Latest content added for Digital Library Partner: UNT Librarieshttps://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=40&sort=title2018-01-27T07:36:46-06:00UNT LibrariesThis is a custom feed for browsing Digital Library Partner: UNT LibrariesA Collisional Mechanism in the Ion-Solid Interaction Which Enhances Scattering Yields Near 180⁰2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc332019/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc332019/"><img alt="A Collisional Mechanism in the Ion-Solid Interaction Which Enhances Scattering Yields Near 180⁰" title="A Collisional Mechanism in the Ion-Solid Interaction Which Enhances Scattering Yields Near 180⁰" src="https://digital.library.unt.edu/ark:/67531/metadc332019/small/"/></a></p><p>In the course of experiments using uniaxial double alignment channeling to investigate radiation damage in single crystals, an anomalously large ion-scattering yield from the near surface of disordered or simulated disordered solid targets was observed. The chronology of the discovery of this new ion-solid effect and its explanation are presented along with experiments detailing the dependence of the new effect upon ion type and energy, as well as target atomic number and density. Targets included a spectrum of polycrystalline elemental targets in a range Z = 29 to Z = 82. Also, the influence of the effect upon scattering yields from an aligned Au(110) single crystal is demonstrated.</p>Complex Numbers in Quantum Theory2016-03-04T16:14:01-06:00https://digital.library.unt.edu/ark:/67531/metadc804988/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc804988/"><img alt="Complex Numbers in Quantum Theory" title="Complex Numbers in Quantum Theory" src="https://digital.library.unt.edu/ark:/67531/metadc804988/small/"/></a></p><p>In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.</p>Complexity as a Form of Transition From Dynamics to Thermodynamics: Application to Sociological and Biological Processes.2008-02-15T14:33:01-06:00https://digital.library.unt.edu/ark:/67531/metadc4209/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc4209/"><img alt="Complexity as a Form of Transition From Dynamics to Thermodynamics: Application to Sociological and Biological Processes." title="Complexity as a Form of Transition From Dynamics to Thermodynamics: Application to Sociological and Biological Processes." src="https://digital.library.unt.edu/ark:/67531/metadc4209/small/"/></a></p><p>This dissertation addresses the delicate problem of establishing the statistical mechanical foundation of complex processes. These processes are characterized by a delicate balance of randomness and order, and a correct paradigm for them seems to be the concept of sporadic randomness. First of all, we have studied if it is possible to establish a foundation of these processes on the basis of a generalized version of thermodynamics, of non-extensive nature. A detailed account of this attempt is reported in Ignaccolo and Grigolini (2001), which shows that this approach leads to inconsistencies. It is shown that there is no need to generalize the Kolmogorov-Sinai entropy by means of a non-extensive indicator, and that the anomaly of these processes does not rest on their non-extensive nature, but rather in the fact that the process of transition from dynamics to thermodynamics, this being still extensive, occurs in an exceptionally extended time scale. Even, when the invariant distribution exists, the time necessary to reach the thermodynamic scaling regime is infinite. In the case where no invariant distribution exists, the complex system lives forever in a condition intermediate between dynamics and thermodynamics. This discovery has made it possible to create a new method of analysis of non-stationary time series which is currently applied to problems of sociological and physiological interest.</p>Complexity as Aging Non-Poisson Renewal Processes2007-09-28T21:50:39-05:00https://digital.library.unt.edu/ark:/67531/metadc3706/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc3706/"><img alt="Complexity as Aging Non-Poisson Renewal Processes" title="Complexity as Aging Non-Poisson Renewal Processes" src="https://digital.library.unt.edu/ark:/67531/metadc3706/small/"/></a></p><p>The search for a satisfactory model for complexity, meant as an intermediate condition between total order and total disorder, is still subject of debate in the scientific community. In this dissertation the emergence of non-Poisson renewal processes in several complex systems is investigated. After reviewing the basics of renewal theory, another popular approach to complexity, called modulation, is introduced. I show how these two different approaches, given a suitable choice of the parameter involved, can generate the same macroscopic outcome, namely an inverse power law distribution density of events occurrence. To solve this ambiguity, a numerical instrument, based on the theoretical analysis of the aging properties of renewal systems, is introduced. The application of this method, called renewal aging experiment, allows us to distinguish if a time series has been generated by a renewal or a modulation process. This method of analysis is then applied to several physical systems, from blinking quantum dots, to the human brain activity, to seismic fluctuations. Theoretical conclusions about the underlying nature of the considered complex systems are drawn.</p>A Comprehensive Model for the Rotational Spectra of Propyne CH₃CCH in the Ground and V₁₀=1,2,3,4,5 Vibrational States2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc332026/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc332026/"><img alt="A Comprehensive Model for the Rotational Spectra of Propyne CH₃CCH in the Ground and V₁₀=1,2,3,4,5 Vibrational States" title="A Comprehensive Model for the Rotational Spectra of Propyne CH₃CCH in the Ground and V₁₀=1,2,3,4,5 Vibrational States" src="https://digital.library.unt.edu/ark:/67531/metadc332026/small/"/></a></p><p>The energy states of C₃ᵥ symmetric top polyatomic molecules were studied. Both classical and quantum mechanical methods have been used to introduce the energy states of polyatomic molecules. Also, it is shown that the vibration-rotation spectra of polyatomic molecules in the ground and excited vibrational states can be predicted by group theory. A comprehensive model for predicting rotational frequency components in various v₁₀ vibrational levels of propyne was developed by using perturbation theory and those results were compared with other formulas for C₃ᵥ symmetric top molecules. The v₁₀=1,2,3 and ground rotational spectra of propyne in the frequency range 17-70 GHz have been reassigned by using the derived comprehensive model. The v₁₀=3 and v₁₀=4 rotational spectra of propyne have been investigated in the 70 GHz, and 17 to 52 GHz regions, respectively, and these spectral components assigned using the comprehensive model. Molecular constants for these vibrationally excited states have been determined from more than 100 observed rotational transitions. From these experimentally observed components and a model based upon first principles for C₃ᵥ symmetry molecules, rotational constants have been expressed in a form which enables one to predict rotational components for vibrational levels for propyne up to v₁₀=5. This comprehensive model also appears to be useful in predicting rotational components in more highly excited vibrational levels but data were not available for comparison with the theory. Several techniques of assignment of rotational spectra for each excited vibrational state are discussed. To get good agreement between theory and experiment, an additional term 0.762(J+1) needed to be added to Kℓ=1 states in v₁₀=3. No satisfactory theoretical explanation of this term has been found. Experimentally measured frequencies for rotational components for J→(J+1)=+1 (0≤J≤3) in each vibration v₁₀=n (0≤n≤4) are presented and compared with those calculated using the results of basic perturbation theory. The v₉=2 rotational spectrum of the propyne molecule was introduced in Appendix A to compare the rotational spectra of the same molecule in different vibrational levels v₉ and v₁₀.</p>A Computer Analysis of Complex Gamma-Ray Spectra2012-12-27T22:03:54-06:00https://digital.library.unt.edu/ark:/67531/metadc130612/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc130612/"><img alt="A Computer Analysis of Complex Gamma-Ray Spectra" title="A Computer Analysis of Complex Gamma-Ray Spectra" src="https://digital.library.unt.edu/ark:/67531/metadc130612/small/"/></a></p><p>The purpose of this investigation was to provide a method for determining the relative intensities of all gamma rays in a particular spectrum, and thereby determine the relative transition probabilities.</p>The Concept of Collision Strength and Its Applications2008-02-15T15:12:39-06:00https://digital.library.unt.edu/ark:/67531/metadc4530/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc4530/"><img alt="The Concept of Collision Strength and Its Applications" title="The Concept of Collision Strength and Its Applications" src="https://digital.library.unt.edu/ark:/67531/metadc4530/small/"/></a></p><p>Collision strength, the measure of strength for a binary collision, hasn't been defined clearly. In practice, many physical arguments have been employed for the purpose and taken for granted. A scattering angle has been widely and intensively used as a measure of collision strength in plasma physics for years. The result of this is complication and unnecessary approximation in deriving some of the basic kinetic equations and in calculating some of the basic physical terms. The Boltzmann equation has a five-fold integral collision term that is complicated. Chandrasekhar and Spitzer's approaches to the linear Fokker-Planck coefficients have several approximations. An effective variable-change technique has been developed in this dissertation as an alternative to scattering angle as the measure of collision strength. By introducing the square of the reduced impulse or its equivalencies as a collision strength variable, many plasma calculations have been simplified. The five-fold linear Boltzmann collision integral and linearized Boltzmann collision integral are simplified to three-fold integrals. The arbitrary order linear Fokker-Planck coefficients are calculated and expressed in a uniform expression. The new theory provides a simple and exact method for describing the equilibrium plasma collision rate, and a precise calculation of the equilibrium relaxation time. It generalizes bimolecular collision reaction rate theory to a reaction rate theory for plasmas. A simple formula of high precision with wide temperature range has been developed for electron impact ionization rates for carbon atoms and ions. The universality of the concept of collision strength is emphasized. This dissertation will show how Arrhenius' chemical reaction rate theory and Thomson's ionization theory can be unified as one single theory under the concept of collision strength, and how many important physical terms in different disciplines, such as activation energy in chemical reaction theory, ionization energy in Thomson's ionization theory, and the Coulomb logarithm in plasma physics, can be unified into a single one -- the threshold value of collision strength. The collision strength, which is a measure of a transfer of momentum in units of energy, can be used to reconcile the differences between Descartes' opinion and Leibnitz's opinion about the "true'' measure of a force. Like Newton's second law, which provides an instantaneous measure of a force, collision strength, as a cumulative measure of a force, can be regarded as part of a law of force in general.</p>A Continuously Sensitive Cloud Chamber2012-08-14T09:01:15-05:00https://digital.library.unt.edu/ark:/67531/metadc97019/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc97019/"><img alt="A Continuously Sensitive Cloud Chamber" title="A Continuously Sensitive Cloud Chamber" src="https://digital.library.unt.edu/ark:/67531/metadc97019/small/"/></a></p><p>A continuous cloud chamber would be a valuable asset to laboratory work in nuclear and atomic physics. For this reason the construction and investigation of a continuously sensitive diffusion cloud chamber has been undertaken. It is the purpose of this paper to report the design and operating characteristics of such a chamber.</p>Cooperation-induced Criticality in Neural Networks2014-04-23T20:20:45-05:00https://digital.library.unt.edu/ark:/67531/metadc283813/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc283813/"><img alt="Cooperation-induced Criticality in Neural Networks" title="Cooperation-induced Criticality in Neural Networks" src="https://digital.library.unt.edu/ark:/67531/metadc283813/small/"/></a></p><p>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.</p>Core-Shell Based Metamaterials: Fabrication Protocol and Optical Properties2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062904/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062904/"><img alt="Core-Shell Based Metamaterials: Fabrication Protocol and Optical Properties" title="Core-Shell Based Metamaterials: Fabrication Protocol and Optical Properties" src="https://digital.library.unt.edu/ark:/67531/metadc1062904/small/"/></a></p><p>The objective of this study is to examine core-shell type plasmonic metamaterials aimed at the development of materials with unique electromagnetic properties. The building blocks of metamaterials under study consist of gold as a metal component, and silica and precipitated calcium carbonate (PCC) as the dielectric media. The results of this study demonstrate important applications of the core-shells including scattering suppression, airborne obscurants made of fractal gold shells, photomodification of the fractal structure providing windows of transparency, and plasmonics core-shell with a gain shell as an active device. Plasmonic resonances of the metallic shells depend on their nanostructure and geometry of the core, which can be optimized for the broadband extinction. Significant extinction from the visible to mid-infrared makes fractal shells very attractive as bandpass filters and aerosolized obscurants. In contrast to the planar fractal films, where the absorption and reflection equally contribute to the extinction, the shells' extinction is caused mainly by the absorption. This work shows that the Mie scattering resonance of a silica core with 780 nm diameter at 560 nm is suppressed by 75% and only partially substituted by the absorption in the shell so that the total transmission is noticeably increased. Effective medium theory supports our experiments and indicates that light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate. Broadband extinction in fractal shells allows as well for a laser photoburning of holes in the extinction spectra and consequently windows of transparency in a controlled manner. Au fractal nanostructures grown on PCC flakes provide the highest mass normalized extinction, up to 3 m^2/g, which has been demonstrated in the broad spectral range. In the nanoplasmonic field active devices consist of a Au nanoparticle that acts as a cavity and the dye molecules attached to it via thin silica shell as the active medium. Such kind of devices is considered as a nano-laser or nano-amplifier. The fabricated nanolasers were studied for their photoluminescence kinetic properties. It is shown that the cooperative effects due to the coupling of dye molecules via Au nanoparticle plasmons result in bi-exponential emission decay characteristics in accord with theory predictions. These bi-exponential decays involve a fast superradiant decay, which is followed by a slow subradiant decay. To summarize, this work shows new attractive properties of core-shell nanoparticles. Fractal Au shells on silica cores prove to be a good scattering suppressor and a band pass filter in a broadband spectral range. They can also be used as an obscurant when PCC is used as the core material. Finally, gold nanoparticles coated with silica with dye results in bi-exponential decays.</p>