1,721,047 research outputs found
Quantum tunnelling and hopping between metallic domains in disordered two-dimensional mesoscopic electron systems
No full textWe investigate transport in 2D mesoscopic electron systems with disorder assuming a percolation mechanism through a network of disconnected conducting metallic domains. The size of the domains is determined by the level of disorder and the strength of the electron correlations. The domains are linked for transport by two competing mechanisms. The first mechanism is familiar thermally activated hopping. The second is quantum tunnelling between adjacent conducting regions bounded by equipotential contours of the same value. This mechanism leads to temperature-independent transmission at low temperatures. We calculate the transmission across the potential barriers separating adjacent domains, and we obtain agreement with recent experimental measurements of temperature-dependent resistivities in mesoscopic 2D systems. We also obtain consistent values for the spatial separation of the domains and the average variation in the random disorder potential. Finally, we show that the effect of quantum coherence can result in a small downturn in the resistivity at low temperatures, again in good agreement with the recent experimental results
Anomalous transport in mesoscopic inhomogeneous two-dimensional electron systems at low temperature
No full textWe analyze transport properties of mesoscopic GaAs two-dimensional 2D electron systems in the insulating
regime. It is well known that disorder can cause the insulating system to break up into connected insulating
domains and isolated conducting domains. In mesoscopic systems the separation between conducting domains
can be very small even when h/e2. In this case, transport at sufficiently low temperatures, T1 K, is
controlled not by conventional hopping but by short-range tunneling between adjacent conducting domains,
and this has a dramatic effect on the low-T transport properties. The resulting T-dependent resistivity T is in
good agreement with measurements of T in 2D electron layers in gated mesoscopic GaAs/AlGaAs structures
and, in particular, T can even exhibit an anomalous metalliclike drop as T→0 even for resistivities
as high as 500h/e2
High-Temperature Superfluidity in Double-Bilayer Graphene
Get PDFExciton bound states in solids between electrons and holes are predicted to form a superfluid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron-hole pairs should be achievable for the first time.
The transition temperatures are well above liquid helium temperatures. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route toward realizing high-temperature superfluidity in existing quality graphene samples
Excitonic superfluidity and screening in electron-hole bilayer systems
No full textSuperfluidity in electron-hole bilayers in graphene and GaAs has been predicted theoretically many times
but not yet definitively observed. A key controversy is the correct approximation for the screening of the
Coulomb interaction for the pairing. Mean-field theories using different approximations for the screening lead to
diametrically contradictory predictions for superfluidity. We test these different approximations against diffusion
quantum Monte Carlo results and find good agreement with the mean-field theory that uses screening in the
superfluid state, but large discrepancies with other approximations for screening. This mean-field theory predicts
no superfluidity in existing devices, provides pointers for new devices to generate the superfluidity, and, very
importantly, it permits calculations for complicated lattices at finite temperatures, impractical in Monte Carlo
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
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
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