1,720,955 research outputs found
MANY-BODY TUNNELING EFFECTS IN NONSTANDARD HUBBARD MODELS
No full textThe standard Hubbard model, while effective in describing interacting many-body systems, has limitations in addressing long-range and complex interparticle interactions. Specifically, the Hubbard model assumes a single-band approximation, neglecting the role of multiple electronic bands that are often critical in real materials. It also focuses only on local on-site Coulomb repulsion, overlooking the longer-range interactions between electrons at different lattice sites. Additionally, the model simplifies electron hopping by assuming uniform tunneling between nearest neighbors, failing to capture the more intricate tunneling processes observed in many materials. These simplifications make the Hubbard model inadequate for describing systems exhibiting multi-orbital effects, nonlocal interactions, or spatially varying tunneling, thus highlighting the need for extensions that account for these complexities.
To address these limitations, nonstandard Hubbard Hamiltonians, incorporating additional terms such as density-induced tunneling and pair tunneling, were proposed long ago. These terms explicitly depend on Wannier functions, which describe electron localization within the system. The standard Hubbard model employs maximally localized Wannier functions, overlooking the full spatial extent of these functions, particularly their tails. In contrast, we show here that the nonstandard Hubbard terms are strongly influenced by these tails, which play a crucial role in generating long-range interactions that significantly impact the system's dynamics.
In this thesis, we present a novel treatment of Wannier functions, developing a perturbative approach that utilizes the barrier penetration coefficient as a perturbation parameter. With these newly defined Wannier functions, we are able to evaluate the nonstandard Hubbard terms and use them to re-derive the nonstandard Hubbard Hamiltonian. Our results demonstrate that enhanced long-range interparticle interactions can lead to a mechanism for repulsive particle pairing, driven by the suppression of single-particle tunneling due to density-induced tunneling. Contrary to predictions from the standard Hubbard model, this suppression does not lead to an insulating state. Instead, it allows the coherent motion of correlated electron pairs via pair tunneling, with these pairs remaining resistant to decay caused by single-electron tunneling transitions.
Using perturbative analytical approximations and extensive numerical simulations, we further investigate nonstandard Hubbard terms and their impact on many-body dynamics, with a particular focus on tunneling dynamics in arbitrary double-well potentials. Our results show that the influence of these nonstandard terms becomes increasingly significant as the interaction strength grows, underscoring their role in driving novel transport behaviors in strongly correlated systems.
These insights are particularly relevant for understanding materials like twisted bilayer graphene and systems undergoing metal-insulator transitions, where strongly correlated interactions are crucial. By validating our model through extensive numerical simulations and comparisons with experimental data, such as second-order tunneling in optical double-well potentials, we provide a more accurate framework for analyzing strongly correlated systems. This work not only enhances our understanding of these complex materials, but also opens new paths for the study of many-body physics.Il modello di Hubbard standard, pur essendo efficace nella descrizione di sistemi interagenti a molti corpi, presenta limitazioni nell'affrontare interazioni a lungo raggio e interazioni interparticellari complesse. In particolare, il modello di Hubbard assume un'approssimazione a banda singola, trascurando il ruolo delle bande elettroniche multiple, spesso critiche nei materiali reali. Inoltre, il modello si concentra solo sulla repulsione coulombiana locale, trascurando interazioni a più lungo raggio tra elettroni in siti reticolari diversi. Inoltre, il modello semplifica l'hopping elettronico ipotizzando un tunneling uniforme tra primi vicini, non riuscendo a cogliere i processi di tunneling più intricati osservati in molti materiali. Queste semplificazioni rendono il modello di Hubbard inadeguato a descrivere sistemi che presentano effetti multiorbitali, interazioni non locali o tunneling che varia nello spazio, evidenziando così la necessità di estensioni che tengano conto di queste complessità.
Per ovviare a queste limitazioni, già da tempo sono state proposte Hamiltoniane di Hubbard non standard, che incorporano termini aggiuntivi come il density-induced tunneling e il pair tunneling. Questi termini dipendono esplicitamente dalle funzioni di Wannier, che descrivono la localizzazione degli elettroni all'interno del sistema. Il modello di Hubbard standard impiega funzioni di Wannier massimamente localizzate, trascurando l'intera estensione spaziale di queste funzioni, in particolare le loro code. Al contrario, questa tesi dimostra che i termini di Hubbard non standard sono fortemente influenzati da queste code, che svolgono un ruolo cruciale nel generare interazioni a lungo raggio che influenzano in modo significativo la dinamica del sistema.
Questa tesi introduce un nuovo approccio alle funzioni di Wannier, sviluppando un approccio perturbativo che utilizza il coefficiente di penetrazione della barriera come parametro perturbativo. Con le funzioni di Wannier appena definite, siamo in grado di valutare i termini di Hubbard non standard e di utilizzarli per rideterminare l'Hamiltoniana di Hubbard non standard. I nostri risultati dimostrano che l'aumento dell'intensità delle interazioni interparticellari a lungo raggio può portare ad un meccanismo di accoppiamento repulsivo delle particelle, guidato dalla soppressione del tunneling di particella singola dovuto al density-induced tunneling. Contrariamente alle previsioni del modello di Hubbard standard, questa soppressione non porta a uno stato isolante. Al contrario, essa consente il movimento coerente di coppie di elettroni correlati tramite pair tunneling, con queste coppie che resistono al decadimento causato dalle transizioni di tunneling di singolo elettrone.
Utilizzando approssimazioni analitiche perturbative e ampie simulazioni numeriche, studiamo ulteriormente i termini di Hubbard non standard e il loro impatto sulla dinamica a molti corpi, con particolare attenzione alla dinamica di tunneling in potenziali arbitrari a doppia buca. I nostri risultati mostrano che l'influenza di questi termini non standard diventa sempre più significativa all'aumentare dell'intensità dell'interazione, sottolineando il loro ruolo nel determinare nuovi comportamenti di trasporto in sistemi fortemente correlati.
Queste intuizioni sono particolarmente importanti per la comprensione di materiali come il twisted bilayer graphene e i sistemi che subiscono transizioni metallo-isolante, dove le interazioni fortemente correlate sono cruciali. Convalidando il nostro modello attraverso ampie simulazioni numeriche e confronti con dati sperimentali, come il tunneling del secondo ordine in potenziali ottici a doppia buca, forniamo un quadro più accurato per analizzare i sistemi fortemente correlati. Questo lavoro di tesi non solo migliora la nostra comprensione di questi materiali complessi, ma apre anche nuove strade per lo studio della fisica a molti corpi
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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