570 research outputs found
Growth control of GaAs nanowires using pulsed laser deposition with arsenic over pressure
Using pulsed laser ablation with arsenic over pressure, the growth conditions
for GaAs nanowires have been systematically investigated and optimized. Arsenic
over pressure with As molecules was introduced to the system by thermal
decomposition of polycrystalline GaAs to control the stoichiometry and shape of
the nanowires during growth. GaAs nanowires exhibit a variety of geometries
under varying arsenic over pressure, which can be understood by different
growth processes via vapor-liquid-solid mechanism. Single-crystal GaAs
nanowires with uniform diameter, lengths over 20 m, and thin surface oxide
layer were obtained and can potentially be used for further electronic
characterization
Molecular monitoring of microbial diversity in expanded granular sludge bed (EGSB) reactors treating oleic acid
A molecular approach was used to evaluate the microbial diversity of bacteria and archaea in two expanded granular sludge bed
(EGSB) reactors fed with increasing oleic acid loading rates up to 8 kg of chemical oxygen demand (COD) mˉ³ dayˉ¹ as the sole carbon source. One of the reactors was inoculated with granular sludge (RI) and the other with suspended sludge (RII). During operation, the
sludge in both reactors was segregated in two layers: a bottom settled one and a top floating one. The composition of the bacterial
community, based on 16S rDNA sequence diversity, was affected most during the oleate loading process in the two reactors. The archaeal
consortium remained rather stable over operation in RI, whereas in RII the relative abundance of Methanosaeta-like organisms became
gradually weaker, starting in the bottom layer. In the range of oleate loads evaluated, 6 kg of COD mˉ³ dayˉ¹ was found as the
maximum value that could be applied to the system. A further increase to 8 kg of oleate-COD mˉ³ dayˉ¹ induced a maximal shift on the
microbial structure of the sludges. At this time point, methanogenic acetoclastic activity was not detected and only very low methanogenic
activity on H2/CO2 was exhibited by the sludges.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI/BD/20326/99.Fundação Calouste Gulbenkian (FCG)
Efficient single-photon emission from electrically driven InP quantum dots epitaxially grown on Si(001)
The heteroepitaxy of III-V semiconductors on silicon is a promising approach
for making silicon a photonic platform for on-chip optical interconnects and
quantum optical applications. Monolithic integration of both material systems
is a long-time challenge, since different material properties lead to high
defect densities in the epitaxial layers. In recent years, nanostructures
however have shown to be suitable for successfully realising light emitters on
silicon, taking advantage of their geometry. Facet edges and sidewalls can
minimise or eliminate the formation of dislocations, and due to the reduced
contact area, nanostructures are little affected by dislocation networks. Here
we demonstrate the potential of indium phosphide quantum dots as efficient
light emitters on CMOS-compatible silicon substrates, with luminescence
characteristics comparable to mature devices realised on III-V substrates. For
the first time, electrically driven single-photon emission on silicon is
presented, meeting the wavelength range of silicon avalanche photo diodes'
highest detection efficiency
Dilaton Domain Walls and Dynamical Systems
Domain wall solutions of -dimensional gravity coupled to a dilaton field
with an exponential potential are shown
to be governed by an autonomous dynamical system, with a transcritical
bifurcation as a function of the parameter when . All
phase-plane trajectories are found exactly for , including
separatrices corresponding to walls that interpolate between and
adS_{d-1} \times\bR, and the exact solution is found for . Janus-type
solutions are interpreted as marginal bound states of these ``separatrix
walls''. All flat domain wall solutions, which are given exactly for any
, are shown to be supersymmetric for some superpotential ,
determined by the solution.Comment: 30 pp, 11 figs, significant revision of original. Minor additional
corrections in version to appear in journa
Image analysis, methanogenic activity measurements, and molecular biological techniques to monitor granular sludge from an EGSB reactor fed with oleic acid
Morphological changes in anaerobic granular sludge fed with increasing loads of oleic acid were quantified by image analysis. The combination of this technique with data on the accumulation of adsorbed long chain fatty acid and with the molecular characterization of microbial community gave insight into the mechanisms of sludge disintegration, flotation and washout. It was found that the bacterial domain was more affected than the archaeal domain during this process. However, no acetoclastic activity and only a residual hydrogenotrophic activity were detected in the sludge at the end of the operation.(undefined
Vanishing Preons in the Fifth Dimension
We examine supersymmetric solutions of N=2, D=5 gauged supergravity coupled
to an arbitrary number of abelian vector multiplets using the spinorial
geometry method. By making use of methods developed in hep-th/0606049 to
analyse preons in type IIB supergravity, we show that there are no solutions
preserving exactly 3/4 of the supersymmetry.Comment: 19 pages, latex. Reference added, and further modification to the
introductio
Tunable Supercurrent Through Semiconductor Nanowires
Nanoscale superconductor-semiconductor hybrid devices are assembled from InAs
semiconductor nanowires individually contacted by aluminum-based superconductor
electrodes. Below 1 K, the high transparency of the contacts gives rise to
proximity-induced superconductivity. The nanowires form superconducting weak
links operating as mesoscopic Josephson junctions with electrically tunable
coupling. The supercurrent can be switched on/off by a gate voltage acting on
the electron density in the nanowire. A variation in gate voltage induces
universal fluctuations in the normal-state conductance which are clearly
correlated to critical current fluctuations. The ac Josephson effect gives rise
to Shapiro steps in the voltage-current characteristic under microwave
irradiation.Comment: 9 pages, 3 figure
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