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MOTT MATERIALS OUT-OF-EQUILIBRIUM: FROM QUANTUM SIMULATIONS TO CONTROL
Full text linkPhase transitions and competing orders in strongly correlated materials emerge from the delicate interplay of many interacting degrees of freedom (including charge, spin, and lattice). This intricate interplay makes these systems highly sensitive to external perturbations, making strongly correlated materials ideal for developing novel technologies and devices leveraging emergent phenomena. Their richness and technological potential, however, are counterbalanced by an inherent complexity originating from the strong intertwining of many degrees of freedom.
In the first part of this thesis, we study non-equilibrium phases in prototypical Mott insulators (LaVO3 and V2O3) induced by means of ultrashort light pulses or application of current, with the aim of tackling open challenges in developing strategies for controlling quantum materials.
When light excitations are employed, quantum coherence could be exploited to achieve enhanced functionalities and ultrafast and reversible manipulation of material properties. The light-induced excitonic population and decoherence dynamics is investigated in LaVO3, where long-range orders in the orbital and spin degrees of freedom strongly influence optical excitations and the evolution of excitonic states. By means of broadband pump-probe and two-dimensional electronic spectroscopy (2DES), we study how the interactions of the LaVO3 excitonic resonance with the ordered background influence the exciton spectral linewidth and decoherence time.
When current is instead employed to control the phase of Mott materials, resistive switching - a sudden drop in resistance caused by a transition from an insulating to a metallic state - can take place. By combining transport measurements with Photo-Electron Emission Microscopy, we image the resistive switching process in V2O3 at the nanoscale. On this length scale, V2O3 displays spatial inhomogeneities resulting from the breaking of the crystal symmetry upon transitioning from the high-temperature metallic phase to the low-temperature insulating one. This experiment provides novel insights into the nature and mechanisms of resistive switching, as well as the role of the nanometric texture of the material, suggesting novel viable routes to control the current-induced insulator-metal transition.
The second part of the thesis is dedicated to the quantum simulation of the physics of strongly correlated materials using artificial platforms. This approach aims to overcome the inherent complexity of quantum materials by employing systems where the phenomena typical of correlated systems can take place in a controlled way, with the relevant parameters that can be tuned on demand. We introduce synthetic lattices composed of lead halide perovskite nanocubes, which we propose as a suitable novel platform for quantum simulations. Pump-probe experiments on CsPbBr3 nanocube superlattices reveal the emergence of several phases relevant for strongly correlated materials (collective superradiant state, exciton gas and electron-hole liquid phases) that can be accessed upon controlling the excitation intensity, thus making the system a suitable platform for the investigation of long-range ordered phases in systems displaying insulator-metal Mott transitions. Nanocube superlattices of the hybrid organic-inorganic compound CH(NH2)2PbI3 are also investigated; 2DES is employed to trace the evolution of optical excitons in this artificial lattice, measure their decoherence time and address how the decoherence process is affected by the structural phase transition taking place in the system.In materiali fortemente correlati, transizioni di fase e la presenza ordini in competizione risultano dal delicato equilibrio tra molteplici gradi di libertà interagenti (tra cui carica, spin e reticolo). Questo complesso intreccio di interazioni rende tali sistemi altamente sensibili a perturbazioni esterne, rendendo i materiali fortemente correlati ideali per lo sviluppo di nuove tecnologie e dispositivi che sfruttano fenomeni emergenti. Tuttavia, la loro ricchezza e il loro potenziale tecnologico sono controbilanciati da una complessità intrinseca dovuta all'interazione tra i molti gradi di libertà.
Nella prima parte di questa tesi, studiamo fasi fuori equilibrio in isolanti di Mott prototipici (LaVO3 e V2O3) indotte mediante impulsi ultracorti di luce o applicazione di corrente, con l’obiettivo di affrontare problemi aperti nello sviluppo di strategie per il controllo dei materiali quantistici.
Quando si utilizzano impulsi di luce come eccitazione, la coerenza quantistica può essere sfruttata per ottenere funzionalità avanzate e una manipolazione ultrarapida e reversibile delle proprietà dei materiali. Le dinamiche della popolazione eccitonica indotta dalla luce e la sua decoerenza sono studiate in LaVO3, dove ordini a lungo raggio nei gradi di libertà orbitali e di spin influenzano fortemente le eccitazioni ottiche e l'evoluzione degli stati eccitonici. Tramite spettroscopia pump-probe e spettroscopia elettronica bidimensionale (2DES), studiamo come le interazioni della risonanza eccitonica di LaVO3 con l’ordine magnetico e orbitale influenzino la larghezza spettrale degli eccitoni e i tempi di decoerenza.
Quando invece si utilizza la corrente per controllare la fase dei materiali di Mott, può verificarsi il resistive switching, ovvero una rapida diminuzione della resistenza causata da una transizione dallo stato isolante a quello metallico. Combinando misure di trasporto e microscopia di emissione fotoelettrica (PEEM), abbiamo osservato il processo di resistive switching in V2O3 su scala nanometrica. Su questa scala spaziale, il V2O3 presenta inomogeneità spaziali derivanti dalla rottura della simmetria cristallina durante la transizione dalla fase metallica ad alta temperatura alla fase isolante a bassa temperatura. Questo esperimento offre nuove informazioni sulla natura e i meccanismi del resistive switching, oltre a evidenziare il ruolo della texture nanometrica del materiale, suggerendo nuove strategie per controllare la transizione isolante-metallo indotta dalla corrente.
La seconda parte della tesi è dedicata alla simulazione quantistica della fisica dei materiali fortemente correlati utilizzando piattaforme artificiali. Questo approccio mira a superare la complessità intrinseca dei materiali quantistici impiegando sistemi nei quali i fenomeni tipici dei sistemi correlati possono verificarsi in modo controllato, con i parametri rilevanti regolabili a piacimento. In questo lavoro, introduciamo reticoli sintetici composti da nanocubi di perovskiti, che proponiamo come nuova piattaforma per simulazioni quantistiche. Esperimenti pump-probe su superreticoli di nanocubi di CsPbBr3 rivelano l’emergere di diverse fasi rilevanti per i materiali fortemente correlati (stato collettivo superradiante, gas di eccitoni e liquido di elettroni e buche) accessibili controllando l’intensità dell’eccitazione, rendendo così il sistema una piattaforma idonea per l’indagine di fasi ordinate a lungo raggio in sistemi che presentano transizioni isolante-metallico di Mott. Anche i superreticoli di nanocubi del composto ibrido organico-inorganico CH(NH2)2PbI3 sono stati investigati; la spettroscopia bidimensionale 2DES è stata utilizzata per tracciare l’evoluzione degli eccitoni ottici in questo reticolo artificiale, misurare il loro tempo di decoerenza e analizzare come il processo di decoerenza sia influenzato dalla transizione di fase strutturale del sistema
Mott materials: unsuccessful metals with a bright future
Full text linkAchieving the full understanding and control of the insulator-to-metal transition in Mott materials is key for the next generation of electronics devices, with applications ranging from ultrafast transistors, volatile and non-volatile memories and artificial neurons for neuromorphic computing. In this work, we will review the state-of-the-art knowledge of the Mott transition, with specific focus on materials of relevance for actual devices, such as vanadium and other transition metal oxides and chalcogenides. We will emphasize the current attempts in controlling the Mott switching dynamics via the application of external voltage and electromagnetic pulses and we will discuss how the recent advances in time- and space-resolved techniques are boosting the comprehension of the firing process. The nature of the voltage/light-induced Mott switching is inherently different from what is attainable by the slower variation of thermodynamic parameters, thus offering promising routes to achieving the reversible and ultrafast control of conductivity and magnetism in Mott nanodevices
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
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