Lithiation of silicon via lithium Zintl-defect complexes

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.Comment: 4 pages, 3 figure

Foraging ecology and community organization in desert seed-eating ants

Journal ArticleGranivorous ants in the southwestern deserts of the United States are characterized by species-specific colony foraging behaviors that determine their efficiencies at utilizing seeds from different density distributions. Workers search for food either in groups or as individuals, and these feeding strategies represent specializations for high and low density resources respectively. While gathering experimental seeds, simultaneously supplied in clumped and dispersed distributions, feeding by group foragers focused on the high density resource patches, while individual foragers harvested seeds predominantly from the dispersed distribution, which required that prey be independently discovered. Collecting native seeds from unmanipulated environments, individual foragers tended to spend proportionately more time searching and experience lower foraging success than did group foragers in the same local habitat. During a period of high seed abundance, group foragers collected a much narrower range of prey types than did individual foragers. Colonies of Pogonomyrmex rugosus exhibited a mixed foraging strategy, with the most distinct feeding columns occurring during a period of peak seed abundance. Among group foragers, greatest activity coincided with periods of relatively high seed densities and low climatic stress, and these species utilized tactics such as seed storage, hibernation, and estivation to weather less favorable periods in a resting state. Individual foragers were active at intermediate levels during less favorable periods. Although high and low density seed resources are not renewed independently of one another, density specialization appears to promote coexistence between group and individual foragers. Their capacity to stably partition resources in this way should depend on (1) the degree of difference in their efficiencies at exploiting different density distributions of seeds and (2) the availability of seeds in the habitat as a function of seed density. In more mesic habitats, where seeds are renewed more frequently and probably also in larger pulses, group foraging species are proportionately more abundant

Galaxy And Mass Assembly (GAMA): Defining Passive Galaxy Samples and Searching for the UV Upturn

Abstract We use data from the GAMA and GALEX surveys to demonstrate that the UV upturn, an unexpected excess of ultraviolet flux from a hot stellar component, seen in the spectra of many early-type galaxies, arises from processes internal to individual galaxies with no measurable influence from the galaxies’ larger environment. We first define a clean sample of passive galaxies without a significant contribution to their UV flux from low-level star formation. We confirm that galaxies with the optical colours of red sequence galaxies often have signs of residual star formation, which, without other information, would prevent a convincing demonstration of the presence of UV upturns. However, by including (NUV−u) and WISE (W2-W3) colours, and FUV data where it exists, we can convincingly constrain samples to be composed of non-star-forming objects. Using such a sample, we examine GALEX photometry of low redshift GAMA galaxies in a range of low-density environments, from groups to the general field, searching for UV upturns. We find a wide range of (NUV−r) colours, entirely consistent with the range seen – and attributed to the UV upturn – in low-redshift red sequence cluster galaxies. The range of colours is independent of group multiplicity or velocity dispersion, with isolated passive galaxies just as likely to have blue UV-to-optical colours, implying significant upturn components, as those in richer groups and in the previous data on clusters. This is supported by equivalent results for (FUV−r) colours which are clear indicators of upturn components

Searching for spatial variations of alpha^2/mu in the Milky Way

(Abridged) A procedure is suggested to explore the value of F = alpha^2/mu, where mu = m_e/m_p is the electron-to-proton mass ratio, and alpha is the fine-structure constant. The fundamental physical constants, which are measured in different physical environments of high (terrestrial) and low (interstellar) densities of baryonic matter are supposed to vary in chameleon-like scalar field models, which predict that both masses and coupling constant may depend on the local matter density. The parameter Delta F/F = (F_obs - F_lab)/F_lab can be estimated from the radial velocity offset, Delta V = V_rot-V_fs, between the low-laying rotational transitions in carbon monoxide 13CO and the fine-structure transitions in atomic carbon [CI]. A model-dependent constraint on Delta alpha/alpha can be obtained from Delta F/F using Delta mu/mu independently measured from the ammonia method. Currently available radio astronomical datasets provide an upper limit on |Delta V| < 110 m/s (1sigma). When interpreted in terms of the spatial variation of F, this gives |Delta F/F| < 3.7*10^-{7}. An order of magnitude improvement of this limit will allow us to test independently a non-zero value of Delta mu/mu = (2.2 +/- 0.4_stat +/- 0.3_sys)*10^{-8} recently found with the ammonia method. Taking into account that the ammonia method restricts the spatial variation of mu at the level of |Delta mu/mu| <= 3*10^{-8} and assuming that Delta F/F is the same in the entire interstellar medium, one obtains that the spatial variation of alpha does not exceed the value |Delta alpha/alpha| < 2*10^{-7}. Since extragalactic gas clouds have densities similar to those in the interstellar medium, the bound on Delta alpha/alpha is also expected to be less than 2*10^{-7} at high redshift if no significant temporal dependence of alpha is present.Comment: 7 pages, 1 table. Accepted for publication in Astronomy and Astrophysic

Modelling foraging movements of diving predators : A theoretical study exploring the effect of heterogeneous landscapes on foraging efficiency

Peer reviewedPublisher PD

Composite random search strategies based on non-directional sensory cues

Many foraging animals find food using composite random search strategies, which consist of intensive and extensive search modes. Models of composite search can generate predictions about how optimal foragers should behave in each search mode, and how they should determine when to switch between search modes. Most of these models assume that foragers use resource encounters to decide when to switch between search modes. Empirical observations indicate that a variety of organisms use non-directional sensory cues to identify areas that warrant intensive search. These cues are not precise enough to allow a forager to directly orient itself to a resource, but can be used as a criterion to determine the appropriate search mode. As a potential example, a forager might use olfactory information, which could help it determine if an area is worth searching carefully. We developed a model of composite search based on non-directional sensory cues. With simulations, we compared the search efficiencies of composite foragers that use resource encounters as their mode-switching criterion with those that use non-directional sensory cues. We identified optimal search patterns and mode-switching criteria on a variety of resource distributions, characterized by different levels of resource aggregation and density. On all resource distributions, foraging strategies based on the non-directional sensory criterion were more efficient than those based on the resource encounter criterion. Strategies based on the non-directional sensory criterion were also more robust to changes in resource distribution. Our results suggest that current assumptions about the role of resource encounters in models of optimal composite search should be re-examined. The search strategies predicted by our model can help bridge the gap between random search theory and traditional patch-use foraging theory

Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle

Context. In the framework of the interstellar polycyclic aromatic hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a fingerprint of single species in this class. Aims. We address the question of detectability of low energy PAH vibrational bands, with respect to spectral contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs). Methods. We extend our extablished Monte-Carlo model of the photophysics of specific PAHs in astronomical environments, to include rotational and anharmonic band structure. The required molecular parameters were calculated in the framework of the Density Functional Theory. Results. We calculate the detailed spectral profiles of three low-energy vibrational bands of neutral naphthalene, and four low-energy vibrational bands of neutral anthracene. They are used to establish detectability constraints based on intensity ratios with ``classical'' AIBs. A general procedure is suggested to select promising diagnostics, and tested on available Infrared Space Observatory data for the Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the far-IR is a challenging, but promising task, especially in view of the forthcoming launch of the Herschel Space Observatory.Comment: 13 pages, 13 figures, accepted for publication in A&

Density-dependent effect on reproductive behaviour of Lysmata amboinensis and L. boggessi (Decapoda: Caridea: Hippolytidae)

We compared the reproductive behaviours of two protandric simultaneous hermaphroditic species (Lysmata amboinensis and L. boggessi) that belong to two groups of Lysmata shrimp with different morphology, geographical distribution, and density. Lysmata amboinensis occurs in tropical waters at low population densities, and L. boggessi is found in aggregation in sub-tropical and temperate areas. Reproductive behaviour of L. boggessi under two densities and L. amboinensis in different habitats were compared. Results show that L. amboinensis was much less active during mating than L. boggessi. Male shrimp of L. amboinensis did not display obvious pre-copulation behaviour. They also took significantly longer to transfer spermatophores and lay eggs after mating than L. boggessi shrimp did. For L. boggessi, moulting time of female shrimp, copulation time and the interval between moulting and mating were significantly shorter when three male shrimp were present than when only one male shrimp was present. Our study suggests that the reproductive behavioural differences in the two shrimp species are possibly the results of density-dependent effect