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This thesis is a study of magnetic properties of new icosahedral quasicrystals Ag50In36Gd14, Zn77Fe7Sc 16 and a 1/1 crysta lline approximant Ag50In36Gd 14. The Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to have a primitive six-dimensional Bravais lattice with a six-dimensional hypercubic lattice constant of, respectively, 7.805(2) and 7.087(1) A. The crystal structure of the approximant Ag50In36Gd14 is found to be of the YbCd6 type (space group lm3¯) with a lattice constant of 15.202(1) A. No evidence is found for a transition to a ground state with long-range magnetic order in the temperature range 2--300 K for all three alloys. These three alloys are shown to be spin glasses. The spin freezing temperature of the Ag50In36Gd14 and Zn77Fe 7Sc16 quasicrystals is, respectively, 4.25(5) and 7.75(2) K. The unusual spin freezing in the approximant is found to occur in two stages: at ∼3.6 K spins develop short-range correlations but they co ntinue to fluctuate at low frequencies, and then long-range freezing is achieved upon further cooling to below ∼2.4 K. The Ag50In36Gd 14 quasicrystal and its 1/1 approximant are shown to belong to a category of strongly geometrically frustrated magnets. The frequency dependence of the freezing temperature in all three alloys is shown to be equally well accounted for by the Vogel-Fulcher law and the power law. Analysis of the ageing effects observed in the Zn77Fe 7Sc16 quasicrystal leads to a major discovery that the nature of the spin-glass state in this quasicrystal is fundamentally different from that of a canonical spin glass. The effective magnetic moment at Gd atoms in the Ag50In 36Gd14 quasicrystal and its 1/1 approximant is found to be close to that of a Gd+3 ion. The effective magnetic moment at Fe atoms in the Zn77Fe7Sc16 quasicrystal is the largest ever found in an Fe-containing quasicrystal. The hyperfine magnetic fields at 155Gd and 57Fe nuclei in the Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to set in at temperatures larger than the corresponding freezing temperatures. The presence of the distribution of the electric quadrupole splitting in the 155Gd Mossbauer spectra of the Ag50In36Gd14 quasicrystal indicate s the existence of the multiplicity of Gd sites. The binomial distribution of the electric quadrupole splitting and the hyperfine magnetic field derived from the 57Fe Mossbauer spectra Zn77Fe 7Sc16 quasicrystal indicates the existence of two classes of Fe sites. The Debye temperatures of the quasicrystals Ag50In 36Gd14, Zn77Fe7Sc16, and the approximant Ag50In36Gd14 determined from the Mossbauer spectra are, respectively, 199(2),443(8), and 199(1) K.

The electronic specific heat of Cadmium has been determined by a number of independent workers; using two basic methods; magnetic and calorimetric. This thesis pursues, the problem by a calorimetric determination using an aquadag thermometer and a thermal valve. Production of low temperatures is necessary as the transition temperature for this superconductor is about 0.56° K. Hence, the interest lies in attempting to determine the heat capacity of the metal both in the normal and superconducting states. The experimental method and results for specific heat measurements for other superconductors is briefly reviewed. The design of the cryostat is discussed in detail in Chapter II. A rather unique method of preparing a paramagnetic salt pill is also given. Resistance thermometers and determination of temperature very important and the temperature range is obtained by adiabatic demagnetisation. The conclusion and results of this research project is given in Chapter III. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The electrical and magnetic properties of magnetite from 100-300°K have been studied. Specimens were obtained from natural crystals of local origin and fabricated in the form of rectangular bars. When this was not possible, due to excessive cracks in the crystal, the electrical properties of the resulting irregularly shaped disc were measured by use of the Van der Pauw theorem. Observation of the temperature variation of the resistivity showed that the resistivity increased as the temperature was decreased and that it increased by a factor 24 over a 10°K temperature interval when the specimen was cooled below 115°K. This phenomenon is related to an order-disorder transition at this temperature, in which the crystalline symmetry changes from cubic to orthorhombic. One specimen did not show the resistivity transition, probably due to excessive impurity content, which would prevent ordering from occurring. The results are explained qualitatively by the Verwey model, which postulates a conduction mechanism involving the jumping of electrons between octahedral sites of the Fe₃O₄ unit cell, as opposed to the conventional band type of conduction. An attempt was made to measure the Hall mobility of the specimens, but a Hall voltage was not detected within the sensitivity of the apparatus. An upper limit of 1 cm²/volt-sec. for the mobility was established. A negative magnetoresistance effect was observed in both specimens and was measured as a function of temperature for both transverse and longitudinal magnetic fields. A pronounced minimum at the transition temperature was observed for one specimen, while the other showed no change here. The AC permeability was observed over the transition region and again a sharp decrease was observed on cooling through the transition. Normal induction curves were obtained for the specimen at various fixed temperatures from the permeability data, from which it was deduced that the specimen was much harder to saturate below the transition. The permeability on warming was found to be independent of the magnetic state of the sample while cooling through the transition, and it was again observed that one crystal did not undergo the phase transition at 115°K. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Small free radicals trapped in solid argon, krypton and carbon tetrachloride at 4.2°K have been studied using electron spin resonance (ESR). An attempt was made to determine whether the methylene radical, produced by the photolysis of diazomethane and ketene trapped in the solid matrix, has a triplet ground state. No signal definitely attributable to the methylene radical was observed. It is postulated that the zero field splitting due to the spin-spin coupling of the unpaired electrons broadened any ESR signal beyond detectability„ The ESR signal of trapped methyl radicals was observed in some experiments, and it is suggested that they were formed by abstraction of hydrogen atoms from another deposited material by methylene radicals. An experiment in which diazomethane was photolysed in the presence of D₂O in an argon matrix at 4.2°K yielded an ESR signal which may possibly have been due to the CH₂D radical. An investigation has been carried out of the populations of the rotational levels of methyl radicals produced by the photolysis of trapped methyl iodide and dimethyl mercury at 4.2°K. For thermal equilibrium freely rotating radicals should populate only the ground state at this temperature, but it was found that the lowest two levels were both populated. It is suggested that either there was not thermal equilibrium, or, more likely, the methyl radicals were undergoing hindered rotation. Room temperature equilibrium mixtures of N₂F₄ - NF₂ were trapped in the three matrices at 4.2°K, and ESR absorption due to the trapped NF₂ radicals was observed. Three lines were observed at this temperature, with the centre one of greater amplitude and smaller line width than the outer two. During warmup the amplitudes and widths of these lines became approximately equal and two further triplets appeared, symmetrically distributed about the centre line. From the warmup spectra the isotropic hyperfine splitting constants for fluorine and nitrogen have been deduced to be 168 and 48 mc./sec. respectively. It is suggested that the radicals underwent slow isotropic rotation at 4.2°K. The degree of s-character of the molecular orbital containing the unpaired electron is discussed in the light of the isotropic hyperfine splitting constants. An unsuccessful attempt to find hyperfine and rotational structure in the ESR signal of the NF₂ radical in the gas phase was carried out. The photolysis of CF₃I in krypton and carbon tetrachloride matrices at 4.2°K yielded a very complicated ESR spectrum. A phase reversal of some of the lines was observed. A broad single line was observed when CF₃I in carbon tetrachloride was irradiated at 77°K. At the time of writing no definite interpretation of the spectra can be suggested. / Science, Faculty of / Chemistry, Department of / Graduate

Short resumés of the theories of propagation of electromagnetic waves in an anisotropic medium, of the Faraday effect, and of the influence of paramagnetic resonance on the Faraday effect are given. The Poincaré sphere is introduced to describe polarized light. A paramagnetic resonance spectrometer is described, which was modified so as to allow the study of magneto-optical phenomena under the influence of paramagnetic resonance. The spectrometer operated in the X-band using a 2K39 Klystron. The samples were located in a transmission type cavity operating in the TE 101 mode, and immersed in liquid helium. The cavity was provided with holes allowing the passage of light through the sample in a direction parallel to the external magnetic field. An optical system provided plane-polarized monochromatic light ( ג = 5461 Å ) incident on the sample. The light emerging from the sample passed through a Glan-Thompson prism analyzer. The relative intensity of the light passing the analyzer could be measured by a photomultiplier circuit and could be displayed as a function of time on an oscilloscope. Experiments are described in detail in which the spin-lattice relaxation time was measured as a function of temperature and external magnetic field. In these experiments, for a given temperature and magnetic field, the Faraday rotation was reduced by pulses of microwave power of varying length applied to the cavity. The return of the Faraday rotation as a function of time to its equilibrium value after the microwave power was cut off could be inferred from the intensity versus time relationship of the light transmitted by the analyzer. Photographic records of this intensity versus time relationship were obtained and the relaxation time was deduced from these records. Results of the measurement of the spin-lattice relaxation time of neodymium ethylsulfate for fields between 780 and 2540 Oerstedt and temperatures between 1.38°K and 4.22°K are given. The relaxation times measured were of the order of .001 to .1 seconds. The relaxation time appeared to be inversely proportional to the third power of the temperature and showed only small field dependence, except for a large dip at a field corresponding to the resonance field for the microwave frequencies used. Within the accuracy of the experiments no effect of the length of the microwave pulses on the relaxation time could be observed. An experiment on cerium ethylsulfate is described, which showed that the spin-lattice relaxation time must be smaller than 1 millisecond for this salt. No accurate determination of the relaxation time could be made in this case. Mention is made of an "overshoot effect" observed in one particular crystal of neodymium ethylsulfate. A possible explanation for this effect is given, by assuming that the crystal in question was twinned. In this case one may infer that the relaxation time is strongly dependent on the orientation of the optical axis of the crystal with respect to the external magnetic field The results were found to disagree with present-day theories of paramagnetic relaxation. Assumptions of doubtful validity in the theory are discussed as possible reasons for such disagreement. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis looks at the transport phenomena in indium arsenide at low temperatures.

A detailed second law analysis of dual fuel LTC is not yet available in the open literature even though dual fuel low temperature combustion (LTC) has been studied before. To address this gap, a previously validated, closed-cycle, multi-zone, simulation of diesel-natural gas dual fuel LTC was used to perform a second law analysis. In the current study, a 2.4-liter single-cylinder research engine operating at a nominal load of 6 bar BMEP and 1700 rpm was used. Zone-wise thermodynamic irreversibilities as well as total cumulative entropy generated and lost available work over the closed cycle were quantified. Subsequently, two convenient second-law parameters were defined: (1) the “lost available indicated mean effective pressure” (LAIMEP), which can be interpreted as an engine-size-normalized measure of available work that is lost due to thermodynamic irreversibilities (analogous to the relationship between indicated mean effective pressure and indicated work); (2) fuel conversion irreversibility (FCI), which is defined as the ratio of lost available work to total fuel chemical energy input. Finally, parametric studies were performed to quantify the effects of diesel start of injection, intake manifold temperature, and intake boost pressure on LAIMEP and FCI. The results show that significant entropy generation occurred in the flame zone (52-61 percent) and the burned zone (31-39 percent) while packets account for less than 6 percent of the overall irreversibilities. Parametric studies showed LAIMEPs in the range of 645-768 kPa and FCIs in the range of 32.8-39.2 percent at different engine operating conditions. Although the present study focused on dual fuel LTC, the conceptual definitions of LAIMEP and FCI are generally applicable for comparing the thermodynamic irreversibilities of IC engines of any size and operating on any combustion strategy.

In this thesis, I demonstrate Doppler and Sisyphus cooling of a cryogenic buffer-gas beam of CaH molecules. I detail the construction and optimization of the experimental apparatus, including the cryogenic source, laser systems, vacuum systems and detection schemes. I demonstrate that the cryogenic source produces a bright and slow beam of CaH molecules via ablation of a solid chemical target and thermalization with a He buffer gas. The molecular beam exits the ablation cell with an average forward velocity of 250 ±200 m/s and a molecular beam flux per ablation pulse of ≈ 1×1010 per steradian per pulse. I present the spectroscopic determination of the molecular transitions necessary to pursue laser cooling. These include the X2Σ+ → A2Π1/2 and the X2Σ+ → B2Σ+ transitions that each contain two spin-rotation states, J = 1/2 and J = 3/2, and a further pair of hyperfine states, F = 0,1 and F = 1,2, respectively. Finally, I describe the vibrational repumping transitions between the four hyperfine states of the J = 1/2 and J = 3/2 branches of the V = 1 vibrational state back to the ground state via decay from an intermediary state, X2Σ+(V = 1) → B2Σ+(V = 0) → X2Σ+(V = 0). I present measurements of the vibrational decay probabilities from the B2Σ+(V = 0) and A2Π1/2(V = 0) excited states to the V = 0,1 and 2 states of the ground X2Σ+ state. Next, I show that we can achieve a high scattering rate of ≈ 1.6E6 photons/second while cycling on the X2Σ+ → A2Π1/2 transition. Finally, I demonstrate the ability to perform transverse cooling of a beam of CaH molecules through both the Doppler mechanism and magnetically assisted Sisyphus mechanism. With the help of a transverse standing wave of laser light, I show that we are able to lower the molecular beam’s transverse temperature from 12.2±1.2 mK to 5.7±1.1mK. This thesis represents a promising start to laser slowing and magneto-optical trapping of CaH molecules, which could provide trapped ultracold samples of atomic hydrogen upon dissociation of the trapped CaH molecules.