Optical properties of transition metal oxide perovskites by the Bethe-Salpeter equation

In questa tesi abbiamo eseguito uno studio sistematico, tramite un approccio computazionale ab-initio, delle proprietà ottiche di un insieme di quattordici diverse perovskiti. Questi quattordici materiali sono stati scelti in modo da formare un insieme rappresentativo delle perovskiti di metalli di transizione, ed includono differenti configurazioni elettroniche, ordinamenti magnetici e caratteristiche strutturali, con bandgap che variano da 0.1 eV a 6.1 eV. Lo studio è stato compiuto utilizzando un approccio basato sulla risoluzione dell'equazione di Bethe-Salpeter, dove le energie di quasi-particella e l'interazione schermata sono state ricavate da una precedente simulazione basata sull'approssimazione GW. Il lavoro svolto è finalizzato alla valutazione del ruolo e del contributo dell'interazione elettrone-lacuna sugli spettri dei materiali considerati. Per ogni perovskite abbiamo inoltre confrontato i risultati ottenuti con le misure sperimentali, laddove presenti, e con precedenti risultati numerici ottenuti nell'ambito dell' Approssimazione di Particelle Indipendenti (ottenute a partire da spettri GW). Infine abbiamo determinato l'origine delle strutture all'interno dello spettro, attribuendole a transizioni tra specifiche bande

Quasiparticle and excitonic properties of monolayer SrTiO3_3

Strontium titanate SrTiO$_3$ is one of the most studied and paradigmatic transition metal oxides. Recently, a breakthrough has been achieved with the fabrication of freestanding SrTiO$_3$ ultrathin films down to the monolayer limit. However, the many-body effects on the quasiparticle and optical properties of monolayer SrTiO$_3$ remain unexplored. Using state-of-the-art many-body perturbation theory in the GW approximation combined with the Bethe-Salpeter equation, we study the quasiparticle band structure, optical and excitonic properties of monolayer SrTiO$_3$. We show that quasiparticle corrections significantly alter the band structure topology; however, the widely used diagonal $G_0W_0$ approach yields unphysical band dispersions. The correct band dispersions are restored only by taking into account the off-diagonal elements of the self-energy. The optical properties are studied both in the optical limit and for finite momenta by computing the electron energy loss spectra. We find that the imaginary part of dielectric function at the long wavelength limit is dominated by three strongly bound excitonic peaks and the direct optical gap is associated to a bright exciton state with a large binding energy of 0.93 eV. We discuss the character of the excitonic peaks via the contributing interband transitions, and reveal that the lowest bound excitonic state becomes optical inactive for finite momenta along $\Gamma$-M, while the other two excitonic peaks disperse to higher energies and eventually merge for momenta close to M.Comment: 10 pages, 4 figure

Excitonic properties of transition metal oxide perovskites and workflow automatization of GW schemes

The Many-Body-Perturbation Theory approach is among the most successful theoretical frameworks for the study of excited state properties. It allows to describe the excitonic interactions, which play a fundamental role in the optical response of insulators and semiconductors. The first part of the thesis focuses on the study of the quasiparticle, optical and excitonic properties of \textit{bulk} Transition Metal Oxide (TMO) perovskites using a G$_0$W$_0$+Bethe Salpeter Equation (BSE) approach. A representative set of 14 compounds has been selected, including 3d, 4d and 5d perovskites. An approximation of the BSE scheme, based on an analytic diagonal expression for the inverse dielectric function, is used to compute the exciton binding energies and is carefully bench-marked against the standard BSE results. In 2019 an important breakthrough has been achieved with the synthesis of ultrathin SrTiO3 films down to the monolayer limit. This allows us to explore how the quasiparticle and optical properties of SrTiO3 evolve from the bulk to the two-dimensional limit. The electronic structure is computed with G0W0 approach: we prove that the inclusion of the off-diagonal self-energy terms is required to avoid non-physical band dispersions. The excitonic properties are investigated beyond the optical limit at finite momenta. Lastly a study of the under pressure optical response of the topological nodal line semimetal ZrSiS is presented, in conjunction with the experimental results from the group of Prof. Dr. Kuntscher of the Augsburg University. The second part of the thesis discusses the implementation of a workflow to automate G$_0$W$_0$ and BSE calculations with the VASP software. The workflow adopts a convergence scheme based on an explicit basis-extrapolation approach [J. Klimeš \textit{et al.}, Phys. Rev.B 90, 075125 (2014)] which allows to reduce the number of intermediate calculations required to reach convergence and to explicit estimate the error associated to the basis-set truncation

Il modello di Ising

L'elaborato fornisce una introduzione al modello di Ising, utilizzato nello studio delle transizioni di fase tra la fase ferromagnetica e quella paramagnetica dei materiali. Nella prima parte viene trattato il modello unidimensionale, di cui viene esposta la soluzione esatta attraverso l'utilizzo delle matrici di trasferimento, dimostrando quindi l'inesistenza di una transizione di fase a temperature finite non nulle. Vengono calcolate le funzioni termodinamiche e se ne dimostra l'indipendenza dalle condizioni al contorno nel limite termodinamico.Viene proposta infine una spiegazione qualitativa del comportamento microscopico, attraverso la lunghezza di correlazione. Nella seconda parte viene trattato il caso a due dimensioni. Inizialmente viene determinata la temperatura critica per reticoli quadrati, attraverso il riconoscimento della presenza di una relazione di dualita tra l'espansione per alte e per basse temperature della funzione di partizione. Successivamente si fornisce la soluzione esatta attraverso una versione modificata del procedimento, originariamente ideato da L.Onsager, di cui e proposta una traccia della dimostrazione. Viene infine brevemente discussa l'importanza che questo risultato ebbe storicamente nella fisica delle transizioni di fase

Optical and excitonic properties of transition metal oxide perovskites by the Bethe-Salpeter equation

We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of 14 compounds has been selected, including 3d (SrTiO3, LaScO3, LaTiO3, LaVO3, LaCrO3, LaMnO3, LaFeO3, and SrMnO3), 4d (SrZrO3, SrTcO3, and Ca2RuO4) and 5d (SrHfO3, KTaO3, and NaOsO3) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural, and magnetic properties. Optical conductivities and optical transitions including electron-hole interactions are calculated through the solution of the Bethe-Salpeter equation (BSE) with quasiparticle energies evaluated by the single-shot G0W0 approximation. The exciton binding energies are computed by means of a model BSE, carefully benchmarked against the full-BSE method, in order to obtain well-converged results in terms of k-point sampling. The predicted results are compared with available measured data, with an overall satisfactory agreement between theory and experiment

Are we ready for a paradigm shift from high-dose conventional to moderate hypofractionated radiotherapy in intermediate-high risk prostate cancer? A systematic review of randomized controlled trials with trial sequential analysis

Aim: to evaluate efficacy and late toxicity of moderate hypofractionated (HFRT) over high-dose (>76 Gy) conventional radiotherapy (CRT) in a non-inferiority perspective. Methods: Randomized controlled trials (RCTs) were included. HFRT regimens were deemed non-inferior to high-dose CRT if the computed CI for the overall RR did not exceed the non-inferiority margin of 7%. Results: When the prespecified margin, corresponding to a critical RR of 0.930 for CCS, OS and BFS, was used all efficacy outcomes satisfied the criteria for the non-inferiority analysis indicating the non-inferiority of HFRT regimens over high-dose CRT in the medium term period. Differently, the evidence concerning the late toxicity was inconclusive. Conclusions: Noninferiority analysis indicates that moderate HFRT regimes are non-inferior over high-dose CRT in the medium-term. Inconclusive is the evidence for the late toxicity. Longer follow-up will provide a more clear answer concerning the non-inferiority of HFRT regimens in the long-term period