1,720,959 research outputs found
Small crystal models for the electronic properties of carbon nanotubes
Full text linkThis thesis develops the basic theoretical aspects of the electronic properties of carbon nanotubes which are necessary for a detailed understanding of optical characterization measurements by photoluminescence and Raman spectroscopy. Starting from a wisely chosen finite portion of the real lattice with proper boundary conditions, the small crystal approach allows to find the minimal set of Brillouin zone points, which are sufficient for computing the essential features of the optical spectra of periodic systems, such as single-walled nanotubes. The real space vision embedded in small crystal approach allows to overcome some limitations inherent to reciprocal space based methods,when dealing with local symmetry breaking effects in the electronic structure of carbon nanotubes, such as electron-electron interactions, point-defects and orientationdependent intertube interactions, the last one in the particular case of double-walled nanotubes. The thesis shows the application of small crystal approach to these issues
and discuss the obtained results with respect to the currently available experimental and theoretical findings.Questa tesi sviluppa gli aspetti teorici basilari delle proprietà elettroniche dei nanotubi di carbonio che sono necessari per una comprensione dettagliata delle misure di caratterizzazione
ottica tramite fotoluminescenza e spettroscopia Raman. Partendo da una porzione finita del reticolo reale opportunamente scelta con condizioni periodiche al contorno appropriate, l’approccio del cristallo piccolo consente di trovare l’insieme più piccolo di punti della zona di Brillouin che sono sufficienti per calcolare il profilo essenziale degli spettri ottici di sistemi periodici, come i nanotubi a parete singola. La visione in spazio reale presente nell’approccio del cristallo piccolo consente di superare le limitazioni inerenti ai metodi in spazio reciproco, quando si debbano considerare effetti di rottura locale di simmetria nella struttura elettronica dei nanotubi di carbonio, come interazioni elettrone-elettrone, difetti puntuali e interazioni intertubo dipendenti dall’orientazione dei tubi costituenti, quest’ultimo
nel caso particolare dei nanotubi a parete doppia. La tesi mostra l’applicazione dell’approccio del cristallo piccolo a questi problemi e i risultati ottenuti vengono discussi in relazione alle attuali conoscenze sperimentali e teoriche
Excitonic properties of armchair graphene nanoribbons from exact diagonalization of the Hubbard model
No full textWe report on excitonic spectra of armchair graphene nanoribbons (AGNRs) obtained from a full many-body exact diagonalization of the Hubbard model within low and intermediate correlation regimes and with a complete characterization of the spin multiplicity of the calculated eigenstates. Our results allow us to group these systems into three different families according to the sequence of the one-and two-photon allowed states and the magnitude of the respective optical oscillator strengths within the investigated correlation regime. The oscillator strengths for the one-photon allowed transitions are found to be lower than those obtained previously for zigzag semiconducting single-walled carbon nanotubes, pointing out a qualitatively different photophysical behaviour of AGNRs
Fluorescence dynamics and fine structure of dark excitons in semiconducting single-wall carbon nanotubes
No full textExact diagonalization results are reported for the bright and dark exciton structure of semiconducting single-wall carbon nanotubes in the framework of the Hubbard model combined with a small crystal approach for several values of the correlation coupling strength U/t. Our findings, in the low-intermediate correlation regime (1 : 5 and can account for reported experimental values of deep triplet states below vertical bar B > and of a K-momentum singlet dark exciton above this state. In order to fit the temporal profile of the photoluminescence (PL) decay, a bottleneck mechanism is considered involving a few dark states, with the respective energy gaps correspondingly obtained in the above-mentioned correlation range. We find that a kinetic model with one dark state above and two below vertical bar B > is able to recover the observed biexponential features of the PL behaviour with a reasonable set of parameters. Within this model we attribute the long tail of the PL to a delayed luminescence process of the bright state caused by the nearby calculated dark states
Exact diagonalization of Hubbard models for the optical properties of single-wall carbon nanotubes
Full text linkExcitonic states of single-walled carbon nanotubes (SWNTs) have usually been calculated using many-body perturbation theories or mean field approaches because a large number of sites cannot be considered within an exact diagonalization (ED) calculation based on the Hubbard model. We use a small crystal approach and show that, for the π structure of nanotubes, an ED calculation is possible. We apply this approach to small-diameter SWNTs and the results show that a crossing of the first bright state with the second excited states occurs when U, the correlation parameter of the Hubbard model, equals 4t, where t is the hopping integral. Two or three strong two-photon absorption (TPA) states are found at energies above the first bright state for U/t≤3. Beyond this value, these states become relevant for TPA below the first bright state. A number of dark states are always calculated below the first bright state at energies that, in the intermediate coupling regime, are of the order of tens to hundreds of meV. This result seems to be consistent with recent experiments
Small crystal approach for the electronic properties of double-wall carbon nanotubes
No full textA small crystal approach (SCA) is a real-space approach that allows one to introduce local symmetry breaking which is difficult to consider in a reciprocal space approach. We apply the SCA to the case of double-wall nanotubes, for which the effects of the orientation-dependent intertube interactions have to be taken into account. The results suggest a solution for the usual difficulties in assigning the resonance Raman spectra of double-wall nanotubes since important variations of the electronic spectra of these carbon nanotubes are found with respect to those of the constituent single-wall nanotubes
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
- …
