1,720,965 research outputs found
Vacuum energy densities of a field in a cavity with a mobile boundary
No full textWe consider the zero-point field fluctuations, and the related field energy densities, inside a one-dimensional and a three-dimensional cavity with a mobile wall. The mechanical degrees of freedom of the mobile wall are described quantum mechanically and they are fully included in the overall system dynamics. In this optomechanical system, the field and the wall can interact with each other through the radiation pressure on the wall, given by the photons inside the cavity or even by vacuum fluctuations. We consider two cases: the one-dimensional electromagnetic field and the three-dimensional scalar field, and use the Green's functions formalism, which allows extension of the results obtained for the scalar field to the electromagnetic field. We show that the quantum fluctuations of the position of the cavity's mobile wall significantly affect the field energy density inside the cavity, in particular at the very proximity of the mobile wall. The dependence of this effect from the ultraviolet cutoff frequency, related to the plasma frequency of the cavity walls, is discussed. We also compare our new results for the one-dimensional electromagnetic field and the three-dimensional massless scalar field to results recently obtained for the one-dimensional massless scalar field. We show that the presence of a mobile wall also changes the Casimir-Polder force on a polarizable body placed inside the cavity, giving the possibility to detect experimentally the new effects we have considered
Field observables near a fluctuating boundary
Get PDFWe review several aspects related to the confinement of a massless scalar field in a cavity with a movable conducting wall of finite mass, free to move around its equilibrium position to which it is bound by a harmonic potential, and whose mechanical degrees of freedom are described quantum mechanically. This system, for small displacements of the movable wall from its equilibrium position, can be described by an e↵ective interaction Hamiltonian between the field and the mirror, quadratic in the field operators and linear in the mirror operators. In the interacting, i.e. dressed, ground state, we first consider local field observables such as the field energy density: we evaluate changes of the field energy density in the cavity with the movable wall with respect to the case of a fixed wall, and corrections to the usual Casimir forces between the two walls. We then investigate the case of two one-dimensional cavities separated by a movable wall of finite mass, with two massless scalar fields defined in the two cavities. We show that in this case correlations between the squared fields in the two cavities exist, mediated by the movable wall, at variance with the fixed-wall case
Spatial correlations of field observables in two half-spaces separated by a movable perfect mirror
Get PDFWe consider a system of two cavities separated by a reflecting boundary of
finite mass that is free to move, and bounded to its equilibrium position by a
harmonic potential. This yields an effective mirror-field interaction, as well
as an effective interaction between the field modes mediated by the movable
boundary. Two massless scalar fields are defined in each cavity. We consider
the second-order interacting ground state of the system, that contains virtual
excitations of both mirror's degrees of freedom and of the scalar fields. We
investigate the correlation functions between field observables in the two
cavities, and find that the squared scalar fields in the two cavities, in the
interacting ground state, are anticorrelated. We discuss the dependence of the
correlation on the distance of the two points considered from the mirror's
average position and on its mass and oscillation angular frequency. These
results show a sort of communication between the two half-spaces separated by
the movable mirror, mediated by its position fluctuations. Observability of
this new phenomenon exploiting two- or many-body dispersion interactions
between polarizable bodies is discussed. The dependence on a cutoff frequency
introduced to regularize the frequency integrations, as well as the case of a
real conductor, are also discussed.Comment: 10 pages, 1 figur
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Apparent tendency to phase variation in an Iranian Grasshopper, Pyrgodera armata (F. W.) (Orthoptera, Acrididae)
No full text2 láminas. Número abril-septiembreThe genus Pyrgodera is easily recognised amongst Old World
genera of the subfamily Oedipodinae, to which it belongs. by
its exceedingly high, arched, laminate pronotal crest with acute
anterior angle, partly covering the occiput. There is only one
species, Pyrgodera armata (F. W.), in the genus and there are
no described subspecies or races. It is widely distributed over the
plains of Western Asia, with the exception of the Arabian Peninsula.
It inhabits alluvial plains and adjoining hills with clay
or stony soils (Uvarov, 1927). There appears to be only one
generation a year and there is an egg diapause in winter. The
hoppers are normally of uniform green or buff colour and are
easily distinguishable from most other Acridid nymphs by their
enormous ,crests.Peer reviewe
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
- …
