1,720,971 research outputs found
Quantum non-locality in systems with open boundaries: From the Wigner-function formalism to non-homogeneous Markovian master equations
No full textWe shall discuss the role of electrical contacts within the conventional treatment of quantum devices based on the Wigner-Function formalism. Our analysis will show that the artificial spatial separation between device active region and external reservoirs, being intrinsically incompatible with the non-local character of quantum mechanics, may produce highly non-physical results like boundary-driven negative probability distributions. To overcome this serious limitation, we shall propose a new strategy to properly separate device and reservoir degrees of freedom, based on the well known projection techniques used in the formal theory of open systems. Our main result is a non-Markovian master equation, where dissipation and dephasing phenomena are also induced by the presence of spatially external particle reservoir
Dissipation and decoherence in nanodevices: a generalized Fermi's golden rule
No full textWe shall revisit the conventional adiabatic or Markov approximation, which—in contrast to the semiclassical case—does not preserve the positive-definite character of the corresponding density matrix, thus leading to highly non-physical results. To overcome this serious limitation, originally pointed out and partially solved by Davies and co-workers almost three decades ago, we shall propose an alternative more general adiabatic procedure, which (i) is physically justified under the same validity restrictions of the conventional Markov approach, (ii) in the semiclassical limit reduces to the standard Fermi's golden rule and (iii) describes a genuine Lindblad evolution, thus providing a reliable/robust treatment of energy-dissipation and dephasing processes in electronic quantum devices. Unlike standard master-equation formulations, the dependence of our approximation on the specific choice of the subsystem (that includes the common partial trace reduction) does not threaten positivity, and quantum scattering rates are well defined even in the case the subsystem is infinitely extended/has a continuous spectru
Microscopic Modeling of Energy Dissipation and Decoherence in Open Quantum Systems: Application to Semiconductor Nanodevices
No full textA general density-matrix description of energy-dissipation and decoherence phenomena in open quantum systems is presented. More specifically, contrary to the conventional single-particle correlation expansion, we shall investigate the effect of the adiabatic or Markov limit, before considering/performing any reduction procedure. Our fully operatorial approach allows us to better identify the general properties of the scattering superoperators entering our effective quantum-transport theory at various description levels. In particular, we shall show that—contrary to the semiclassical case—the conventional Markov limit does not preserve the positive-definite character of the corresponding density matrix, thus leading to highly non-physical results. To overcome this serious limitation, originally pointed out and partially solved by Davies and co-workers almost three decades ago, we shall propose an alternative and more general adiabatic procedure, which (i) in the semiclassical limit reduces to the standard Fermi's golden rule, and (ii) describes a genuine Lindblad evolution, thus providing a reliable/robust treatment of energy-dissipation and dephasing processes in semiconductor quantum device
Transport in quantum devices: Modelling contacts in the Wigner formalism
No full textWe shall discuss the role of the contacts as they are modelled in the conventional treatment of quantum devices based on the Wigner-Function formalism. Our analysis will show that the artificial spatial separation between device active region and external reservoirs is intrinsically incompatible with the non-local character of quantum mechanics. More specifically, by means of an exactly-solvable semiconductor model, we shall show that the application of the usual Up-Wind boundary scheme to the Wigner transport equation may produce highly non-physical results, like boundary-driven negative probability distribution
Quantum non-locality in systems with open boundaries: limitations of the Wigner function formalism
No full textQuantum non-locality is here put at variance with spatial contacts, as modelled within the Wigner-Function formalism: we shall show how these two concepts are indeed intrinsically incompatible when applied to transport through quantum devices. More specifically, by means of an exactly-solvable semiconductor model, we show that the application of the usual Up-Wind boundary scheme to the Wigner transport equation may produce highly non-physical results, like boundary-driven negative probability distributions. © 2007 American Institute of Physic
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
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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