1,721,011 research outputs found
Bulk and collective properties of a dilute Fermi gas in the BCS-BEC crossover
Full text linkWe investigate the zero-temperature properties of a dilute two-component Fermi gas with attractive interspecies interaction in the BCS-BEC crossover. We build an efficient parametrization of the energy per particle based on Monte Carlo data and asymptotic behavior. This parametrization provides, in turn, analytical expressions for several bulk properties of the system such as the chemical potential, the pressure and the sound velocity. In addition, on the basis of a local polytropic equation of state, we determine the collective modes of the Fermi gas under harmonic confinement in the framework of the hydrodynamic theory. The calculated collective frequencies are compared to experimental data on confined vapors of Li atoms and with other theoretical predictions
Calculated rate coefficients between CI-MS reagent ions and organosulfur compounds causing food taints and off-flavours
No full textVolatile sulfur compounds play a crucial role in the aroma profile of food and fermented beverages. We explore chemical-ionization mass spectrometry (CI-MS) ion-molecule reaction kinetics of commonly used reagent ions to a list of volatile organic sulfur compounds (VOSCs). We compute the rate coefficients of ion-molecule reactions, useful for the accurate identification and quantification of trace gases, using capture collision models based on the electric dipole moment and polarizability of the neutral VOSCs. To this aim, we evaluate molecular properties, such as the electric dipole moment, polarizability, proton affinity (PA), and ionization energy (IE) for each VOSC, by means of hybrid density functional theory (DFT) simulations. The PA and IE values are useful in the selection of appropriate reagent ions to be used in CI-MS. We thoroughly investigate collision rate coefficients at effective temperatures and internal energies, as relevant for highly energetic proton transfer reactio..
Ab initio calculation of the proton transfer reaction rate coefficients to volatile organic compounds related to cork taint in wine
No full textWe compute the proton transfer rates to a range of volatile organic compounds (VOCs) related to cork taint in wine. These rates are useful to support quantification in proton-transfer-reaction mass spectrometry (PTR-MS) and in selected-ion flow-tube mass spectrometry (SIFT-MS). We apply the average dipole orientation theory and the parameterized trajectory method to evaluate the rate coefficients for proton transfer occurring in ion–molecule collision, from both H3O+ and NH (Formula presented.) to the VOCs. The main input ingredients for these methods are the electric dipole moment and polarizability of the VOC molecules, which we evaluate by means of quantum chemical calculations based on density functional theory. We provide new data for proton transfer rate coefficients of compounds responsible for cork taint and off-flavor in wine such as chloroanisoles, bromoanisoles, methylisoborneol, guaiacol, and terpenes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Condensate fraction of a Fermi gas in the BCS-BEC crossover
Full text linkWe investigate the Bose-Einstein condensation of fermionic pairs in a uniform two-component Fermi gas, obtaining an explicit formula for the condensate density as a function of the chemical potential and the energy gap. We analyze the condensate fraction in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of weakly interacting Cooper pairs to the Bose-Einstein condensate of molecular dimers. By using the local-density approximation we study confined Fermi vapors of alkali-metal atoms for which there is experimental evidence of condensation also on the BCS side of the Feshbach resonance. Our theoretical results are in agreement with these experimental data and give the behavior of the condensate on both sides of the Feshbach resonance at zero temperature
Phase diagram of a model of correlated hopping of electrons in a lattice of Berry molecules
No full textThe one-dimensional (ID) phase diagram of a model for correlated hopping of electrons in a lattice of Berry phase molecules is presented. Electrons hop in the presence of an extra orbital degree of freedom at each site. This is mimicked as a spin-1 variable whose allowed states depend on the electron occupancy so as to take into account the orbital degeneracies of different molecular occupancies. In the ID case we find that at low electron densities n much less than 1 there is a region with dominant superconducting correlations surviving an additional repulsive on-site interaction U as strong as the bandwidth, W=4t. The critical value U-c of U below which superconductivity is found to be dominant decreases with increasing density n. For n=1/2 we find U-c/t approximate to 1, whereas at n=1 (half-filling) our (less accurate) results are compatible with U-c/t approximate to 0. For U>U-c(n) and away from half-filling (n not equal 1) the system is metallic with dominant 2k(F) charge density wave correlations. At half-filling a charge gap opens for U>U-c and the system becomes an insulator. A spin gap characterizes the phase diagram for all densities and for all values of U, even in the metallic regime U>U-c
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