52 research outputs found
A Study of the Interfacial Configuration of Alq3 and Co Bilayer in Organic Spin Valves
No full textThe interfacial electronic structure of the organic material- tris(8-hydroxyquinolinato)aluminum (Alq3) forming an interface with cobalt metal has been investigated in this research. The primary characterization method used in this research was near-edge X-ray absorption fine structure (NEXAFS) spectroscopy which probes the unoccupied molecular orbitals of a material. Density functional theory (DFT) calculations have also been employed to calculate the partial density of states (PDOS) of all constituent elements present in Alq3 molecule. The DFT calculations helped to determine the molecular orbital structure of Alq3 and to understand how the orbital structure is influenced by forming an interface with ferromagnetic Co layer. The experimental NEXAFS spectra measured in total fluorescence yield (TFY) showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 states of Alq3 were not affected by the presence of Co when Co is deposited onto Alq3. On the other hand, a charge transfer between Co and Alq3 led the loss or reduction of LUMO+2 state for a Co(top)/Alq3 bilayer sample when compared to pristine Alq3 reference sample (without Co deposition). This selective effect of Co on the orbital configuration of Alq3 suggests that Co atoms diffuse into Alq3 and interact with preferred sites in Alq3. By comparing the spectral change in the experimental NEXAFS spectra to the calculated PDOS of Alq3, the preferred interaction sites between Co and Alq3 could be successfully determined. This work suggests that the spectroscopic approach using synchrotron-radiation X-ray spectroscopy can serve as a powerful means for studying the interfacial electronic structure between magnetic metals and organic semiconductors and can contribute to the research and development of high performance organic spintronics
Fabrication and characterization of nanocrystalline silicon LEDs : a study of the influence of annealing
No full textThis thesis describes the fabrication of a set of bright, visible light-emitting silicon LEDs. These devices were fabricated in-house at the University of Saskatchewan using a custom plasma ion implantation tool, an annealing furnace, and a physical vapour deposition system. A high-fluence (F = 4 × 1015 cm^−2) implantation of molecular hydrogen ions extracted from an RF inductively coupled plasma at an energy of 5 keV was used to create a heavily damaged region in the silicon centered approximately 40 nm below the silicon surface with a width of approximately 56 nm. A matrix of annealing (e.g. thermal processing) processes at 400 ºC and 700 ºC and different durations (30 minutes and 2 hours) as well as an aluminum gettering procedure were tested with the goal of increasing the output electroluminescence intensity. Current-voltage characterization was used to extract information about the defect-rich nanocrystalline, light-emitting layer as well as the Schottky barrier height. This enabled comparison of these new devices with previous silicon LEDs based on porous silicon and other approaches. The processes which were used to fabricate these devices are compatible with standard CMOS processing techniques and could provide one solution to the problem of optical interconnect on multi-core chips. The scientific significance of this work is the demonstration of bright, visible light emission at mean photon energies ∼1.84 eV corresponding to a photon wavelength of λ ≈ 675 nm. This is remarkable given that ordinary crystalline silicon is an indirect bandgap material with a bandgap energy of 1.1 eV, in which band-to-band radiative recombination is forbidden by momentum conservation. The devices fabricated in this thesis have light emission properties similar to previous silicon LEDs based on nanocrystalline or nanoporous silicon. They have the advantage of being easily electrically driven. The nanocrystalline region which is the source of the light emission was nucleated from the ion-implanted layer below the surface of the silicon. This makes these devices mechanically robust and insensitive to environmental conditions. The engineering significance of this work is the production of CMOS compatible light emitters. This study demonstrated increased light emission efficiency at higher annealing temperatures which is likely due to enhanced diffusion and nucleation of silicon nanocrystals in the ion-implant damaged layer
Fubini-Study geometries in the higher-dimensional gravity
No full textWe construct approximate solutions to the Einstein-Maxwell theory with
uplifting the four dimensional Fubini-Study Kahler manifold. We find the
solutions can be expressed as the integrals of two special functions. The
solutions are regular almost everywhere except a bolt structure on a single
point in any dimensionality. We also show that in the context of considered
ansatzes for the metric function and the Maxwell field, the solutions are
unique and can not be non-trivially extended to include the cosmological
constant in any dimensions.Comment: 21 pages, 9 figures, typos corrected, references added, some revisio
Holography for Black Holes in General Relativity and Beyond
No full textBoth extremal and non-extremal Kerr black holes have been considered to be holographically dual to two-dimensional (2D) conformal field theories (CFTs). In this thesis, we study the holography to the case of a rotating Janis-Newman-Winicour (JNW) black holes, a rotating Brans-Dicke-Kerr (BDK) black hole, and an asymptotically anti-de Sitter (AdS) rotating charged black holes in () gravity, where , where is a constant. Firstly, we find that the rotating JNW solution does not satisfy the Einstein field equation. Thus, we could not establish a well-defined Kerr/CFT correspondence in this theory. Secondly, we find that the scalar wave equation in the background of BDK black hole is not separable. The existence of the symmetry can be found in the radial equation of the scalar probe around the non-extremal black hole. Therefore, the inseparability of the scalar wave equation eliminates the possibility of any holography aspect for BDK black hole. Thirdly, we find that the scalar wave radial equation at the near-horizon region implies the existence of the 2D conformal symmetries. We note that the identification of the azimuthal angle in the black hole line element, corresponds to a spontaneous breaking of the conformal symmetry by left and right temperatures and , respectively. We show that choosing proper central charges for the dual CFT, we produce exactly the macroscopic Bekenstein-Hawking entropy from the microscopic Cardy entropy for the dual CFT. These observations suggest that the rotating charged AdS black hole in () gravity is dual to a 2D CFT at finite temperatures and for a specific value of mass , rotational, charge, and () parameters, , , and \abs{\alpha}, respectively
Study of the electron density in the high latitude ionosphere with incoherent scatter radars and Swarm satellites
No full textDespite century-long efforts in assessing the electron density distribution in the Earth’s ionosphere
and significant progress in identification of its major features, there are still many uncertainties
with respect to the solar cycle’s seasonal and diurnal trends at various latitudes. Knowledge of
these variations is needed for robust forecasting of the state of the ionosphere for the operation of
practical radio systems, for example communication via high frequency radio waves.
This thesis utilizes data from the incoherent scatter radar (ISR) at Poker Flat (Alaska, USA)
to assess the diurnal and seasonal variations of three parameters of the ionospheric F2 region, the
peak density, the height of the peak, and the thickness of the layer. These parameters are assessed
for relatively high solar activity in 2014, and relatively low solar activity in 2016. Daytime electron
densities were found to be largest during winter and spring and nighttime electron densities were
found to be smallest in winter. Electron densities during the higher solar activity year were found
to be greater than those during the lower solar activity year by a factor of 2-5, depending on the
time of day, as expected. Details of the diurnal variations in electron density for various seasons
are further discussed.
ISR electron density data are also used for the validation of electron density measurements
from Langmuir probes onboard the Swarm satellites in the topside ionosphere (~500 km). This
work is an expansion of previous studies that use a different mode of ISR operation and a different
approach to both ISR and Swarm satellite data handling. In addition to observations over Poker
Flat (geographic latitude of ~60° N), observations over Resolute Bay, Canada at extreme high
latitudes of ~80° are also considered. It is shown that, overall, the ratio of Swarm electron density
measurements to those measured by ISRs is ~0.5-0.6 and that smaller ratios are observed at larger
electron densities, usually during the daytime. At low electron densities less than 3∙1010 m-3
, the
ratios are typically greater than 1, indicating an overestimation effect. The overestimation effect is
stronger at night and at higher altitude. It is also more evident during lower solar activity when the
electron density in the topside ionosphere is smaller. The conclusions on the electron density
underestimation and overestimation by the Swarm Langmuir probes are overall consistent with
previous reports, but this thesis confirms that these effects also occur at high latitudes
Toroidal Flow Velocity Measurement in the STOR-M Tokamak by Ion Doppler Spectroscopy
No full textAn Ion Doppler Spectrometer (IDS) was built for the purpose of toroidal ion flow velocity measurement in the STOR-M tokamak. Emission lines from carbon and oxygen ions C_III, O_V and C_VI were used for spectral profiles measurement. Based on the Doppler effect, the ion flow velocities were calculated from the spectral line shift.
The IDS system was built based on a Czery-Turner spectrometer. A cylindrical rod lens was used to magnify the grating dispersion. The spectrum was recorded by a photomultiplier tube (PMT) detector, whose output current was converted to voltage through a current preamplifier.
The IDS achieved a time resolution of 0.1 ms defined by the RC time, and the ion flow velocity resolution is 1 ~ 2 km/s. Profile measurement shows the impurity emission lines at different ionization stage populated at different radial positions in the chamber. The flow velocities were measured with various operation modes. A flow velocity shear was discovered for different ion species. The ions in the edge region are flowing in the same direction as plasma current, while the ions in the plasma center are flowing in the anti-current direction. The impurity flow directions are inversed with inversed plasma current configuration. It was found that the impurity flow velocities can be changed by compact torus (CT) injection, resonant magnetic perturbation (RMP) field, as well as rapid hydrogen puffing
Structure and atomic dynamics in condensed matter under pressure and Li-ion battery materials
No full textThe main goal of this research was to apply first-principles electronic structure calculations to investigate atomic motions in several condensed materials. This thesis consists of five separate but related topics that are classified into two main categories: structure of materials under pressure and Li ion dynamics in lithium battery materials.
The atomic structure of liquid gallium was investigated in order to resolve a controversy about an anomalous structural feature observed in the x-ray and neutron scattering patterns. We explored the pressure effect when modifying the liquid structure close to the solid-liquid melting line. The atomic trajectories obtained from first-principles molecular dynamics (FPMD) calculations were examined. The results clarified the local structure of liquid gallium and explained the origin of a peculiar feature observed in the measured static structure factor. We also studied the structure of a recently discovered phase-IV of solid hydrogen over a broad pressure range near room temperature. The results revealed novel structural dynamics of hydrogen under extreme pressure. Unprecedented large amplitude fluxional atomic dynamics were observed. The results helped to elucidate the complex vibrational spectra of this highly-compressed solid.
The atomic dynamics of Li ions in cathode, anode, and electrolyte materials - the three main components of a lithium ion battery - were also studied. On LiFePO4, a promising cathode material, we found that in addition to the commonly accepted one-dimensional diffusion along the Li channels in the crystal structure, a second but less obvious multi-step Li migration through the formation of Li-Fe antisites was identified. This discovery confirms the two-dimensional Li diffusion model reported in several Li conductivity measurements and illustrates the importance of the distribution of intrinsic defects in the enhancement of Li transport ability. The possibility of using type-II clathrate Si136 as an anode material was investigated. It was found that lithiated Si-clathrates are intrinsic metals and their crystal structures are very stable. Calculations revealed the charge and discharge voltages are very low and almost independent of the Li concentrations, an ideal property for an anode material. Significantly, migration pathways for Li ions diffusing through the cavities of the clathrate structures were found to be rather complex. Finally, the feasibility of a family of Li3PS4 crystalline and nanoporous cluster phases were studied for application as solid electrolytes. It was found that the ionic conductivity in the nanocluster is much higher than in crystalline phases. It is anticipated that the knowledge gained in the study of battery materials will assist in future design of new materials with improved battery charge and discharge performance
Vector Charmonium Meson-Hybrid Mixing: a Field Theoretical Analysis
No full textQuantum chromodynamics (QCD) is the quantum field theory describing strong interactions.
Hybrids in QCD, which are bound states consisting of a charm and an anticharm quark
with a constituent gluon, have been theorized for some time. In this thesis we begin to
explore the idea that perhaps these particles exist as quantum mechanical superpositions
of hybrid and pure mesonic states (which are bound states consisting of a quark and an
antiquark). In particular, we will be interested in vector charmonium (charm-anticharm)
meson-hybrid mixing. Here we do a field theoretical analysis of charmonium meson-hybrid mixing in the JPC = 1−− channel; the two point cross-correlator has been calculated to
leading order in the strong coupling (αs). We include the perturbative, four dimensional
and six dimensional condensate contributions. The perturbative contribution was found
to contain non-polynomial divergences which were addressed through the introduction of
operator mixing. The results of this calculation are presented in a form that is ready for a
QCD sum rules analysis
PeV Scale Dark Skyrmions
No full textThe background information relevant to understanding skyrmions and dark matter is first reviewed. PeV scale skyrmions as a coherent state of dark heavy mesons with an Anderson-Higgs portal coupling to the Standard Model are analyzed as dark matter. Their creation would have been non-thermal via a chiral phase transition in the early universe. It turns out that it does not matter whether the transition is First Order or Second Order. Constraints are put on the skyrmion parameter g_V^2M_S, where g_V is the Skyrme coupling and M_S is the skyrmion mass, which indicate that it is possible for the skyrmions to be PeV scale and for g_V ≲ 1. For the dark mesons to not contribute significantly to dark matter they must be TeV scale if g_wh, their coupling to the Anderson-Higgs field, is to be perturbative. Considering the dark skyrmions as coherent states of the dark mesons gives an expression for the skyrmion-nucleon scattering cross section in terms of g_V, g_wh, and M_S. Direct dark matter detection experiment results are used to constrain g_V in terms of g_wh for 100TeV ≲ M_S ≲ 10PeV. It is possible for g_V to be of perturbative strength, especially for skyrmion masses closer to 10PeV. Future direct detection experiments will yield even more stringent constraints
New Class Of Exact Solutions To Einstein-Maxwell-Dilaton Theory On Four-Dimensional Bianchi Type IX Geometry
No full textWe construct new classes of cosmological solution to the five dimensional Einstein-Maxwell-dilaton theory, that are non-stationary and almost conformally regular everywhere. The base geometry for the solutions is the four-dimensional Bianchi type IX geometry.
In the theory, the dilaton field is coupled to both the electromagnetic field and the cosmological constant term, with two different coupling constants. We consider all possible solutions with different values of the coupling constants, where the cosmological constant takes any positive, negative or zero values.
In the ansatzes for the metric, dilaton and electromagnetic fields, we consider dependence on time and two spatial directions. We also consider a special case of the Bianchi type IX geometry, in which the geometry reduces to that of Eguchi-Hanson type II geometry and find a more general solution to the theory
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