1,721,368 research outputs found
Stability, dynamical properties, and melting of a bi-layer Wigner crystal
No full textWe investigate the stability, dynamical properties, and melting of a two-dimensional ~2D! Wigner crystal~WC! of classical Coulombic particles in a bilayer structure. Compared to the single-layer WC, this systemshows a rich phase diagram. Five different crystalline phases are stable; the energetically favored structure canbe tuned by changing either the interlayer distance or the particle density. Phase boundaries consist of bothcontinuous and discontinuous transitions. We calculated the phonon excitations of the system within theharmonic approximation and we evaluated the melting temperature of the bilayer WC by use of a modifiedLindemann criterion, appropriate to 2D systems. We minimized the harmonic free energy of the system withrespect to the lattice geometry at different values of temperature/interlayer distance and we found notemperature-induced structural phase transition
Hole subbands and effective masses in p-doped [113]-grown heterojunctions
No full textWe calculate the hole band structure and effective masses in a two-dimensional hole gas (2DHG) in p-doped [113]-grown heterojunctions. A Hamiltonian for the 2DHG is derived which is formally similar to the Luttinger Hamiltonian which describes the hole gas in conventional [001] heterojunctions. We use a limited analytical basis set of uncoupled heavy-hole and light-hole states which allow us to obtain a qualitative description of the differences between [113]- and [001]-grown heterojunctions. We avoid the self-consistent calculation of the electrostatic confining potential by use of an analytical model which proved to be very accurate. In addition to numerical calculations, we derive analytical expressions of the hole subbands which are valid at small in-plane wave vectors. We compute the classical effective masses of the 2DHG as a function of the charge density and we compare with those obtained for conventional [001]-grown structures. We found that the latter are systematically larger in a broad range of carrier concentrations
Superconducting nanofilms: molecule-like pairing induced by quantum confinement
No full textQuantum confinement of the perpendicular motion of electrons in single-crystalline metallic
superconducting nanofilms splits the conduction band into a series of single-electron
subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the
energetic position of each subband can vary significantly with changing nanofilm thickness,
substrate material, protective cover and other details of the fabrication process. It can occur
that the bottom of one of the available subbands is situated in the vicinity of the Fermi level.
We demonstrate that the character of the superconducting pairing in such a subband changes
dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known
crossover from the Bardeen–Cooper–Schrieffer regime to Bose–Einstein condensation
(BCS–BEC) observed in trapped ultracold fermions. For Pb nanofilms with thicknesses of 4
and 5 monolayers (MLs) this will lead to a spectacular scenario: up to half of all the Cooper
pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few
nanometers. As a result, the superconducting condensate will be a coherent mixture of almost
molecule-like fermionic pairs with ordinary, extended Cooper pairs
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
Enhancement of electron-hole superfluidity in double few-layer graphene
Full text linkWe propose two coupled electron-hole sheets of few-layer graphene as a new nanostructure to observe
superfluidity at enhanced densities and enhanced transition temperatures. For ABC stacked few-layer
graphene we show that the strongly correlated electron-hole pairing regime is readily accessible
experimentally using current technologies. We find for double trilayer and quadlayer graphene sheets
spatially separated by a nano-thick hexagonal boron-nitride insulating barrier, that the transition
temperature for electron-hole superfluidity can approach temperatures of 40 K.
The prediction of electron-hole superfluidity in spatially separated electron and hole layers
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
- …
