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Author: Ilya Valmianski Publisher: ISBN: Category : Languages : en Pages : 92
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: Ilya Valmianski Publisher: ISBN: Category : Languages : en Pages : 92
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: Siming Wang Publisher: ISBN: 9781321324044 Category : Languages : en Pages : 126
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.
Author: David Michael Lamb Publisher: ISBN: Category : Optoelectronic devices Languages : en Pages : 128
Book Description
Transition metal oxides have been the subject of intense study by material scientists and chemists for many years. They represent a unique solid state material which can undergo reversible phase transitions from insulators (or semiconductors) to metals when they undergo changes in ambient conditions. It has long been established that vanadium oxides undergo these reversible phase transitions. When irradiating infrared light on thin films of VO2, a phase transition from a semiconductor to a metal causes the IR light to go from transmitting to reflecting. This study investigates phase transitions of metal oxide thin films in which visible light can be changed from transmitting to reflecting. Such a device would broaden the field of optoelectronics, optical neural networks, and advanced detector systems. An optoelectronic device from a thin film heterostructure of vanadium and titanium oxides is fabricated in this study. When a voltage was applied to the device, HeNe light responded to a reversible phase transition from a semiconductor to a more metallic material. Light on the transmission side experienced a significant drop while simultaneously experiencing a corresponding increase in reflectivity.
Author: Kunal Tiwari Publisher: ISBN: Category : Languages : en Pages :
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: Tyler J. Huffman Publisher: ISBN: Category : Physics Languages : en Pages :
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: Ilya Valmianski
Author: Ilya Valmianski
Author: Siming Wang
Author: Suhas Kumar
Author: David Michael Lamb
Author: Kunal Tiwari
Author: Jaime Monzon Reyes
Author: Joyeeta Nag
Author: Scott H. Beasor
Author: Tyler J. Huffman
Author: Hermen Folken Pen