5,867 research outputs found
Thermodynamic behavior of a one-dimensional Bose gas at low temperature
We show that the chemical potential of a one-dimensional (1D) interacting
Bose gas exhibits a non-monotonic temperature dependence which is peculiar of
superfluids. The effect is a direct consequence of the phononic nature of the
excitation spectrum at large wavelengths exhibited by 1D Bose gases. For low
temperatures , we demonstrate that the coefficient in expansion of the
chemical potential is entirely defined by the zero-temperature density
dependence of the sound velocity. We calculate that coefficient along the
crossover between the Bogoliubov weakly-interacting gas and the Tonks-Girardeau
gas of impenetrable bosons. Analytic expansions are provided in the asymptotic
regimes. The theoretical predictions along the crossover are confirmed by
comparison with the exactly solvable Yang-Yang model in which the
finite-temperature equation of state is obtained numerically by solving
Bethe-{\it ansatz} equations. A 1D ring geometry is equivalent to imposing
periodic boundary conditions and arising finite-size effects are studied in
details. At we calculated various thermodynamic functions, including the
inelastic structure factor, as a function of the number of atoms, pointing out
the occurrence of important deviations from the thermodynamic limit.Comment: 14 pages, 16 figure
Ab-initio calculation of the vibrational modes of SiH4, H2SiO, Si10H16, and Si10H14O
We have studied the normal modes of hydrogenated and oxidized silicon
nanocrystals, namely SiH4 (silan), H2SiO (silanon), Si10H16 and Si10H14O. The
small clusters (SiH4 and H2SiO) have been used for convergence tests and their
bondlengths and frequencies have been compared with experimental and
theoretical reference data. For the large clusters (Si10H16 and Si10H14O) we
have investigated the vibrational density of states where we have identified
the oxygen-related spectral features. The vibrational modes have been also
analyzed with respect to the displacement patterns. The calculations have been
carried out within the density-functional and density-functional perturbation
theory using the local-density approximation.Comment: 9 pages, 7 figure
Androgens and Hypertension in Men and Women: a Unifying View.
This review was designed to revaluate the androgen role on the mechanisms of hypertension and cardiovascular risks in both men and women. Sex steroids are involved in the regulation of blood pressure, but pathophysiological mechanism is not well understood. Androgens have an important effect on metabolism, adipose and endothelial cell function, and cardiovascular risk in both men and women. A focal point in this contest is represented by the possible gender-specific regulation of different tissues and in particular of the adipose cell. Available data confirm that androgen deficiency is linked to increased prevalence of hypertension and cardiovascular diseases. Adipocyte dysfunction seems to be the main involved mechanism. Androgen replacement reduces inflammation state in man, protecting by metabolic syndrome progression. In women, androgen excess has been considered as promoting factor of cardiovascular risk. However, recent data suggest that excessive androgen production has little effect per se in inducing hypertension in young women of reproductive age. Also in postmenopausal women, data on relative androgen excess and hypertension are missing, while adrenal androgen deficiency has been associated to increased mortality.
RECENT FINDINGS:
Molecular mechanisms linking androgen dysregulation to hypertension are almost Unknown, but they seem to be related to increased visceral fat, promoting a chronic inflammatory state through different mechanisms. One of these may involve the recruitment and over-activation of NF-kB, a ubiquitous transcription factor also expressed in adipose cells, where it may cause the production of cytokines and other immune factors. The NF-kB signalling pathway may also influence brown adipogenesis leading to the preferential enlargement of visceral adipocytes. Chronic inflammation and adipocyte dysfunction may alter endothelial function leading to hypertension. Both in men and in women, particularly in the post-menopausal period, hypoandrogenism seems to be a major determinant of the increased prevalence of hypertension. The relationship between androgen signalling and NF-kB might explain the pathophysiological mechanism leading to the development of endothelium dysfunction and hypertension
First-Principles Study of Electronic and Vibrational Properties of BaHfN
The transition metal nitride BaHfN, which consists of weakly bonded
neutral slabs of closed shell ions, has structural and chemical similarities to
other layered nitrides which have impressive superconducting T when
electron doped: AHfNCl, AZrNCl, ATiNCl, with ,
and K, respectively for appropriate donor (A) concentrations . These
similarities suggest the possibility of BaHfN being another relatively high
T nitride upon doping, with effects of structure and the role of specific
transition metal ions yet to be understood. We report first-principles
electronic structure calculations for stoichiometric BaHfN using density
functional theory with plane-wave basis sets and separable dual-space Gaussian
pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest
conduction band is primarily of Hf 5 character, similar to
-ZrNCl and -TiNCl. The two N sites, one in the Hf layer and
another one in the Ba layer, were found to have very anisotropic Born effective
charges (BEC):deviations from the formal charge (-3) are opposite for the two
sites, and opposite for the two orientations (in-plane, out of plane). LO-TO
splittings and comparison of BECs and dielectric constant tensors to those of
related compounds are discussed, and the effect of electron doping on the
zone-center phonons is reported.Comment: 11 pages, 5 figure
Investigating the potential of emerging porous organic-based photocatalysts for CO2 reduction
The conversion of CO2 to synthetic fuels via photocatalysis represents a unique opportunity for sustainable energy production, addressing two of the most pressing problems of our time, namely climate change and the global energy crisis. In order to overcome the current constraints regarding performance for industrial applications, more efficient photocatalysts must be developed. In this thesis, I investigate the potential of emerging porous organic-based photocatalysts for CO2 photoreduction in the gas phase. The first part of my thesis focuses on better understanding the factors which govern charge transfer across semiconductor/metal-organic framework (MOF) heterojunction interfaces. By measuring the electronic structure of the individual heterojunction components and taking into account band bending, I built a band model of the heterojunction interface, which helped rationalize the photocatalytic enhancements and losses observed in MOF-based heterojunctions. This model highlights the importance of considering band bending when constructing MOF-based heterojunctions, an aspect that is not commonly investigated. The second part of my thesis focuses on investigating for the first time the ability of hypercrosslinked polymers (HCPs) to photoreduce CO2. I show that HCPs are promising visible-light active photocatalysts for the conversion of CO2 to CO. The leading HCP of this study outperforms TiO2 P25 by a factor of 7.5 when using only visible light and sacrificial H2O, without using additional sacrificial agents or co-catalysts. Lastly, the final part of my thesis focused on investigating the CO2 photoreduction mechanism, as well as the CO2 and H2O adsorption process on a triazine-biphenyl HCP. This was achieved by coupling in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), modulation excitation experiments, density functional theory (DFT) calculations, and advanced processing techniques. I found that CO2 and H2O adsorb on the same HCP sites albeit with different adsorption strengths. The primary amines of the triazines were identified as favouring strong CO2 binding interactions. No intermediate species were found under transient light irradiation. However, I observed partial CO2 and H2O desorption and a redistribution of interactions between the CO2/H2O molecules that remain adsorbed at the HCP adsorption sites. Overall, this thesis presents new opportunities for the use and rational design of emerging porous organic-based photocatalysts for CO2 reduction.Open Acces
A homogeneous Rayleigh quotient with applications in gradient methods
Given an approximate eigenvector, its (standard) Rayleigh quotient and harmonic Rayleigh quotient are two well-known approximations of the corresponding eigenvalue. We propose a new type of Rayleigh quotient, the homogeneous Rayleigh quotient, and analyze its sensitivity with respect to perturbations in the eigenvector. Furthermore, we study the inverse of this homogeneous Rayleigh quotient as stepsize for the gradient method for unconstrained optimization.The notion and basic properties are also extended to the generalized eigenvalue problem
Prenatal and postnatal drug exposure: focus on persistent central effects
Clinical studies indicate significant use of prescription, nonprescription and social/recreational drugs by women during pregnancy; however, limited knowledge exists about the detrimental effects that this practice may have on the developing central nervous system of the fetus. Importantly, few experimental and clinical data are available on how gestational exposure could exacerbate the effects of the same or a different drug consumed by the offspring later in life. The present review summarizes recent findings on the central toxicity elicited by several classes of drugs, administered prenatally and postnatally in experimental animals and humans, focusing on prescription and nonprescription analgesics, anti-inflammatory agents, alcohol and nicotine
Limited memory gradient methods for unconstrained optimization
The limited memory steepest descent method (Fletcher, 2012) for unconstrained
optimization problems stores a few past gradients to compute multiple stepsizes
at once. We review this method and propose new variants. For strictly convex
quadratic objective functions, we study the numerical behavior of different
techniques to compute new stepsizes. In particular, we introduce a method to
improve the use of harmonic Ritz values. We also show the existence of a secant
condition associated with LMSD, where the approximating Hessian is projected
onto a low-dimensional space. In the general nonlinear case, we propose two new
alternatives to Fletcher's method: first, the addition of symmetry constraints
to the secant condition valid for the quadratic case; second, a perturbation of
the last differences between consecutive gradients, to satisfy multiple secant
equations simultaneously. We show that Fletcher's method can also be
interpreted from this viewpoint
A homogeneous Rayleigh quotient with applications in gradient methods
Given an approximate eigenvector, its (standard) Rayleigh quotient and
harmonic Rayleigh quotient are two well-known approximations of the
corresponding eigenvalue. We propose a new type of Rayleigh quotient, the
homogeneous Rayleigh quotient, and analyze its sensitivity with respect to
perturbations in the eigenvector. Furthermore, we study the inverse of this
homogeneous Rayleigh quotient as stepsize for the gradient method for
unconstrained optimization. The notion and basic properties are also extended
to the generalized eigenvalue problem
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