244 research outputs found
New Fourfolds from F-Theory
In this paper, we apply Borcea-Voisin's construction and give new examples of
fourfolds containing a del Pezzo surface of degree six, which admit an elliptic
fibration on a smooth threefold. Some of these fourfolds are Calabi-Yau
varieties, which are relevant for the compactification of Type IIB string
theory known as -Theory. As a by-product, we provide a new example of a
Calabi--Yau threefold with Hodge numbers .Comment: 12 Pages, 1 Figure, to appear in Math. Nac
Particle motion in a rotating dust spacetime: the Bonnor solution
We investigate the geometrical properties, spectral classification,
geodesics, and causal structure of the Bonnor's spacetime [Journal of Physics A
Math. Gen., \textbf{10}, 1673 (1977)], i.e., a stationary axisymmetric solution
with a rotating dust as a source. This spacetime has a directional singularity
at the origin of the coordinates (related to the diverging vorticity field of
the fluid there), which is surrounded by a toroidal region where closed
timelike curves (CTCs) are allowed, leading to chronology violations. We use
the effective potential approach to provide a classification of the different
kind of orbits on the symmetry plane as well as to study the motion parallel to
the symmetry axis. In the former case we find that as a general feature test
particles released from a fixed space point and directed towards the
singularity are repelled and scattered back as soon as they approach the CTC
boundary, without reaching the central singularity.Comment: 11 pages, 5 figures (10 eps files
High pressure chemistry of red phosphorus by photo-activated simple molecules
High pressure (HP) is very effective in reducing intermolecular distances and inducing unexpected chemical reactions. In addition the photo-activation of the reactants in HP conditions can lead to very efficient and selective processes. The chemistry of phosphorus is currently based on the white molecular form. The red polymeric allotrope, despite more stable and much less toxic, has not attracted much attention so far. However, switching from the white to the red form would benefit any industrial procedure, especially from an environmental point of view. On the other side, water and ethanol are renewable, environmental friendly and largely available molecules, usable as reactants and photo-activators in HP conditions. Here we report a study on the HP photo-induced reactivity of red phosphorus with water and ethanol, showing the possibility of very efficient and selective processes, leading to molecular hydrogen and valuable phosphorus compounds. The reactions have been studied by means of FTIR and Raman spectroscopy and pressure has been generated using membrane Diamond (DAC) and Sapphire (SAC) anvil cells. HP reactivity has been activated by the two-photon absorption of near-UV wavelengths and occurred in total absence of solvents, catalysts and radical initiators, at room T and mild pressure conditions (0.2–1.5 GPa)
Multiscale Sample Entropy of Cardiovascular Signals: Does the Choice between Fixed- or Varying-Tolerance among Scales Influence Its Evaluation and Interpretation?
Multiscale entropy (MSE) quantifies the cardiovascular complexity evaluating Sample Entropy (SampEn) on coarse-grained series at increasing scales Ď„. Two approaches exist, one using a fixed tolerance r at all scales (MSEFT), the other a varying tolerance r(Ď„) adjusted following the standard-deviation changes after coarse graining (MSEVT). The aim of this study is to clarify how the choice between MSEFT and MSEVT influences quantification and interpretation of cardiovascular MSE, and whether it affects some signals more than others. To achieve this aim, we considered 2-h long beat-by-beat recordings of inter-beat intervals and of systolic and diastolic blood pressures in male (N = 42) and female (N = 42) healthy volunteers. We compared MSE estimated with fixed and varying tolerances, and evaluated whether the choice between MSEFT and MSEVT estimators influence quantification and interpretation of sex-related differences. We found substantial discrepancies between MSEFT and MSEVT results, related to the degree of correlation among samples and more important for heart rate than for blood pressure; moreover the choice between MSEFT and MSEVT may influence the interpretation of gender differences for MSE of heart rate. We conclude that studies on cardiovascular complexity should carefully choose between fixed- or varying-tolerance estimators, particularly when evaluating MSE of heart rate
Spray-loading: A cryogenic deposition method for diamond anvil cell.
An efficient loading technique has been developed for flammable, toxic, or explosive gases which can be condensed at liquid nitrogen temperature and ambient pressure in membrane diamond anvil cells (DACs). This cryogenic technique consists in a deposition of small quantities of the desired gas directly into the sample chamber. The deposition is performed using a capillary that reaches the space between the diamond anvils. The DAC is kept under inert gas overpressure during the whole process, in order to avoid contamination from atmospheric O2, CO2, and H2O. This technique provides significant advantages over standard cryo-loading and gas-loading when the condensation of dangerous samples at liquid nitrogen temperature raises safety concerns because it allows dealing with minimum quantities of condensed gases. The whole procedure is particularly fast and efficient. The "spray-loading" has been successfully used in our laboratory to load several samples including acetylene, ammonia, ethylene, and carbon dio..
Dataset on gait patterns in degenerative neurological diseases
We collected the gait parameters and lower limb joint kinematics of patients with three different types of primary degenerative neurological diseases: (i) cerebellar ataxia (19 patients), (ii) hereditary spastic paraparesis (26 patients), and (iii) Parkinson's disease (32 patients). Sixty-five gender-age matched healthy subjects were enrolled as control group. An optoelectronic motion analysis system was used to measure time-distance parameters and lower limb joint kinematics during gait in both patients and healthy controls
A null frame for spacetime positioning by means of pulsating sources
We introduce an operational approach to the use of pulsating sources, located
at spatial infinity, for defining a relativistic positioning and navigation
system, based on the use of four-dimensional bases of null four-vectors, in
flat spacetime. As a prototypical case, we show how pulsars can be used to
define such a positioning system. The reception of the pulses for a set of
different sources whose positions in the sky and periods are assumed to be
known allows the determination of the user's coordinates and spacetime
trajectory, in the reference frame where the sources are at rest. We describe
our approach in flat Minkowski spacetime, and discuss the validity of this and
other approximations we have considered.Comment: 19 pages, revised to match the version accepted for publication in
Advances in Space Researc
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