The accurate determination of site populations from $\sp{57}$Fe Mossbauer spectroscopy requires that spectra be corrected for absorber thickness effects and that site-specific Mossbauer recoilless fractions (f-factors) be known. This thesis uses a novel method to perform thickness-effect corrections, then uses the temperature dependence of the thickness-corrected spectral areas of selected peaks to obtain site-specific f-factors at 80 K and 300 K for the iron sites in true trioctahedral micas. A preliminary study of a biotite single crystal showed the site-specific nature of the f-factors. Five samples--three biotites rich in octahedral Fe$\sp{3+}$, a phlogopite rich in tetrahedral Fe$\sp{3+}$, and a synthetic annite--were then studied to quantify the site-specific f-factors. Our final values, taken to be representative of all true trioctahedral micas, are:++$$\vbox{\halign{#\hfil&&\quad#\hfil\cr&octahedral Fe$\sp{2+}$ sites&f$\sp{\lbrack 2+\rbrack}$ (300 K)&= 0.56 $\pm$ 0.06\cr&&f$\sp{\lbrack 2+\rbrack}$ (80 K)&= 0.82 $\pm$ 0.02\cr&octahedral Fe$\sp{3+}$ sites&f$\sp{\lbrack 3+\rbrack}$ (300 K)&= 0.69 $\pm$ 0.07\cr&&f$\sp{\lbrack 3+\rbrack}$ (80 K)&= 0.87 $\pm$ 0.07\cr&tetrahedral Fe$\sp{3+}$ sites&f$\sp{\langle 3+\rangle}$ (300 K)&= 0.75 $\pm$ 0.10\cr&&f$\sp{\langle 3+\rangle }$ (80 K)&= 0.90 $\pm$ 0.03\cr}}$$(TABLE/EQUATION ENDS)These f-factors are used in conjunction with thickness-corrected sub-spectral areas to obtain the first accurate iron site populations for annite:$$\eqalign{\rm\lbrack Fe\sp{2+}\rbrack /Fe&= (90.5 \pm 1.0)\ \%\cr \lbrack\rm Fe\sp{3+}\rbrack /Fe&= (5.3 \pm 1.0)\ \%\cr \langle\rm Fe\sp3+\rangle /Fe&= (4.2 \pm 0.3)\ \%\cr}$$Experimental uncertainties related to each part of the work are considered in detail, and evaluated where possible.

The effects of an applied bias in the longitudinal or growth direction on four In$\sb{\rm x}$Ga$\sb{\rm 1-x}$As-GaAs strained single quantum wells and three strained layer superlattices have been studied using photocurrent and electroreflectance spectroscopy at liquid helium temperature. Weak applied electric fields on the quantum well samples gives rise to a red quadratic shift to the lowest interband transition between the first confined electron (E1) and heavy-hole (H1) levels, the quantum confined Stark effect (QCSE). The magnitude of the QCSE increases with well width. This field dependence becomes subquadratic at high applied fields due to carrier accumulation on the low energy side of the wells. Superlattices with relatively small periods, i.e. 10 nm, exhibit interwell coupling giving rise to a miniband structure under flatband conditions. The application of an electric field removes the interwell coupling giving rise to a ladder like progression in energy for the interband transition energies, called Wannier Stark ladders. The measured exciton transition energies follow a linear field dependence given by the product of the Stark ladder index, the superlattice period, and the electric field. The low field behaviour is more complex due to the Coulomb interaction between the electrons and heavy-holes. The measured field dependent exciton transition energies for the quantum wells agree well with single particle model calculations, while for the superlattice samples the exciton Stark ladder calculations of Dignam and Sipe have yielded good agreement with the measured data.

Resonant and non-resonant tunneling processes in p-type Si/Si$\sb{\rm 1-x}$Ge$\sb{\rm x}$/Si double barrier structures were investigated at cryogenic temperatures in both planar and perpendicular magnetic fields. A differential conductance measurement technique was devised to observe non-resonant tunneling features superimposed on a rapidly increasing current background. An additional resonant feature was observed at high bias voltage and is attributed to tunneling of holes from the two-dimensional heavy hole subband in the emitter to a third quasi-bound state in the quantum well. Experimental evidence for non-resonant tunneling processes is presented for the first time in p-type double barrier structures. Inter-Landau level transitions between the emitter and the quantum well occur either elastically by scattering from impurities or inelastically with the emission of optical phonons. Both processes have been observed in our samples. Data above 4.2 K suggest that tunneling processes also take place from the three-dimensional valence band edge states in the emitter. A novel spectroscopy of the quasi-bound states in the well was performed by measuring the shift in energy of the resonant tunneling features as a function of applied planar magnetic field.

We present a mean field model for the magnetism of random alloys. The reader is introduced to the philosophy of the mean field approach with an example of a simple ferromagnet. The special problem presented by random arrangement of alloy components on a lattice is discussed before we describe the technique we have developed. Our approach simplifies the treatment of these complicated systems by classifying sites according to the characteristics of their local environments. We apply this technique to both a fictional 1-D chain and to a real 3-D alloy series (fcc Fe-Ni). Fair success in modelling the complex magnetic behaviour characteristic of mixed exchange alloys (such as flattened magnetization curves, departure of the saturation moment from the Slater-Pauling curve, a complex phase diagram, and para-process susceptibility) is achieved.

Dilute magnetic semiconductor alloys based on I-III-VI$\sb2$ chalcopyrite structure compounds and containing either iron or manganese as the magnetic component, were investigated in the context of a comparative study. The implications and effects of crystallographic ordering were of particular interest to this research. As a result of these experimental investigations, the phase diagram of the CuIn$\sb{\rm 1-x}$Fe$\sb{\rm x}$S$\sb2$ system was proposed, and certain modifications to the previously accepted phase diagrams of the systems containing manganese were given. For the iron containing system, the magnetic behaviour was interpreted in terms of a Curie-Weiss law, and the applicability of ligand field theory was investigated as a theoretical basis for understanding the results so obtained. It was determined that a transition similar to the $\sp6$A$\sb1$-$\sp4$T$\sb1$ crossover of crystal field theory may occur as a function of iron concentration in this material. Various computer fits and simulations were carried out in order to analyze the diffraction, and magnetic susceptibility measurements made on the alloys containing manganese. Besides displaying the usual non-magnetic-ion type of cationic ordering as in the transition from zincblende to chalcopyrite structure, these systems have been shown to exhibit behaviour characteristic of magnetic-ion crystallographic ordering (Mn-ordering). These ordering properties have been shown to be highly sensitive to sample heat treatment. The diffraction techniques were found to be insensitive to the types of Mn- ordering suspected to occur, however these alloys yielded direct evidence for the Mn-ordering in the form of their static, low field magnetic susceptibility behaviour. Of particular interest, some of the low temperature annealed, high concentration magnetic ion materials were found not to display the low temperature spin-glass cusp characteristic of the rapidly cooled samples, and this was interpreted to be the clearest signature of the Mn-ordering. The manganese systems may deviate from the chalcopyrite structure in the low temperature region of the phase diagram. The magnetic behaviour of these alloys was shown to be consistent with a long-range exchange, mean-field analysis of the planar ordering model. (Abstract shortened by UMI.)

We present new evidence for cross flow and cutting of nonparallel flux lines in type II superconductors. A dramatic reversal is observed in the evolution of $B\sb{z}\sb{hole}$, the axial flux density in the cavity of a hollow cylinder when the magnitude of a helical magnetic field, $\vec H\sb{ext} = \\phi H\sb\phi + \ z H\sb{\rm II}$ is increased or decreased along the surfaces of the specimen. This turn around constitutes a 180$\sp\circ$ degree deviation from the predictions of the "classical" critical state framework (no flux line cutting). Measurements of the concurrent evolution of $\langle B\sb{z}\rangle \sb{wall}$, the axial flux density threading the wall of the hollow cylinder, complement the above data. The behaviour of $\langle B\sb{z}\rangle\sb{wall}$ is also in drastic disagreement with the "classical" picture of "one way" traffic of flux lines dictated solely by the change in magnitude of $\vec H\sb{ext}.$ These observations also provide strong support for our interpretation. Our account of these unexpected phenomena is based on the ideas of two way traffic of sublattices of nonparallel flux lines traversing each other via flux line cutting processes. We briefly review the classical critical state concept. The essential features of the flux cutting process, cross traversal of flux line sheets and attendant "breathing" modes are also outlined. Then the Generalized Critical State Model incorporating a phenomenological framework based on Maxwell's equations, standard physical constraints and two separate energy dissipation mechanisms is summarized. Finally we present our data curves and show in qualitative detail that the observed behaviour demonstrates that flux line cutting occurs and associated breathing in and out of non-parallel flux lines takes place across the surface of type II superconductors subjected to a varying $\vec H\sb{ext}.$

The photoelectromagnetic (PEM) effect was used to perform experiments on photocarriers' transport properties in GaAs substrates and epitaxial layers. For the undoped semi-insulating substrate, the carriers' mobility $\mu$ at T = 77K was measured by PEM effect method, giving a value of $\mu$ = 40,000 cm$\sp2$ V$\sp{-1}$s$\sp{-1}$. It was found that the carriers' diffusion length is L$\sb{\rm D}$ = 0.27 $\mu$m at T = 300K and L$\sb{\rm D}$ = 0.44 $\mu$m at T = 77K. For the 3.4$\mu$m thick epitaxial layer grown by metal-organic chemical vapour deposition (MOCVD), the carriers' diffusion length was measured to be L$\sb{\rm D}$ = 2.6 $\mu$m at T = 300K, L$\sb{\rm D}$ = 3.1 $\mu$m at T = 77K and L$\sb D \approx$ 3.5 $\mu$m at T = 5K. A photocurrent (PC) spectroscopy study of In$\sb{\rm x}$Ga$\sb{\rm 1-x}$As/GaAs quantum wells on Cr-doped and undoped substrates has been performed. Several well-resolved structures related to inter-subband transitions are observed in the photoconductivity spectra for both T = 300K and T = 77K, and they are in good agreement with calculation taking into account the strain-induced splitting of the bands. Both Cr-doped and undoped substrates have been found to influence the results, as shown by optical quenching techniques.

Zn-doped, p-type, GaAs and $\rm Ga\sb{0.85}In\sb{0.15}As$ samples grown by low-pressure metal organic vapour phase epitaxy with free carrier concentrations in the range of n = 4.3 $\times$ 10$\sp {\rm cm}\sp{-3}$ (nominally undoped)--p = 1.95 x 10$\sp{20} {\rm cm}\sp{-3}$ at room temperature have been studied by temperature-dependent photoluminescence. At low doping levels, recombinations involving discrete impurity states and free excitons provided measurement of both the 5 K band gap ($E\sb{g}(5)=(1.296\pm0.003)$ eV) and the zinc acceptor binding energy $(E(Zn\sp0)=(0.025\pm0.003)$ eV) in the $\rm Ga\sb{0.85}In\sb{0.15}As$ alloy. At high concentrations, the discrete acceptor levels are replace by an impurity band which merges with the valence band above the Mott$\sp{\lbrack 1\rbrack}$ transition. This gives rise to a density of states band tail extending into the gap and containing both extended and localised states. In the presence of such a high density of impurities, potential fluctuations and interparticle interactions result in a band gap shrinkage $\vert \Delta E\sb{g}\vert$ which has been observed with photoluminescence experiments. A model based on the presence of Kane$\sp{\lbrack 2\rbrack}$ band tails and on the assumption of a constant matrix element for the relevant optical transitions has been fitted to the photoluminescence spectra of heavily doped layers of GaAs and $\rm Ga\sb{0.85}In\sb{0.15}As$ in the range of $p=1.6\times 10\sp $ cm$\sp{-3}-p=1.95\times 10\sp{20}$ cm$\sp{-3}.$ This model provided a good description of the experimental results. The 5 K band gap shrinkage has been found to be $\vert\Delta E\sb{g}\vert=2.7\times 10\sp{-8}p\sp{1/3}$ for GaAs and $\vert\Delta E\sb{g}\vert=1.4\times 10\sp{-8}p\sp{1/3}$ for $\rm Ga\sb{0.85}In\sb{0.15}As$ with $\vert\Delta E\sb{g}\vert$ in Ev and p in cm$\sp{-3}.$

The Bayesian statistical inference theory is studied and applied to two problems in applied physics: spectral analysis and parameter estimation in time series data and hyperfine parameter extraction in Mossbauer spectroscopy. The applications to spectral analysis and parameter estimation for both single- and multiple-frequency signals are presented in detail. Specifically, the marginal posterior probabilities for the amplitudes and frequencies of the signals are obtained by using Gibbs sampling without performing the integration, no matter whether the variance of the noise is known or unknown. The best estimates of the parameters can be inferred from these probabilities together with the corresponding variances. When the variance of the noise is unknown, an estimate about the variance of the noise can also be made. Comparisons of our results have been made with results using the Fast Fourier Transformation (FFT) method as well as Bretthorst's method. The same numerical approach is applied to some complicated models and conditions, such as periodic but non-harmonic signals, signals with decay, and signals with chirp. Results demonstrate that even under these complicated conditions the Bayesian inference and Gibbs sampling can still give very accurate results with respect to the true result. Also through the use of the Bayesian inference methods it is possible to choose the most probable model based on known prior information of data, assuming a model space. The Bayesian inference theory is applied to hyperfine parameter extraction in Mossbauer spectroscopy for the first time. The method is a free-form model extraction approach and gives full error analysis of hyperfine parameter distributions. Two applications to quadrupole splitting distribution analysis in Fe-57 Mossbauer spectroscopy are presented. One involves a single site of Fe3+ and the other involves two sites for Fe3+ and Fe2+. In each case the method gives a unique solution to the distributions with arbitrary shape and is not sensitive to the elemental doublet parameters. The Bayesian inference theory is also applied to the hyperfine field distribution extraction. Because of the complexity of the elemental lineshape, all the other extraction methods can only use the first order perturbation sextet as the lineshape function. We use Blaes' exact lineshape model to extract the hyperfine field distribution. This is possible because the Bayesian inference theory is a free-form model extraction method. By using Blaes' lineshape function, different cases of orientations between the electric field gradient principle axis directions and the magnetic hyperfine field can be studied without making any approximations. As an example the ground state hyperfine field distribution of Fe65Ni35 Invar is extensively studied by using the method. Some very interesting features of the hyperfine field distribution are identified.

Experimental results concerning ballistic excitonic transport in high quality cuprite single crystals are presented and discussed. The onset conditions of this anomalous transport, in the form of a packet propagating at near sound velocity, is compatible with theoretical predictions for Bose-Einstein condensation of n = 1 paraexcitons. Experiments involving the interaction between two packets reveal coherence properties, in agreement with an interpretation assuming an exciton Bose-Einstein condensate as the origin of the packet. The amplification of the packet by thermal excitons and the creation of a stationary condensate in the form of a filament are discussed in terms of radiationless exciton scattering induced by the condensate.