Latest content added for UNT Digital Library Partner: UNT Librarieshttp://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=402014-03-24T20:07:29-05:00UNT LibrariesThis is a custom feed for browsing UNT Digital Library Partner: UNT LibrariesSynchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277794/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277794/"><img alt="Synchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra" title="Synchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra" src="http://digital.library.unt.edu/ark:/67531/metadc277794/thumbnail/"/></a></p><p>Four aspects of the dynamics of continuous-time dynamical systems are studied in this work. The relationship between the Lyapunov exponents of the original system and the Lyapunov exponents of induced Poincare maps is examined. The behavior of these Poincare maps as discriminators of chaos from noise is explored, and the possible Poissonian statistics generated at rarely visited surfaces are studied.</p>An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278369/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278369/"><img alt="An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide" title="An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide" src="http://digital.library.unt.edu/ark:/67531/metadc278369/thumbnail/"/></a></p><p>A double modulation microwave spectrometer is used to evaluate the linewidth parameters for some excited rotational components in the bending vibration v_8 of 13CH3 13C 15N and 13CH3C15N isotopomers of methyl cyanide. The linewidth parameters for self-broadening of the ΔJ=2←1 rotational components for the ground v_8 , 1v_8, and the 2v_8 vibrations were determined over a pressure range of 1 to 13 mtorr and at a temperature of 300 K. The double modulation technique is used to explore the high eighth derivative of the line shape profile of the spectral line. This technique proved to give good signal-to-noise ratios and enabled the recovery of weak signals. An experimental method is developed to correct for source modulation broadening. The tests of the ratios of the two inner peak's separation of the eighth derivative of the line showed that they were up to 95% similar to those for a Lorentzian line shape function. The line shapes were assumed to be Lorentzian for the theoretical analysis of the derivative profiles and comparisons were made between experiment and theory on this basis. Dipole moments for vibrationally excited states were calculated from linewidth parameters and show systematic decrease with the increase of excitation. Impact parameters were calculated using the "hard sphere" model of the kinetic theory of gases. The results were many times larger than the size of the molecule itself. This suggests that the dominant interaction is a long range dipole-dipole force interaction.</p>Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277907/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277907/"><img alt="Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells" title="Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells" src="http://digital.library.unt.edu/ark:/67531/metadc277907/thumbnail/"/></a></p><p>In this thesis the author describes the use of transient grating techniques to study the transport of electrons and holes perpendicular to the layers of a GaAs/AlGaAs multiple quantum well (MQW).</p>Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278165/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278165/"><img alt="Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy" title="Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy" src="http://digital.library.unt.edu/ark:/67531/metadc278165/thumbnail/"/></a></p><p>The nonlinear dynamics of three physical systems has been investigated. Diode resonator systems are experimentally shown to display a period doubling route to chaos, quasiperiodic states, periodic locking states, and Hopf bifurcation to chaos. Particularly, the transition from quasiperiodic states to chaos in line-coupled systems agrees well with the Curry-Yorke model. The SPICE program has been modified to give realistic models for the diode resonator systems.</p>Transport Processes in Synchrotrons2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277803/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277803/"><img alt="Transport Processes in Synchrotrons" title="Transport Processes in Synchrotrons" src="http://digital.library.unt.edu/ark:/67531/metadc277803/thumbnail/"/></a></p><p>This thesis examines the evolution of beams in synchrotrons. Following an introduction to accelerator physics in Chapter 1, in Chapter 2 I describe the Fermilab E778 'diffusion' experiment. Families of sextupoles were powered to drive the 2/5 resonance, and a beam was then kicked to populate a nonlinear region of the transverse phase space. The beam was then observed over periods of approximately 30 minutes for a variety of kick amplitudes and physical apertures. In Chapter 3 comments about the analytic treatment of such systems are discussed, including the assumptions inherent in the conventional treatment. I motivate my use of a simplified model in Chapter 4 after examining common computational methods. Deriving the model from the formalism of traditional accelerator physics, I discuss its implementation on a massively parallel computer, the Intel iPSC/860 hypercube, and examine the performance of this algorithm in detail. Using the simple model to perform the numerical experiment equivalent to E778 is the subject of Chapter 5. I derive the parameters needed for the simple model based upon the physical experiment. Both three dimensional cases and cases with reduced dimensionality are run. From power supply ripple data and an electrical model of the magnet string, I compute tune modulation depths, and a subset of these are run. I conclude that tune modulation from power supply ripple is not a significant source of transport for this system. In Chapter 6, the intensities of the beams are used to compare the experimental and numerical runs, using both exponential and algebraic decays, and the algebraic form is seen to provide a better fit. The agreement between numerical and experimental results is best for fully three-dimensional runs, but the numerical results show slower decay than the experimental. Individual particles are examined, whose motion consists of stochastic motion interspersed with regular motion, suggestive of a Continuous Time Random Walk process. A pausing time distribution is extracted which is algebraic in time, which is consistent with dispersive transport observed elsewhere.</p>Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278287/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278287/"><img alt="Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes" title="Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes" src="http://digital.library.unt.edu/ark:/67531/metadc278287/thumbnail/"/></a></p><p>This dissertation deals with the characterization of the nonlinear absorption and refraction of two representative metallophthalocyanine dyes: chloro aluminum phthalocyanine dissolved in methanol, referred to as CAP, and a silicon naphthalocyanine derivative dissolved in toluene, referred to as SiNc. Using the Z-scan technique, the experiments are performed on both the picosecond and nanosecond timescales at a wavelength of 0.532 μm.</p>A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation2014-02-01T18:14:03-06:00http://digital.library.unt.edu/ark:/67531/metadc271849/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc271849/"><img alt="A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation" title="A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation" src="http://digital.library.unt.edu/ark:/67531/metadc271849/thumbnail/"/></a></p><p>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 of low fluence low energy silicon and carbon implantation on the creation of defects in n-type 4H-SiC. The characterization of the defects in these implanted samples was performed using the techniques, RBS-channeling and Raman spectroscopy. We have also utilized these characterization techniques to analyze defects created in much deeper layers of the SiC due to implantation of high energy nitrogen ions. The experimentally determined depths of the Si damage peaks due to low energy (60 keV) Si and C ions with low fluences (< 1015 cm-2) are consistent with the SRIM-2011 simulations. From RBS-C Si sub-lattice measurements for different fluences (1.1×1014 cm-2 to 3.2×1014 cm-2) of Si implantation in 4H-SiC, the Si vacancy density is estimated to range from 1.29×1022 cm-3 to 4.57×1022 cm-2, corresponding to average vacancy distances of 4.26 Å to 2.79 Å at the damage peak (50±5 nm). Similarly, for C implanted fluences (1.85×1014 cm-2 to 1×1015 cm-2), the Si vacancy density varies from 1.37×1022 cm-3 to 4.22×1022 cm-3 with the average vacancy distances from 4.17 Å to 2.87 Å at the damage peak (110±10 nm). From the Raman spectroscopy, the implantation-induced lattice disorders calculated along the c-axis (LO mode) and perpendicular to c-axis (TO mode) in 4H-SiC are found to be similar. Furthermore, the results obtained from SQUID measurements in C implanted n-type 4H-SiC sample with fluences ranging from 1×1012 to 1.7×1016 ions/cm2 have been discussed. The implanted samples showed diamagnetism similar to the unimplanted sample. To date, to our best of knowledge, no experimental work has been reported on investigating defect induced magnetism for self-ion implantation in n-type 4H-SiC. These first reports of experimental results can provide useful information in future studies for a better understanding of self-ion implantation in SiC-based DMS.</p>Zinc Oxide Nanoparticles for Nonlinear Bioimaging, Cell Detection and Selective Cell Destruction2014-02-01T18:14:03-06:00http://digital.library.unt.edu/ark:/67531/metadc271908/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc271908/"><img alt="Zinc Oxide Nanoparticles for Nonlinear Bioimaging, Cell Detection and Selective Cell Destruction" title="Zinc Oxide Nanoparticles for Nonlinear Bioimaging, Cell Detection and Selective Cell Destruction" src="http://digital.library.unt.edu/ark:/67531/metadc271908/thumbnail/"/></a></p><p>Light matter interactions have led to a great part of our current understanding of the universe. When light interacts with matter it affects the properties of both the light and the matter. Visible light, being in the region that the human eye can "see," was one of the first natural phenomenon we used to learn about our universe. The application of fundamental physics research has spilled over into other fields that were traditionally separated from physics, being considered two different sciences. Current physics research has applications in all scientific fields. By taking a more physical approach to problems in fields such as chemistry and biology, we have furthered our knowledge of both. Nanocrystals have many interesting optical properties. Furthermore, the size and properties of nanocrystals has given them applications in materials ranging from solar cells to sunscreens. By understanding and controlling their interactions with systems we can utilize them to increase our knowledge in other fields of science, such as biology. Nanocrystals exhibit optical properties superior to currently used fluorescent dyes. By replacing molecular dyes with nanoparticles we can reduce toxicity, increase resolution and have better cellular targeting abilities. They have also shown to have toxicity to cancer and antibacterial properties. With the understanding of how to target specific cells in vitro as well as in vivo, nanoparticles have the potential to be used as highly cell specific nanodrugs that can aid in the fight against cancer and the more recent fight against antibiotic resistant bacteria. This dissertation includes our work on bioimaging as well as our novel drug delivery system. An explanation of toxicity associated with ZnO nanoparticles and how we can use it and the nonlinear optical properties of ZnO for nanodrugs and nanoprobes is presented.</p>Criticality in Cooperative Systems2014-02-01T18:14:03-06:00http://digital.library.unt.edu/ark:/67531/metadc271910/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc271910/"><img alt="Criticality in Cooperative Systems" title="Criticality in Cooperative Systems" src="http://digital.library.unt.edu/ark:/67531/metadc271910/thumbnail/"/></a></p><p>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 system, and then observe the information transmission for different disturbance values. I proved that at criticality the transfer of information gets the maximal efficiency. As last step, the flock model has been shown that, despite its simplicity, is sufficiently a realistic model as proved via the use of 3D simulations and computer animations.</p>Broad-band Light Emission From Ion Implanted Silicon Nanocrystals Via Plasmonic and Non-plasmonic Effects for Optoelectronics2013-08-13T14:47:25-05:00http://digital.library.unt.edu/ark:/67531/metadc177255/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc177255/"><img alt="Broad-band Light Emission From Ion Implanted Silicon Nanocrystals Via Plasmonic and Non-plasmonic Effects for Optoelectronics" title="Broad-band Light Emission From Ion Implanted Silicon Nanocrystals Via Plasmonic and Non-plasmonic Effects for Optoelectronics" src="http://digital.library.unt.edu/ark:/67531/metadc177255/thumbnail/"/></a></p><p>Broad band light emission ranging from the ultraviolet (UV) to the near infrared (NIR) has been observed from silicon nanoparticles fabricated using low energy (30-45 keV) metal and non-metal ion implantation with a fluence of 5*1015 ions/cm2 in crystalline Si(100). It is found from a systematic study of the annealing carried out at certain temperatures that the spectral characteristics remains unchanged except for the enhancement of light emission intensity due to annealing. The annealing results in nucleation of metal nanoclusters in the vicinity of Si nanoparticles which enhances the emission intensity. Structural and optical characterization demonstrate that the emission originates from both highly localized defect bound excitons at the Si/Sio2 interface, as well as surface and interface traps associated with the increased surface area of the Si nanocrystals. The emission in the UV is due to interband transitions from localized excitonic states at the interface of Si/SiO2 or from the surface of Si nanocrystals. The radiative efficiency of the UV emission from the Si nanoparticles can be modified by the localized surface plasmon (LSP) interaction induced by the nucleation of silver nanoparticles with controlled annealing of the samples. The UV emission from Si nanoclusters are coupled resonantly to the LSP modes. The non-resonant emission can be enhanced by electrostatic-image charge effects. The emission in the UV (~3.3 eV) region can also be significantly enhanced by electrostatic image charge effects induced by Au nanoparticles. The UV emission from Si nanoclusters, in this case, can be coupled without LSP resonance. The recombination of carriers in Si bound excitons is mediated by transverse optical phonons due to the polarization of the surface bound exciton complex. The low energy side of emission spectrum at low temperature is dominated by 1st and 2nd order phonon replicas. Broad band emission ranging from the UV to the NIR wavelength range can be obtained from Ag implanted onto a single silicon substrate.</p>