Since the discovery of a giant magnetocaloric effect in Gd5Si 2Ge2 that is associated with a first-order magnetostructural transition, the R5SixGe4- x materials (where R=Y,La-Lu) have come under intensive study. Systematic structural data has now been obtained for most of the R5Si xGe4-x compounds (Y, Pr, Nd, Gd, Tb, Er, and recently Yb), and from this structural data, trends have been begun to emerge. The Yb5SixGe 4-x system however defies all conventional wisdom and adopts the orthorhomic Gd5Si4 structure, independently of x, a unique feature in the R5SixGe 4-x family. The magnetic transition temperatures of the Yb5SixGe4- x compounds have been found to be monotonically increasing from 2.4 K to 3.2 K with increasing Ge content contrary to trend measured in other R5SixGe4- x compounds. Moreover, magnetic susceptibility suggests the presence of both Yb2+ and Yb3+ in all compounds at a 60:40 ratio. / In this study, we have performed cold-source 170Yb Mossbauer spectroscopy on samples that have been previously fully characterized, and analyzed our data with an exact solution to the full Hamiltonian. / We conclusively confirm the presence of both Yb2+ and Yb3+ ions in all samples, however with a ratio closer to 50:50. Previous attempts to assign specific crystallographic sites for them fail. We also find the magnetic transition temperature to be far lower than was indicated by the susceptibility data. More strikingly we find that the magnetic behavior of the x = 0 and 4 members is similar, with a single transition temperature at similar temperatures, whereas the middle members, x = 2, 3, have not only considerably different magnetic behaviour (we find two separate magnetic components with different ordering temperatures), but also lower transition temperatures. This system is considerably more complex than was originally thought.

Observations from a broad range of astrophysical scales have forced us to the realization that the well understood matter comprising the stars and galaxies we see around us accounts for only a small fraction of the total mass of the Universe. An amount roughly five times larger exist in the form of dark matter about which we have virtually no direct evidence apart from its large scale gravitational effects. It is also known that the largest contribution to the energy density of the universe is the dark energy, a negative pressure form of energy which will not be dealt with here. I will present a candidate for the dark matter which is based completely in known physics and which presents several possible observational signatures. In this model the dark matter is composed of dense nuggets of baryonic matter and antimatter in a colour superconducting state. If these object are sufficiently massive their low number density will make them effectively dark in the sense that collisions with visible matter become infrequent. This work presents the basics of dark matter as a colour superconductor and then uses the physical properties of the quark nuggets to extract observational consequences.

I have used an analytic model of tidal interactions to predict the evolution of a substructure in a static dark matter halo. Given the initial conditions of the satellite and background halo, the model predicts with high accuracy the mass loss of the satellite and also its density evolution. The main phenomena taken into account in the model are tidal truncation at the tidal radius of the satellite and heating due to tidal shocks at the pericenter of its orbit. To calibrate and test the model, it has been compared with numerical simulations of a satellite orbiting in a static dark matter halo. The model predicts a set of tidal radii for the satellite in different stages of its evolution. The mass of the satellite is accurately calculated at each stage by truncating an NFW (Navarro, Frenk and White) profile at the tidal radius. The mass lost beyond the tidal limit is scaled by half the instantaneous orbital period of the satellite. The model can also be used to predict analytically the new density profile of the satellite. This new profile is given by a modification of the NFW density profile as a function of radius. The tidal radius is the only parameter going into this modification. The effect of numerical relaxation has been studied and quantified by performing the same simulations in lower resolutions. I find that substructures with less than 1000 particles are artificially relaxed and this process affects their mass loss and results in their premature disruptions. This underlines the utility of an analytic model predicting the evolution of substructures in minor mergers.

<p>Multifunctional polymer nanocomposites (PNCs) are attractive for the design of tunable RF and microwave components such as flexible electronics, attenuators, and antennas due to cost-effectiveness and durability of polymeric matrices. In this work, three separate PNCs were synthesized. Magnetite (Fe₃O₄) and cobalt ferrite (CFO) nanoparticles, synthesized by thermal decomposition, were used as PNC fillers. Polymers used in this work were a commercial polymer provided by the Rogers Corporation (RP) and polyvinylidene fluoride (PVDF). PNCs in this thesis consist of Fe₃O₄ in RP, CFO in RP, and Fe₃O₄ in PVDF. Characterization techniques for determining morphology of the nanoparticles, and their resulting PNCs, include x-ray diffraction, transmission electron microscopy and magnetometry. </p><p> All magnetometry measurements were taken using a Quantum Design Physical Property Measurement System with a superconducting magnet. Temperature and external magnetic field magnetization measurements revealed that all samples exhibit superparamagnetic behavior at room temperature. Blocking temperature, coercivity and reduced remnant magnetization do not vary with concentration. Tunable saturation magnetization, based on nanoparticle loading, was observed across all PNCs, regardless of polymer or nanoparticle choice, indicating that this is an inherent property in all similar PNC materials. </p><p> Tunability studies of the magneto-dielectric PNCs were carried out by adding the PNC to cavity and microstrip linear resonator devices, and passing frequencies of 1&ndash;6 GHz through them in the presence of transverse external magnetic fields of up to 4.5 kOe, provided by an electromagnet. Microwave characteristics were extracted from scattering parameters of the PNCs. In all cases, losses were reduced, quality factor was increased, and tunability of the resonance frequency was demonstrated. Strong magnetic field dependence was observed across all samples measured in this study. </p>

Measurements of the complex dielectric constant of ceramic barium titanate have been made at temperatures ranging from 300&deg;K down to 2.4&deg;K. The frequencies employed ranged from 50 kilocycles per second up to 2 megacycles per second with the majority of the measurements at 500 kilocycles per second. The real part of the dielectric constant, K ', had a value of about 1100 at room temperature, decreasing to approximately 100 in the liquid helium temperature region (2--4&deg;K). The slope of K' against the temperature had a high positive value of 0.01 per &deg;K in agreement with Wul. The imaginary part of the dielectric constant, K&Prime;, had a value of about 4 at room temperature, rising to a very broad maximum of approximately 15 at 200&deg;K and steadily decreasing below 80&deg;K to room temperature values below 4 in the liquid helium region. The experimental work was done in the Collins cryostat of the low temperature laboratory of The Rice Institute, and the impedance measurements were made with a Boonton Q-meter and a General Radio Type 916-A impedance bridge. In addition to the pure barium titanate measurements, measurements were also made on specimens diluted with 15% and 30% strontium titanate respectively. The curves obtained for diluted specimens had the same general appearance as those for the pure barium titanate except that the sharp maximum in the K' curve, normally occurring at 390&deg;K, was brought near room temperature in case of the 15% dilution and to 274&deg;K for the 30% dilution. This lowering of the temperature of the peak is in accord with results reported by von Hipple, Coursey and Brand, and Jackson and Reddish.

Abstract Not Available.

Abstract Not Available.

Although Zachariasen was able to determine the gross structure of sodium bicarbonate, his assumptions preclude any conclusions to be drawn concerning the nature of the carbonate group itself. It is therefore the purpose of this paper to reinvestigate the sodium bicarbonate structure employing modern Weissenberg photography and least squares refinement techniques with particular emphasis placed on the determination of the carbonate group configuration. Zachariasen's parameters and bond distances will appear in conjunction with the results reported here to facilitate ease of comparison.

The problem set up as the objective of the present thesis is the detection and measurement of adiabatic thermal changes in barium titanate ceramic at low temperatures. Concurrent with this is the problem of devising experimental techniques suitable for the investigation of this phenomenon in other dielectrics at liquid helium temperatures, a temperature region in which no previous measurements of this type have been made.

Although it was in fact the study of the Wigner distribution function that suggested to the author the general problem discussed in Part I, a consideration of the question of the formation of quantum mechanical operators corresponding to dynamical variables, being of fundamental importance to quantum theory, should properly precede a discussion of the Wigner distribution function. In both Parts I and II spin and relativistic effects will be ignored. A word about notation is in order. A symbol such as A will denote the quantum mechanical operator corresponding to the dynamical quantity A. For example, H is the Hamiltonian operator; q is the coordinate operator and p the conjugate momentum operator. Whenever there is a possibility of confusion, O(A) is used to represent the operator corresponding to the dynamical quantity A. For example, O(pq) is the operator corresponding to the product pq, and O(H2) is the operator corresponding to the square of the classical Hamiltonian. A subscript appearing on a differential operator, such as in 66PH , indicates the function on which the operator acts. Unless otherwise indicated all integrations are to be taken from -infinity to infinity. As usual, h denotes Planck's constant divided by 2pi. Since any physical investigation is directly dependent on the quality of the tools used in the investigation, mathematical rigor is much to be desired in physical theory. That rigor has not been achieved in this work is all too obvious, but it was nonetheless a constant goal.