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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Arrays of interacting nanostructures: Ferromagnetic and superconducting cases

Ursache, Andrei E 01 January 2007 (has links)
Arrays of ultra high-density (1.2 Tera/in2) vertical ferromagnetic cobalt nanowires with aligned shape and crystal magnetic-anisotropy axes are fabricated by electro-deposition in nanoporous polymer templates. The nanoporous films are derived from self-assembling PS-b-PMMA diblock-copolymers, which can provide lateral structure dimensions on the order of 10 nm, small enough to make cobalt nanowires in magnetic single-domain regime. An optimized fabrication procedure is developed for arrays of cobalt nanowires with enhanced perpendicular-to-plane magnetic anisotropy, by combining effects of shape anisotropy with perpendicularly oriented uniaxial magneto-crystalline anisotropy of hcp Co. Special conditions of electrolyte pH (pH>5.1) are required to obtain the desired c-axis crystal orientation, and pulse electrodeposition conditions are found to improve the overall perpendicular magnetic anisotropy, resulting in array-coercivities as large as 2.7 kOe at 300 K. X-ray diffraction and SQUID magnetometry measurements are used to characterize their structural and magnetic properties. A numerical model for the magnetization dynamics of an array of single-domain particles with vertically-aligned anisotropy axes is presented, which includes finite temperature effects, and magnetostatic interactions in a mean-field approximation. The model can qualitatively describe most features of the magnetization behavior, such as the temperature dependence of array-coercivity, "shearing" of the magnetization curves and the reduction of array-coercivity due to array demagnetization effects. Qualitative agreement is also obtained with the observed slow-relaxation magnetization decay of the cobalt nanowire arrays. To investigate particle non-uniformity effects, the model was further extended to include a distribution in energy barriers and particle volumes. The last part is concerned with the experimental investigation of the collective behavior of arrays of superconducting lead (Pb) nanowires with diameters smaller than the coherence length, which are coupled to each other by Andreev reflection at the S-N point contact interfaces with an underlying normal metal film. The system is characterized by magnetization and electrical transport measurements and is found to behave like an effective-medium type II superconductor with vortex pinning.
22

Superfluid and superconducting transitions in low-dimensional systems

Balabanyan, Karen G 01 January 2007 (has links)
Superfluid and superconducting phase transitions of interacting bosons are observed in diverse physical systems such as helium, superconducting materials, ultra-cold atomic gases, etc. For all these systems one-, two-, and three-dimensional (1D, 2D, and 3D) realizations are experimentally accessible. However, the current experimental focus is shifting to the low dimensional systems (e.g., films and wires) due to facilitated measurement techniques and a constant demand of technological applications. From a theoretical standpoint large fluctuations in 1D and 2D systems make them a challenging problem, especially when disorder is present. Here we study peculiarities associated with disorder in low-dimensional cases. Specifically, we are interested in the class of the systems where superfluid or superconducting transitions can be mapped to the finite temperature 2D superfluid-normal Kosterlitz-Thouless (KT) transition: (a) 2D bosonic system with a strong diagonal disorder, (b) 1D system of lattice bosons with off-diagonal disorder in the limit of large integer filling factor at zero temperature, and (c) 3D layered superconductors with no tunnelling between layers in zero external magnetic field.
23

Adsorption to carbon nanotubes

Kahng, Yung Ho 01 January 2005 (has links)
We have probed the adsorption property of single-wall carbon nanotube (SWNT) bundles using the temperature-programmed desorption technique. The SWNT sample cleanliness effect on the 4He adsorption was investigated. Room air contacting significantly decreased the 4He adsorption capacity. The 4He adsorption vs. pump-out temperature on SWNT samples and on charcoal was obtained. A two-state binding site model did not fit well to the SWNT data, while it fit well to the charcoal data indicating the 4He binding energy on charcoal to be 400 ± 32 K which agreed with other group's value. Using the desorption rate isotherm analysis technique, we obtained coverage dependant 4He binding energies on SWNT bundles. Our values agreed with other group's results at near 400 K where the coverages overlapped, and our energy value increased to a much higher value at near 900 K at lower coverages beyond the lowest coverage of other group. The 4He addition temperature was changed from 273 K to lower values in the 8–40 K range for three SWNT samples and a charcoal sample. While the 4He adsorption was not sensitive on the addition temperature on charcoal, it was different on SWNT samples. Some sites were not accessible for 4He atoms at low temperatures. The 4He access to these sites increased as the gas addition temperature increased, and at 35 K and above a full 4He access to a 273 K dosed level was observed. An activated diffusion model fit to the 4He amount, vs. gas addition temperature data yielded the activation energy for diffusion to be 28 ± 14 K and 47 ± 6 K on two samples. One sample showed more restricted 4He access for 4He at 15 K. This sample had more impurities. Codesorption measurements were done on SWNT samples. Xe in smaller quantity (6% level) than 4He and H2, suppressed the adsorption of other gases to the background level. H2 suppressed 4 He to the background level, when added in equal amount at 273 K. However when 4He was added at 273 K and H2 was added later at 19 K, H2 did not suppress the 4He adsorption. Equal mixture doses of 4He and 3He at 273 K yielded 8.4 times more 4He binding than 3He. This strong isotope selectivity agreed with the predicted quantum sieving effect.
24

Dynamics of polyelectrolyte solutions and diblock copolymer in an electric field

Ashok, Balakrishnan 01 January 2004 (has links)
We present a theoretical treatment of the dynamic properties of polymer and polyelectrolyte solutions, incorporating the screening associated with hydrodynamic, excluded-volume and electrostatic interactions. The importance of inter-chain interactions in influencing properties like the viscosity of a polyelectrolyte solution is emphasized. This treatment is then extended to cover the case of living polymers, specifically, the rheological properties of wormlike micelles. We also investigate the effect of simple shear and elongational flows on the configurational and rheological properties of a single polymer chain. The coil-stretch transition of a polymer under elongational flow was investigated, results indicating a possible first order transition. We also study the behavior of confined diblock copolymers in lamellar and cylindrical morphologies when placed in an external electric field. The crucial parameter influencing the value of the critical electric field above which the diblock orients itself parallel to the field, is found to be the mismatch in the interfacial tensions between polymer and plate, with a mismatch in dielectric constants of the copolymer components also playing a role.
25

Inelastic gas: An experimental study of vibro-fluidized dilute granular media

Feitosa, Klebert Bezerra 01 January 2004 (has links)
We conduct an experimental study of a two dimensional vibro-fluidized dilute granular medium. The system is composed of spherical beads confined to move in a vertical plane and excited by intense vertical vibrations. We perform full-field tracking of positions and orientations of the spheres by high speed photography. In steady-state, the motion of the grains resembles that of a molecular gas, thus the name granular gas. We study the distribution of linear velocities in the granular gas. The investigation shows that the distributions are non-gaussian, best fitted by the function P(v) ∼ exp(−β: v:/σ)1.5), and insensitive to number density, driving parameters and particle inelasticity. The distribution is a one parameter distribution, parameterized by the mean square velocity; which defines a granular temperature. T = ½ ⟨v 2⟩. We study binary mixtures of the granular media. We find that, in general, the granular temperature is not equal for the two types of spheres. However, the temperature ratio is constant in the bulk. The ratio depends strongly on the mass ratio of the spheres, but not on their inelasticity. The ratio is also insensitive to compositional parameters of the mixture such as number fraction and number density. We also investigate the statistics of the power flux into a subsystem of the granular gas. The power shows large fluctuations, including frequent large negative fluctuations. The relative probabilities of positive and negative fluctuations in the power flux are in close accord with the Fluctuation Theorem of Gallavotti and Cohen (Gallavotti & Cohen, 1995b). We also compare the effective temperature that emerges from this analysis to the kinetic granular temperature. Finally, we study the rotational dynamics of the granular gas. We find that the granular temperature is not equipartitioned between translational and rotational degrees of freedom. We also demonstrate that the ratio of rotational to translational energy is independent of the vibration intensity and concentration of particles, if we isolate the component of rotational energy that is fed only by inter-particle collisions.
26

Studies of helium films on disordered and patterned substrates

Luhman, Dwight Ryan 01 January 2006 (has links)
The results of a series of experiments involving rough surfaces, adsorbed helium films, and helium films adsorbed on rough surfaces are reported. A comprehensive study using atomic force microscopy of the surface roughness of thermally deposited CaF2 of varying film thicknesses indicates that the surface roughness of the CaF2 film increases systematically with CaF2 film thickness while the porosity of the film remains constant. The roughness, growth, and dynamic exponents are found to be α = 0.88±0.03, β = 0.75±0.03, and z = α/β = 1.17±0.06 respectively. In addition, we find evidence indicating that the growth dynamics may be dominated by noise described by a power-law probability distribution. The nature of the superfluid or Kosterlitz-Thouless transition of helium films adsorbed on rough CaF2 surfaces was studied. For T = 1.672 K the signature of decoupled mass observed on crossing the Kosterlitz-Thouless transition as a function of 4He film thickness decreases and becomes increasingly difficult to identify as the surface roughness is increased. A peak in the dissipation, indicative of the onset of superfluidity, changes little with roughness. In a separate experiment with smaller 4He film coverages and thus lower temperatures, the presence of surface roughness does not have a significant effect on the frequency shift at the Kosterlitz-Thouless transition. These results are interpreted as an indication that a varying film thickness, as would be present in higher temperature experiments due to capillary condensation and absent in the lower temperature experiments, induces disordered into the system which in turn broadens the transition. Continuous third sound experiments on substrates patterned with a disordered and ordered array of rough CaF2 scatterers are reported. In the disordered case we compare the experimental results to theoretical predictions of classical wave localization in such systems. The application of a continuous superfluid flow across the substrate breaks the time-reversal symmetry of the system and is predicted to effect the transmission properties. The application of a flow is found experimentally to have no effect on the transmission properties of the substrates. The evolution of third sound pulse shapes in the presence of a superfluid flow was also studied.
27

Cluster algorithms and computational complexity

Li, Xuenan 01 January 2002 (has links)
Cluster algorithms for the 2D Ising model with a staggered field have been studied and a new cluster algorithm for path sampling has been worked out. The complexity properties of Bak-Seppen model and the Growing network model have been studied by using the Computational Complexity Theory. The dynamic critical behavior of the two-replica cluster algorithm is studied. Several versions of the algorithm are applied to the two-dimensional, square lattice Ising model with a staggered field. The dynamic exponent for the full algorithm is found to be less than 0.5. It is found that odd translations of one replica with respect to the other together with global flips are essential for obtaining a small value of the dynamic exponent. The path sampling problem for the 1D Ising model is studied using both a local algorithm and a novel cluster algorithm. The local algorithm is extremely inefficient at low temperature, where the integrated autocorrelation time is found to be proportional to the fourth power of correlation length. The dynamic exponent of the cluster algorithm is found to be zero and therefore proved to be much more efficient than the local algorithm. The parallel computational complexity of the Bak-Sneppen evolution model is studied. It is shown that Bak-Sneppen histories can be generated by a massively parallel computer in a time that is polylog in the length of the history, which means that the logical depth of producing a Bak-Sneppen history is exponentially less than the length of the history. The parallel dynamics for generating Bak-Sneppen histories is contrasted to standard Bak-Sneppen dynamics. The parallel computational complexity of the Growing Network model is studied. The growth of the network with linear kernels is shown to be not complex and an algorithm with polylog parallel running time is found. The growth of the network with γ ≥ 2 super-linear kernels can be realized by a randomized parallel algorithm with polylog expected running time.
28

Dielectric susceptibility of the supercooled liquids: Glycerol and triphenyl phosphite

Win, Kyaw Zin 01 January 2006 (has links)
We measure the dielectric susceptibility &egr;(ν) of supercooled liquid glycerol and map it out as a function of pressure, temperature and volume. We build a high pressure cell made of beryllium copper to exert high pressure (up to 900 MPa) on glycerol at low temperature (near glass transition temperature). The frequency range is from 0.01 Hz to 100 kHz. We also measure the susceptibility of triphenyl phosphite, TPP, to study its polyamorphic phases. We find that the glass transition temperature, the fragility, and the width of the dielectric loss spectrum of glycerol increase with pressure. We introduce the notion of the generalized fragility to separate the effects due to volume and temperature on the glass transition and conclude that both volume and temperature are comparably important for glycerol. We point out a connection between the generalized fragility and a recently discovered volume-temperature scaling exponent and find that our conclusion holds for many glass formers. The relation between the traditional fragility and the width is compatible with a correlation found between these quantities at 1 atmosphere. The width, too, changes more quickly as a function of fractional changes in volume than the corresponding change in temperature. We verify the Nernst-Einstein relation between the ionic conductivity and the relaxation frequency at various pressures and find that the effective hydrodynamic radius in this relation decreases with density. We have measured &egr;(ν) of TPP from 0.01 Hz to 10 kHz. The glacial phase of TPP has a longer relaxation time, wider loss spectrum, and lower dielectric constant compared to the supercooled liquid. The width of &egr;(ν) in the glacial phase increases with temperature. The dielectric function is measured as a function of time while TPP is transforming from the supercooled to the glacial and the glacial to the crystal. We present a differential effective medium calculation to investigate the possibility that the glacial phase is a mixture of liquid and crystal. We have also videotaped, in real time, the phase transformation from the supercooled phase to the glacial. Based on these investigations we conclude that the glacial phase is a new amorphous phase, distinct from the supercooled liquid.
29

EXPERIMENTAL INVESTIGATIONS OF CARBON-DIOXIDE AND SULFUR-HEXAFLUORIDE IN THE VICINITY OF THEIR CRITICAL POINTS

PUGLIELLI, VINCENT GEORGE 01 January 1970 (has links)
Abstract not available
30

THE EMISSIVITY OF TUNGSTEN IN THE NEAR INFRARED.

SAPP, EDWIN ROBERT 01 January 1975 (has links)
Abstract not available

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