6,796 research outputs found
Density functional approach to the band gaps of finite and periodic two-dimensional systems
No full textWe present an approach based on density functional theory for the calculation of fundamental gaps of both finite and periodic two-dimensional (2D) electronic systems. The computational cost of our approach is comparable to that of total energy calculations performed via standard semilocal forms. We achieve this by replacing the 2D local density approximation with a more sophisticated—yet computationally simple—orbital-dependent modeling of the exchange potential within the procedure by Guandalini et al. [Phys. Rev. B 99, 125140 (2019)]. We showcase promising results for semiconductor 2D quantum dots and artificial graphene systems, where the band structure can be tuned through, e.g., Kekulé distortion
Eccitazioni cariche e neutre non lineari in sistemi a bassa dimensionalità e molecolari mediante l'utilizzo di approcci basati sui funzionali della densità
No full textLe eccitazioni elettroniche assumono un ruolo di fondamentale importanza in diverse proprietà fisiche dei materiali, ad esempio nel trasporto quantistico, trasporto termico, conducibilità e proprietà ottiche. A seconda della carica elettrica dello stato finale, possiamo distinguere tra eccitazioni cariche e neutre. Nelle eccitazioni cariche, consideriamo elettroni che vengono aggiunti o sottratti al sistema. Questi processi possono essere studiati tramite esperimenti di fotoemissione diretta o inversa. Invece, nelle eccitazioni neutre, la carica totale del sistema viene conservata durante il processo di eccitazione. Questo tipo di eccitazioni possono essere rilevate attraverso misure di assorbimento ottico, sia in regime lineare che non lineare.
La teoria del funzionale della densità (DFT) e la sua estensione dipendente dal tempo (TDDFT) sono spesso il primo quadro teorico di riferimento nella descrizione a primi principi dei processi di eccitazione. Tuttavia, è noto che la DFT e la TDDFT danno luogo ad errori e limiti di applicabilità a causa delle approssimazioni funzionali che devono essere necessariamente utilizzate in pratica. Pertanto, lo sviluppo di approssimazioni più accurate ed altre estensioni teoriche è un campo di ricerca interessante e attivo.
In questo lavoro, mi occupo dello sviluppo di nuovi metodi per il calcolo delle eccitazioni cariche e neutre.
Nella prima parte, mostro come il gap fondamentale dei punti quantici bidimensionali può essere accuratamente stimato al costo computazionale di un calcolo di stato fondamentale, seguito da uno step non autoconsistente dal costo computazionale trascurabile, il tutto eseguito all'interno dell'approssimazione della densità locale. Nonostante ciò, la procedura include formalmente la discontinuità del potenziale di scambio espressa attraverso il metodo del potenziale effettivo orbitale.
Nella seconda parte, ricavo un'approssimazione del potenziale che include esplicitamente un gap di scambio diverso da zero per sistemi bidimensionali sia finiti che periodici. Sebbene la forma funzionale utilizzata coinvolga direttamente gli orbitali a singola particella, il costo computazionale è paragonabile ai calcoli DFT standard. L'approssimazione del potenziale è applicata al grafene artificiale, in cui una distorsione di Kekulé utilizzata per aprire un gap nei punti di Dirac.
Nella terza parte di questo lavoro mi occupo di studiare le eccitazioni neutre non lineari. In particolare, derivo la sezione d'urto ottica per un sistema a più elettroni soggetto a un campo elettrico impulsivo nel regime non perturbativo, cioè per valori arbitrari dell'intensità del campo incidente, partendo dallo stato fondamentale. Successivamente mostro che la sezione d'urto include assorbimenti da stati eccitati all'aumentare dell'intensità del campo elettrico. Queste transizioni ottiche non sono possibili all'interno del regime lineare. Come esempio di applicazione, considero il caso di un sistema modello unidimensionale composto da due elettroni interagenti. L'analisi rivela che gli stati eccitati di tipo gerade, che non partecipano alle proprietà ottiche nel regime lineare, sono popolati nel regime non lineare a causa dell'assorbimento da stato eccitato. Questa analisi aiuta a interpretare le simulazioni di TDDFT in tempo reale che impiegano campi elettrici impulsivi oltre il regime lineare, come per lo studio dei processi coinvolti nei fenomeni di limitazione ottica.
I risultati ottenuti in questa tesi di dottorato contribuiscono allo sviluppo di metodi accurati e praticabili per studiare le eccitazioni elettroniche nei sistemi quantistici e, in particolare, allo sviluppo teorico nel campo degli approcci a principi primi basati sui funzionali della densità.Electronic excitations play a prominent role in a large variety of physical properties of materials, e.g., quantum transport, heat transport, conductivity, and optical properties. Depending on the electric charge of the final state, we may distinguish between charged and neutral excitations. In charged excitations, we consider electrons that are added or subtracted to the system. Direct or inverse photoemission experiments are a primary tool for the experimental observation of such processes.
Instead, in neutral excitations, the total charge of the system is conserved during the excitation process. These can be probed through optical absorption measurements, both in the linear and nonlinear regimes.
Density functional theory (DFT) and its time-dependent extension (TDDFT) are often the theoretical framework of first choice in the first-principles description of excitation processes. However, it is well known that DFT and TDDFT show failures and limitations due to the functional approximations which are necessary in practice. Thus, the development of more accurate approximations and theoretical extensions is an interesting and intense field of research.
In this work, I develop new advances in the calculation of charged and neutral excitations.
In the first part, it is shown that the fundamental gap of two-dimensional quantum dots can be accurately estimated at the effort of a standard ground-state calculation supplemented with a non-self-consistent step of negligible cost, all performed at the level of the local-density approximation. Yet, the procedure formally exploits the exchange discontinuity as expressed through the orbital-effective-potential method.
In the second part, I derive an approximate potential that can capture non-vanishing exchange gaps both infinite and periodic two-dimensional systems. Although the procedure involves single-particle orbitals directly, the computational cost is comparable to standard DFT calculations. The potential approximation is applied to the artificial graphene, Kekulé distorted to open a gap at the Dirac points.
In the third part of this work, nonlinear neutral excitations are investigated. In particular, I derive the optical cross section of a many-electron system subject to an impulsive electric field in the nonperturbative regime, i.e. for arbitrary values of the field strength, starting from the ground state. I show that the cross section includes absorptions from excited states for increasing intensities of the electric field - which are optical transitions that cannot be captured within the linear regime.
As an example, I consider the case of a 1D two-electron model system. The analysis reveals that gerade excited states, which are dark in the linear regime, are populated in the nonlinear regime due to excited-state absorption. This analysis helps to interpret real-time TDDFT simulations which employ impulsive electric fields beyond the linear regime, as for studying processes in optical limiting phenomena.
The results obtained in this Ph.D. thesis contribute to the development of accurate and feasible methods to investigate electronic excitations in quantum systems, and, more generally, to the theory development of first-principles density-functional approaches
Efficient GW calculations in two dimensional materials through a stochastic integration of the screened potential
No full textAbstract Many-body perturbation theory methods, such as the G 0 W 0 approximation, are able to accurately predict quasiparticle (QP) properties of several classes of materials. However, the calculation of the QP band structure of two-dimensional (2D) semiconductors is known to require a very dense BZ sampling, due to the sharp q-dependence of the dielectric matrix in the long-wavelength limit (q → 0). In this work, we show how the convergence of the QP corrections of 2D semiconductors with respect to the BZ sampling can be drastically improved, by combining a Monte Carlo integration with an interpolation scheme able to represent the screened potential between the calculated grid points. The method has been validated by computing the band gap of three different prototype monolayer materials: a transition metal dichalcogenide (MoS2), a wide band gap insulator (hBN) and an anisotropic semiconductor (phosphorene). The proposed scheme shows that the convergence of the gap for these three materials up to 50meV is achieved by using k-point grids comparable to those needed by DFT calculations, while keeping the grid uniform
[Poesia] Três poemas de Alberto Secama
No full textThree poems by Alberto Secama. About the author: Alberto Secama is an Angolan poet who has poems published on many websites and on facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secamaTres poemas de Alberto Secama. Sobre el autor: Alberto Secama es un poeta angoleño que tiene poemas publicados en varios sitios y en el facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secamaTrês poemas de Alberto Secama. Sobre o autor: Alberto Secama é um poeta angolano que possui poemas publicados em vários sites e no facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secam
On the estimation of the concentration curve under complex sampling designs
No full textThis paper focuses on the estimation of the concentration curve of a finite population, when data are collected according to a complex sampling design with different inclusion probabilities. The asymptotic law of the finite population version of the concentration process is first studied. Then, a resampling scheme able to approximate such a law is constructed. Finally, an application to the construction of confidence bands is considered
Fundamental gaps of quantum dots on the cheap
No full textWe show that the fundamental gaps of quantum dots can be accurately estimated at the computational effort of a standard ground-state calculation supplemented with a non-self-consistent step of negligible cost, all performed within density-functional theory at the level of the local-density approximation
Orizzonti mantovani. Spunti e dinamiche paesaggistiche ne L'Illustrissimo di Alberto Cantoni
No full textIn the literary production of Alberto Cantoni, short story writer and novelist between the nineteenth and twentieth centuries, the novel L'Illustrissimo is highly important both because it is the last publication of the author, from Pomponesco, a small town a few kilometers south of Mantua, both because it summarizes in a single text the different nuances and different directions that his writing has taken over the course of his literary career, also due to a writing and processing time that embraces the entire span of years of his career itself. In the foreground, in addition to the numerous and brilliant characters, one of the protagonists is the Mantuan landscape which, not a simple background, becomes a true literary parameter which in different and significant ways affects the purposes and mechanisms of the novel
Plenary Session: Luis Alberto Urrea
No full texta) Plenary Session: Luis Alberto Urrea, Mexican American Chicago Writer
Luis Alberto Urrea is a 2005 Pulitzer Prize finalist for non-fiction and member of the Latino Literature Hall of Fame. He is a prolific and acclaimed writer who has used his dual-culture life border experiences to explore the complex and interconnected Mexican-US American reality. The critically acclaimed and best-selling Mexican-born author of 13 books, Urrea has won numerous awards for his poetry, fiction and essays.
Moderator: Dr. Héctor García, Loyola University Chicag
O duplo significado do sentir e do pensar nas obras de Alberto Caeiro e Descartes
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Comunicação e Expressão. Programa de Pós-Graduação em LiteraturaA pretensão desta dissertação intitulada O duplo significado do sentir e do pensar nas obras de Alberto Caeiro e Descartes é investigar a relação entre o sentir e o puro pensar, usando como referência a poesia O Guardador de Rebanhos, de Alberto Caeiro, que apresenta de forma evidente o tema; e como este assunto é tratado nas Meditações Metafísicas de René Descartes, também de forma claramente visível. Podemos perceber em ambos os autores, uma diplopia, uma espécie de duplo olhar sobre a relação sentir e puro pensar. No primeiro capítulo vou falar sobre a tese do duplo olhar de Descartes, a respeito da relação sentir e puro pensar na leitura de Merleau-Ponty, salientando a visão de natureza. No segundo capítulo, parto da leitura das três primeiras Meditações Metafísicas para caracterizar a tensão que se pode verificar entre Descartes e Alberto Caeiro quando se referem ao puro pensar. O foco principal de Descartes, nas três primeiras Meditações, é o pensar puro, e neste ponto Alberto Caeiro critica o pensar puro. No terceiro capítulo, analisando as três últimas Meditações, percebe-se que há uma virada no pensamento de Descartes, no qual o autor inverte seu ponto de vista, aproximando-se da idéia de Alberto Caeiro. No quarto capítulo, mostrarei a leitura que outros autores fazem acerca do sentir e do pensar na obra de Alberto Caeiro, mostrando que assim como Descartes, este também ostenta uma diplopia, não do sentir, mas do pensar. The pretension of this work entitled O duplo significado do sentir e do pensar nas obras de Alberto Caeiro e Descartes is to investigate the relationship between the feeling and the pure thinking, using as reference the poetry O Guardador de Rebanhos, by Alberto Caeiro, that presents in an evident way this theme; and as this subject is treated in René Descartes' Metaphysical Meditations, also in a clearly visible way. It is possible to notice in both authors, the diplopy, a sort of double look about the relationship between feeling and pure thinking. The thesis about Descartes'double look regarding the relation of feeling and pure thinking, in Merleau-Ponty reading, pointing out the nature vision, will be treated in the first chapter. In the second chapter, I start from the reading of the first three Metaphysical Meditations to characterize the tension that can be verified between Descartes and Alberto Caeiro when they refer to the pure thinking. Descartes' main focus in the first three Meditations is pure thinking and, in this point, Alberto Caeiro criticizes pure thinking. In the third chapter, analyzing the last three Meditations, it is possible to notice that there is an alteration in Descartes' thought, in which the author inverts his point of view, approaching of Alberto Caeiro's idea. In the fourth chapter, I will show the reading that other authors do concerning feeling and thinking in Alberto Caeiro's work, showing that as well as Descartes, this also shows a diplopy, not of feeling, but of thinking
Lobbying for change as a new theory and practice of active citizenship: author interview with Alberto Alemanno
No full textDo you believe that lobbying is a vehicle for representing the interests of some rather than all, meaning its actors can never be a force for good? Alberto Alemanno, author of Lobbying for Change: Find Your Voice to Create a Better Society, might convince you of the contrary. This interview with Barbara Aubin, LSE Public Affairs, reflects on the importance of engaging with our political representatives, holding them accountable and lobbying for the causes we deeply care about as fundamental elements of a healthy democracy
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