Composition-induced structural transitions in mixed rare-gas clusters

The low-energy structures of mixed Ar--Xe and Kr--Xe Lennard-Jones clusters are investigated using a newly developed parallel Monte Carlo minimization algorithm with specific exchange moves between particles or trajectories. Tests on the 13- and 19- atom clusters show a significant improvement over the conventional basin-hopping method, the average search length being reduced by more than one order of magnitude. The method is applied to the more difficult case of the 38-atom cluster, for which the homogeneous clusters have a truncated octahedral shape. It is found that alloys of dissimilar elements (Ar--Xe) favor polytetrahedral geometries over octahedra due to the reduced strain penalty. Conversely, octahedra are even more stable in Kr--Xe alloys than in Kr_38 or Xe_38, and they show a core-surface phase separation behavior. These trends are indeed also observed and further analysed on the 55-atom cluster. Finally, we correlate the relative stability of cubic structures in these clusters to the glassforming character of the bulk mixtures.Comment: 14 pages, 8 figures, 5 tables PRB vol 70, in pres

Enhancing single-parameter quantum charge pumping in carbon-based devices

We present a theoretical study of quantum charge pumping with a single ac gate applied to graphene nanoribbons and carbon nanotubes operating with low resistance contacts. By combining Floquet theory with Green's function formalism, we show that the pumped current can be tuned and enhanced by up to two orders of magnitude by an appropriate choice of device length, gate voltage intensity and driving frequency and amplitude. These results offer a promising alternative for enhancing the pumped currents in these carbon-based devices.Comment: 3.5 pages, 2 figure

Organic waste materials for Bioengineering works

Bioengineering uses plant and biodegradable materials of natural origin, stones, steel, additives and synthetic products in various combinations and as support for the growth of plants. The lack of available resources and progressive increasing of desertification in Sicily, led to the search for alternative materials. Objective of the work is testing organic waste materials for the realization of bioengineering works in the several areas of application: terrestrial, fluvial and coastline. To this aim, is proposed the use of innovative techniques that involve the construction of low-cost brushwood, environmentally friendly materials made: the pruning of vines and the oceanic Posidonia oceanica beached (banquette). The use of these two organic materials, which are a special solid waste, widely present in Sicily, is part of an efficient use of resources while respecting the environment. Assembled by hand or mechanically in the form of fascine or biocarpet, the residues of the vine pruning will constitute the modular element to achieve anti-erosion linear works while the residues of Posidonia oceanica (previously leached), constitute the growing media which, along sowing or planting of native species, make it "alive" the artefact

Naturalistic Hydroseeding

The applications usually want to restore an adequate level of vegetation cover, to contrast effectively erosion and slope instability, however, often neglecting the naturalistic appearance. Really we should combine the technical aspect with the naturalistic, aiming to eliminate non-native species from vegetation context and to guide the choice of usable herbaceous species. The study, carried out in a dump of a southern Italian town (Termini Imerese - Sicily), provided for the reconstitution of plant cover of 7.4 hectares through hydroseeding applied directly on the soil of the final covering, with waste often exposed. The aim of this study was the use of alternative hydroseeding with the use of native plants for the recovery of the vegetation cover and increase the biodiversity of the intervention zone. So have been selected 21 species with high biotechnical value: Graminaceae species (genus: Ampelodesmos, Oryzopsis, Cynodon, Festuca , Brachipodium, Lolium, Lygeum) and Leguminosae species (genus: Hedysarum, Lotus, Medicago). In addition, were used seeds of wild flowers, fertilizers, mulch of wood fiber and cellulose, organic and synthetic adhesives. Moreover, a search was conducted on the morphology and resistance to tensile efforts of the roots of three species of referring: Ampelodesmos mauritanicus (Poir.) Dur. & Schinz, Oryzopsis miliacea (L.) Asch. & Schweinf and Hedysarum coronarium L. The first results confirm that the natural hydroseeding with native plants creates an effective plant cover in deprived areas; it also helps increase biodiversity. Therefore, the methodology used appears to be a more sustainable alternative to traditional hydroseeding using a mixture of commercial seeds and nationality is in doubt. The validity of the approach is also confirmed by the good results of biotechnical reference species both in terms of speed of growth, that of radical density (RAR) and the tension breaking roots

Uniform non-stoichiometric titanium nitride thin films for improved kinetic inductance detector array

We describe the fabrication of homogeneous sub-stoichiometric titanium nitride films for microwave kinetic inductance detector (mKID) arrays. Using a 6 inch sputtering target and a homogeneous nitrogen inlet, the variation of the critical temperature over a 2 inch wafer was reduced to <25 %. Measurements of a 132-pixel mKID array from these films reveal a sensitivity of 16 kHz/pW in the 100 GHz band, comparable to the best aluminium mKIDs. We measured a noise equivalent power of NEP = 3.6e-15 W/Hz^(1/2). Finally, we describe possible routes to further improve the performance of these TiN mKID arrays.Comment: 7 pages, 4 figures, submitted to Journal of low temperature physics, Proceedings of LTD-1

Measuring two-photon orbital angular momentum entanglement

We put forward an approach to estimate the amount of bipartite spatial entanglement of down-converted photon states correlated in orbital angular momentum and the magnitude of the transverse (radial) wave vectors. Both degrees of freedom are properly considered in our framework, which only requires azimuthal local linear optical transformations and mode selection analysis with two fiber detectors. The coincidence distributions predicted by our approach give an excellent fit to the distributions measured in a recent experiment aimed to show the very high-dimensional transverse entanglement of twin photons from a down-conversion source. Our estimate for the Schmidt number is substantially lower but still confirms the presence of high-dimensional entanglement.Comment: Extended paper of a published version in PRA, with some extra appendice

Melting of aluminium clusters

The melting of Al clusters in the size range 49 <= N <= 62 has been studied using two model interatomic potentials. The results for the two models are significantly different. The glue potential exhibits a smooth relatively featureless heat capacity curve for all sizes except for N = 54 and N = 55, sizes at which icosahedral structures are favoured over the polytetrahedral. Gupta heat capacity curves, instead, show a well-defined peak that is indicative of a first-order-like transition. The differences between the two models reflect the different ground-state structures, and neither potential is able to reproduce or explain the size dependence of the melting transition recently observed in experiments

Exchange Monte Carlo for Molecular Simulations with Monoelectronic Hamiltonians

We introduce a general Monte Carlo scheme for achieving atomistic simulations with monoelectronic Hamiltonians including the thermalization of both nuclear and electronic degrees of freedom. The kinetic Monte Carlo algorithm is used to obtain the exact occupation numbers of the electronic levels at canonical equilibrium, and comparison is made with Fermi-Dirac statistics in infinite and finite systems. The effects of a nonzero electronic temperature on the thermodynamic properties of liquid silver and sodium clusters are presented