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Author: Kunal Tiwari
Publisher:
ISBN:
Category :
Languages : en
Pages :
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"The insulator-to-metal transition of vanadium dioxide has attracted the interest of condensed matter physicists for over half a century. In its high-temperature phase, VO2 is metallic with tetragonal rutile crystallography. In its low-temperature phase, it has correlated semiconducting electronic character and a charge-density-wave- like paired monoclinic lattice structure. Determining the relative roles of electron-electron and electron-phonon interactions in the electronic structure of the low temperature phase has been the source of the physics community's interest in VO2.Over the past two decades, it has been shown that the insulator to metal transition may be photoinduced with ultrafast laser pulses. In this thesis we present ultrafast electron diffraction and ultrafast time resolved terahertz spectroscopy measurements of this photoinduced phase transition. Our ultrafast electron diffraction results reveal, at low fluences, a novel metastable phase. This phase has the crystallography of the insulating state, but a dramatically collapsed band gap. A reorganization of valence charge density accompanies this modulated spectroscopic activity.These results have twofold significance. They show that the insulating behavior of the low temperature phase is affected primarily by electron-electron correlations, not by lattice structure. Importantly, they also show that ultrafast electron diffraction may be used to probe both electronic and lattice structure dynamics--it is sensitive to valence charge density reorganizations.Our time resolved terahertz spectroscopy results complement these ultrafast electron diffraction data. We show that, in the novel metastable monoclinic phase, the band gap does not collapse below 50 meV. We also show that dynamics in the time resolved terahertz conductivity through the full photoinduced phase transition occur on two timescales--one fast (240 femtosecond) timescale, characteristic of the coherent athermal photoinduced phase transition; and one slow (picosecond) timescale, characteristic of the astructural transition to the metastable monoclinic phase. In conjunction with our ultrafast electron diffraction measurements, these results suggest that the slow dynamics of the astructural phase transition, and the structural phase transition may be affected by the same underlying mechanism." --
Author: Kunal Tiwari
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"The insulator-to-metal transition of vanadium dioxide has attracted the interest of condensed matter physicists for over half a century. In its high-temperature phase, VO2 is metallic with tetragonal rutile crystallography. In its low-temperature phase, it has correlated semiconducting electronic character and a charge-density-wave- like paired monoclinic lattice structure. Determining the relative roles of electron-electron and electron-phonon interactions in the electronic structure of the low temperature phase has been the source of the physics community's interest in VO2.Over the past two decades, it has been shown that the insulator to metal transition may be photoinduced with ultrafast laser pulses. In this thesis we present ultrafast electron diffraction and ultrafast time resolved terahertz spectroscopy measurements of this photoinduced phase transition. Our ultrafast electron diffraction results reveal, at low fluences, a novel metastable phase. This phase has the crystallography of the insulating state, but a dramatically collapsed band gap. A reorganization of valence charge density accompanies this modulated spectroscopic activity.These results have twofold significance. They show that the insulating behavior of the low temperature phase is affected primarily by electron-electron correlations, not by lattice structure. Importantly, they also show that ultrafast electron diffraction may be used to probe both electronic and lattice structure dynamics--it is sensitive to valence charge density reorganizations.Our time resolved terahertz spectroscopy results complement these ultrafast electron diffraction data. We show that, in the novel metastable monoclinic phase, the band gap does not collapse below 50 meV. We also show that dynamics in the time resolved terahertz conductivity through the full photoinduced phase transition occur on two timescales--one fast (240 femtosecond) timescale, characteristic of the coherent athermal photoinduced phase transition; and one slow (picosecond) timescale, characteristic of the astructural transition to the metastable monoclinic phase. In conjunction with our ultrafast electron diffraction measurements, these results suggest that the slow dynamics of the astructural phase transition, and the structural phase transition may be affected by the same underlying mechanism." --
Author: K. Nasu
Publisher: World Scientific
ISBN: 9789812565723
Category : Science
Languages : en
Pages : 360
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Book Description
A new class of insulating solids was recently discovered. Whenirradiated by a few visible photons, these solids give rise to amacroscopic excited domain that has new structural and electronicorders quite different from the starting ground state. This occurrenceis called photoinduced phase transition, and this multi-authoredbook reviews recent theoretical and experimental studies of this newphenomenon.
Author: Jaime Monzon Reyes
Publisher:
ISBN:
Category : Electric conductivity
Languages : en
Pages : 456
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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: 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: 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: Paul Corkum
Publisher: Springer Science & Business Media
ISBN: 3540687793
Category : Science
Languages : en
Pages : 853
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Book Description
This volume is a collection of papers presented at the Fifteenth International Conference on Ultrafast Phenomena held at the Asilomar Conference Grounds, Pacifc Grove, CA, USA, from July 31 – August 4, 2006. The Ultrafast P- nomena conferences are held every two years and provide a forum for disc- sion of the latest results in ultrafast optics and their applications in science and engineering. These meetings bring together researchers spanning several felds of science and engineering to discuss and debate the latest advances in ult- fast science. This unique forum provides a conduit for the greater dissemi- tion of the latest advances using ultrashort coherent pulses of light. More than 280 papers were presented. Signifcant progress in creating ever shorter pulses of light was reported in the attosecond range, with new applications in high harmonic generation and frequency comb metrology. Multidimensional sp- troscopy is rapidly evolving to provide new insights into quantum coherence and interactions in complex systems. Improvements in time resolved electron and x-ray diffraction provide better atomic scale perspectives on structural dynamics. These examples are but a small subset of the collected works ga- ered in this volume, which provides a valuable synopsis of the recent advances and impact of ultrafast technology in illuminating fundamental processes in physics, chemistry, and biology. There were 323 attendees at the meeting, more than one third of which were graduate and postdoctoral students. Increased s- dent attendance energized the proceedings.
Author: Yanan Dai
Publisher: Springer Nature
ISBN: 3030528367
Category : Science
Languages : en
Pages : 124
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This thesis presents significant advances in the imaging and theory of the ultrafast dynamics of surface plasmon polariton fields. The author details construction of a sub-10 femtosecond and sub-10 nanometer spatiotemporal resolution ultrafast photoemission microscope which is subsequently used for the discovery of topological meron and skyrmion-like plasmonic quasiparticles. In particular, this enabled the creation of movies of the surface plasmon polariton fields evolving on sub-optical wavelength scales at around 0.1 femtosecond per image frame undergoing vortex phase evolution. The key insight that the transverse spin of surface plasmon polaritons undergoes a texturing into meron or skyrmion-like topological quasiparticles (defined by the geometric charge of the preparation) follows. In addition, this thesis develops an analytical theory of these new topological quasiparticles, opening new avenues of research, while the ultrafast microscopy techniques established within will also be broadly applicable to studies of nanoscale optical excitations in electronic materials.
Author: Yi Long
Publisher: CRC Press
ISBN: 1000393577
Category : Science
Languages : en
Pages : 416
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Book Description
The usage of building energy accounts for 30–40% of total energy consumption in developed countries, exceeding the amount for industry or transportation. Around 50% energy for building services is contributed by heating, ventilation, and air-conditioning (HVAC) systems. More importantly, both building and HVAC energy consumptions are predicted to increase in the next two decades. Windows are considered as the least energy-efficient components of buildings. Therefore, smart windows are becoming increasingly important as they are capable of reducing HVAC energy usage by tuning the transmitted sunlight in a smart and favoured way: blocking solar irradiation on hot days, while letting it pass through on cold days. Compared with other type of smart windows, thermochromic windows have the unique advantages of cost-effectiveness, rational stimulus, and passive response. This book covers fabrication of vanadium dioxide–based smart windows, discusses various strategies to enhance their performance, and shares perspectives from the top scientists in this particular field.
Author: Siming Wang
Publisher:
ISBN: 9781321324044
Category :
Languages : en
Pages : 126
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Book Description
Vanadium oxides are a prototypical family of materials that exhibit first order metal insulator transitions (MIT). In the past 15 years, the research has been focused on the role of different driving forces and the inhomogeneity in the phase transitions of vanadium oxides. Multiple stimuli, such as voltage, current and laser pulses, have been used to induce a MIT in vanadium oxides. Inhomogeneity can give rise to phase coexistence and multiple avalanches in mesoscopic scale vanadium oxides. In this thesis, I will focus on understanding the MIT of mesoscopic vanadium oxides. I will address the phase transition mechanism through resistance - temperature (R-T) and current - voltage (I-V) characteristics. I will present the R-T characteristic of nano-sized vanadium oxide devices, which exhibits multiple avalanches over two orders of magnitude. Statistics on the avalanches indicate different MIT mechanisms for different vanadium oxides. The I-V characteristic of micro-sized vanadium oxide devices has been previously interpreted as evidence for a voltage induced transition, a non-thermal pure electronic transition in vanadium oxides. I will present a comprehensive study of the I-V characteristic supported by various techniques, including fluorescent local temperature measurement, low temperature scanning electron microscopy and numerical simulation. The results prove that Joule heating plays a significant role in the voltage induced transition of vanadium oxides. I will also discuss the other important aspect of the phase transition, the structural phase transition (SPT) in vanadium oxides. The SPT can be used to manipulate the magnetic properties of ferromagnetic materials, e.g. coercivity and magnetization. In a vanadium oxide/ferromagnet bilayer, the coercivity increases as the SPT occurs, due to the stress anisotropy induced by the SPT. In the special case of a V2O3/Ni bilayer with a smooth interface, a large coercivity enhancement appears at the middle of the V2O3 SPT. This effect is attributed to the phase coexistence in V2O3 at the nanoscale and supported by micromagnetic simulations.
