1,721,004 research outputs found
The BEC-BCS crossover in ultracold Fermi gases beyond the contact-potential approximation
No full textIn this work we apply a theoretical and computational method to the calculation of the observables of 6Li and 40K gases along the crossover from the BCS to the BEC states. This approach – based on the mean-field Bogoliubov-de Gennes equations – enables us to go beyond the Fermi contact interaction that is generally used to study ultracold Fermi gases, and investigate the effect of the finite potential range on the principal observables in these systems. Furthermore, a detailed description of the logical steps and of the ensuing numerical algorithms is discussed
A scattering view of the Bogoliubov-de Gennes equations
No full textWe advocate the use of the T -matrix of the pair potential to study the properties of ultracold Fermi gases in the mean-field approximation. Our approach does not require renormalization procedures even in the limit of contact interaction, and it provides a rigorous definition of the range of the potential. We also rewrite the Bogoliubov-de Gennes equation for the pairing function as a function of the T-matrix, and use it to investigate finite-range effects on the main thermodynamic observables in a gas of 6Li atoms at unitarity, calculating the pair potential with ab initio quantum chemical methods
SURPRISES: When ab initio meets statistics in extended systems
No full textThe surface photoelectron and inner shell electron spectroscopy (SURPRISES) program suite performs ab initio calculations of photoionization and non-radiative decay spectra in nanoclusters and solid state systems by using a space-energy similarity procedure to reproduce the band-like part of the spectra. This approach provides an extension of Fano resonant multichannel scattering theory dealing with the complexity arising from condensed matter calculations at a computational cost comparable to that of molecules. The bottleneck of electron spectroscopy ab initio calculations in condensed matter is the size of the Hilbert space where the wavefunctions are expanded and the increase in number of final decay states in comparison to that of atoms and molecules. In particular, the diagonalization of the interchannel interaction to take into account the correlation between the double ion and the escaping electron is impracticable when hole delocalization on valence bands and electronic excitations are included in the model. To overcome this problem SURPRISES uses a 'space-energy similarity' approach, which allows the spreading of the Auger probability over the bands without tuning semi-empirical parameters. Furthermore, a completely new feature in the landscape of ab initio resonant decay processes calculations is represented by including energy loss through a statistical approach. Using the calculated lineshape as electron source, a Monte Carlo routine simulates the effect of inelastic losses on the original lineshape. In this process, the computed spectrum can be directly compared with acquired experimental spectra, thus avoiding background subtraction, a procedure not free from uncertainty. The program can exploit the symmetry of the system under investigation to reduce the calculation scaling and may compute photoemission and Auger decay angular distribution patterns including energy loss for the electrons emitted in resonance-affected photoionization processes. In this paper, we present general methods, computational techniques and a number of numerical tests applied to the calculation of Si K–LL and O K–LL Auger spectra from different SiO2 nanoclusters
Lithium abundances in AGB stars and a new estimate for the7Be life-time
Full text linkIn most cases RGB and AGB stars with M <= 2M(circle dot) destroy Li (which is instead synthesized trough electron-captures on Be-7). This occurs through the combined operation of mixing processes and proton captures, when H-burning operates close to the envelope. Observed Li abundances are however difficult to explain, as they cover a wide spread. Various uncertainties affect model attempts, but so far the largest one concerns the processes of bound and free e-captures on Be-7, hence its life-time, whose known estimates are valid only for solar conditions. RGB and AGB stages have temperatures and densities below the envelope covering a wide range and differing from solar by up to a factor of five for T and up to five orders of magnitudes for rho, hence extrapolations are unreliable. Recently, we presented an estimate of the Be-7 half-life based on a fully quantistic method that goes beyond the Debye-Huckel approximation. Here we discuss its consequences on Li nucleosynthesis in low mass AGB stars
Electron spectroscopies and inelastic processes in nanoclusters and solids: Theory and experiment
No full textThe recent, very significant developments in high intensity and brightness electron and
photon sources have opened new possibilities of applying electron spectroscopies, such
as photoemission, Auger and electron energy loss, to the study of many interesting
features in the dynamics of atoms, molecules and condensed-matter systems. In the
last few years it has become possible to obtain electron spectra with an overall energy
resolution (electron/photon source and electron spectrometer) considerably smaller than
the linewidth of the investigated level and to study quantitatively the combined effects of
the intrinsic dynamical properties of the system, of features of the incident beam and of
the electron spectrometer on the spectral lineshape. For all these reasons, it is important
to have theoretical methods that are able to analyze the dynamics of systems at any level
of aggregation under the influence of an incident radiation and, simultaneously, to predict
spectral lineshapes quantitatively by correlating their features with internal dynamics of
the perturbed system.
In this report, we present experiments and a critical overview of theoretical methods
for interpreting electron spectra of atoms, molecules and solid-state systems. The general
theoretical framework for this analysis is resonant multichannel scattering theory. Electron
spectroscopies are, in fact, based on scattering processes in which the initial state consists
of a projectile, typically photons or electrons, exciting a target to a resonant state, which
has long lifetimes if compared to the collision time. This metastable state is embedded
in the continuum of final states characterized by the presence of a few fragments, whose
observation provides useful information on the properties of the system under study. Even
if the general theory of scattering and decay phenomena has been largely developed, its
specific application to electron spectroscopies in condensed matter and, in several cases
also to atoms and molecules, presents difficulties that have hindered the production of high
quality theoretical spectra until recently. This is mainly due to computational problems
related to treating a large number of decay channels, which prevent one from using
numerical techniques for representing the electron as it moves outward through the field
of the ionized system. Furthermore, another issue is represented by the need to account for
shake processes and extrinsic energy losses due to the coupling with collective excitations.
In this work we present a theoretical method which does not suffer from the limitations
of previous approaches, and allows one accurately to reproduce the experimental results
in solids. This method provides an extension to condensed matter of Fano's formulation of the interaction between discrete and continuum states. It includes the combined effects of
intrinsic and extrinsic features on spectral lineshapes so that computed spectra are directly
comparable to acquired spectra, avoiding background subtraction or deconvolution
procedures. This approach is sufficiently general to be applied not only to the analysis and
interpretation of autoionization, Auger and photoemission spectra, but also to the study of
other processes since its central feature is the ability of calculating accurate wavefunctions
for continuum states of extended systems
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A Computational Perspective on Multichannel Scattering Theory with Applications to Physical and Nuclear Chemistry
No full textIn this chapter we discuss a general theoretical framework, based on the generalization
of Fano’s approach to discreteecontinuum interaction, able to describe a variety of reso-
nant phenomena and decay events, ranging from core-electron spectroscopies to the
description of universal properties in ultracold Fermi gases, and from the investigation
of b-decay in heavy nuclei to the nucleosynthesis of the elements in stars. This method
is capable of analyzing the dynamics of atoms, molecules, and solids under the influ-
ence of incident radiation or electronic beams and, simultaneously, to predict quanti-
tatively the spectral line shapes correlating their features with the internal dynamic ofC
the perturbed system. On the computational side, we demonstrate by using the
concept of the multichannel scattering T-matrix that our approach is amenable to effi-
cient program implementations for determining numerically continuum wave functions
including the main correlation effects. For each application we present a detailed des-
cription of the main algorithms and computational procedures used to implement our
scattering approach and to limit the computational cost of the calculations
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