524 research outputs found
Dilatonic black holes in a S-duality model
We find exact charged black hole solutions of a string effective action that
is invariant under S-duality transformations. These black hole solutions have
the same causal structure as the Reissner-Nordstrom (RN) solutions. They reduce
to the RN solutions for self-dual configurations of the dilaton and to the
Garfinkle-Horowitz-Strominger (GHS) solution in the weak (or strong) coupling
regime. Using the purely magnetic solutions of the S-duality model we also
generate dyonic black hole solutions of the GHS model, which have the causal
structure of the RN solutions.Comment: 10 pages, Plain-Tex, no figure
BPS-like bound and thermodynamics of the charged BTZ black hole
The charged Banados-Teitelboim-Zanelli (BTZ) black hole is plagued by several
pathologies: a) Presence of divergent boundary terms in the action, hence of a
divergent black hole mass; b) Once a finite, renormalized, mass M is defined
black hole states exist for arbitrarily negative values of M; c) There is no
upper bound on the charge Q. We show that these pathological features are an
artifact of the renormalization procedure. They can be completely removed by
using an alternative renormalization scheme leading to a different definition
M_0 of the black hole mass, which is the total energy inside the horizon. The
new mass satisfies a BPS-like bound M_0\ge (\pi/2)Q^2 and the heat capacity of
the hole is positive. We also discuss the black hole thermodynamics that arises
when M_0 is interpreted as the internal energy of the system. We show, using
three independent approaches (black hole thermodynamics, Einstein equations,
Euclidean action formulation) that M_0 satisfies the first law if a term
describing the mechanical work done by the electrostatic pressure is
introduced.Comment: Two references and a footnote adde
Clean and Dirty Superconductivity in Pure, Al doped, and Neutron Irradiated MgB2: a Far-Infrared Study
The effects of Al substitution and neutron irradiation on the conduction
regime (clean or dirty) of the - and -band of MgB have been
investigated by means of far-infrared spectroscopy. The intensity reflected by
well characterized polycrystalline samples was measured up to 100 cm in
both normal and superconducting state. The analysis of the superconducting to
normal reflectivity ratios shows that only the effect of the opening of the
small gap in the dirty -band can be clearly observed in pure MgB,
consistently with previous results. In Al-doped samples the dirty character of
the -band is increased, while no definitive conclusion on the conduction
regime of the -band can be drawn. On the contrary, results obtained
for the irradiated sample show that the irradiation-induced disorder drives the
-band in the dirty regime, making the large gap in -band
observable for the first time in far-infrared measurements.Comment: 11 pages, 1 figur
The 2D analogue of the Reissner-Nordstrom solution
A two-dimensional (2D) dilaton gravity model, whose static solutions have the
same features of the Reissner-Nordstrom solutions, is obtained from the
dimensional reduction of a four-dimensional (4D) string effective action
invariant under S-duality transformations. The black hole solutions of the 2D
model and their relationship with those of the 4D theory are discussed.Comment: 5 pages, Plain-Tex, no figure
HORIZONTAL TWO-PHASE FLOW PATTERN RECOGNITION
In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-phase flow in a test section consisting of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water. The observed flow patterns are stratified-bubble-slug/plug-annular. The WMS consists of two planes of parallel wire grids (16x16) that are placed across the channel at 1.5 mm and span over the measuring cross section. The wires of both planes cross under an angle of 90°, with a diameter Dwire of 70 Όm and a pitch equal to 1.3 mm. The void fraction profiles are derived from the sensor data and their evolution in time and space is analyzed and discussed. The dependence of the signals on the measured fluid dynamic quantities is discussed too. The main task is to predict which flow pattern will exist under any set of operating conditions as well as to predict the value of characteristic flow parameters
Fluxon dynamics in Li-Al codoped MgB2 by microwave surface resistance measurements
The magnetic-field-induced variations of the microwave surface resistance,
R_s, have been investigated in ceramic Mg_{1-x}(LiAl)_xB_2, with x in the range
0.1 - 0.4. The measurements have been performed on increasing and decreasing
the DC magnetic field, H_0, at fixed temperatures. At low temperatures, we have
observed a magnetic hysteresis in the R_s(H_0) curves in all the investigated
samples. On increasing the temperature, the range of H_0 in which the
hysteretic behavior is visible shrinks; however, in the sample with x = 0.1 it
is present up to temperatures close to T_c. We show that the field dependence
of R_s can be quantitatively justified taking into account the critical-state
effects on the fluxon lattice only in the sample with x = 0.4. On the contrary,
in the samples with x < 0.4 the hysteresis exhibits an unusual shape, similar
to that observed in others two-gap MgB_2 samples, which cannot be justified in
the framework of the critical-state models.Comment: 8 pages, 3 embedded figures; proceedings of the Sixth International
Conference on Vortex Matter in Nanostructured Superconductors (VORTEX VI)
17-24 September 2009, Rhodes, Greec
Seminal fluid metabolomic markers of oligozoospermic infertility in humans
Infertility affects 12â15% of couples worldwide, and male factors are the cause of nearly half of all cases. Studying seminal fluid composition could lead to additional diagnostic accuracy and a better understanding of the pathophysiology of male factor infertility. Metabolomics offers a new opportunity to evaluate biomarkers and better understand pathological mechanisms. The aim of the study was to identify new markers or therapeutic targets to improve outcomes in male factor or idiopathic infertility patients. Semen samples were obtained from 29 men with a normal spermogram test, and from 18 oligozoospermic men. Samples were processed and analyzed by Nuclear Magnetic Resonance spectroscopy and, subsequently, multivariate and univariate statistical analyses. Receiving Operator Curves (ROC) and Spearman correlations were also performed. An Orthogonal Partial Least Square Discriminant Analysis supervised multivariate model was devised to compare the groups. The levels of fructose, myoâinositol, aspartate and choline were altered. Moreover, Spearman Correlation associated fructose, aspartate and myoâinositol with the total amount of spermatozoa, total motile spermatozoa, % of immotility and % of âin situâ spermatozoic motility respectively. NMRâbased metabolomics allowed the identification of a specific metabolic fingerprint of the seminal fluids of patients affected by oligozoospermia
Redox activity as a powerful strategy to tune magnetic and/or conducting properties in benzoquinone-based metal-organic frameworks
Multifunctional molecular materials have attracted material scientists for several years as they are promising materials for the future generation of electronic devices. Careful selection of their molecular building blocks allows for the combination and/or even interplay of different physical properties in the same crystal lattice. Incorporation of redox activity in these networks is one of the most appealing and recent synthetic strategies used to enhance magnetic and/or conducting and/or optical properties. Quinone derivatives are excellent redox-active linkers, widely used for various applications such as electrode materials, flow batteries, pseudo-capacitors, etc. Quinones undergo a reversible two-electron redox reaction to form hydroquinone dianions via intermediate semiquinone radical formation. Moreover, the possibility to functionalize the six-membered ring of the quinone by various substituents/functional groups make them excellent molecular building blocks for the construction of multifunctional tunable metal-organic frameworks (MOFs). An overview of the recent advances on benzoquinone-based MOFs, with a particular focus on key examples where magnetic and/or conducting properties are tuned/switched, even simultaneously, by playing with redox activity, is herein envisioned
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