17,576 research outputs found
Improved spectral algorithm for the detection of network communities
We review and improve a recently introduced method for the detection of
communities in complex networks. This method combines spectral properties of
some matrices encoding the network topology, with well known hierarchical
clustering techniques, and the use of the modularity parameter to quantify the
goodness of any possible community subdivision. This provides one of the best
available methods for the detection of community structures in complex systems.Comment: 4 pages, 1 fugure; to appear in the Proceedings of the 8th Granada
Seminar - Computational and Statistical Physic
The mechanisms of arsenic detoxification by the green microalgae chlorella vulgaris
The mechanisms of arsenic interaction with the green microalga Chlorella vulgaris (C. vulgaris) and the potential for its bio-remediation from water were investigated. This was made possible by the development of an improved arsenic extraction from C. vulgaris, leading to successful glutathione and phytochelatins (GSH/PC) complex speciation analysis with 71.1% efficiency. The response of C. vulgaris when challenged by As(III), As(V) and dimethylarsinic acid (DMA) was assessed through experiments on adsorption, efflux and speciation of arsenic (reduction, oxidation, methylation and chelation with GSH/PC). At high phosphate concentration (1.62 mM of PO4-3), poor adsorption of As(V) led to low intracellular uptake; at low phosphate concentration (3.2 μM of PO4-3), an increase in the level of free thiols was observed as well as a moderate decrease in intracellular pH with no evidence for signals of oxidative stress. Chlorella vulgaris cells did not produce any As-GS/PC complex when exposed to As(V). This may indicate that a reduction step is needed for As(V) complexation with GSH/PC. Chlorella vulgaris cells formed DMASV-GS upon exposure to DMA. The formation of this complex in vivo has only been reported once in Brassica oleracea plants. This complex is perhaps a fragment of a bigger molecule and thus part of another detoxification mechanism since its formation was not related to the concentration of DMA in media or the exposure time. It was found that As(III) triggers the formation of arsenic complexes with PC and homophytochelatins (hPC) and their compartmentalisation to vacuoles. It is the first time that, as a result of the newly developed extraction method using sonication, such intact complexes have been identified in C. vulgaris exposed to arsenic and their hPC complexes have been reported in any organism. The potential of C. vulgaris to bio-remediate arsenic from water is highly selective and effective for the more toxic As(III) (for human life) without the potential hazard to reduce As(V) to As(III). This was possible to assess because of the following empirical observations:
• At low phosphate (3.2 μM of PO4-3) and in the presence of As(V), C. vulgaris are not likely to grow and be efficient at bio-remediating arsenic.
• At high phosphate (1.62 mM of PO4-3) and in the presence of As(V), C. vulgaris are highly likely to grow but are not likely to be efficient at bio-remediating arsenic. However the potential to transform As(V) into more toxic As(III) is very low.
• Under any phosphate concentration and in the presence of As(III), C. vulgaris has high potential to bio-remediate arsenic, by storing As(III) into the cell biomass while retaining significantly high growth rates
Disordered graphene Josephson junctions
A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method
is used to describe disordered single-layer graphene Josephson junctions.
Scattering by vacancies, ripples or charged impurities is included. We compute
the Josephson current and investigate the nature of multiple Andreev
reflections, which induce bound states appearing as peaks in the density of
states for energies below the superconducting gap. In the presence of single
atom vacancies, we observe a strong suppression of the supercurrent that is a
consequence of strong inter-valley scattering. Although lattice deformations
should not induce inter-valley scattering, we find that the supercurrent is
still suppressed, which is due to the presence of pseudo-magnetic barriers. For
charged impurities, we consider two cases depending on whether the average
doping is zero, i.e. existence of electron-hole puddles, or finite. In both
cases, short range impurities strongly affect the supercurrent, similar to the
vacancies scenario
True airspeed measured by airborne laser Doppler velocimeter
Velocimeter utilizing carbon dioxide laser measures true airspeed of aircraft. Results of flight tests indicate that clear-weather airspeeds can be measured with accuracy better than 0.1% at altitudes up to 3000 meters; measurements can be made at much greater altitudes in cloudy or turbid air
Tight-binding description of intrinsic superconducting correlations in multilayer graphene
Using highly efficient GPU-based simulations of the tight-binding
Bogoliubov-de Gennes equations we solve self-consistently for the pair
correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by
considering a finite intrinsic s-wave pairing potential. We find that the two
different stacking configurations have opposite bulk/surface behavior for the
order parameter. Surface superconductivity is robust for ABC stacked multilayer
graphene even at very low pairing potentials for which the bulk order parameter
vanishes, in agreement with a recent analytical approach. In contrast, for
Bernal stacked multilayer graphene, we find that the order parameter is always
suppressed at the surface and that there exists a critical value for the
pairing potential below which no superconducting order is achieved. We
considered different doping scenarios and find that homogeneous doping strongly
suppresses surface superconductivity while non-homogeneous field-induced doping
has a much weaker effect on the superconducting order parameter. For multilayer
structures with hybrid stacking (ABC and ABA) we find that when the thickness
of each region is small (few layers), high-temperature surface
superconductivity survives throughout the bulk due to the proximity effect
between ABC/ABA interfaces where the order parameter is enhanced.Comment: 7 page
Effect of spin-orbit interaction on a magnetic impurity in the vicinity of a surface
We propose a new mechanism for surface-induced magnetic anisotropy to explain
the thickness-dependence of the Kondo resistivity of thin films of dilute
magnetic alloys. The surface anisotropy energy, generated by spin-orbit
coupling on the magnetic impurity itself, is an oscillating function of the
distance d from the surface and decays as 1/d^2. Numerical estimates based on
simple models suggest that this mechanism, unlike its alternatives, gives rise
to an effect of the desired order of magnitude.Comment: 4 pages, 4 figure
- …