1,720,973 research outputs found
Resonance energy transfer between two atoms in a conducting cylindrical waveguide
No full textWe consider the energy transfer process between two identical atoms placed inside a perfectly conducting cylindrical waveguide. We first introduce a general analytical expression of the energy transfer amplitude in terms of the electromagnetic Green's tensor; we then evaluate it in the case of a cylindrical waveguide made of a perfect conductor, for which analytical expressions of the Green's tensor exist. We numerically analyze the energy transfer amplitude when the radius of the waveguide is such that the transition frequency of both atoms is below the lower cutoff frequency of the waveguide, so that the resonant photon exchange is strongly suppressed. We consider both cases of atomic dipoles parallel and orthogonal to the axis of the guide. In both cases, we find that the energy transfer is modified by the presence of the waveguide. In the near zone, that is when the atomic separation is smaller than the atomic transition wavelength, the change, with respect to the free-space case, is small for axial dipoles, while it is larger for radial dipoles; it grows when the intermediate region between near and far zone is approached. In the far zone, we find that the energy transfer amplitude is strongly suppressed by the waveguide, becoming virtually zero. A physical interpretation of these results is discussed. Finally, we discuss the resonance interaction energy and force between two identical correlated atoms in the waveguide, one excited and the other in the ground state, prepared in their symmetric or antisymmetric superposition
Dispersion Interaction between Two Hydrogen Atoms in a Static Electric Field
Full text linkWe consider the dispersion interaction between two ground-state hydrogen atoms, interacting with the quantum electromagnetic field in the vacuum state, in the presence of an external static electric field, both in the nonretarded and in the retarded Casimir-Polder regime. We show that the presence of the external field strongly modifies the dispersion interaction between the atoms, changing its space dependence. Moreover, we find that, for specific geometrical configurations of the two atoms with respect to the external field and/or the relative orientation of the fields acting on the two atoms, it is possible to change the character of the dispersion force, turning it from attractive to repulsive, or even make it vanishing. This new finding clearly shows the possibility to control and tailor interatomic dispersion interactions through external actions. By a numerical estimate of the field-modified interaction, we show that at typical interatomic distances the change of the interaction's strength can match or even outmatch the unperturbed interaction; this can be obtained for values of the external field that can be currently achieved in the laboratory, and sufficiently weak to be taken into account perturbatively
Reply to "Comment on 'Dispersion Interaction between Two Hydrogen Atoms in a Static Electric Field' "
No full textIn their Comment on our Letter Dispersion Interaction between Two Hydrogen Atoms in a Static Electric Field, P. P. Abrantes et al. address one of the main points discussed in our Letter, that is, the possibility to manipulate interatomic interactions through an external static electric field. In our Letter, we have shown that the interaction between two ground-state atoms can be significantly modified, exploiting an external static electric field, and even turned from attractive to repulsive, depending on the strength of the external field and the geometrical configu- ration. In their Comment, Abrantes et al. point out that it is the electrostatic contribution between the electric dipoles induced in both atoms by the external field that is dominant and can become repulsive, overcoming the usual attractive dispersion force. They write the interatomic force as the sum of a classical electrostatic dipole-dipole interaction and a dispersion interaction modified by the external field and point out that it is the total force that changes its sign. As we discuss below in more detail, we partially agree with their interpretation of this result. Essential points, in our opinion, are the exact definition of the dispersion interaction and how it is separated from the (classical) electrostatic con- tribution when the atoms are polarized by the static electric field, clarifying which quantity is evaluated at any step
A Software Defined Radio Platform Implementing a WiFi and ZigBee Receiver
No full textA successful attempt to design and implement a multi-standard compliant Basebnd Processor is here presented. By exploiting the potential of FPGA's reconfigurability, the received signal from RF stage have been processed in order to properly decode frames of IEEE 802.11 (WiFi) and IEEE 802.15.4 (ZigBee) protocols. both falling within the ISM band (centered at 2.45GHz). The experimental implementation carried out is a practical demonstration of the Software Defined Radio concept
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
Test procedures for use of wireless partial discharges instrumentation in HV networks
No full textOn-field diagnostic methods based on Partial Discharge (PD) measurements for asset assessment in High Voltage (HV) sectors are experiencing a constant growth in recent years. This rise of interest motivated the research toward faster, reliable and less expensive methods and technologies to perform PD measurements. Recently, a portable instrument based on a wideband antenna sensor, intended for accurate partial discharge measurements, has been developed by the authors. This sensor allows to capture electromagnetic waves in a bandwidth ranging from 0.1-100 MHz as well as the electric field of the AC supply voltage to be used as a phase reference. This approach completely avoid the need of a direct connection with the parts under test, also allowing to perform the measure at a distance, in some cases. The instrument also integrates a digital acquisition units allowing to obtain a Phase Resolved PD (PRPD) pattern in real-time. These features makes this device particularly suitable as a portable instrumentation for on-field operations, especially in on-line tests. The effectiveness of this new approach to PD measurement, as well as the related new possibilities, has been thoroughly studied and assessed. The portable instrument has been characterized through both laboratory and on-site experiments, carried out on different types of HV components (mainly terminations, joints and transformers). The instrument performances were compared to traditional sensing techniques such as capacitive couplers and High Frequency Current Transformers (HFCT). In general a very good agreement was found performing concurrent measurements. Another aim of this research was to study standardised procedures to perform on-line PD measurements when using portable instruments based on electromagnetic sensors, in order to obtain repeatable and reliable results. In this case in fact some factors like position of the sensor, distance from the components, calibration and sequence of operations, need to be evaluated, considering the impact on the measurement results. These experiments and related results are presented and discussed in the paper. © 2014 CIGRE. All rights reserved
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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