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Author: Tyler J. Huffman
Publisher:
ISBN:
Category : Physics
Languages : en
Pages :
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Book Description
The salient feature of the familiar structural transition accompanying the thermally-driven metal-insulator transition in bulk vanadium dioxide (VO2) is a pairing of all the vanadium ions in the monoclinic M¬1 insulating phase. Whether this pairing (unit cell doubling) alone is sufficient to open the energy gap has been the central question of a classic debate which has continued for almost sixty years. Interestingly, there are two less familiar insulating states, monoclinic M2 and triclinic, which are accessible via strain or chemical doping. These phases are noteworthy in that they exhibit distinctly different V-V pairing. With infrared and optical photon spectroscopy, we investigate how the changes in crystal structure affect the electronic structure. We find that the energy gap and optical inter-band transitions are insensitive to changes in the vanadium-vanadium pairing. This result is confirmed by DFT+U and HSE calculations. Hence, our work conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO2. VO2 is a candidate material for novel technologies, including ultrafast data storage, memristors, photonic switches, smart windows, and transistors which move beyond the limitations of silicon. The attractiveness of correlated materials for technological application is due to their novel properties that can be tuned by external factors such as strain, chemical doping, and applied fields. For advances in fundamental physics and applications, it is imperative that these properties be measured over a wide range of regimes. Towards this end, we study a single domain VO2 crystal with polarized light to characterize the anisotropy of the optical properties. In addition, we study the effects of compressive strain in a VO2 thin film in which we observe remarkable changes in electronic structure and transition temperature. Furthermore, we find evidence that electronic correlations are active in the metallic rutile phase as well. VO2 films exhibit phase coexistence in the vicinity of the metal-insulator transition. Using scanning near-field infrared microscopy, we have studied the patterns of phase coexistence in the same area on repeated heating and cooling cycles. We find that the pattern formation is reproducible each time. This is an unexpected result from the viewpoint of classical nucleation theory that anticipates some degree of randomness. The completely deterministic nature of nucleation and growth of domains in a VO2 film with imperfections is a fundamental finding. This result also holds promise for producing reliable nanoscale VO2 devices.
Author: Tyler J. Huffman
Publisher:
ISBN:
Category : Physics
Languages : en
Pages :
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Book Description
The salient feature of the familiar structural transition accompanying the thermally-driven metal-insulator transition in bulk vanadium dioxide (VO2) is a pairing of all the vanadium ions in the monoclinic M¬1 insulating phase. Whether this pairing (unit cell doubling) alone is sufficient to open the energy gap has been the central question of a classic debate which has continued for almost sixty years. Interestingly, there are two less familiar insulating states, monoclinic M2 and triclinic, which are accessible via strain or chemical doping. These phases are noteworthy in that they exhibit distinctly different V-V pairing. With infrared and optical photon spectroscopy, we investigate how the changes in crystal structure affect the electronic structure. We find that the energy gap and optical inter-band transitions are insensitive to changes in the vanadium-vanadium pairing. This result is confirmed by DFT+U and HSE calculations. Hence, our work conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO2. VO2 is a candidate material for novel technologies, including ultrafast data storage, memristors, photonic switches, smart windows, and transistors which move beyond the limitations of silicon. The attractiveness of correlated materials for technological application is due to their novel properties that can be tuned by external factors such as strain, chemical doping, and applied fields. For advances in fundamental physics and applications, it is imperative that these properties be measured over a wide range of regimes. Towards this end, we study a single domain VO2 crystal with polarized light to characterize the anisotropy of the optical properties. In addition, we study the effects of compressive strain in a VO2 thin film in which we observe remarkable changes in electronic structure and transition temperature. Furthermore, we find evidence that electronic correlations are active in the metallic rutile phase as well. VO2 films exhibit phase coexistence in the vicinity of the metal-insulator transition. Using scanning near-field infrared microscopy, we have studied the patterns of phase coexistence in the same area on repeated heating and cooling cycles. We find that the pattern formation is reproducible each time. This is an unexpected result from the viewpoint of classical nucleation theory that anticipates some degree of randomness. The completely deterministic nature of nucleation and growth of domains in a VO2 film with imperfections is a fundamental finding. This result also holds promise for producing reliable nanoscale VO2 devices.
Author: Yin Shi
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Vanadium dioxide (VO2) is a strongly correlated system which exhibits an intriguing metal-insulator transition (MIT) accompanied by a structural transition at a temperature slightly above the room temperature. It offers potential novel device applications such as sensors, Mott field-effect transistors, and memristors, which desire guidance from mesoscopic theoretical modeling. Based on symmetry consideration, we formulate a mesoscopic phase-field model of the MIT explicitly incorporating both structural and electronic instabilities as well as free electrons and holes. We employ this model to investigate the MIT in mesoscale VO2 subject to various stimuli such as heat, stress/strain, electric field, doping, electric current, and light. First, the temperature-stress/strain phase diagrams of VO2 nanobeams and thin films under different mechanical boundary conditions are calculated consistently, which show good agreement with existing experimental observations. We also calculate the temperature-radius phase diagrams of VO2 nanoparticles and nanofibers. Second, in a VO2 slab under an electric field in an open-circuit configuration, an abrupt universal resistive transition is shown to occur inside the supercooling region, in sharp contrast to the conventional Landau-Zener smooth electric breakdown. Third, the temperature-dopant-concentration phase diagrams of VO2 doped with various metal ions are calculated consistent with the experiments. Furthermore, hole doping in VO2 may induce a metastable metallic monoclinic phase, which could be stabilized through geometrical confinement and the size effect in VO2-VO_{2-delta} bilayers leading to the decoupling of the electronic and structural phase transitions. Fourth, we demonstrate that the electric current may drive the MIT isothermally via the current-induced electron correlation weakening, inducing a few-nanosecond ultrafast resistive switching consistent with experimental measurements. The isothermal temperature-current phase diagram is further calculated and the current is also found able to drive domain walls to move. Fifth, dynamic processes of the MIT in VO2 illuminated by femtosecond laser pulses are simulated, showing the emergence of the transient metallic monoclinic phase and the bias-induced shrinkage of the photoinduced metallic phase. We also prove that during a generic metal-insulator transition, a nonequilibrium homogeneous state may be unstable against charge density modulations with certain wavelengths, and thus evolves to the equilibrium phase through transient electronic phase separation. This transient electronic phase separation is shown to take place in VO2 upon photoexcitation.
Author: Jaime Monzon Reyes
Publisher:
ISBN:
Category : Electric conductivity
Languages : en
Pages : 456
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Author: Artur Braun
Publisher: Walter de Gruyter GmbH & Co KG
ISBN: 3110629941
Category : Science
Languages : en
Pages : 456
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Book Description
This book uses an array of different approaches to describe photosynthesis, ranging from the subjectivity of human perception to the mathematical rigour of quantum electrodynamics. This interdisciplinary work draws from fields as diverse as astronomy, agriculture, classical and quantum optics, and biology in order to explain the working principles of photosynthesis in plants and cyanobacteria.
Author: Ilya Valmianski
Publisher:
ISBN:
Category :
Languages : en
Pages : 92
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Book Description
Vanadium oxides are a prototypical family of highly correlated oxides. In his dissertation, I present the study of two vanadium oxides in particular, V2O3 and VO2, which undergo simultaneously both a structural phase transition and a metal to insulator transition. While traditionally these phase transitions were studied in equilibrium, bulk, or in meso/macro-scale devices, in my work I focused on different modalities: fast, small, and strained. In my work on fast time scales during photoexcitation of V2O3 we found a novel meta-stable intermediate state that appears due to symmetry change in the monoclinic phase. This change occurs in the proximity of high temperature rhombohedral domains on length scales similar to those of electronic correlation. Our finding shows that the electronic and structural transitions in V2O3 have similar length scales but very different time scales. In VO2 and V2O3 nanoscale devices, we found a length-scale competition between Joule heating and electric field driven current induced metal to insulator transition. We proposed a novel thermoelectric model and performed simulations using finite element methods. Our modeling showed that the transition is highly inhomogeneous and the resulting filaments are surface bound with thermal gradients generating Seebeck electric fields on the order of 1000 V/cm. Finally, we studied pressurized and strained thin films in V2O3 and discovered strong strain relaxation for pressures of up to 500 MPa, which cause a deviation of thin film Pressure-Temperature phase diagram from bulk behavior. This strain relaxation relies on the difference between the structural and morphological length scales, which allows the formation of strain relaxing creases. Once those creases are fully strained, the thin films respond similarly to bulk samples.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 22
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Vanadium oxides are very interesting compounds which exhibit exotic transport phenomena. In particular vanadium dioxide (VO2) undergoes a first-order transition from a high-temperature metallic phase to a low-temperature insulating phase at almost the room temperature (T = 340 K). The resistivity jumps by several orders of magnitude through this transition, and the crystal structure changes from rutile (R-phase) at high-temperature to monoclinic (so-called M1-phase) at low-temperature. The latter is characterized by a dimerization of the vanadium atoms into pairs, as well as a tilting of these pairs with respect to the c-axis. VO2 has also attracted a great deal of attention for its ultrafast optical response, switching between the R and the M1 phase. Despite the large number of experimental studies focusing on this material the physics driving this phase transition and the resulting optical properties is still mysterious. There are intensive reports around the world to make devices such as switches, transistors, detectors, varistors, phase change memory, exploiting the unique properties of VO2. Two physical effects, Peierls, i.e. dimerization, and the Mott mechanism due to strong Coulomb repulsion are important in the metal-insulator transition (MIT) of VO2. Understanding the detailed interplay and the relative importance of both Peierls and Mott mechanism is important for controlling this material with an eye towards applications. For example, whether the driving force of this transition is electronic (i.e. occurring on femtosecond timescales) or structural (occurring on the picosecond timescale) is important to understand the speed of the switching from the M1 to the rutile phase. The insights obtained in this study together with the computational machinery developed, will serve as a basis for rational material design of VO2 based applications.
Author: Suhas Kumar
Publisher:
ISBN: 9783659924958
Category :
Languages : en
Pages : 92
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Author: Robert Marvel
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 173
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Author:
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ISBN:
Category :
Languages : en
Pages :
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Author: Ryan Hogan
Publisher:
ISBN: 9781392184004
Category :
Languages : en
Pages :
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"Le dioxyde de vanadium (VO2) est un matériau avec une transition de phase qui se passe vers une température de 68°C. C'est un matériau important qui a fait l'objet de nombreuses études au cours des dernières décennies, et plusieurs applications ont été démontrées en optique. À ce jour, les études ont trouvé que la transition de phase peut être induite par le chauffage ambiant; par courant électrique (l'effet Joule); par excitation optique, et par stress mécanique. Dans la présente thèse, nous démontrons que le frottement est une méthode alternative pour induire la transition de phase dans le VO2. En effet, le flux thermique créé à l'interface entre le VO2 et un matériau qui glisse peut être suffisant pour induire la transition de phase. Ainsi, nous avons pu mesurer l'effet du frottement sur les propriétés du VO2, en particulier la résistivité (électrique) et la réflectance (optique). Le flux minimal détectable est de l'ordre de 1 mW/cm2, ce qui ouvre la porte à d'intéressantes applications, comme la détection du glissement. Cependant, la majorité de données ont été faites avec la réflectance, et ce; pour deux raisons : la sensibilité est meilleure, et la fabrication plus facile pour des échantillons de haute qualité. Nous avons étudié et simulé théoriquement les principales variables qui affectent la transition de phase par frottement, comme la pression de frottement, la vitesse de glissement, le coefficient de frottement, la température d'opération et la conductivité thermique du substrat. Nous avons fait une modélisation de la diffusion de chaleur dans un échantillon pour voir les effets la température induite. Nous avons créé plusieurs échantillons sous forme de couches minces (environ 100 nm d'épaisseur) sur des substrats de verre et de saphir. En comparant le modèle avec les données expérimentales, nous avons trouvé une bonne corrélation. Par exemple, la théorie prédit que la réponse de la chaleur serait plus rapide pour le saphir. Avec les résultats expérimentaux, nous avons obtenu un temps caractéristique de relaxations pour le saphir qui est 75 fois plus rapide que le verre. Les travaux futurs possibles considéreront des matériaux comme le silicium pour augmenter la conductivité thermique et la diffusivité thermique et ainsi augmenter la rapidité. L'utilisation de faisceaux sondes polarisés du faisceau de sondes pourraient améliorer la sensibilité. Aussi, l'épaisseur de la couche de VO2 et l'angle d'incidence du faisceau sonde pourraient être optimisés. Pour les applications, la méthode de détection par résistivité électrique pourrait permettre un design tout-électronique de capteur capable de mesurer les petits mouvements de glissement. Enfin, un nouvelle méthode pour mesurer les coefficients de frottement sans directement mesurer la force de frottement pourrait découler de cetterecherche."--Résumé.
