1,720,979 research outputs found
A brief review on point interactions.
No full textWe review properties and applications of point interaction Hamiltonians.
This class of operators is first defined following a classical presentation and then generalized
to cases in which some dynamical and/or geometrical parameters are varying
with time. We recall their relations with smooth short range potentials
A one-dimensional model for ionization induced by scattering with a heavy particle
No full textThe ionization probability for a one-dimensional model atom perturbed by a moving repulsive scatterer is considered. The moving scattering centre is meant to mimic a second quantum particle crossing the region where a much lighter particle is initially bound in the atom. We compute the first three terms (of order t3/2, t2, t5/2 respectively) in the expansion for small times of the ionization probability and we deduce that the first term showing explicit dependence on the velocity of the scatterer is of order t5/2. A possible application of the model for the description of a quantum particle in a cloud chamber is also outlined
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
Joint excitation probability for two harmonic oscillators in dimension one and the Mott problem.
No full textWe analyze a one dimensional quantum system consisting of a test particle interacting with two harmonic oscillators placed at the positions a_1, a_2, with a_1 >0, |a_2|>a_1, in the two possible situations: a_2>0 and a_2 <0. At time zero the harmonic oscillators are in their ground state and the test particle is in a superposition state of two wave packets centered in the origin with opposite mean momentum.
Under suitable assumptions on the physical parameters of the model,
we consider the time evolution of the wave function and we compute
the probability \mathcal{P}^{-}_{n_1 n_2} (t) (resp. \mathcal{P}^{+}_{n_1 n_2}(t)) that both oscillators are in the excited states labelled by n_1, n_2 >0 at time t > |a_2| v_0^{-1} when a_2 0).
We prove that \mathcal{P}_{n_1 n_2}^{-} (t) is negligible with respect to \mathcal{P}_{n_1 n_2}^{+} (t), up to second order in time dependent perturbation theory.
The system we consider is a simplified, one dimensional version of the original model of a cloud chamber introduced by Mott in \cite{m}, where the result was argued using euristic arguments in the framework of the time independent perturbation theory for the stationary Schr\"{o}dinger equation.
The method of the proof is entirely elementary and it is essentially based on a stationary phase argument. We also remark that all the computations refer to the Schr\"{o}dinger equation for the three-particle system, with no reference to the wave packet collapse postulate
Resonances in models of spin-dependent point interactions
No full textIn dimension d = 1, 2, 3 we define a family of two-channel Hamiltonians obtained as point perturbations of the generator of the free decoupled dynamics. Within the family we choose two Hamiltonians, \hat{H}_0 and \hat{H}_\varepsilon , giving rise respectively to the unperturbed and to the perturbed evolution. The Hamiltonian \hat{H}_0 does not couple the channels and has an eigenvalue embedded in the continuous spectrum. The Hamiltonian \hat{H}_\varepsilon is a small perturbation, in resolvent sense, of \hat{H}_0 and exhibits a small coupling between the channels. We take advantage of the complete solvability of our model to prove with simple arguments that the embedded eigenvalue of \hat{H}_0 shifts into a resonance for \hat{H}_\varepsilon . In dimension three we analyze details of the time behavior of the projection onto the region of the spectrum close to the resonance
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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