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  Access Rights: Public
 Department: Department of Physics
 Collection: UNT Theses and Dissertations
Cooperation-induced Criticality in Neural Networks

Cooperation-induced Criticality in Neural Networks

Date: August 2013
Creator: Zare, Marzieh
Description: The human brain is considered to be the most complex and powerful information-processing device in the known universe. The fundamental concepts behind the physics of complex systems motivate scientists to investigate the human brain as a collective property emerging from the interaction of thousand agents. In this dissertation, I investigate the emergence of cooperation-induced properties in a system of interacting units. I demonstrate that the neural network of my research generates a series of properties such as avalanche distribution in size and duration coinciding with the experimental results on neural networks both in vivo and in vitro. Focusing attention on temporal complexity and fractal index of the system, I discuss how to define an order parameter and phase transition. Criticality is assumed to correspond to the emergence of temporal complexity, interpreted as a manifestation of non-Poisson renewal dynamics. In addition, I study the transmission of information between two networks to confirm the criticality and discuss how the network topology changes over time in the light of Hebbian learning.
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A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation

A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation

Date: May 2013
Creator: Kummari, Venkata Chandra Sekhar
Description: SiC has become an attractive wide bandgap semiconductor due to its unique physical and electronic properties and is widely used in high temperature, high frequency, high power and radiation resistant applications. SiC has been used as an alternative to Si in harsh environments such as in the oil industry, nuclear power systems, aeronautical, and space applications. SiC is also known for its polytypism and among them 3C-SiC, 4H-SiC and 6H-SiC are the most common polytypes used for research purposes. Among these polytypes 4H-SiC is gaining importance due to its easy commercial availability with a large bandgap of 3.26 eV at room temperature. Controlled creation of defects in materials is an approach to modify the electronic properties in a way that new functionality may result. SiC is a promising candidate for defect-induced magnetism on which spintronic devices could be developed. The defects considered are of room temperature stable vacancy types, eliminating the need for magnetic impurities, which easily diffuse at room temperature. Impurity free vacancy type defects can be created by implanting the host atoms of silicon or carbon. The implantation fluence determines the defect density, which is a critical parameter for defect induced magnetism. Therefore, we have studied the influence ...
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Theoretical and Experimental Investigations of Peg Based Thermo Sensitive Hydro Microgel

Theoretical and Experimental Investigations of Peg Based Thermo Sensitive Hydro Microgel

Date: December 2012
Creator: Chi, Chenglin
Description: Poly ethylene glycol (PEG) based microgels were synthesized and investigated. The PEG microgel has the same phase transition as the traditional poly N-isopropylacrylamide (PNIPAM). As a good substitute of PNIPAM, PEG microgel exhibits many advantages: it is easier to control the lower critical solution temperature (LCST) of the microgel by changing the component of copolymers; it has a more solid spherical core-shell structure to have a double thermo sensitivity; it is straightforward to add other sensitivities such as pH, magnetic field or organic functional groups; it readily forms a photonic crystal structure exhibiting Bragg diffraction; and, most importantly, the PEG microgel is biocompatible with human body and has been approved by FDA while PNIPAM has not. PEG microgels with core-shell structure are synthesized with a two-step free radical polymerization and characterized with DLS, SLS and UV–Vis. The dynamic mechanics of melting and recrystallizing of the PEG core-shell microgel are presented and discussed. Photonic crystals of PEG microgels were synthesized and characterized. The crystal can be isolated in a thin film or a bulk column. The phase transition of PEG microgel was simulated with the mean field theory. The enthalpy and entropy of phase transition can be estimated from the best ...
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A Non-equilibrium Approach to Scale Free Networks

A Non-equilibrium Approach to Scale Free Networks

Date: August 2012
Creator: Hollingshad, Nicholas W.
Description: Many processes and systems in nature and society can be characterized as large numbers of discrete elements that are (usually non-uniformly) interrelated. These networks were long thought to be random, but in the late 1990s, Barabási and Albert found that an underlying structure did in fact exist in many natural and technological networks that are now referred to as scale free. Since then, researchers have gained a much deeper understanding of this particular form of complexity, largely by combining graph theory, statistical physics, and advances in computing technology. This dissertation focuses on out-of-equilibrium dynamic processes as they unfold on these complex networks. Diffusion in networks of non-interacting nodes is shown to be temporally complex, while equilibrium is represented by a stable state with Poissonian fluctuations. Scale free networks achieve equilibrium very quickly compared to regular networks, and the most efficient are those with the lowest inverse power law exponent. Temporally complex diffusion also occurs in networks with interacting nodes under a cooperative decision-making model. At a critical value of the cooperation parameter, the most efficient scale free network achieves consensus almost as quickly as the equivalent all-to-all network. This finding suggests that the ubiquity of scale free networks in nature ...
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Theoretical and Experimental Investigations Concerning Microgels of Varied Spherical Geometries

Theoretical and Experimental Investigations Concerning Microgels of Varied Spherical Geometries

Date: August 2012
Creator: Wahrmund, Joshua Joseph
Description: Polymer gels have been studied extensively due to their ability to simulate biological tissues and to swell or collapse reversibly in response to external stimuli. This work presents a variety of studies using poly-N-isopropylacrylamide (PNIPA) hydrogels. The projects have been carried out both in the lab of Dr. Zhibing Hu and in collaboration with others outside of UNT: (1) an analysis of the swelling kinetics of microgel spherical shells prepared using a novel design of microfluidic devices; (2) a comparison of the drug-release rates between nanoparticle structures having either core or core-with-shell (core-shell) designs; (3) an investigation into the thermodynamics of swelling for microgels of exceedingly small size.
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Ultrafast Spectroscopy of Hybrid Ingan/gan Quantum Wells

Ultrafast Spectroscopy of Hybrid Ingan/gan Quantum Wells

Date: August 2012
Creator: Mahat, Meg Bahadur
Description: Group III nitrides are efficient light emitters. The modification of internal optoelectronic properties of these materials due to strain, external or internal electric field are an area of interest. Insertion of metal nanoparticles (MNPs) (Ag, Au etc) inside the V-shaped inverted hexagonal pits (IHP) of InGaN/GaN quantum wells (QWs) offers the potential of improving the light emission efficiencies. We have observed redshift and blueshift due to the Au MNPs and Ag MNPs respectively. This shift could be due to the electric field created by the MNPs through electrostatic image charge. We have studied the ultrafast carrier dynamics of carriers in hybrid InGaN/GaN QWs. The change in quantum confinement stark effect due to MNPs plays an important role for slow and fast carrier dynamics. We have also observed the image charge effect on the ultrafast differential transmission measurement due to the MNPs. We have studied the non-linear absorption spectroscopy of these materials. The QWs behave as a discharging of a nanocapacitor for the screening of the piezoelectric field due to the photo-excited carriers. We have separated out screening and excitonic bleaching components from the main differential absorption spectra of InGaN/GaN QWs.
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Criticality in Cooperative Systems

Criticality in Cooperative Systems

Date: May 2012
Creator: Vanni, Fabio
Description: Cooperative behavior arises from the interactions of single units that globally produce a complex dynamics in which the system acts as a whole. As an archetype I refer to a flock of birds. As a result of cooperation the whole flock gets special abilities that the single individuals would not have if they were alone. This research work led to the discovery that the function of a flock, and more in general, that of cooperative systems, surprisingly rests on the occurrence of organizational collapses. In this study, I used cooperative systems based on self-propelled particle models (the flock models) which have been proved to be virtually equivalent to sociological network models mimicking the decision making processes (the decision making model). The critical region is an intermediate condition between a highly disordered state and a strong ordered one. At criticality the waiting times distribution density between two consecutive collapses shows an inverse power law form with an anomalous statistical behavior. The scientific evidences are based on measures of information theory, correlation in time and space, and fluctuation statistical analysis. In order to prove the benefit for a system to live at criticality, I made a flock system interact with another similar ...
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Effects of Dissipation on Propagation of Surface Electromagnetic and Acoustic Waves

Effects of Dissipation on Propagation of Surface Electromagnetic and Acoustic Waves

Date: May 2012
Creator: Nagaraj, Nagaraj
Description: With the recent emergence of the field of metamaterials, the study of subwavelength propagation of plane waves and the dissipation of their energy either in the form of Joule losses in the case of electomagnetic waves or in the form of viscous dissipation in the case of acoustic waves in different interfaced media assumes great importance. with this motivation, I have worked on problems in two different areas, viz., plasmonics and surface acoustics. the first part (chapters 2 & 3) of the dissertation deals with the emerging field of plasmonics. Researchers have come up with various designs in an efort to fabricate efficient plasmonic waveguides capable of guiding plasmonic signals. However, the inherent dissipation in the form of Joule losses limits efficient usage of surface plasmon signal. a dielectric-metal-¬dielectric planar structure is one of the most practical plasmonic structures that can serve as an efficient waveguide to guide electromagnetic waves along the metal-dielectric boundary. I present here a theoretical study of propagation of surface plasmons along a symmetric dielectric-metal-dielectric structure and show how proper orientation of the optical axis of the anisotropic substrate enhances the propagation length. an equation for propagation length is derived in a wide range of frequencies. ...
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Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures

Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures

Date: May 2012
Creator: Llopis, Antonio
Description: III-V nitrides have been put to use in a variety of applications including laser diodes for modern DVD devices and for solid-state white lighting. Plasmonics has come to the foreground over the past decade as a means for increasing the internal quantum efficiency (IQE) of devices through resonant interaction with surface plasmons which exist at metal/dielectric interfaces. Increases in emission intensity of an order of magnitude have been previously reported using silver thin-films on InGaN/GaN MQWs. the dependence on resonant interaction between the plasmons and the light emitter limits the applications of plasmonics for light emission. This dissertation presents a new non-resonant mechanism based on electrostatic interaction of carriers with induced image charges in a nearby metallic nanoparticle. Enhancement similar in strength to that of plasmonics is observed, without the restrictions imposed upon resonant interactions. in this work we demonstrate several key features of this new interaction, including intensity-dependent saturation, increase in the radiative recombination lifetime, and strongly inhomogeneous light emission. We also present a model for the interaction based on the aforementioned image charge interactions. Also discussed are results of work done in the course of this research resulting in the development of a novel technique for strain measurement ...
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Modification of Graphene Properties: Electron Induced Reversible Hydrogenation, Oxidative Etching and Layer-by-layer Thinning

Modification of Graphene Properties: Electron Induced Reversible Hydrogenation, Oxidative Etching and Layer-by-layer Thinning

Date: May 2012
Creator: Jones, Jason David
Description: In this dissertation, I present the mechanism of graphene hydrogenation via three different electron sources: scanning electron microscopy, e-beam irradiation and H2 and He plasma irradiation. in each case, hydrogenation occurs due to electron impact fragmentation of adsorbed water vapor from the sample preparation process. in the proposed model, secondary and backscattered electrons generated from incident electron interactions with the underlying silicon substrate are responsible for the dissociation of water vapor. Chemisorbed H species from the dissociation are responsible for converting graphene into hydrogenated graphene, graphane. These results may lead to higher quality graphane films having a larger band gap than currently reported. in addition, the dissertation presents a novel and scalable method of controllably removing single atomic planes from multi-layer graphene using electron irradiation from an intense He plasma under a positive sample bias. As the electronic properties or multi-layer graphene are highly dependent on the number of layers, n, reducing n in certain regions has many benefits. for example, a mask in conjunction with this thinning method could be used for device applications.
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