Alumni Dissertations

 

Alumni Dissertations

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  • Magnetic Resonance Studies of Energy Storage Materials

    Author:
    Rafael Vazquez Reina
    Year of Dissertation:
    2013
    Program:
    Physics
    Advisor:
    Steven Greenbaum
    Abstract:

    Abstract Magnetic Resonance Studies of Energy Storage Materials by Rafael Vázquez Reina Adviser: Professor Steven G. Greenbaum In today's society there is high demand to have access to energy for portable devices in different forms. Capacitors with high performance in small package to achieve high charge/discharge rates, and batteries with their ability to store electricity and make energy mobile are part of this demand. The types of internal dielectric material strongly affect the characteristics of a capacitor, and its applications. In a battery, the choice of the electrolyte plays an important role in the Solid Electrolyte Interphase (SEI) formation, and the cathode material for high output voltage. Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) spectroscopy are research techniques that exploit the magnetic properties of the electron and certain atomic nuclei to determine physical and chemical properties of the atoms or molecules in which they are contained. Both EPR and NMR spectroscopy technique can yield meaningful structural and dynamic information. Three different projects are discussed in this dissertation. First, High energy density capacitors where EPR measurements described herein provide an insight into structural and chemical differences in the dielectric material of a capacitor. Next, as the second project, Electrolyte solutions where an oxygen-17 NMR study has been employed to assess the degree of preferential solvation of Li+ ions in binary mixtures of EC (ethylene carbonate) and DMC (dimethyl carbonate) containing LiPF6 (lithium hexafluo- rophosphate) which may be ultimately related to the SEI formation mechanism. The third project was to study Bismuth fluoride as cathode material for rechargeable batteries. The objective was to study 19F and 7Li MAS NMR of some nanocomposite cathode materials as a conversion reaction occurring during lithiation and delithation of the BiF3/C nanocomposite.

  • An experimental investigation into the mechanisms of bacterial evolution

    Author:
    Zhenmao Wan
    Year of Dissertation:
    2014
    Program:
    Physics
    Advisor:
    Mark Hillery
    Abstract:

    This thesis studies the two fundamental mechanisms of bacterial evolution — horizontal gene transfer and spontaneous mutation, in the bacterium Escherichia coli through novel experimental assays and mathematical simulations. First, I will develop a growth assay utilizing the quantitative polymerase chain reaction (qPCR) to provide real-time enumeration of genetic marker abundance within bacterial populations. Second, I will focus on horizontal gene transfer in E. coli occurring through a process called conjugation. By fitting the qPCR data to a resource limited, logistic growth model, I will obtain estimated values of several key parameters governing the dynamics of DNA transfer through conjugation under two different conditions: i) in the absence of selection; ii) in the presence of negative selection pressure — bacteriophage infection. Last, I will investigate spontaneous mutation through qPCR assay of competition between wild-type and mutator phenotype E. coli. Mutator phenotype has an elevated mutation rate due to defects in DNA proofreading and repairing system. By introducing antibiotic selective pressure, I will examine the fixation probability of mutators competing with wild-type in novel environment. I also will utilize simulations to study the impact of three parameters on the fixation probability.

  • POTENTIAL ENERGY LANDSCAPE OF PARTICULATE MATTER

    Author:
    Kun Wang
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Hernan Makse
    Abstract:

    The application of concepts from equilibrium statistical mechanics to out of equilibrium systems has a long history of describing diverse systems ranging from glasses to granular materials. These systems are considered "complex" since equilibrium statistics is insufficient in its attempt to describe the system dynamics. An appealing approach for understanding these complex systems is to study the properties of the system's "potential energy landscape" (PEL), described by the 3N-coordinates of all particles in the multi-dimensional configuration space, or landscape, of the potential energy of the system (N is the number of particles). For dissipative jammed systems- granular materials or droplets- a key concept introduced by S. Edwards in 1989 is to replace the energy ensemble describing conservative systems by the volume ensemble. However, this approach is no able to describe the jamming critical point (J-point) for deformable particles like emulsions, whose geometric configurations are influenced by the applied external stress. Therefore, the volume ensemble requires augmentation by the ensemble of stresses. Just as volume fluctuations in the Edwards ensemble can be described by compactivity, the stress fluctuations give rise to an angoricity, another analogue of temperature in equilibrium systems. In this Thesis, we test the combined volume-stress ensemble for granular matter by comparing the statistical properties of jammed configurations obtained by dynamics with those averaged over the ensemble as a test of ergodicity. Agreement between both methods suggests the idea of "thermalization" at a given angoricity and compactivity. These intensive variables elucidate the thermodynamic order of the jamming phase transition by showing the absence of critical fluctuations above jamming in static observables like pressure and volume. Our results demonstrate the possibility of calculating important observables such as the entropy, volume, pressure, coordination number and the distribution of interparticle forces to fully characterize the scaling laws near the jamming transition from a statistical mechanics point of view. We also study the energy-landscape network. We find the stable basins and the first order saddles connecting them, and identify them with the network nodes and links, respectively. We analyze the network properties and model the system's evolution.

  • QUASI-NORMAL MODES IN RANDOM MEDIA

    Author:
    Jing Wang
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Azriel Genack
    Abstract:

    This thesis is an experimental study of microwave transmission through quasi-one-dimensional random samples via quasi-normal modes. We have analyzed spectra of localized microwave transmitted through quasi-one-dimensional random samples to obtain the central frequency, linewidth and field speckle pattern of the modes for an ensemble of samples at three lengths. We find strong correlation between modal field speckle patterns. This leads to destructive interference between modes which explain strong suppression of steady state transmission and of pulsed transmission at early times. At longer times, the rate of decay of transmission slows down because of the increasing prominence of long-lived modes. We have also studied the statistics of mode spacings and widths in localized samples. The distribution of mode spacings between adjacent modes is close to the Wigner surmise predicted for diffusive waves, which exhibit strong level repulsion. However, a deviation from Wigner distribution can be seen in the distribution of spacings beyond the nearest ones. A weakening in the rigidity of the modal spectrum is also observed as the sample length increases because of reduced level repulsion for more strongly localized waves. In contrast to residual diffusive behavior for level spacing statistics, the distribution of level widths are log-normal as predicted for localized waves. But the residual diffusive behavior can be seen from the smaller variance of the normalized mode width as compared to predictions for strongly localized waves. We also measured the steady state and dynamic fluctuations and correlation of localized microwave transmitted through random waveguides. We find the degree of intensity correlation first increases, and then decays with time delay, before increasing dramatically. The variation in the spatial correlation of intensity with time delay is due to the changing effective number of modes that contribute to transmission. A minimum in correlation is reached when the number of modes contributing appreciably to transmission peaks. At long times, the degree of intensity correlation and the variance of total transmission increase dramatically. This reflects the reduced role of short-lived overlapping states and the growing weight of long-lived spectrally isolated modes.

  • STATISTICAL MECHANICS OF JAMMED MATTER

    Author:
    Ping Wang
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Hernan Makse
    Abstract:

    In a thermal system, the Brownian motion of the constituent particles implies that the system dynamically explores the available energy landscape, such that the notion of a statistical ensemble applies. For densely packed systems of interest in this study, in which enduring contacts between particles are important, the potential energy barrier prohibits an equivalent random motion. At first sight it seems that the thermal statistical mechanics do not apply to these systems as there is no mechanism for averaging over the configurational states. Hence, these systems are inherently out of equilibrium. On the other hand, if the granular material is gently tapped such that the grains can slowly explore the available configurations, the situation becomes analogous to the equilibrium case scenario. It has been shown that the volume of the system is dependent on the applied tapping regime, and that this dependence is reversible, implying ergodicity. This result gives support to the proposed statistical ensemble valid for dense, static and slowly moving granular materials which was first introduced by Edwards and Oakeshott in 1989. Through this approach, notions of macroscopic quantities such as entropy and compactivity were also introduced to granular matter.

  • Dipolar interactions, long range order and random fields in a single molecule magnet, Mn12-acetate

    Author:
    Bo Wen
    Year of Dissertation:
    2013
    Program:
    Physics
    Advisor:
    Myriam Sarachik
    Abstract:

    In this thesis, I will present an experimental study of two single molecule magnets, Mn12–ac and Mn12–ac–MeOH. I will show that in both systems, the temperature dependence of the inverse susceptibility yields a positive intercept on the temperature axis (a positive Weiss temperature), implying the existence of a ferromagnetic phase at low temperature. Applying a magnetic field in the transverse direction moves the Weiss temperature downward towards zero. This implies that the transverse field triggers mechanisms in the system that compete with the dipolar interaction and suppress the long–range ordering. I will then show that the suppression in Mn12–ac is considerably stronger than that expected for a pure TFIFM (Transverse Field Ising Ferromagnetic) model system. By contrast, the behavior of Mn12–ac–MeOH is consistent with the model. We attribute the difference between the two systems to the presence of randomness in Mn12–ac associated with isomer disorder. Thus, in addition to spin–canting and thermal fluctuations, which contribute to the suppression of long–range order in both materials in the same way, the random fields due to isomer disorder that exist in Mn12–ac and not in Mn12–ac–MeOH causes further suppression of ferromagnetism in Mn12–ac. The behavior observed for Mn12–ac is consistent with a random field model calculated for this system by Millis et al.

  • Time Reversal Optical Tomography and Decomposition Methods for Detection and Localization of Targets in Highly Scattering Turbid Media

    Author:
    Binlin Wu
    Year of Dissertation:
    2013
    Program:
    Physics
    Advisor:
    Swapan Gayen
    Abstract:

    New near-infrared (NIR) diffuse optical tomography (DOT) approaches were developed to detect, locate, and image small targets embedded in highly scattering turbid media. The first approach, referred to as time reversal optical tomography (TROT), is based on time reversal (TR) imaging and multiple signal classification (MUSIC). The second approach uses decomposition methods of non-negative matrix factorization (NMF) and principal component analysis (PCA) commonly used in blind source separation (BSS) problems, and compare the outcomes with that of optical imaging using independent component analysis (OPTICA). The goal is to develop a safe, affordable, noninvasive imaging modality for detection and characterization of breast tumors in early growth stages when those are more amenable to treatment. The efficacy of the approaches was tested using simulated data, and experiments involving model media and absorptive, scattering, and fluorescent targets, as well as, "realistic human breast model" composed of ex vivo breast tissues with embedded tumors. The experimental arrangements realized continuous wave (CW) multi-source probing of samples and multi-detector acquisition of diffusely transmitted signal in rectangular slab geometry. A data matrix was generated using the perturbation in the transmitted light intensity distribution due to the presence of absorptive or scattering targets. For fluorescent targets the data matrix was generated using the diffusely transmitted fluorescence signal distribution from the targets. The data matrix was analyzed using different approaches to detect and characterize the targets. The salient features of the approaches include ability to: (a) detect small targets; (b) provide three-dimensional location of the targets with high accuracy (~within a millimeter or 2); and (c) assess optical strength of the targets. The approaches are less computation intensive and consequently are faster than other inverse image reconstruction methods that attempt to reconstruct the optical properties of every voxel of the sample volume. The location of a target was estimated to be the weighted center of the optical property of the target. Consequently, the locations of small targets were better specified than those of the extended targets. It was more difficult to retrieve the size and shape of a target. The fluorescent measurements seemed to provide better accuracy than the transillumination measurements. In the case of ex vivo detection of tumors embedded in human breast tissue, measurements using multiple wavelengths provided more robust results, and helped suppress artifacts (false positives) than that from single wavelength measurements. The ability to detect and locate small targets, speedier reconstruction, combined with fluorophore-specific multi-wavelength probing has the potential to make these approaches suitable for breast cancer detection and diagnosis.

  • RENORMALIZATION OF QCD UNDER LONGITUDINAL RESCALING

    Author:
    JING XIAO
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Peter Orland
    Abstract:

    Under a longitudinal rescaling of coordinates x0,3 → λ x0,3 , λ << 1, the classical QCD action simplifies dramatically. This is the high-energy limit, as λ ∼ s-1/2 where s is the center-of-mass energy squared of a hadronic collision. We find the quantum corrections to the rescaled action at one loop, in particular finding the anomalous powers of λ in this action, with λ < 1. The method is an integration over high-momentum components of the gauge field. This is a Wilsonian renormalization procedure, and counterterms are needed to make the sharp-momentum cut-off gauge invariant. Our result for the quantum action is found, assuming |lnλ| << 1, which is essential for the validity of perturbation theory. If λ is sufficiently small (so that |lnλ| >> 1), then the perturbative renormalization group breaks down. This is due to uncontrollable fluctuations of the longitudinal chromomagnetic field.

  • Crystal Growth and Neutron Scattering Studies of High Temperature Superconductors

    Author:
    Zhijun Xu
    Year of Dissertation:
    2011
    Program:
    Physics
    Advisor:
    Jiufeng Tu
    Abstract:

    Since the discovery of the first high temperature superconductor in the 1980's, there have been continuing efforts to understand the mechanism of high-TC superconductivity. Studies on the cuprate systems seem to suggest that there is an intimate relationship between superconductivity and magnetism, and recently this has also shown to be the case for the newly discovered Fe-based superconductors. Neutron scattering is a powerful tool for studying magnetism in superconductors, which can provide important information about the momentum and energy dependence of magnetic correlations. The work presented in this thesis is divided into two main sections. Since high-quality large-size single crystals are necessary for the neutron scattering experiments. The first section is about sample preparation, where I will introduce single crystal growths via the Floating-zone technique as well as unidirectional solidi¯cation methods. The second section is neutron scattering experiments, which will show neutron scattering and transport measurements results in two high-temperature superconductor systems: La2-xBaxCuO4 (LBCO), and Fe1+yTe1-xSex (FeTeSe). In the LBCO system, we found that static magnetic order competes with bulk superconductivity. In addition, applying a magnetic field to or adding Zn impurities in the sample will enhance the static magnetic order and suppress the superconductivity. In the FeTeSe system, we found that spin resonance is associated with superconductivity, while resonance and superconductivity are simultaneously suppressed by an applied magnetic field or adding Fe impurities. Our results suggest that the magnetic correlations are important for the superconductivity, and proper tuning of these correlations may be a key for superconductivity.

  • A Classical And Quantum Noise Model

    Author:
    Yejun Yang
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Leon Cohen
    Abstract:

    We develop detailed statistics of a noise model that consists of $N$ independent harmonic oscillators where the total force is given by the sum of the individual forces. This model was first proposed in the paper by Ford, Kac, and Mazur that was aimed at deriving the Langevin equation from first principles. We extend the model and calculate relevant probability distributions and other statistical quantities such as the autocorrelation function. In the usual model one assumes that the initial position and momentum values are stochastic variables that determine the statistical features of the force by ensemble averaging over those quantities. We extend that by also treating the mass and frequency as statistical quantities. We consider both the equilibrium case, that is the canonical distribution for the initial positions and momenta, for the but we also consider other initial distributions and show that this leads to non-stationary autocorrelation functions. One of our basic aims is to also develop this model for the quantum case and compare the results with the classical case. The general approach we use for the calculation of the statistical quantities is by way of the characteristic function. We use the characteristic function approach because the oscillators are independent of each other. However, the quantum characteristic function present unique difficulties because the initial momentum and position operators do not commute. We use the Weyl correspondence to define the quantum characteristic function and we derive explicit expressions for both the pure case and mixtures. We show that many of the statistical quantities can be expressed in terms of the Wigner distribution. In addition, we consider the time-frequency Wigner spectrum of momentum governed by the Langevin equation when the random driving term is quantum noise. We obtaine an explicit equation. The equation is solved exactly and includes both the transient and the stationary part. The time-dependent Wigner spectrum generalizes the result of Wang and Uhlenbeck wherein they showed that for the white noise driving force the power spectrum in the stationary state regime is Lorenzian. We show that our solution reduces to the classical solution when the parameters of the quantum noise are such that the white noise limit is approached and when the long time limit is taken.