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1
Madaras, Scott. « Insulator To Metal Transition Dynamics Of Vanadium Dioxide Thin Films ». W&M ScholarWorks, 2020. https://scholarworks.wm.edu/etd/1616444322.
Texte intégralRésumé :
Vanadium Dioxide (VO2) is a strongly correlated material which has been studied for many decades. VO2 has been proposed for uses in technologies such as optical modulators, IR modulators, optical switches and Mott memory devices. These technologies are taking advantage of VO2’s insulator to metal transition (IMT) and the corresponding changes to the optical and material properties. The insulator to metal transition in VO2 can be accessed by thermal heating, applied electric field, or ultra-fast photo induced processes. Recently, thin films of VO2 grown on Titanium Dioxide doped with Niobium (TiO2:Nb), have shown promise as a possible UV photo detector with high quantum efficiency which utilizes a heterostructure between these two materials. In this work, the dynamics of the IMT on thin films of VO2 is explored. We show that surface plasmons generated in an Au thin film can induce the insulator to metal transition in a thin film of VO2 due to the enhanced electric field as well as help detect the IMT via changes in its resonance condition. Time resolved pump probe studies were also done on thin films of VO2 grown on TiO2 and TiO2:Nb, using UV photon energy of 3.1 eV (400nm wavelength). The fluence threshold of the IMT at 3.1 eV was significantly lower than published values for the 1.55 eV pump fluence. The time response of the IMT shows uncommon reflectivity dynamics in these samples. The response was partially attributed to internal interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2, and can be elucidated by a diffusion model with respect to its optical properties. Finally, the photocurrent generation time constants for the sample with highest quantum efficiency are given and compared to its ultrafast photo induced IMT time constants.
2
Proskuryakov, Yuri. « Interactions, localisation and the metal to insulator transition in two-dimensional semiconductor systems ». Thesis, University of Exeter, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.288367.
Texte intégral
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Liu, Mengkun. « Ultrafast far-infrared studies of Vanadates - multiple routes for an insulator to metal transition ». Thesis, Boston University, 2012. https://hdl.handle.net/2144/12484.
Texte intégralRésumé :
Thesis (Ph.D.)--Boston University
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.The metal insulator transition in vanadates has been studied for decades and yet new discoveries still spring up revealing new physics, especially among two of the most studied members: Vanadium sesquioxide (V203) and Vanadium dioxide (V02). Although subtleties abound, both of the materials have first order insulator to metal phase transitions that are considered to be related to strong electron-electron (e-e) correlation. Further, ultrafast spectroscopy of strongly correlated materials has generated great interest in the field given the potential to dynamically distinguish the difference between electronic (spin) response versus lattice responses due to the associated characteristic energy and time scales. In this thesis, I mainly focus on utilizing ultrafast optical and THz spectroscopy to study phase transition dynamics in high quality V203 and V02 thin films epitaxially grown on different substrates. The main findings of the thesis are: (1) Despite the fact that the insulator to metal transition (IMT) in V203 is electron-correlation driven, lattice distortion plays an important role. Coherent oscillations in the far-infrared conductivity are observed resulting from coherent acoustic phonon modulation of the bandwidth W. The same order of lattice distortion induces less of an effect on the electron transport in V02 in comparison to V203. This is directly related to the difference in latent heat of the phase transitions in V02 and V203. (2) It is possible for the IMT to occur with very little structural change in epitaxial strained V02 films, like in the case of Cr doped or strained V203. However, in V02, this necessitates a large strain which is only possible by clamping to a substrate with larger c axis parameter through epitaxial growth. This is demonstrated for V02 films on Ti02 substrates. (3) Initiating an ultrafast photo-induced insulator-to-metal transition (IMT) is not only possible with above bandgap excitation, but also possible with high-field far-infrared excitation. With the help of the field enhancement in metamaterial split ring resonator gaps, we obtain picosecond THz electric field transients of several MVIem which is sufficient to drive the insulator to metal transition in V02.
4
Spitzig, Alyson. « The importance of Joule heating on the voltage-triggered insulator-to-metal transition in VO₂ ». Thesis, University of British Columbia, 2017. http://hdl.handle.net/2429/62808.
Texte intégralRésumé :
The large change in resistivity in the material VO₂ has attracted considerable attention since it was first discovered in 1959. Recently, the ability to trigger the insulator-to-metal transition (IMT) with a strong electric field has been observed, but there has been debate about whether the transition is due to field-effects. We apply a voltage bias across a VO₂ thin film via a conductive atomic force microscope (CAFM) tip and measure the resultant current. We observe the IMT as a jump in the measured current in the IV curves. We fit the IV curves to the Poole-Frenkel (PF) conduction mechanism in the insulating state, immediately preceding the IMT. The PF conduction mechanism describes the thermal excitation of electrons into the conduction band in insulators, facilitated by strong electric fields. The PF mechanism is temperature dependent, and we use the temperature dependence to calculate the local temperature of the film before the transition. We directly compare the local electric field and local temperature of the film immediately preceding the IMT. We determine that the transition is not solely due to the applied electric field, but rather that the tip has locally warmed the film close to its IMT temperature through Joule heating.Science, Faculty of
Physics and Astronomy, Department of
Graduate
5
Weerasinghe, Hasitha C. « Electrical characterization of metal-to-insulator transition in iron silicide thin films on sillicone substrates ». [Tampa, Fla] : University of South Florida, 2006. http://purl.fcla.edu/usf/dc/et/SFE0001677.
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Watson, Deborah Lee. « Quantum interference effects in the magnetoresistance of semiconductor structures near the metal to insulator transition ». Thesis, University of Exeter, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286547.
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O'Neal, Jared. « A Numerical Study of a Disorder-driven 2D Mott Insulator-to-Metal Quantum Phase Transition ». The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492701913534985.
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8
Read, Daniel Edward. « Electrical and magnetic properties of n-Cd(_1-x)Mn(_x) Te close to the metal-insulator transition ». Thesis, Durham University, 2001. http://etheses.dur.ac.uk/3783/.
Texte intégralRésumé :
Electrical transport and magnetic measurements have been made on n-Cd(_1-x)Mn(_x) Te (0.047 < X < 0.197) for samples doped with hi or In,Al. Results are presented for measurements made as a function of temperature (40 mK < T < 300 K), applied magnetic field (0 T < B < 6 T) and photogenerated carrier density. Low field magnetic susceptibility measurements have identified a transition from a paramagnetic phase to a spin glass phase at low temperatures. The measured temperature dependence of the magnetic susceptibility is consistent with a cluster glass model. Measurements of the spin glass freezing temperature have been carried out on four samples having different Mn fractions The results obtained are in agreement with the limited number of previous measurements on nominally undoped samples. At low temperatures photo-induced changes in magnetism have been measured in both the paramagnetic and the spin glass phase. The change in magnetisation on illumination is due to an increased number of bound magnetic polarons formed around quasi localised s-spins. High temperature susceptibility measurements have been used to examine the parameters characterising the paramagnetic phase, and their variation with Mn fraction. Electrical transport measurements at very low temperatures (T < 800 mK) have shown a strong temperature dependent electron localisation. This has resulted in the confirmation of insulating behaviour in a sample (x = 0.047) having n. At low temperatures and in zero field an activated form of the conductivity is observed. In applied magnetic fields (B > 50 mT) Efros-Shklovskii variable range hopping is observed in the insulating phase. These results are attributed to the formation of a hard gap in the density of states, having a magnetic origin. At higher fields an insulator-metal phase transition occurs. In the metallic phase the conductivity can be described by a quantum correction to the zero temperature conductivity due to the effect of electron-electron interactions. Results obtained before and after illumination are consistent with scaling theory of electron localisation, having a critical exponent close to unity, indicative of the importance of electron-electron interactions. A reduction in the value of the critical field is seen after increasing the carrier density (B(_c) = 2.0 and 1.3 T for n = 3.3 xl0(^17)cm(^-3) and 3.8 x10(^17) cm(^-3) respectively).At low temperatures an anisotropy in the resistivity has been measured for samples in the spin glass phase. Magnetoresistance measurements have shown results consistent with previous measurements, in addition to a large, low field component that is attributed to the effect of magnetic field on electrons in the variable range hopping regime.
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Humbert, Vincent. « Etude des états fondamentaux dans des systèmes supraconducteurs désordonnés de dimension 2 ». Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS149/document.
Texte intégralRésumé :
Un matériau 3D, initialement supraconducteur, peut avoir différents états fondamentaux dépendamment de son degré de désordre : supraconducteur, métallique ou isolant. A dimension réduite (2D), la localisation d’Anderson interdit théoriquement tout état métallique. La modification du désordre induit alors une Transition directe Supraconducteur-Isolant (TSI). La présence de fortes interactions électroniques, non prises en compte dans les théories conventionnelles, pourrait cependant remettre en cause ce paradigme et laisser émerger des états métalliques 2D, complexifiant l’image généralement admise de la TSI. Ainsi, des travaux récents ont révélé la présence de deux phases métalliques distinctes dans les films minces de a-NbxSi1-x, s’intercalant entre les états supraconducteur et isolant.Durant cette thèse, nous avons étudié les propriétés de transport électronique à basse fréquence et à très basse température (T<1K) de films minces de NbxSi1-x amorphes afin de caractériser l’évolution de l’état fondamental en fonction du désordre. Celui-ci a été modifié dans nos films en jouant sur la température de recuit, l’épaisseur et la composition. Nous nous sommes alors attardés sur la destruction de ces états métalliques vers un état isolant. L’analyse des lois de conduction dans le régime isolant nous a permis de quantifier l’évolution de ses propriétés – notamment des énergies caractéristiques – en fonction du désordre. Nous avons alors pu conclure que la phase isolante pouvait être essentiellement décrite par un modèle fermionique. A moindre désordre, dans la phase métallique 2D adjacente à l’isolant, nous avons mis en évidence des signes précurseurs de l’état isolant qui évoluent continument jusqu’à et à travers la Transition Métal 2D-Isolant. Nous proposons une interprétation de l’ensemble de nos résultats impliquant deux canaux parallèles dont l’importance relative est déterminée par le désordre : l’un est fermionique, l’autre gouverné par des fluctuations supraconductrices qui persistent même lorsque la cohérence macroscopique a disparu. L’état métallique est alors dominé par ces dernières, alors que, dans l’isolant, la localisation des excitations fermioniques l’emporte.Une seconde partie de la thèse s’est concentrée sur le développement expérimental d’un dispositif de calibration large bande dédié à l’étude de films minces en réflectométrie haute fréquence (GHz) à basses températures (T<4K). Ce dispositif a pour but, lors d’une unique mise à froid, de mesurer successivement la réflexion de références connues ainsi que de l’échantillon. La calibration obtenue permet ainsi de s’affranchir de l’environnement micro-onde et d’obtenir la valeur absolue de l’impédance complexe de ces films. Les résultats obtenus sur des films minces supraconducteurs de Vanadium, comparés aux théories de la supraconductivité, permettent une première validation du dispositif et de son principe de fonctionnement en vue d’une utilisation sur des systèmes plus complexes, tels que les films minces proches de la TSIAn initially superconducting 3D material can have different ground states, depending on its disorder : superconducting, metallic or insulating. At lower dimensionality, Anderson localization theoretically forbids any metallic state. A change in disorder then induces a direct Superconductor-to-Insulator Transition (SIT). The presence of strong Coulomb interactions, which are not taken into account in conventional theories, may disrupt this paradigm and enable the emergence of 2D metallic phases, thus complicating the generally admitted picture for the SIT. Indeed, recent work has revealed the existence of two distinct metallic phases in a-NbxSi1-x thin films, in between the superconducting and insulating states.During this work, we have studied the low frequency transport properties of amorphous NbxSi1-x films at low temperatures (T<1K), in order to characterize the evolution of their ground state with disorder. In our films, disorder has been tuned by varying the heat treatment temperature, the thickness or the composition. We have then focused on the destruction of these metallic states, giving rise to an insulating state. Through the analysis of conduction laws in the insulating regime, we have quantified the evolution of its properties – in particular its characteristic energies – as disorder is varied. We could then conclude that the insulating phase can essentially be accounted for by a fermionic model. At lower disorder level, in the 2D metallic phase neighboring the insulator, we have evidenced precursor signs of the insulating state which continuously evolve until and through the 2D Metal-to-Insulator Transition. We offer an interpretation of all our results implying the existence of two parallel channels which relative importance is determined by the sample disorder level : one is fermionic, the other governed by superconducting fluctuations which persist even when the macroscopic phase coherence is lost. The metallic state is then dominated by the latter, whereas, in the insulator, fermionic excitations prevail.In a second part, we report on the experimental development of a calibration device for the broadband reflectometry measurement of thin films at microwave frequencies (GHz) and low temperatures (T<4K). This apparatus aims at measuring, during a single cool down, the reflection of known references as well as of the sample. The obtained calibration enables to obtain the absolute value of the films complex impedance, independently of the microwave environment. The results obtained on superconducting Vanadium films, compared with theories of superconductivity, enabled a first validation of the setup and of its working principle. This calibration device is therefore operational to measure more complex systems, such as thin films in the vicinity of the SIT
10
Golalikhani, Maryam. « Structure and electronic properties of atomically-layered ultrathin nickelate films ». Diss., Temple University Libraries, 2015. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/353844.
Texte intégralRésumé :
PhysicsPh.D.
This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials.
Temple University--Theses
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Seidemann, Johanna. « Iontronic - Étude de dispositifs à effet de champ à base des techniques de grilles liquides ioniques ». Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY075/document.
Texte intégralRésumé :
Les liquides ioniques sont des fluides non volatiles, constitués de cations et d’anions, qui sont conducteurs ioniques, isolants électriques, et peuvent avoir des valeurs de capacité très élevées. Ces liquides sont susceptibles non seulement de remplacer les électrolytes solides, mais également de susciter des champs électriques intenses (>SI{10}{megavoltpercentimetre}) au sein d’une couche dite double couche électronique (electric double layer, EDL) à l’interface entre le liquide et le matériau sur lequel il est déposé. Ceci conduit à une injection de porteurs de charge bidimensionelle avec des densités allant jusqu’à SI{e15}{cm^{-2}}. Cet effet de grille remarquablement fort des liquides ioniques est réduit en présence d’états piégés ou de rugosité de surface. À cet égard, les dicalchogénures de métaux de transitions, de très haute qualité cristalline et atomiquement plats, font partis des semi-conducteurs les plus adaptés aux grilles EDL.Nous avons réalisé des transistors à effet de champ avec des EDL dans des nanotubes multi-couches de ce{WS2}, avec des performances comparables à celles de transistors EDL sur des ilots de ce{WS2}, et meilleurs que celles de nanotubes de ce{WS2} avec une grille solide. Nous avons obtenu des mobilités allant jusqu’à SI{80}{squarecentimetrepervoltpersecond} pour les porteurs n et p, et des ratios de courants on/off dépassant SI{e5}{} pour les deux polarités. Pour de forts dopages de type électron, les nanotubes ont un comportement métallique jusqu’à basse température. De plus, utiliser un liquide ionique permet de créer une jonction pn de manière purement électrostatique. En prenant avantage de cet effet, nous avons pu réaliser un transistor photoluminescent dans un nanotube.La possibilité de susciter de très forte densités de charges donne la possibilité d’induire des phases métalliques ou supraconductrices dans des semi-conducteurs a large bande interdite. Nous avons ainsi réussi à induire par effet de champ une phase métallique à basse température dans du diamant intrinsèque avec une surface hydrogénée, et nous avons obtenu un effet de champ dans du silicone dopé métallique.Les liquides ioniques offrent beaucoup d’avantages, mais leur champ d’application est encore réduit par l’instabilité du liquide, ainsi que par les courants de fuites et l’absorption graduelle d’impuretés. Un moyen efficace de s’affranchir de ces inconvénients, tout en conservant la possibilité d’induire de très fortes densités de porteurs, est de gélifier le liquide ionique. Nous sommes allés plus loin en fabriquant des gels ioniques modifiés, avec les cations fixés sur une seule surface et les anions libres de se mouvoir au sein du gel. Cet outil nous a permis de réaliser une nouvelle diode à effet de champ de faible puissanceIonic liquids are non-volatile fluids, consisting of cations and anions, which are ionically conducting and electrically insulating and hold very high capacitances. These liquids have the ability to not only to replace solid electrolytes, but to create strongly increased electric fields (>SI{10}{megavoltpercentimetre}) in the so-called electric double layer (EDL) on the electrolyte/channel interface, which leads to the injection of 2D charge carrier densities up to SI{e15}{cm^{-2}}. The remarkably strong gate effect of ionic liquids is diminished in the presence of trapped states and roughness-induced surface disorder, which points out that atomically flat transition metal dichalcogenides of high crystal quality are some of the semiconductors best suited for EDL-gating.We realised EDL-gated field-effect transistors based on multi-walled ce{WS2} nanotubes with operation performance comparable to that of EDL-gated thin flakes of the same material and superior to the performance of backgated ce{WS2} nanotubes. For instance, we observed mobilities of up to SI{80}{squarecentimetrepervoltpersecond} for both p- and n-type charge carriers and our current on-off ratios exceed SI{e5}{} for both polarities. At high electron doping levels, the nanotubes show metallic behaviour down to low temperatures. The use of an electrolyte as topgate dielectric allows the purely electrostatic formation of a pn-junction. We successfully fabricated a light-emitting transistor taking advantage of this utility.The ability of high charge carrier doping suggests an electrostatically induced metal phase or superconductivity in large gap semiconductors. We successfully induced low temperature metallic conduction into intrinsic diamond with hydrogen-terminated surface via field-effect and we observed a gate effect in doped, metallic silicon.Ionic liquids have many advantageous properties, but their applicability suffers from the instability of their liquid body, gate leakage currents and absorption of impurities. An effective way to bypass most of these problems, while keeping the ability of ultra-high charge carrier injection, is the gelation of ionic liquids. We even went one step further and fabricated modified ion gel films with the cations fixed on one surface and the anions able to move freely through the film. With this tool, we realised a novel low-power field-effect diode
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Meling, Artur. « Scattering of vibrationally excited NO from vanadium dioxide ». Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2020. http://hdl.handle.net/21.11130/00-1735-0000-0005-12F9-E.
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Masunaga, Sueli Hatsumi. « Preparação e caracterização de manganitas (La,Pr)CaMnO ». Universidade de São Paulo, 2005. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-04012011-123031/.
Texte intégralRésumé :
Amostras policristalinas de La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; foram produzidas pelo método da mistura estequiométrica de óxidos e tratadas termicamente ao ar a 1400 oC. As amostras foram caracterizadas através de medidas de difração de raios-X, resistividade elétrica rho(T), susceptibilidade magnética chi(T) e magnetorresistividade rho(T, H = 50 kOe). Os resultados das análises dos diagramas de raios-X indicaram que os materiais são de fase única e que houve uma substituição efetiva de La por Pr no sítio A ao longo da série. Medidas de rho(T) e chi(T) revelaram que a temperatura de transição de fase metal-isolante TMI e temperatura de Curie TC decrescem com o aumento da concentração y e que a resistividade residual rho0 (rho(T = 10 K)) é consideravelmente alta em amostras com y 0.35. Ainda, com o decréscimo de T, as amostras com y 0.35 transitam para uma fase de ordenamento de carga em TOC ~ 194 K e, em seguida, para uma fase metálica em TMI. Essas medidas também sugerem a coexistência de fases ferromagnética-metálica FMM e de ordenamento de carga isolante OCI nesses materiais. Nas propriedades físicas macroscópicas, a fase FMM mostrou ser a dominante para os compostos com pequenas concentrações de Pr (y 0.25) e a fase OCI dominante para os compostos com altas concentrações de Pr (y 0.40). As medidas de rho(T, H = 50 kOe) mostram que a magnitude da resistividade elétrica decresce drasticamente nas vizinhanças de TMI sob a aplicação de um campo magnético externo. A magnitude de MR (MR = (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe)) entre os extremos da série (y = 0 e 0.625) varia até sete ordens de grandeza, sendo que o máximo valor de MR para amostras com y = 0 é de ~ 0.75 e naquelas com y = 0.625 é ~ 3.4x106 . O diagrama de fases deste composto evidencia uma região crítica (0.30 y 0.40) onde os valores de TMI, TC, MR e 0 variam abruptamente como função de y, sendo que em outras regiões tal variação é mais suave. A variação significativa desses quatro valores indica uma competição mais forte entre as fases coexistentes ocorre na região crítica. Algumas características marcantes podem ser observadas nas amostras da região crítica tais como: a presença de um segundo pico, abaixo de TMI, em ~ 90 K e ~ 72 K na curva de rho(T) de amostras com y = 0.30 e 0.35, histerese térmica mais pronunciada em rho(T) e chi(T), MR torna-se colossal, relaxação significativa da resistividade elétrica com o tempo, entre outras. Assim, as propriedades de transporte e magnéticas nessa região crítica são dominadas pela forte competição entre as fases coexistentes.Polycrystalline samples of La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; were synthesized by the solid-state reaction method and sintered in air at 1400 oC. These compounds were studied by measurements of X-ray powder diffraction, magnetic susceptibility chi(T), and electrical resistivity rho(T, H). X-ray powder diffraction measurements indicated single phase materials and an effective substitution of La by Pr. Results from rho(T) and chi(T) revealed that increasing y in this series results in a rapid reduction of both the insulator to metal transition temperature TMI and the Curie temperature TC. Such a rapid decrease in TMI with increasing y is also accompanied by the occurrence of a new transition temperature, termed TCO, which is related to the transition to the charge ordered CO state. Such a temperature, which is essentially independent of y, occurs at TCO ~ 194 K and is mainly observed in samples with y 0.35. The other feature is the presence of a large residual resistivity electrical rho(0 = (10 K)) for large y (y 0.35) at low-T even though rho(T) suggests a metallic behavior below TMI. The temperature for the maximum magnetoresistance effect occurs near TMI, that shifts to higher T with increasing field. The MR is defined here as (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe) and is enhanced by seven orders of magnitude from ~ 0.75 up to ~ 3.4x106 in samples with y = 0 and y = 0.625, respectively. Some features like the thermal hysteresis observed in both rho(T) and chi(T) curves indicate the coexistence of different phases in a range of y concentration, i. e., the ferromagnetic-metallic FMM and the charge ordered-insulating COI domains. The FMM is stable for y 0.25, but the COI state becomes dominant for y 0.40. There is a critical region in the phase diagram, ranging from y = 0.30 to 0.40, where the magnitude of the TMI, TC, MR, and 0 were found to display abrupt changes with increasing y. Some anomalous features like a second peak in rho(T) below TMI, a two-step increasing in chi(T), a colossal MR effect and others are observed for compositions belonging to this critical region. Our combined data suggest that the general physical properties of these compounds in such a critical region are dominated by the strong competition between coexisting ferromagnetic-metallic and charge ordered-insulating phases.
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Lange, Matthias Michael [Verfasser], et Dieter [Akademischer Betreuer] Kölle. « A High-Resolution Polarizing Microscope for Cryogenic Imaging : Development and Application to Investigations on Twin Walls in SrTiO3 and the Metal-Insulator Transition in V2O3 / Matthias Michael Lange ; Betreuer : Dieter Kölle ». Tübingen : Universitätsbibliothek Tübingen, 2018. http://d-nb.info/1174583703/34.
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Doyle, Suzanne Martha. « Phase transitions in low dimensional materials close to the metal-insulator boundary ». Thesis, University of Bristol, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.358783.
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Nájera, Ocampo Oscar. « Study of the dimer Hubbard Model within Dynamical Mean Field Theory and its application to VO₂ ». Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS462/document.
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J'étudie en détail la solution d'un modèle simplifié d'électrons fortement corrélés, à savoir le modèle de Hubbard dimérisé. Ce modèle est la réalisation la plus simple d'un problème de cluster DMFT. Je fournis une description détaillée des solutions dans une région de coexistence où l'on trouve deux états (méta) stables des équations DMFT, l'un métallique et l'autre isolant. De plus, je décris en détail comment ces états disparaissent à leurs lignes critiques respectives. Je clarifie le rôle clé joué par la corrélation intra-dimère, qui agit ici en complément des corrélations de Coulomb.Je passe en revue la question importante du passage continue entre unisolant Mott et un isolant Peierls où je caractérise une variété de régimes physiques. Dans un subtil changement de la structure électronique, lesbandes de Hubbard évoluent des bandes purement incohérentes (Mott) à desbandes purement cohérentes (Peierls) à travers un état inattendu au caractère mixte. Je trouve une température d'appariement singulet T* en-dessous de laquelle les électrons localisés à chaque site atomique peuvent se lier dans un singulet et minimiser leur entropie. Ceci constitue un nouveau paradigme d'un isolant de Mott paramagnétique.Enfin, je discute la pertinence de mes résultats pour l'interprétation de différentes études expérimentales sur VO₂. Je présente plusieurs arguments qui me permettent d'avancer la conclusion que la phase métallique, à vie longue (métastable) induite dans les expériences pompe-sonde, et l'état métallique métastable M₁, thermiquement activé dans des nano-domaines, sont identiques. De plus, ils peuvent tous être qualitativement décrits, dans le cadre de notre modèle, par un métal corrélé dimériséWe study in detail the solution of a basic strongly correlated model,namely, the dimer Hubbard model. This model is the simplest realization ofa cluster DMFT problem.We provide a detailed description of the solutions in the ``coexistentregion'' where two (meta)stable states of the DMFT equations are found, onea metal and the other an insulator. Moreover, we describe in detail howthese states break down at their respective critical lines. We clarify thekey role played by the intra-dimer correlation, which here acts in additionto the onsite Coulomb correlations.We review the important issue of the Mott-Peierls insulator crossoverwhere we characterize a variety of physical regimes. In a subtle change inthe electronic structure the Hubbard bands evolve from purely incoherent(Mott) to purely coherent (Peierls) through a state with unexpected mixedcharacter. We find a singlet pairing temperature T* below which thelocalized electrons at each atomic site can bind into a singlet and quenchtheir entropy, this uncovers a new paradigm of a para-magnetic Mottinsulator.Finally, we discuss the relevance of our results for the interpretation ofvarious experimental studies in VO₂. We present a variety of argumentsthat allow us to advance the conclusion that the long-lived (meta-stable)metallic phase, induced in pump-probe experiments, and the thermallyactivated M₁ meta-stable metallic state in nano-domains are the same.In fact, they may all be qualitatively described by the dimerizedcorrelated metal state of our model
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Le, Bourdais David. « Microcapteurs de pression à base de manganites épitaxiées ». Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112021/document.
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Les oxydes sont des matériaux complexes possédant une physique riche et toujours au centre de nombreuses recherches. Parmi ces oxydes, les manganites ont retenu notre attention car ils présentent une transition métal-isolant abrupte en température, générant un très fort coefficient en température en conditions d’environnement standards. L’objectif de ce travail est de démontrer que ce fort coefficient peut être exploité pour l’amélioration des performances des jauges de pression de type Pirani qui subissent un certain essoufflement dans leur développement. La voie menant à l’aboutissement d’une telle jauge à base d’oxydes pose en revanche un certain nombre de limites technologiques à lever et auxquelles nous avons répondu. La première de ces limites concerne l’intégration des oxydes monocristallins sur silicium, que nous avons reproduite et étendue au cas des substrats de type SOI et GaAs. Nos procédés proposent de passer par deux techniques, l’épitaxie par jets moléculaire et l’ablation laser, pour assurer une croissance optimale de nos films sur ces substrats et d’assurer la reproductibilité de leur réponse en température, notamment la position de leur température de transition en accord avec l’état de l’art. L’épitaxie de ces oxydes génère un niveau de contrainte non négligeable qui n’a jamais été mesuré. En concevant divers dispositifs autosupportés, et en s’appuyant sur les considérations théoriques et des modélisations par éléments finis, nous avons pu quantifier la relaxation de cette contrainte importante et assurer près de 100% de reproductibilité des systèmes suspendus. Ces mêmes systèmes nous permettent de caractériser pour la première fois le facteur de jauge des manganites monocristallines par l’application d’une contrainte contrôlée par nanoindentation. Il est également démontré qu’ils constituent des jauges de pression Pirani à la sensibilité accrue de deux ordres de grandeur pour une consommation en puissance réduite. Des solutions permettant d’améliorer l’ensemble des aspects de ces jauges sont étudiéesFunctional perovskite oxides are of great interest for fundamental and applied research thanks to the numerous physical properties and inherent mechanisms they display. With the maturation of thin film deposition techniques, research teams are able to reproduce oxide films and nanostructures of great crystalline quality with some of the most remarkable properties found in physics, a state leading now to upper-level thoughts like their ability to fulfill industrial needs. This thesis work is an answer to some of the problematics that arise when considering the oxide transition from the research to the industrial world, by focusing on their integration for micromechanical devices (MEMS) such as sensors. In order to ease the access to MEMS manufacturing, it is of importance to allow the deposition of thin oxide films on semiconductor substrates. A first study show that these access bridges can be crossed when using appropriate buffer layers such as SrTiO3 deposited on Silicon or gallium arsenide – produced in close collaboration with INL by Molecular Beam Epitaxy - and yttria-stabilized zirconia directly grown on silicon by pulsed laser deposition, which adapts the surface properties of the substrate to perovksite-based materials. Formation of thin epitaxial and monocristalline films of functional oxides is thus allowed on such buffer layers. As an example, characterization of two mixed-valence manganites La0.80Ba0.20MnO3 and La0.67Sr0.33MnO3 demonstrates that both materials are of excellent crystalline quality on these semiconducting substrates and that their physical characteristics match the one found on classical oxide substrates like SrTiO3. Stress evolution in thin films, which has a major effect in epitaxial materials, is then addressed to quantify its impact on oxide microstructure viability. This work gives an identification of the most significant factors favoring stress generation in the case of the films we produced. Then, based on the deformation measurement of free-standing cantilevers made of manganites on pseudo-substrates, and with the support of appropriate analytical models, a new state of equilibrium is established, giving new information about the evolution of static stress from deposition to MEMS device manufacturing. Solutions to manage their reproducibility is then studied. From another perspective, free-standing microstructures made of monocristaline manganites were used to display the effect of dynamical strain on their electrical resistivity (piezoresistivity) and their inherent structures.Finally, a specific example of the capabilities of reproducible free-standing microbridges made of manganites is presented through the conception of a pressure gauge based on Pirani effect. Indeed, it is shown that the abrupt resistivity change this material exhibits near their metal-to-insulating transition creates high temperature coefficients in standard application environments that can be taken as an advantage to improve the sensibility and power consumption of such gauges whose development had significantly slowed down over the past years. A set of improvements on their sensitivity range and their signal acquisition is also presented. Combined to a specific and innovative package, it is also demonstrated that Pirani gauge capabilities can be enhanced and that the complete devices fulfill embedded application requirements
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FRANCESCHINI, PAOLO. « NOVEL SCHEMES FOR ULTRAFAST MANIPULATION OF QUANTUM MATERIALS ». Doctoral thesis, Università Cattolica del Sacro Cuore, 2022. http://hdl.handle.net/10280/111822.
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La possibilità di controllare le proprietà elettroniche on-demand su una scala di tempo ultraveloce rappresenta una delle sfide più intriganti verso la realizzazione di dispositivi fotonici ed elettronici di nuova generazione. Stimolata da questo, negli ultimi decenni la ricerca scientifica ha concentrato la propria attenzione su diverse piattaforme a stato solido. Tra tutte, nanostrutture dielettriche (e metamateriali) e materiali correlati si presentano come i più promettenti candidati per la realizzazione di dispositivi dotati di nuove funzionalità. Al di là delle caratteristiche specifiche che rendono i dielettrici più adatti ad applicazioni in fotonica e i materiali correlati ai dispositivi elettronici, entrambe le categorie manifestano nuove funzionalità se soggetti ad uno stimolo esterno sotto forma di impulsi di luce con durata più breve della scala di tempo caratteristica del rilassamento dei gradi di libertà interni al sistema. Infatti, lo stato fuori equilibrio raggiunto a seguito di una foto-eccitazione presenta proprietà elettroniche ed ottiche di gran lunga differenti da quelle all'equilibrio. Pertanto, l'obiettivo di questo lavoro di tesi consiste nello sviluppo di nuovi metodi ed approcci sperimentali in grado di indurre, misurare e controllare nuove funzionalità in materiali complessi su una scala di tempo ultraveloce.The possibility to control the electronic properties on-demand on an ultrafast time scale represents one of the most exciting challenges towards the realization of new generation photonic and electronic devices. Triggered by this, in the last decades the research activity focused its attention to different solid-state platforms. Among all, dielectric nanostructures (and metamaterials) and correlated materials represent the most promising candidate for the implementation of devices endowed by new functionalities. Apart from the specific features making dielectrics more suitable for photonic applications and correlated materials for electronic devices, both categories exhibit new functionalities if subjected to an external stimulus in the form of excitation light pulses shorter than the relaxation timescale of the internal degrees of freedom of the system. Indeed, the out-of-equilibrium state achieved upon photoexcitation exhibits electronic and optical properties highly different from those at equilibrium. Therefore, the aim of this thesis work consists in the development of new methods and experimental approaches capable to induce, measure, and control new functionalities in complex materials on an ultrafast time scale.
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Amow, Gisele. « Structural and physical properties of the vacancy-doped systems R¦1¦-¦x TiO (R = Nd for 0.00 < ; or equal to x < ; or equal 0.33 and Sm for 0.00 < ; or equal to x < ; or equal to 0.17), an investigation of metal-insulator transitions ». Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0019/NQ50976.pdf.
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Santos, Ana Filipa Alves Rodrigues dos. « Investigation of VO2 Metal-to-Insulator Transition for application in memory devices ». Master's thesis, 2020. http://hdl.handle.net/10362/129483.
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Vanadium oxides have been extensively studied due to their multiple forms. In particular,
VO2 presents a reversible and ultrafast metal-to-insulator transition (MIT) occurring at ~68ºC that
changes the material from monoclinic to tetragonal rutile structure. An external input (thermal,
chemical or electronic) can trigger this transition, changing it from a high resistance state to a low
resistance state, meaning that it can be used as an electronic switch.
In this project, the optimized conditions to produce VO2 thin films (~200 nm of thickness)
were studied and films were deposited by e-beam evaporation and rf magnetron sputtering (with
different O2 pressures), followed by a Rapid Thermal Annealing (RTA) treatment at different
temperatures. The structural, chemical, electronic and morphological properties of VO2 thin films
were characterized by means of X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy
(XPS) and Atomic Force Microscope (AFM).
E-beam deposited films were not very reproducible due to contaminations of the tungsten
crucible and it was difficult to evaporate the VO2 pellets. The films were amorphous after
deposition and the annealing performed shown that VO2 monoclinic phase was achieved at
500ºC. The substrate used was Glass/ITO. As RTA temperature increased and O2 pressure
decreased, the crystallization and roughness of the films increased.
The optimized conditions for sputtering were attained with 3x10-5 mbar of O2 pressure
and RTA temperature of 450ºC in a N2 environment with a base pressure of 250 mbar. MIM
devices were fabricated with sputtering where Molibdenium (Mo) metal was on top of VO2, using
shadow masks with circular contacts and using ITO films as the back contact.
An in-situ heating characterization was performed in XRD and XPS to analyze the
transition in terms of phase changes as well as chemical and electronic properties. Preliminary
electrical characterization was performed to explore the MIT on optimized VO2 thin films.Os óxidos de vanádio têm sido bastante estudados devido às suas múltiplas formas e fases. Em particular, o VO2 apresenta uma transição metal-isolante (MIT) reversível e ultrarrápida que ocorre a ~68ºC que altera o material de estrutura monoclínica para tetragonal rutile. Um estímulo externo (térmico, químico ou elétrico) pode provocar esta transição, alterando-a de um estado de alta resistência para baixa resistência, significando que pode ser usado como interruptor elétrico. Neste trabalho, foram estudadas as condições ótimas para produzir filmes finos de VO2 (~200 nm de espessura). Foram depositados por e-beam evaporation e rf magnetron sputtering (a diferentes pressões de O2), seguidos de um tratamento RTA a diferentes temperaturas. A composição estrutural dos filmes finos de VO2 foi caracterizada por XRD, XPS e AFM. Os filmes depositados por e-beam não são reproduzíveis devido a contaminações do cadinho de tungsténio e foi difícil evaporar os “pellets” de VO2. Os filmes eram amorfos depois da deposição e o recozimento realizado mostrou que a fase monoclínica de VO2 foi atingida com sucesso a 500ºC. O substrato utilizado foi Vidro/ITO. À medida que a temperatura aumentava e a pressão de O2 diminuía, a cristalização e rugosidade dos filmes aumentava. As condições ótimas por sputtering foi obtida para 3x10-5 mbar de pressão de O2 e temperatura de 450ºC num ambiente de N2 com uma pressão base de 250 mbar. Dispositivos MIM foram fabricados por sputtering onde Molibdénio (Mo) foi o metal depositado por cima do VO2, usando máscaras com contactos circulares e usando filmes de ITO como contacto de baixo. A caracterização in-situ com aquecimento foi realizada no XRD e XPS para se analisar a transição in termos de diferenças de fase assim como as propriedades químicas e elétricas. A caracterização elétrica preliminar foi realizada para explorar a MIT em filmes finos de VO2 otimizados.
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Kądzielawa, Andrzej. « First-principle approach to electronic states and metal-insulator transition in selected correlated model systems ». Praca doktorska, 2015. https://ruj.uj.edu.pl/xmlui/handle/item/278054.
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Kübler, Carl [Verfasser]. « Femtosecond terahertz studies of many body correlations : from ultrafast phonon plasmon dynamics to an insulator metal transition / vorgelegt von Carl Kübler ». 2007. http://d-nb.info/986197246/34.
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« Metal-insulator Transition And Cross Over From Coherent Band-like Transport To Incoherent Transport In Ferrimagnetic Epitaxial Spinel Nico2o4 Thin Films ». Tulane University, 2014.
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Teubert, Jörg [Verfasser]. « Interaction between extended and localized electronic states in the region of the metal to insulator transition in semiconductor alloys / vorgelegt von Jörg Teubert ». 2009. http://d-nb.info/993324347/34.
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Chaitanya, Lekshmi I. « In-situ Synthesis Of AxWO3(A=Na,K), SrMoO3, La1-xSrxVO3, LaNi1-x(Mn3Co)xO3 And La1-xCexNiO3 Thin Films By Pulsed Laser Deposition : Study Of Electrical Conductivity And Metal To Insulator Transition ». Thesis, 2004. http://etd.iisc.ernet.in/handle/2005/1362.
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Messina, Troy Christopher. « Steric effects in the metallic-mirror to transparent-insulator transition in YHx ». Thesis, 2002. http://wwwlib.umi.com/cr/utexas/fullcit?p3110658.
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Shamin, Saquib. « Electrical Transport in Si:P and Ge:P δ-doped Systems ». Thesis, 2015. http://etd.iisc.ernet.in/2005/3915.
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Doped semiconductor systems have for decades provided an excellent platform to study novel concepts in solid state physics such as quantum hall effect, metal-to-insulator transition (MIT), weak localization and many body interaction effects. Doped Si, in particular and doped Ge has been studied extensively to study MIT as a function of dopant concentration or uniaxial stress. Spin transport phenomena have also been probed in bulk doped Si. All the previous studies involved bulk doped semiconductors where the dopants are spread through the bulk of the material. However spatial confinement of dopants in one or more dimensions may lead to a range of exotic quantum phenomena such as an absence of Anderson localization in one and two dimensions, hole-mediated (Nagaoka) ferromagnetism and new modes of quantum transport, when the Fermi energy lies at or close to centre of the band. Since many of these phenomena are inherent to lower dimensions, it has been hard to observe these experimentally in bulk doped crystals of Si and Ge. Recent advances in the monolayer doping techniques with atoms that closely pack on a surface, has made it possible to design a new class of 2D electron systems (2DES) in elemental semiconductors, such as Si and Ge, where the dopant (P) atoms are confined within a few atomic planes. The uniqueness of these systems lies not merely in the planar doping profile in bulk semiconductors that allow versatile designs of nanodevices, such as 1D wires, tunnel gaps and quantum dots, but also that it is now possible to study the interplay of wavefunction overlap and commensurability effects in 2D with unprecedented control. From an application perspective as well these systems are technologically important as they are aimed at being the building blocks of a solid state quantum computer. This thesis deals with investigating the electrical transport properties, both average (resistance) and dynamic (noise) of doped semiconductor systems in 2D delta layers, 1D wires and 0D quantum dots.
We find that the 2D δ-layers shows suppressed low frequency noise and the Hooge parameter of delta doped Si is about five to six orders of magnitude lower when compared to bulk doped Si in metallic regime. At low temperatures, the noise arises in these systems due to universal conductance fluctuations. For 1D wires as well we find that the Hooge parameter is one of the lowest among various 1D systems including carbon
nanotubes. We identify that charge traps in the Si/SiO2 are responsible for causing noise in δ-doped systems. Then we study the noise and transport in 2D delta layers as a function of doping density (and hence carrier density and interaction). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P delta layers, suggesting a spontaneous breaking of time reversal symmetry driven by strong Coulomb interactions. At low doping density we observe metal-like dependence of resistance on temperature at low temperatures, raising the possibility of a metallic ground state in 2D at 0 K in doped semiconductors. Finally we probe the low density devices (with broken time reversal symmetry) using superconducting Al as ohmic contacts. Anomalous increase in resistance below the superconducting transition temperature of Al and magnetoresistance with a sharp peak at 0 T is observed. Additionally we find that when the Al is superconducting, there exists a non-local resistance in low doped devices.
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TANZI, LUCA. « One-dimensional disordered bosons from weak to strong interactions : the Bose glass ». Doctoral thesis, 2014. http://hdl.handle.net/2158/850906.
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Münzenberg, Markus. « X-ray magnetic circular dichroism in iron/rare-earth multilayers and the impact of modifications of the rare earth's electronic structure ». Doctoral thesis, 2000. http://hdl.handle.net/11858/00-1735-0000-0006-B411-2.
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