Python/C library called FragMent.

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

We study the fragmentation of the nearest high line-mass filament, the integral shaped filament (ISF, line-mass $\sim$ 400 M$_\odot$ pc$^{-1}$) in the Orion A molecular cloud. We have observed a 1.6 pc long section of the ISF with the Atacama Large Millimetre/submillimeter Array (ALMA) at 3 mm continuum emission, at a resolution of $\sim$3" (1 200 AU). We identify from the region 43 dense cores with masses about a solar mass. 60% of the ALMA cores are protostellar and 40\% are starless. The nearest neighbour separations of the cores do not show a preferred fragmentation scale; the frequency of short separations increases down to 1 200 AU. We apply a two-point correlation analysis on the dense core separations and show that the ALMA cores are significantly grouped at separations below $\sim$17 000 AU and strongly grouped below $\sim$6 000 AU. The protostellar and starless cores are grouped differently: only the starless cores group strongly below $\sim$6 000 AU. In addition, the spatial distribution of the cores indicates periodic grouping of the cores into groups of $\sim$30 000 AU in size, separated by $\sim$50 000 AU. The groups coincide with dust column density peaks detected by Herschel. These results show hierarchical, two-mode fragmentation in which the maternal filament periodically fragments into groups of dense cores. Critically, our results indicate that the fragmentation models for lower line-mass filaments ($\sim$ 16 M$_\odot$ pc$^{-1}$) fail to capture the observed properties of the ISF. We also find that the protostars identified with Spitzer and Herschel in the ISF are grouped at separations below $\sim$17 000 AU. In contrast, young stars with disks do not show significant grouping. This suggests that the grouping of dense cores is partially retained over the protostar lifetime, but not over the lifetime of stars with disks.

Development of observational learning during school formation in jack mackerel Trachurus japonicus juveniles

We assessed whether the development of observational learning in jack mackerel Trachurus japonicus juveniles corresponds with that of their schooling behaviour. Schooling behaviour was quantitatively analyzed by nearest neighbour distance and separation angle in two size classes of fish, 20-mm and 40-mm in body length. Observer and non-observer fish with matching sizes were conditioned to pellets by temporarily stopping aeration. Observer fish were provided with five observation trials of other individuals feeding near an air stone when aeration was stopped. After the observation trial, fish were conditioned to pellets with the stop of aeration, and then the learning process was evaluated by the increase in the association with the feeding area when aeration was stopped. In 20-mm fish, which were at an immature stage of schooling behaviour, there was no difference in the learning process between observer and non-observer fish. In contrast, 40-mm fish were confirmed to have a well-developed schooling behaviour, and the observer learnt the feeding area more efficiently than the non-observer. This study provides evidence that observational learning develops along with the development of the social interaction.

Environments of strong/ultrastrong, ultraviolet Fe ii emitting quasars

We have investigated the strength of ultraviolet Fe II emission from quasars within the environments of Large Quasar Groups (LQGs) in comparison with quasars elsewhere, for 1.1 <= <z_LQG> <= 1.7, using the DR7QSO catalogue of the Sloan Digital Sky Survey. We use the Weymann et al. W2400 equivalent width, defined between the rest-frame continuum-windows 2240-2255 and 2665-2695 Ang., as the measure of the UV Fe II emission. We find a significant shift of the W2400 distribution to higher values for quasars within LQGs, predominantly for those LQGs with 1.1 <= <z_LQG> <= 1.5. There is a tentative indication that the shift to higher values increases with the quasar i magnitude. We find evidence that within LQGs the ultrastrong emitters with W2400 >= 45 Ang. (more precisely, ultrastrong-plus with W2400 >= 44 Ang.) have preferred nearest-neighbour separations of ~ 30-50 Mpc to the adjacent quasar of any W2400 strength. No such effect is seen for the ultrastrong emitters that are not in LQGs. The possibilities for increasing the strength of the Fe II emission appear to be iron abundance, Ly-alpha fluorescence, and microturbulence, and probably all of these operate. The dense environment of the LQGs may have led to an increased rate of star formation and an enhanced abundance of iron in the nuclei of galaxies. Similarly the dense environment may have led to more active blackholes and increased Ly-alpha fluorescence. The preferred nearest-neighbour separation for the stronger emitters would appear to suggest a dynamical component, such as microturbulence. In one particular LQG, the Huge-LQG (the largest structure known in the early universe), six of the seven strongest emitters very obviously form three pairings within the total of 73 members.

Moderate turbidity enhances schooling behaviour in fish larvae in coastal waters

AbstractWe evaluated the effects of turbidity on school formation in ayu (Plecoglossus altivelis) [24.5 ± 2.2 mm standard length (Ls)], Japanese anchovy (Engraulis japonicus) (29.1 ± 3.1 mm Ls) larvae, which often live in turbid coastal waters, and yellowtail (Seriola quinqueradiata) juveniles (37.1 ± 2.5 mm Ls), which live in clear offshore waters. Fish were introduced into experimental tanks at one of five turbidity levels obtained by dissolving 0, 5, 20, 50, or 300 mg l−1 of kaolin in seawater. Their behaviour was video recorded, and the nearest neighbour distance (DNN) and separation angle (AS) were compared among turbidity levels. Mean DNN of ayu was significantly smaller at 20 and 50 mg l−1 than any other level of turbidity, as was AS at 20 mg l−1 compared with 0 mg l−1. Mean AS of anchovy was smaller at 50 mg l−1 of turbidity than any others. In contrast, mean DNN of yellowtail was larger at 300 mg l−1 than any others. These results suggest that moderate turbidities enhance schooling behaviour in ayu and Japanese anchovy larvae, whereas turbidity has an inhibitive effect on schooling of yellowtail juveniles, corresponding well to the habitat characteristics of each species.

Computer simulation of the influence of hydrogen on stress–order correlations in amorphous silicon

This paper reports the computational simulation of the correlations between atomic-level stress and local structure fluctuations in a computational model of amorphous silicon. A single parameter has been identified, which uniquely characterises the structural order in these structures. This parameter is the linear combination of the SDs of the first and second nearest neighbour separations. The stress fluctuations, under progressive hydrogen incorporation, show two clear dependences on this parameter, and therefore on the structural order. This dual dependency clearly alludes to structural network configurations that contain low and high hydrogen concentrations. The implications of the results on the local geometry of tetrahedrally bonded amorphous solids are discussed.

Low-energy electron diffraction (LEED) study of an aperiodic thin film of Cu on 5-fold i-Al-Pd-Mn

Thin films of copper grown on five-fold i-AlPdMn at room temperature consist of domains that are rotationally aligned with the five primary symmetry directions of the substrate and which have one-dimensional aperiodic order. This aperiodic order is evident in scanning tunnelling microscopy images as wide and narrow rows that are spaced according to a Fibonacci sequence. A low-energy electron diffraction (LEED) study of this film indicates that the structure within the domains is periodic along the rows, with a repeat distance equal to the nearest-neighbour separation in bulk Cu. To determine the complete structure, a dynamical LEED experiment was performed for a five-layer Cu film at a sample temperature of 85 K. The analysis was performed using two different computational methods, one based on quasicrystalline slabs and the other on periodic approximants. Of the model structures tested, the film is found to be most consistent with a structure based on the Cu{100} surface structure, but having aperiodic displacements, both in-plane and out-of-plane, along a ⟨110⟩ direction.

Schooling behaviour and retinomotor response of juvenile Pacific bluefin tuna Thunnus orientalis under different light intensities

Schooling behaviour and histological retinal light adaptation in juvenile Pacific bluefin tuna Thunnus orientalis were examined under various light intensities to determine the effect of light intensity on behaviour. After monitoring the schooling behaviour of juveniles 35–36 and 45–46 days post hatching, schooling variables such as nearest neighbour distance and separation swimming index were measured under different light intensities. Furthermore, retinal indices of light adaptation were investigated histologically for each experimental light intensity. Under intensities &gt;5 lx, schooling variables in the two juvenile growth stages were nearly constant, allowing schooling. In contrast, the schooling variables indicated that the fish gradually swam more widely and randomly with decreasing light intensities &lt;5 lx. The retinal indices also showed a shift from light adaptation to dark adaptation at light levels &lt;5 lx. From 5 to 0·01 lx, retinal adaptation and fish schooling behaviour changed with light intensity. These data suggest that the schooling behaviour of juvenile Pacific bluefin tuna is greatly affected by retinal adaptation.

Ab initio investigation of the topology and properties of three-dimensional clusters of water (H 2O) n

Quantum mechanical studies were carried out in order to understand and describe the nature and strength of the hydrogen bond topology in three dimensional water cluster systems (H 2O) n , in the size range 6–20 molecules. Optimal structures and vibrational frequencies of cage like water clusters are computed at the Hartree–Fock (HF) and the electronic density functional theory (DFT) level of theory with a detailed data analysis. Theoretical results are compared to experimental data when available. The general tendency is a decrease of the global minimum energy as the cluster size increases. The nearest neighbour separation between oxygen atoms decreases exponentially when the cluster size increases. The present work suggests that DFT results are in a good agreement with the corresponding HF data, however slight differences remain with regard to values of the nearest neighbour separation between oxygen atoms. The Morokuma’s decomposition of the total interaction energy performed on two and three-dimensional clusters lead to a better understanding of the cluster formation with a strong polarization energy component.

Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon

The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles.

Power-law island size distributions and the Avrami model

The island size distribution in the early stages of thin-film deposition is simulated using the Avrami model and is shown to follow closely a power-law form for both two- and three-dimensional island growth. This model applies to systems where the time-averaged monomer density is constant across the substrate, as occurs for example when the mean free path of the diffusing monomers is limited. It offers a possible explanation for recently observed island size distributions which are continuously decreasing functions of size in certain systems. The distribution of nearest-neighbour separations of the islands is also considered and it is shown that this distribution can be used to distinguish between island arrays formed in the Avrami model and those of deposition-diffusion-aggregation models.

Statistical studies of level correlations and chaotic phenomena in spectroscopy

This review covers some modern applications of statistical methods in the analysis of complicated molecular spectra, obtained usually with laser excitation. Among the available fluctuation measures, special emphasis is given to the nearest neighbour separation and spectral rigidity measures, and attention is called to the power spectrum (or statistical Fourier-transform) method.

Structural transitions in ultra-thin nickel films on Rh{111}

For the first three monolayers, ultra-thin nickel films on Rh{111} grow in a close to layer-by-layer fashion. A nickel monolayer film is “pseudomorphic” with the underlying substrate. An automated tensor LEED I( V) analysis indicates the clean Rh{111} surface to be laterally unreconstructed with a small amplitude damped oscillatory relaxation of the outermost two atomic layer spacings of Δd z 12 = −2.7 ± 1.4% and Δd z 23 = + 0.4 ± 1.4%, resulting in a Pendry R-factor of 0.16. For the nickel adsorption site is found to be the fcc threefold hollow, with a nickel to rhodium interlayer spacing of 2.06 Å, slightly shorter than the sum of metallic hard sphere radii, with the outermost rhodium interlayer spacing relaxed to Δd z 12 = −1.4 ± 3.2%. The minimum Pendry R-factor of 0.28 was obtained. Spot profile analysis indicated significant beam broadening upon initiation of second layer growth with asymmetry at high parallel momentum transfer consistent with formation of small domains of incommensurate nickel with a nearest-neighbour separation intermediate between rhodium and nickel, finally yielding a complex pattern due to multiple scattering between the pseudomorphic p(1 × 1) slab and the incommensurate film for thicknesses beyond 1 ML. Model pair potential calculations indicate a layer dependent gradual relaxation of the nickelickel interlayer spacing towards that of bulk nickel in the outermost three atomic layers. Alloying occurs on an experimental time scale at temperatures above 600 K leading to formation of a substitutionally disordered RhNi alloy of variable composition.

Mono- and dilayer modifications of lithium lattice parameters

Systematic local density treatment (via the all-electron, full-potential, linear combination of gaussian orbitals fitting function (LCGTO-FF) algorithms) of the cohesive properties of mono- and dilayer Li when combined with recent calculations for bulk Li of similar high quality, yields a prediction of modest intraplanar lattice expansion (dilayer: 2.8%, monolayer: 1.6%) and substantial interplanar contraction (dilayer c/a=1.46 versus calculated bulk c/a=1.64). The differences between these predictions and the limited experimental data for Li overlayers on graphite (which exhibit about 6.1% expansion in nearest-neighbour separation) suggest possible substrate or bonding effects in the experiment. The total dilayer cohesive energy is 79.8% of the bulk cohesive energy while for the monolayer the fraction is 63.1%. The interplanar contribution to the dilayer cohesive energy is -0.29 eV. The dilayer uniaxial (c-axis) compressibility is 2.5 times as large as the calculated value for the HCP Li crystal (the latter value is in quite good agreement with available measurements). Mono- and dilayer energy bands (at the level of bare Kohn-Sham eigenvalues) are basically consistent with those calculated self-consistently for the crystal using the same LDA model. However, calculated work function values are larger than measured crystalline values, by more than 0.6 eV, for both the mono- and dilayers.

Relations between vacancy-solute binding energies and bulk properties in metals

H.-J. Rockosch (1983) has recently obtained a set of vacancy-solute binding energies for seven Cu-based and five Ag-based alloys from an analysis of diffusion data. The author investigates the extent to which these binding energies correlate with (i) the difference between solute and host valences Delta Z, (ii) the slope of the solidus curve, dTm/dCs, and (iii) the lattice constant change da/dCs. D. Lazarus (1954) and A.D. LeClaire (1978) have previously developed a model relating binding energies to Delta Z; in this work, the author presents new models that predict correlations (ii) and (iii). Specifically, the author derives the approximate relations Evs=-(E1v/z)(dTm/dCs) (where E1v is the monovacancy formation energy in the pure solvent and z is the coordination number), and Evs=-(9K Omega /z)(d ln nat(R1)/d ln R1)-1 d ln a/dCs (where K is the bulk modulus, nat is the atomic charge density of host atoms, R1 is the nearest-neighbour separation and Omega is the atomic volume). The correlation between the Rockosch binding energies and either Delta Z or dTm/dCs is only fair; the correlation with da/dCs, however, is quite close.

Application of a finite initial separation limit to the concentration gradient model of fluorescence quenching in liquids

The Stern-Volmer equation in the Smoluchowski-Collins-Kimball approximation is modified by the use of a most probable nearest neighbour separation as an upper integration limit of the time-independent flux equation. The resulting relationship provides a description of the experimental relative fluorescence yield for 111 data points in 12 systems with an overall average deviation of 2.3%, and affords realistic estimates of the relative diffusion coefficient and (in the diffusion-controlled limit) the encounter diameter.

AB initio hartree-fock study of the MgO(001) surface

The MgO(001) surface has been studied with an ab initio Hartree-Fock crystalline orbital LCAO program. An optimized basis set containing nine atomic orbitals (three s and six p) per atom has been used. The semi-infinite crystal has been simulated by a slab containing three planes (six atoms per cell). In agreement with the most recent LEED experiments, no relaxation is found and the “rumpling” is very small (1% of the nearest neighbour separation in the bulk). The analysis of the electron charge distribution and of the ion multipoles shows that the fully ionic character found for the bulk is maintained at the surface, and that the anion deformation is very small. A surface energy of 1.43 J/m 2 was obtained.

The α*-constant, equilibrium state and bearing netplanes in polymers, biopolymers and catalysts

Debye's idea of an 'a priori probability function' of interatomic distances in a liquid is generalised by the theory of paracrystals. In the case of a square lattice this generalised model is studied using a two-dimensional coin model. Between the nearest-neighbour separations the only correlation is with respect to their directions. The netplanes formed by nearest neighbours are 'bearing netplanes', so-called because of the bearing role they play in microparacrystals. In the expression for free enthalpy Delta G(N) a new term 3/2NA0g2 takes into account the minimum given by the alpha *-law, where N defines the number of bearing netplanes in a spatial direction, g is the nearest-neighbour-distance fluctuation and A0 the enthalpy density given by the tangential potentials. Delta G depends on the number of valence bonds within a lattice cell and therefore increases with the distance d0 between the bearing netplanes. alpha * then decreases automatically and reaches values of alpha *=0.15+or-0.05 for d0<or approximately=3 AA and alpha * approximately=0.05 for d0>3 AA. Even larger alpha *-values have been observed in the DuPont Kevlar fibre; this can be partly explained in terms of a non-equilibrium state. The bearing netplanes build up the surface of a microparacrystal in an equilibrium state and thus replace Wulff's rule.