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    Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites
    (Weinheim : Wiley-VCH, 2023) Rao, Shraddha M.; Kiligaridis, Alexander; Yangui, Aymen; An, Qingzhi; Vaynzof, Yana; Scheblykin, Ivan G.
    Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality.
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    Defect-Induced Magnetism in Nonmagnetic Oxides: Basic Principles, Experimental Evidence, and Possible Devices with ZnO and TiO2
    (Weinheim : Wiley-VCH, 2020) Esquinazi, Pablo David; Hergert, Wolfram; Stiller, Markus; Botsch, Lukas; Ohldag, Hendrik; Spemann, Daniel; Hoffmann, Martin; Adeagbo, Waheed A.; Chassé, Angelika; Nayak, Sanjeev K.; Ben Hamed, Hichem
    The magnetic moment and the magnetic order induced by localized defects, like vacancies, interstitials, and/or nonmagnetic (NM) ions, in a NM oxide atomic lattice are discussed. When the defect concentration is of the order of or larger than ≈3 at%, magnetic order at room temperature can appear. Herein, the theoretical basic principles needed to understand and compute this new magnetic phenomenon in solids are developed in detail. In particular, the main results of density functional theory (DFT) calculations are used to estimate the magnetization and X-ray magnetic circular dichroism (XMCD) values. The main experimental evidences on this phenomenon are reviewed, especially magnetization, the element-specific XMCD, and transport properties in two selected oxides, ZnO and TiO2. Emphasis is given on the simplicity and efficiency ion irradiation methods have to trigger magnetic order in these oxides as well as a very sensitive method to characterize magnetic impurities. Two possible applications of this phenomenon are discussed, namely spin filtering at magnetic/NM interfaces in ZnO and perpendicular magnetic anisotropy triggered in TiO2 anatase microstructures. The existing literature on defect-induced magnetism in oxides is shortly reviewed, which provides further evidence on the robustness of this phenomenon in solids.
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    Dynamical systems with multiple, long delayed feedbacks: Multiscale analysis and spatio-temporal equivalence
    (Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2015) Yanchuk, Serhiy; Giacomelli, Giovanni
    Dynamical systems with multiple, hierarchically long delayed feedback are introduced and studied. Focusing on the phenomenological model of a Stuart-Landau oscillator with two feedbacks, we show the multiscale properties of its dynamics and demonstrate them by means of a space-time representation. For sufficiently long delays, we derive a normal form describing the system close to the destabilization. The space and temporal variables, which are involved in the space-time representation, correspond to suitable timescales of the original system. The physical meaning of the results, together with the interpretation of the description at different scales, is presented and discussed. In particular, it is shown how this representation uncovers hidden multiscale patterns such as spirals or spatiotemporal chaos. The effect of the delays size and the features of the transition between small to large delays is also analyzed. Finally, we comment on the application of the method and on its extension to an arbitrary, but finite, number of delayed feedback terms.