1,720,966 research outputs found
Role of electron-electron correlation in the valence states of YBa2Cu3O7: Low-energy excitations and Fermi surface
No full textWe have studied the role of on-site correlation in the low-energy excitations of YBa2Cu3O7 in the normal state, calculating self-energies and spectral functions according to the three-body-scattering approach. The method allows to include all the details of realistic ab initio band-structure calculations and to augment them with the inclusion of electron-electron correlations, getting quasiparticle energies for one-particle removal. It is found that correlation effects modify the energy dispersion of hole quasiparticle states, both in the high and low binding energy region, and strongly modify the Fermi surface topology
Quantum dynamics of polaron formation with the Wigner-function approach
No full textIn compound semiconductor crystals with a substantial degree of ionicity Froelich interaction of charge carriers with longitudinal optical (LO) phonons is strong enough to produce detectable polaron effects. In this paper polaron effects are analysed using a quantum theory of electron transport based on the momentum- and frequency-dependent Wigner function f(w) (p, omega), defined starting from the G(<) Green function, simply related to the electron spectral function A(p,omega). The theoretical approach considers the dynamical evolution of the electron Wigner function in the presence of phonon scattering. An elaboration of the quantum dynamical equation in terms of Wigner paths formed by free flights and scattering events is used. These paths are especially suitable for a Monte Carlo solution of the transport equation for the Wigner function very similar to the semiclassical traditional Monte Carlo solution of the Boltzmann equation. Numerical results for GaAs and CdTe in a variety of physical conditions are presented
A BUBBLY UNIVERSE
No full textWe propose to explain the present large scale structure of the universe in terms of a first order phase transition in a two field inflation: the seeds of structure are assumed to be the ensuing strong, non-Gaussian, bubblelike inhomogeneities generated by the tunneling field. Along with this, of course, the ordinary zero-point fluctuations of the slow rolling inflaton are also present: they are seen as Gaussian and small perturbations of the microwave background on the large angular scales. We describe a biparametric model of bubbles in the matter dominated era (MDE) in which caustics form at a redshift z* in the surrounding shells and we assume that the caustics themselves are the loci of galaxy formation, i.e., the places where light is turned on. (Most likely z* will then define also the epoch of reionization.) The two parameters are then determined by the bubble’s two main features, present depth and z*. The caustics will evolve into the shells of galaxies observed today around the nearly empty and spherical voids. Among the possible scenarios we focus on two that yield late or early caustic formation. In the MDE the shells born with the caustics experience a strong overcomoving growth (the larger the deeper is the central cavity): this phenomenon may turn bubbles substantially subdominant at decoupling (i.e., filling then only a small fraction of the available space) into the dominant features by the present time, as the observations require. For compensated voids, from the Sachs-Wolfe, adiabatic, and Doppler effects, we find that the largest present radii compatible with COBE amount to ≈100h−1 Mpc in either scenario. Thus, if the large scale structure were generated by bubbles, the present luminous universe could look bubbly up to scales of the order of one hundred Mpc mimicking a fractal with dimension D≈2 without conflicting with the isotropy of the microwave background, because homogeneity is restored thereabove
A Probe of Planck Energy Physics
Get PDFLarge scale voids are a very prominent feature in recent redshift surveys: here we attempt an explanation in terms of a first order phase transition occurring during the slow roll epoch of a two field inflation. The ensuing bubble like perturbations -- nucleated at a number of e-folds Napprox 50 before reheating -- are thought to be the precursors of the voids we observe today. They pass the CMB constraints. Furthermore, the claim, in apparent conflict with inflation, that the Universe is open, may also be explained if we live in a superhorizon bubble nucleated around Napprox 60 and if the Universe itself be a collection of such bubbles. Our model predicts sharply peaked bubble spectra, e.g. at Omega0 ≅.2, so that our probability of living in the right bubble is close to one. Finally, one can even envision a sequence of two phase transitions, at N approx 60 and approx 50, where bubbles are born within bubbles. In conclusion, the study of the large scale structure may turn out to be a powerful probe -- maybe the only one -- of Planck physics
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
Quenching of majority-channel quasiparticle excitations in cobalt
No full textThe low-energy electronic excitations in cobalt are studied by a theoretical method that includes many-body effects and a realistic description of the band structure. Angle-resolved photoemission spectra measured on a thick film of hexagonal close-packed Co on Cu(111) agree well with calculated spectral functions. Because of many-body effects no sharp quasiparticle peaks exist for binding energies larger than 2 eV and in this energy region the spectrum is essentially incoherent. The many-body corrections are much stronger in the majority-spin channel and drastically affect the spin polarization of the spectra
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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