357 research outputs found
Timelike structures of ten-dimensional supersymmetry
In several contexts, supersymmetry can be reformulated in terms of
calibrations, namely forms whose integrals measure minimal energies. It has
been conjectured that this should be possible in general. For type II
supergravity, we present a new system of equations which realizes this
expectation. Besides the customary D-brane calibrations, it also includes
NS5-brane and KK5-monopole calibrations. It is equivalent to supersymmetry
under the assumption that the Killing vector associated to supersymmetry is
timelike. No assumption is made on a factorization of spacetime. We also obtain
a version of the system which is manifestly S-invariant and we present an
application to near-horizon backgrounds. Using calibration, a definition of
central charges in purely gravitational terms is given.Comment: 61 pages; v2: matches published version, comments adde
Measurement of the surface susceptibility and the surface conductivity of atomically thin by spectroscopic ellipsometry
We show how to correctly extract from the ellipsometric data the surface
susceptibility and the surface conductivity that describe the optical
properties of monolayer . Theoretically, these parameters stem from
modelling a single-layer two-dimensional crystal as a surface current, a truly
two-dimensional model. Currently experimental practice is to consider this
model equivalent to a homogeneous slab with an effective thickness given by the
interlayer spacing of the exfoliating bulk material. We prove that the error in
the evaluation of the surface susceptibility of monolayer , owing to
the use of the slab model, is at least 10% or greater, a significant
discrepancy in the determination of the optical properties of this material.Comment: Keywords: Ellipsometry, graphene, MoS2, two dimensional crystals,
optical contrast, absorption, transition metal dichalcogenide monolayer
Toxicological effects and bioaccumulation of fullerene C60 (FC60) in the marine bivalve Ruditapes philippinarum.
Abstract Fullerene C60 (FC60), with its unique physical properties, has been used in many applications in recent decades. The increased likelihood of direct release into the environment has raised interest in understanding the biological effects of FC60 to aquatic organisms. Nowadays, only few studies have analysed FC60 effects and bioaccumulation in marine organisms following in vivo exposure. To provide new data about FC60 toxicity, Ruditapes philippinarum was selected as target species to assess potential adverse effects of the contaminant. Clams were exposed for 1, 3 and 7 days to predicted environmental concentrations of FC60 (1 and 10 μg/L) and cellular and biochemical responses were evaluated in clams' gills, digestive gland and haemolymph. The FC60 content in gills and digestive gland was determined in all experimental conditions after 7 days of exposure. Results showed an increase in oxidative stress. In particular, a significant modulation in antioxidant enzyme activities, and changes in glutathione S-transferase activity were observed in gills. Moreover, damage to lipids and proteins was detected in FC60-treated (10 µg/L) clams. In digestive gland, slighter variations in antioxidant enzyme activities and damage to molecules were detected. CAT activity was significantly affected throughout the exposure, whereas damage to lipids was evident only at the end of exposure. FC60 accumulation was revealed in both gills and digestive gland, with values up to twelve-fold higher in the latter. Interestingly, haemolymph parameters were slightly affected by FC60 compared to the other tissues investigated. Indeed, only Single Cell Gel Electrophoresis and Neutral Red uptake assays showed increased values in FC60-exposed clams. Moreover, volume and diameter of haemocytes, haemocyte proliferation, and micronucleus assay highlighted significant variations in treated clams, but only in the first phases of exposure, and no changes were detected after 7 days. Our results suggested clam gills as the target tissue for FC60 toxicity under the exposure conditions tested: the high damage detected to lipids and proteins could contribute to long-term problems for the organism
The general (2,2) gauged sigma model with three--form flux
We find the conditions under which a Riemannian manifold equipped with a
closed three-form and a vector field define an on--shell N=(2,2) supersymmetric
gauged sigma model. The conditions are that the manifold admits a twisted
generalized Kaehler structure, that the vector field preserves this structure,
and that a so--called generalized moment map exists for it. By a theorem in
generalized complex geometry, these conditions imply that the quotient is again
a twisted generalized Kaehler manifold; this is in perfect agreement with
expectations from the renormalization group flow. This method can produce new
N=(2,2) models with NS flux, extending the usual Kaehler quotient construction
based on Kaehler gauged sigma models.Comment: 24 pages. v2: typos fixed, other minor correction
Investigation of surface inclination effect during dropwise condensation of flowing saturated steam
When a pure vapor condenses over a surface, it can form a continuous liquid film or a multitude of discrete droplets, thus realizing the so-called dropwise condensation (DWC). In the literature, most of the experimental data refer to DWC on vertical condensing surfaces with quiescent vapor. However, in many applications, the condensing vapor usually has a non-zero flow velocity with a consequent effect on the sliding motion of droplets. Moreover, the drag force due to vapor velocity may be the only mechanism for liquid removal on a horizontal surface or in space applications. A systematic investigation of the effects of vapor drag and surface inclination on the heat transfer and droplet population during DWC is needed and is addressed in the present paper.
Here, DWC of flowing steam is experimentally studied on sol-gel silica-based coated aluminium substrates at three different inclinations: vertical, inclined at 45°, and horizontal. Heat transfer coefficient (HTC) and droplet population measurements are performed in a wide range of heat flux (260–610 kW m−2) and average vapor velocity (3.3–13.8 m s−1). When decreasing the tilt angle, from vertical to horizontal, due to the lower contribution of the gravity force, the average droplet size increases, and a strong HTC reduction is observed above all at low vapor velocities. Because of the vapor drag force, the HTC increases with steam velocity and, at the highest mass velocity, the HTC is independent from the surface inclination. A model for the droplet departing radius in the presence of vapor velocity, initially proposed by the present authors for the sole case of vertical surfaces, is here modified to account also for the effect of surface inclination and then assessed against the present experimental data. Hence, we propose to predict the heat flux during DWC by coupling the new equation for the departing radius with the available models of heat transfer through a single droplet and drop-size distribution. The developed calculation method is found to provide satisfactory predictions of the HTC for the whole range of vapor velocity, heat flux and surface inclination
Cobalt Spinel Nanocubes on N-Doped Graphene: A Synergistic Hybrid Electrocatalyst for the Highly Selective Reduction of Carbon Dioxide to Formic Acid
Carbon
dioxide reduction into useful chemical products is a key
technology to address urgent climate and energy challenges. In this
study, a nanohybrid made by Co<sub>3</sub>O<sub>4</sub> and graphene
is proposed as an efficient electrocatalyst for the selective reduction
of CO<sub>2</sub> to formate at low overpotential. A comparison between
samples with different metal oxide to carbon ratios and with or without
doping of the graphene moiety indicates that the most active catalyst
is formed by highly dispersed and crystalline nanocubes exposing {001}
oriented surfaces, whereas the nitrogen doping is critical to obtain
a controlled morphology and to facilitate a topotactic transformation
during electrocatalytic conditions to CoO, which results in the true
active phase. The nanohybrid made up by intermediate loading of Co<sub>3</sub>O<sub>4</sub> supported on nitrogen-doped graphene is the
most active catalyst, being able to produce 3.14 mmol of formate in
8 h at −0.95 V vs SCE with a Faradaic efficiency of 83%
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Effects of the prewhitening method, the time granularity and the time segmentation on the Mann-Kendall trend detection and the associated Sen's slope
The most widely used non-parametric method for trend analysis is the Mann-Kendall test associated with the Sen's slope. The Mann-Kendall test requires serially uncorrelated time series, whereas most of the atmospheric processes exhibit positive autocorrelation. Several prewhitening methods have been designed to overcome the presence of lag-1 autocorrelation. These include a prewhitening, a detrending and/or a correction for the detrended slope and the original variance of the time series. The choice of which prewhitening method and temporal segmentation to apply has consequences for the statistical significance, the value of the slope and of the confidence limits. Here, the effects of various prewhitening methods are analyzed for seven time series comprising in-situ aerosol measurements (scattering coefficient, absorption coefficient, number concentration and aerosol optical depth), Raman Lidar water vapor mixing ratio and the tropopause and zero degree levels measured by radio-sounding. These time series are characterized by a broad variety of distributions, ranges and lag-1 autocorrelation values and vary in length between 10 and 60 years. A common way to work around the autocorrelation problem is to decrease it by averaging the data over longer time intervals than in the original time series. Thus, the second focus of this study is evaluation of the effect of time granularity on long-term trend analysis. Finally, a new algorithm involving three prewhitening methods is proposed in order to maximize the power of the test, to minimize the amount of erroneous detected trends in the absence of a real trend and to ensure the best slope estimate for the considered length of the time series
Reformulating Supersymmetry with a Generalized Dolbeault Operator
The conditions for N=1 supersymmetry in type II supergravity have been
previously reformulated in terms of generalized complex geometry. We improve
that reformulation so as to completely eliminate the remaining explicit
dependence on the metric. Doing so involves a natural generalization of the
Dolbeault operator. As an application, we present some general arguments about
supersymmetric moduli. In particular, a subset of them are then classified by a
certain cohomology. We also argue that the Dolbeault reformulation should make
it easier to find existence theorems for the N=1 equations.Comment: 30 pages, no figures. v2: minor correction
Perturbing gauge/gravity duals by a Romans mass
We show how to produce algorithmically gravity solutions in massive IIA (as
infinitesimal first order perturbations in the Romans mass parameter) dual to
assigned conformal field theories. We illustrate the procedure on a family of
Chern--Simons--matter conformal field theories that we recently obtained from
the N=6 theory by waiving the condition that the levels sum up to zero.Comment: 30 page
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