1,720,960 research outputs found
Automatizzazione di un'accurata descrizione delle funzioni di risposta nei sistemi a bassa dimensionalità tramite teoria delle perturbazioni dei sistemi a molti corpi
No full textNella scienza dei materiali, la combinazione di simulazioni teoriche con tecniche High-throughput(HT) sta guadagnando maggiore attenzione, in quanto può accelerare la ricerca e la scoperta di nuovi materiali con le proprietà desiderate. Nell'ultimo decennio, i metodi HT sono stati applicati principalmente alla density functional theory (DFT) con grande successo. Tuttavia, la DFT manca di accuratezza quando si tratta di eccitazioni e spettroscopie, come ad esempio nella previsione delle proprietà delle quasiparticelle (esperimenti ARPES) o degli spettri di assorbimento ed electron energy loss(EELS), dove è necessaria una descrizione accurata degli effetti eccitonici.
In questa tesi vogliamo combinare la potenza dello screening HT con l'accuratezza della many-body-perturbation-theory (MBPT) per studiare le proprietà elettroniche e ottiche di sistemi a bassa dimensionalità. In particolare, adottiamo l'approssimazione GW per studiare la struttura elettronica delle bande, e l'equazione di Bethe-Salpeter per calcolare gli spettri ottici.
La prima parte del lavoro è stata dedicata allo sviluppo di una serie di workflow per eseguire studi di convergenza automatizzati dei principali parametri computazionali che entrano nei calcoli MBPT. Essendo i test di convergenza in MBPT di fondamentale importanza per ottenere risultati significativi, rappresentando anche la parte che richiede più tempo umano nei calcoli GW/BSE, questo passaggio è stato essenziale nello sforzo di unire MBPT con gli schemi HT. I workflow sopra menzionati sono stati prima validati su sistemi ben noti come hBN o TiO2, e poi applicati per studiare gli effetti eccitonici in C3N, un sistema 2D recentemente realizzato.
Il C3N è un semiconduttore a gap indiretto 2D emergente con interessanti proprietà meccaniche, termiche ed elettroniche. In questa tesi abbiamo eseguito una descrizione completa delle proprietà elettroniche e dielettriche di C3N, concentrandoci sulla struttura a bande eccitonica. Come per altri materiali 2D, troviamo una dispersione eccitonica lineare non analitica prossima a q=0, che però mostra una convessità verso il basso legata alla natura indiretta del band gap.
Successivamente, questi workflow sono stati utilizzati per studiare l'accuratezza della Godby-Needs plasmon-pole approximation (GN-PPA) implementata nel codice Yambo sul set GW100, un set di 100 molecole utilizzate per confrontare i principali codici MBPT mediante G0W0. Per tutte le molecole abbiamo calcolato il potenziale di ionizzazione (IP) e l'affinità elettronica (EA) e confrontato i risultati con gli altri codici MBPT con i dati GW100 disponibili. I risultati di questo studio ci hanno permesso di concludere che lo schema GN-PPA supera altre approssimazioni comunemente usate, come il modello Hybertsen-Louie, e fornisce risultati in un accordo ragionevole con implementazione più sofisticate del GW come full-frequency e contour-deformation. Recentemente, il gruppo EPFL-THEOS (Lausanne, CH), guidato dal Prof. Nicola Marzari, ha eseguito una HT computational exfoliation di composti 3D sperimentalmente conosciuti, scoprendo più di 1800 materiali 2D esfoliabili.
La parte finale di questa tesi è dedicata allo studio delle proprietà elettroniche e ottiche di un sottoinsieme di questi sistemi, al fine di identificare possibilmente materiali candidati che possano realizzare la tanto ricercata fase di isolante eccitonico (EI). La fase EI è una fase correlata della materia, proposta più di 50 anni fa da L. Keldysh, W. Kohn, in cui lo stato fondamentale di un sistema, al di sotto di una temperatura critica, è formato da un condensato di eccitoni di Bose. Per fare previsioni sulle possibili instabilità eccitoniche nello stato fondamentale, sono indispensabili una stima molto accurata del band gap elettronico e delle energie di legame eccitoniche, e workflow per automatizzare calcoli MBPT costituiscono uno strumento estremamente efficace per questa ricerca.In materials science, the combination of theoretical simulations with high-throughput (HT) techniques is gaining larger attention, as it can speed up the search and discovery of novel materials expressing desired properties. In the last decade, HT methods have been mostly applied to density functional theory (DFT) demonstrating great success. Nevertheless, DFT lacks of predictive accuracy when dealing with excitations and spectroscopies, as for instance in the prediction of quasiparticle properties (ARPES experiments), or absorption and electron energy loss spectra (EELS) where an accurate description of the excitonic effects is needed.
In this Thesis we want to combine the power of HT screening with the accuracy of many-body perturbation theory (MBPT) to study the electronic and optical properties of low-dimensional systems. In particular, we adopt the GW approximation to study the electronic band structure and the Bethe-Salpeter equation to compute optical spectra.
The first part of the work has been devoted to developing a set of workflows to perform automated convergence studies of the main computational parameters entering in MBPT calculations. Being convergence tests in MBPT of paramount importance to obtain meaningful results, also representing the most human time-consuming part of a GW/BSE calculation, this step has been essential in the effort of joining MBPT with HT schemes. The above mentioned workflow was first validated on well-known systems like hBN or TiO2, and then applied to study excitonic effects in C3N, a recently synthesized 2D system.
Monolayer C3N is an emerging 2D indirect gap semiconductor with interesting mechanical, thermal, and electronic properties. In this thesis we have performed a complete description of C3N electronic and dielectric properties, focusing on the momentum-resolved excitonic band structure. As for other 2D materials, we find a linear non-analytic excitonic dispersion close to q=0, which however shows a downward convexity related to the indirect nature of the band gap.
Next, these workflows were used to study the accuracy of the Godby-Needs plasmon-pole approximation (GN-PPA) implemented in the Yambo code on the GW100 set, a set of 100 molecules used to benchmark the main MBPT codes by means of G0W0 approximation. For all the molecules we computed converged ionization potential (IP) and electron affinity (EA), and compared the results with the other MBPT codes with available GW100 data. The results of this study allowed us to conclude that the GN-PPA scheme outperforms other commonly used approximations such as the Hybertsen-Louie plasmon pole model, and provides results in a reasonable agreement with more sophisticated implementation of GW as the full-frequency and contour deformation methods.
Recently, the EPFL-THEOS (Lausanne, CH) group, led by Prof. Nicola Marzari, performed a HT computational exfoliation of experimentally known 3D compounds, discovering more than 1800 exfoliable layered materials.
The final part of this Thesis is dedicated to the investigation of the electronic and optical properties of a subset of these recently discovered 2D systems, in order to possibly identify candidate materials that can realize the long sought excitonic insulator (EI) phase.The EI phase is a correlated phase of matter, proposed more than 50 years ago by L. Keldysh, W. Kohn, in which the ground state of a system, below a critical temperature, is formed by a Bose condensate of excitons. To make prediction on the possibile excitonic instabilities in the ground state, very accurate estimation of the electronic band gap and excitonic binding energies are indispensable and workflows capable of making automatic calculations based on MBPT constitute an extremely effective tool for this research
Automated computational workflows for muon spin spectroscopy
No full textPositive muon spin rotation and relaxation spectroscopy is a well established experimental technique for studying materials. It provides a local probe that generally complements scattering techniques in the study of magnetic systems and represents a valuable alternative for materials that display strong incoherent scattering or neutron absorption. Computational methods can effectively quantify the microscopic interactions underlying the experimentally observed signal, thus substantially boosting the predictive power of this technique. Here, we present an efficient set of algorithms and workflows devoted to the automation of this task. In particular, we adopt the so-called DFT+m procedure, where the system is characterized in the density functional theory (DFT) framework with the muon modeled as a hydrogen impurity. We devise an automated strategy to obtain candidate muon stopping sites, their dipolar interaction with the nuclei, and hyperfine interactions with the electronic ground state. We validate the implementation on well-studied compounds, showing the effectiveness of our protocol in terms of accuracy and simplicity of use
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
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