Recent Submissions

  • Infrared Spectroscopy of Ge:Mn Thick Films Prepared by Ion Implantation and Post-annealing

    Obied, Laila Hassan; Department of Physics
    An infrared transmission study of Ge:Mn systems is presented in this work. Various Ge:Mn samples have been prepared by both single (Mn2+ ion energy 4.76 MeV, dose 2x10^16 /cm^2) or multiple (dose 1x10^16 /cm^2) into high resistivity (100) Ge substrates. Both conventional and flash lamp annealing procedures have been used to prepare a set of samples that were characterized by X-ray Diffraction (XRD), Secondary Ion Mass Spectrometry (SIMS) and magnetometry as well as infrared transmission spectroscopy (60-8000 cm^-1). After implantation, the Ge:Mn layers are about 3 mm thick and amorphous as revealed by XRD. Samples conventionally annealed at temperatures below 250C maintained their amorphous structure. Samples annealed at temperature above 330C recrystallized into a polycrystalline structure. SIMS measurements show that the diffusion of Mn is greatly affected by the starting distribution and that diffusion is much higher in samples prepared with the lower total Mn dose. The presence of multiple magnetic phases in all the annealed samples was indicated by field and temperature dependent measurements of the total moment. Our diverse preparations suggest that the formation of secondary phases in Ge:Mn system in unavoidable. Magnetic measurements suggest that the maximum volume fraction of metallic Mn5Ge3 inclusions is of the order of 1x10^-3, much lower than the percolation threshold of metallic inclusions in an insulating matrix. Infrared transmission measurements on the same samples show the presence of a low frequency Drude like absorption with high hole concentration (p~10^ 18 /cm^3) which is greater than the critical density for metal-insulator (MI) transition in Ge. This observation suggests Mn has crystallized in substitutional sites to a far greater degree than was achieved with traditional equilibrium crystal growth. The MI transition has been investigated in the Ge:Mn system by studying the temperature dependent transmission between 4K and 300K and comparing to similar measurements on a sample of Ga-doped Ge with p = 1.5x10^16 /cm^3 which is on the insulating side of the MI transition. Finally, absorption was observed that may possibly be attributed to Mn5Ge3 inclusions in the Ge:Mn matrix.
  • Nonlinear dynamics of granular assemblies

    Przedborski, Michelle; Department of Physics
    In this work we investigate granular chains, which are one-dimensional systems of discrete macroscopic particles interacting via the intrinsically nonlinear Hertz law. Such systems support the propagation of solitary waves (SWs), which are non-dispersive, mobile bundles of energy. A comprehensive analysis into the dynamical behaviour of these systems and the properties of SW propagation is presented, and several interesting new results are obtained. First, we find that the transition to the quasi-equilibrium (QEQ) phase in granular chains can be manipulated by altering the material properties of the system. We further use these results to develop a novel shock absorption device with a predictable and tunable frequency response, making it useful also for energy harvesting applications. Second, we show for the first time that granular chains with various nonlinearities of the contact potential can achieve thermal equilibrium at sufficiently long times, and thus QEQ is an intermediate phase of these systems. We characterize the equilibrium phase by deriving approximate distribution functions for grain velocity and kinetic energy and system kinetic energy in a microcanonical ensemble of interacting particles. As a by-product, we derive the equilibrium specific heat, and a size-dependent correction term, for such systems. We also show how these ideas extend to heterogeneous systems such as diatomic, tapered, and random-mass chains. Furthermore, we look closely at the transition to equilibrium by using statistical tests to show that the long-term dynamics is ergodic, and by examining the behaviour of various correlation functions close to the onset of the transition. Third, we solve a highly nonlinear, fourth-order wave equation that models the continuum theory of long-wavelength pulses in weakly compressed, homogeneous granular chains with a general power-law contact interaction, to characterize all travelling wave solutions admitted by the equation. This involves deriving conservation laws admitted by the wave equation, followed by a modified energy analysis. We find that the wave equation admits various types of travelling wave solutions, including SW solutions as well as nonlinear periodic wave solutions. Not only have the SW solutions not appeared before in the literature on granular chains, but they are also a new addition to the literature on SWs in general.
  • A Multi-Scale Molecular Dynamic Approach to the Study of the Outer Membrane of the Bacteria Psudomonas Aeruginosa PA01 and the Biocide Chlorhexidine

    Van Oosten, Brad; Department of Physics
    The introductory chapters of this thesis contains an explanation to the methods and basic theory of the molecular dynamics approach. Together with the appendix section, in which a step by step tutorial how to set up and run basic simulations using the gromacs software is presented, this thesis can serve as an introductory aid in performing molecular dynamics simulations. In the research portion of this thesis, I provide several uses for the molecular dynamics approach applied to the biocide chlorhexidine as well as the study of membranes, including a mimic of the bacteria membrane of Pseudomonas Aeruginosa PA01. The motivation for this research was previous work done in our lab which determined that chlorhexidine has a high affinity for DMPC and found the depth at which it resides in a model DMPC membrane. From this information, an all-atom representation of chlorhexidine was made, which was proven to reproduce the experimental results. While we learned much about chlorhexidine in a model DMPC membrane, this study lacked the destruction of the membrane as well as the study of chlorhexidine in a biologically relevant membrane. For these reasons coarse grained versions of the all-atom chlorhexidine models as well as a new lipopolysaccharide molecule was created. With the coarse grained model of chlorhexidine and the ability to create a bacterial membrane mimic, the study of chlorhexidine and other antibacterial agents can be further studied.
  • Conservation laws of magnetohydrodynamics and their symmetry transformation properties

    Pshenitsin, Dmitry; Department of Physics
    All kinematic conservation laws along with their symmetry transformation properties are derived for the system of magnetohydrodynamic equations governing incompressible viscous plasmas (or any other conducting fluid) in which the dynamic and magnetic viscosities are constant. Reductions of this system under translation symmetries, axial rotation symmetries, and helical symmetries are considered. For each reduced system, all kinematic conservation laws and point symmetries are obtained. The results yields several new conservation laws which are expected to be relevant for physical applications of magnetohydrodynamics.
  • Theory and Application of a Pure-sampling Quantum Monte Carlo Algorithm

    Ospadov, Egor; Department of Physics
    The objective of pure-sampling quantum Monte Carlo is to calculate physical properties that are independent of the importance sampling function being employed in the calculation, save for the mismatch of its nodal hypersurface with that of the exact wave function. To achieve this objective, we describe a pure-sampling algorithm that combines features of forward-walking methods of pure-sampling and reptation quantum Monte Carlo. The importance sampling is performed by using a single-determinant basis set composed of Slater-type orbitals. We implement our algorithm by systematically increasing an algorithmic parameter until the properties sampled from the electron distributions converge to statistically equivalent values, extrapolated in the limit of zero time-step. In doing so, we are able to unambiguously determine the values for the ground-state fixed-node energies and one-electron properties of various molecules. These quantities are free from importance sampling bias, population control bias, time-step bias, extrapolation-model bias, and the finite-field approximation. We applied our algorithm to the ground-states of lithium hydride, water and ethylene molecules, and found excellent agreement with the accepted literature values for the energy and a variety of other properties for those systems. Some of our one-electron properties of ethylene had not been calculated before at any level of theory. In a detailed comparison, we found reptation quantum Monte Carlo, our closest competitor, to be less efficient by at least a factor of two. It requires different sets of time-steps to accurately determine the ground-state energy and one-electron properties, whereas our algorithm can achieve the same objective by using a single set of time-step values.
  • The Effects of Magnetic Dilution and Applied Pressure on Several Frustrated Spinels

    Korobanik, Jory; Department of Physics
    The effects of magnetic dilution and applied pressure on frustrated spinels GeNi2O4, GeCo2O4, and NiAl2O4 are reported. Dilution was achieved by substitution of Mg2+ in place of magnetically active Co2+ and Ni2+ ions. Large values of the percolation thresholds were found in GeNi(2-x)MgxO4. Specifically, pc1 = 0.74 and pc2 = 0.65 in the sub-networks associated with the triangular and kagome planes, respectively. This anomalous behaviour may be explained by the kagome and triangular planes behaving as coupled networks, also know as a network of networks. In simulations of coupled lattices that form a network of networks, similar anomalous percolation threshold values have been found. In addition, at dilution levels above x=0.30, there is a T^2 dependency in the magnetic heat capacity which may indicate two dimensional spin glass behaviour. Applied pressures in the range of 0 GPa to 1.2 GPa yield a slight decrease in ordering temperature for both the kagome and triangular planes. In GeCo(2-x)MgxO4, the long range magnetic order is more robust with a percolation threshold of pc=0.448. Similar to diluted nickel germanate, at low temperatures, a T^2 magnetic heat capacity contribution is present which indicates a shift from a 3D ordered state to a 2D spin glass state in the presence of increased dilution. Dynamic magnetic susceptibility data indicate a change from canonical spin glass to a cluster glass behaviour. In addition, there is a non-linear increase in ordering temperature with applied pressure in the range P = 0 to 1.0 GPa. A spin glass ground state was observed in Ni(1-x)MgxAl2O4 for (x=0 to 0.375). Analysis of dynamic magnetic susceptibility data yield a characteristic time of tau* = 1.0x10^(-13) s, which is indicative of canonical spin glass behaviour. This is further corroborated by the linear behaviour of the magnetic specific heat contribution. However, the increasing frequency dependence of the freezing temperature suggests a trend towards spin cluster glass formation.
  • Structural, Magnetic and Thermal Studies of Ce1-xEuxCrO3 Nano-Powders

    Taheri, Maryam; Department of Physics
    A new series of nano-sized Ce1-xEuxCrO3 (x = 0.0 to 1.0) with an average particle size of 50 - 80 nm were synthesized using a solution combustion method. Nano-powders Ce1-xEuxCrO3 with the canted antiferromagnetic property exhibited interesting magnetic behaviours including the reversal magnetization and the exchange bias effect. The effect of europium doping as the ion with the smaller radius size and different electron con figuration on structural, magnetic and thermal properties of Ce1-xEuxCrO3 were investigated using various experimental techniques, i.e. DC/AC magnetic susceptibility, heat capacity, thermal expansion, Raman scattering, X-ray photoemission spectroscopy, transmission/scanning electron microscopy, X-ray powder diffraction and neutron scattering. An exchange bias effect, magnetization irreversibility and AC susceptibility dispersion in these samples confirmed the existence of the spin disorder magnetic phase in Ce1-xEuxCrO3 compounds. The exchange bias phenomenon, which is assigned to the exchange coupling between glassy-like shell and canted antiferromagnetic core, showed the opposite sign in CeCrO3 and EuCrO3 at low temperatures, suggesting different exchange interactions at the interfaces in these compounds. The energy level excitation of samples were examined by an inelastic neutron scattering which was in good agreement with the heat capacity data. Neutron scattering analysis of EuCrO3 was challenging due to the large neutron absorption cross-section of europium. All diffraction patterns of Ce1-xEuxCrO3 showed the magnetic peak attributed to the antiferromagnetic Cr3+ spins while none of the diffraction patterns could detect the magnetic ordering of the rare-earth ions in these samples.
  • Investigation of Frustrated Quasi-One-Dimensional Quantum Spin-Chain Materials

    Caslin, Kevin; Department of Physics
    Copper arsenite CuAs2O4 and Copper antimonite CuSb2O4 are S=1/2 (Cu2+ 3d9 electronic configuration) quasi-one-dimensional quantum spin-chain compounds. Both compounds crystallize with tetragonal structures containing edge sharing CuO6 octahedra chains which experience Jahn-Teller distortions. The basal planes of the octahedra link together to form CuO2 ribbon-chains which harbor Cu2+ spin-chains. These compounds are magnetically frustrated with competing nearest-neighbour and next-nearest-neighbour intrachain spin-exchange interactions. Despite the similarities between CuAs2O4 and CuSb2O4, they exhibit very different magnetic properties. In this thesis work, the physical properties of CuAs2O4 and CuSb2O4 are investigated using a variety of experimental techniques which include x-ray diffraction, magnetic susceptibility measurements, heat capacity measurements, Raman spectroscopy, electron paramagnetic resonance, neutron diffraction, and dielectric capacitance measurements. CuAs2O4 exhibits dominant ferromagnetic nearest-neighbour and weaker antiferromagnetic next-nearest-neighbour intrachain spin-exchange interactions. The ratio of the intrachain interactions amounts to Jnn/Jnnn = -4.1. CuAs2O4 was found to order with a ferromagnetic groundstate below TC = 7.4 K. An extensive physical characterization of the magnetic and structural properties of CuAs2O4 was carried out. Under the effect of hydrostatic pressure, CuAs2O4 was found to undergo a structural phase transition at 9 GPa to a new spin-chain structure. The structural phase transition is accompanied by a severe alteration of the magnetic properties. The high-pressure phase exhibits dominant ferromagnetic next-nearest-neighbour spin-exchange interactions and weaker ferromagnetic nearest-neighbour interactions. The ratio of the intrachain interactions in the high-pressure phase was found to be Jnn/Jnnn = 0.3. Structural and magnetic characterizations under hydrostatic pressure are reported and a relationship between the structural and magnetic properties was established. CuSb2O4 orders antiferromagnetically below TN = 1.8 K with an incommensurate helicoidal magnetic structure. CuSb2O4 is characterized by ferromagnetic nearest-neighbour and antiferromagnetic next-nearest-neighbour spin-exchange interactions with Jnn/Jnnn = -1.8. A (H, T) magnetic phase diagram was constructed using low-temperature magnetization and heat capacity measurements. The resulting phase diagram contains multiple phases as a consequence of the strong intrachain magnetic frustration. Indications of ferroelectricity were observed in the incommensurate antiferromagnetic phase.
  • alpha-Tocopherol's Antioxidant Role: A Biophysical Perspective

    Marquardt, Drew TC; Department of Physics (Brock University, 2014-10-30)
    I present evidence of an antioxidant mechanism for vitamin E that correlates strongly with its physical location in a model lipid bilayer. These data address the overlooked problem of the physical distance between the vitamin's reducing hydrogen and lipid acyl chain radicals. The combined data from neutron diffraction, NMR and UV spectroscopy experiments, all suggest that reduction of reactive oxygen species and lipid radicals occurs specifically at the membrane's hydrophobic-hydrophilic interface. The latter is possible when the acyl chain adopts conformations in which they snorkel to the interface from the hydrocarbon matrix. Moreover, not all model lipids are equal in this regard, as indicated by the small differences in the vitamin's location. The present result is a clear example of the importance of lipid diversity in controlling the dynamic structural properties of biological membranes. Importantly, these results suggest that measurements of alpha-tocopherol oxidation kinetics, and its products, should be revisited by taking into consideration the physical properties of the membrane in which the vitamin resides.