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- <p>Ferromagnetic materials -- Fluctuations<br />Neutrons -- Diffraction<br />Calcium titanate<br />Transition metal compounds<br />Magnetoresistance<br />Energy levels (Quantum mechanics)</p> (1)
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Articles 1 - 8 of 8
Full-Text Articles in Physical Sciences and Mathematics
Quantum Phase Transitions In Disordered Magnets, David Nozadze
Quantum Phase Transitions In Disordered Magnets, David Nozadze
Doctoral Dissertations
"We study the effects of quenched weak disorder on quantum phase transitions in disordered magnets. The presence of disorder in the system can lead to a variety of exotic phenomena, e.g., the smearing of transitions or quantum Griffiths singularities. Phase transitions are smeared if individual spatial regions can order independently of the bulk system. In paper I, we study smeared quantum phase transitions in binary alloys A₁₋ₓBₓ that are tuned by changing the composition x. We show that in this case the ordered phase is extended over all compositions x < 1. We also study the composition dependence of observables. In paper II, we investigate the influence of spatial disorder correlations on smeared phase transitions. As an experimental example, we demonstrate in paper III, that the composition-driven ferromagnetic-toparamagnetic quantum phase transition in Sr₁₋ₓCaₓ RuO₃ is smeared. When individual spatial regions cannot order but fluctuate slowly, the phase transition is characterized by strong singularities in the quantum Griffiths phase. In paper IV, we develop a theory of the quantum Griffiths phases in disordered ferromagnetic metals. We show that the quantum Griffiths singularities are stronger than the usual power-law quantum Griffiths singularities in insulating magnets. In paper V, we present an efficient numerical method for studying quantum phase transitions in disordered systems with O(N) order parameter symmetry in the large-N limit. Our algorithm solves iteratively the large-N self-consistent equations for the renormalized distances from criticality. Paper VI is devoted to the study of transport properties in the quantum Griffiths phase associated with the antiferromagnetic quantum phase transition in a metal. We find unusual behavior of transport properties which is in contrast to the normal Fermi-liquid behavior"--Abstract, page v.
Optical Spectroscopic Studies Of Two Star Forming Regions, Kristen Leilani Erickson
Optical Spectroscopic Studies Of Two Star Forming Regions, Kristen Leilani Erickson
Doctoral Dissertations
"The ρ Ophiuchi and Serpens molecular clouds are sites of low mass star formation. The goals of this study were to identify young stellar objects (YSOs) and estimate ages and masses in order to infer an initial mass function (IMF), investigate disk evolution, and determine the star-forming history. Optical spectroscopic surveys of an unbiased sample of candidate young stars have been completed. Optical images taken in different photometric bands were used to create color-magnitude diagrams from which sources were selected for spectroscopic observation. In combination with published data, 135 association members in ρ Ophiuchi and 63 association members plus 16 …
Phase Transitions In Disordered Systems, Fawaz Y. Hrahsheh
Phase Transitions In Disordered Systems, Fawaz Y. Hrahsheh
Doctoral Dissertations
"Disorder can have a wide variety of consequences for the physics of phase transitions. Some transitions remain unchanged in the presence of disorder while others are completely destroyed. In this thesis we study the effects of disorder on several classical and quantum phase transitions in condensed matter systems. After a brief introduction, we study the ferromagnetic phase transition in a randomly layered Heisenberg magnet using large-scale Monte-Carlo simulations. Our results provide numerical evidence for the exotic infinite-randomness scenario. We study classical and quantum smeared phase transitions in substitutional alloys A₁₋ₓBₓ. Our results show that the disorder completely destroys the phase …
Quantum Phase Transitions In Impurity Models And Percolating Lattices, Manal M. Al-Ali
Quantum Phase Transitions In Impurity Models And Percolating Lattices, Manal M. Al-Ali
Doctoral Dissertations
"This thesis investigates the influence of random disorder and dissipation on zero-temperature quantum phase transitions. Both phenomena can fundamentally change the character of the phases of a quantum many-particle system and of the transitions between them. If dissipation and disorder occur simultaneously in a system undergoing a quantum phase transition, particularly strong effects can be expected. In the first paper reproduced in this thesis, we study a single quantum rotor coupled to a sub-Ohmic dissipative bath. We find that this system undergoes a quantum phase transition from a delocalized phase to a localized phase as the dissipation strength is increased. …
Structural And Magnetic Properties Of Ni And Cr Substituted La₀.₇Sr₀.₃Mno₃, Thomas Franklin Creel Jr.
Structural And Magnetic Properties Of Ni And Cr Substituted La₀.₇Sr₀.₃Mno₃, Thomas Franklin Creel Jr.
Doctoral Dissertations
"The La₁₋ₓSrₓMn1-yByO₃ perovskite systems where B represents a 3d transition metal such as Cr, Fe, Co, Ni, Cu, and Zn, are of significant interest due to their large magnetoresistance, and electronic transport properties that change significantly, depending on values of x and which transition metal used. Their applications in technology include cathodes for solid oxide fuel cells, resistance random access memory and catalysis. At certain doping concentrations on the B-site, metal-to-semiconductor or metal-to-insulator transitions coupled with colossal magnetoresistance occur, decreasing the resistivity (up to six orders of magnitude) upon the application of an external magnetic field. …
Transport In Correlated And Uncorrelated Random Media, With Applications To Molecularly Doped Polymers, Nilanka Praveena Gurusinghe
Transport In Correlated And Uncorrelated Random Media, With Applications To Molecularly Doped Polymers, Nilanka Praveena Gurusinghe
Doctoral Dissertations
"The commonly observed Poole-Frenkel field dependence; of the mobility of photo-injected charges in molecularly-doped polymers has been shown to arise from the spatially-correlated, Gaussian energy distribution of transport sites encountered by charges moving through the material. Experimental current-time transients obtained for molecularly-doped polymers exhibit universality with respect to electric field and a metal-insulator-like transition from non-dispersive to dispersive transport, features usually identified with multiple trapping models that assume an uncorrelated exponential distribution of trap energies. For materials that exhibit both sets of features the possibility arises that both kinds of disorder coexist. We study here, analytically and numerically, transport in …
Study Of Artificially Disordered Optical Fibers, Herath Mudiyanselage Sumudu Rasika Kumari Herath
Study Of Artificially Disordered Optical Fibers, Herath Mudiyanselage Sumudu Rasika Kumari Herath
Doctoral Dissertations
"Light transmission through photo-sensitive multi-mode fibers (MMF) with controlled volume disorder is investigated. Experiment shows that a segment of disordered MMF as short as 10 cm is sufficient to distribute power uniformly over all co-propagating modes and the intensity at the output surface of the fiber follows the Rayleigh negative exponential function. To explain the experimental findings, a comprehensive theoretical model is developed with three main results. First, statistical properties of all components of the dielectric tensor are obtained and analyzed in the framework a microscopical model of photo-sensitivity in a germano-silicate glasses. Secondly, it is shown that induced birefringence …
Characterization Of Nano-Scaled Metal-Hydrides Confined In Nano-Porous Carbon Frameworks, David Edward Peaslee
Characterization Of Nano-Scaled Metal-Hydrides Confined In Nano-Porous Carbon Frameworks, David Edward Peaslee
Doctoral Dissertations
"Metal hydrides are currently being studied to provide hydrogen for use in fuel cells and for transportation applications. Hydrogen can be stored in chemical compounds at higher density and lower volume than liquid H₂ or compressed gas. Thermodynamic properties of metal hydrides differ between bulk and nano-sized particles. Many metal hydrides with useful volumetric and gravimetric capacities have high decomposition temperatures, but when placed in nano-sized frameworks (or templates) desorption and adsorption temperatures can be fine-tuned to meet engineering requirements for real-world systems. Additionally, some metal hydrides have shown a change in the decomposition pathway when infiltrated into these frameworks, …