The peaks and valleys observed by Nowak et al (Phys. Rev. B54, R12725 (1996)) in the local magnetization M(x) and local relaxation rate S(x) in a single crystal of $\rm Tl\sb2Ba\sb2CuO\sb{6+\delta}$ containing columnar defects are well reproduced by an empirical model where the critical current density $j\sb{C}$ is fractionally enhanced in the vicinity of the matching field $B\sb\phi$ and of a multiple of $B\sb\phi$ by superimposing two broad triangular peaks on a continuous dependence of $j\sb{C}$ on the magnetic flux density B. The model fits the variety of M(x) and S(x) data taking $B\sb{S1}$ and $B\sb{S2}$, the flux density at the first and second peaks in $j\sb{C}$, to have a ratio $B\sb{S2}/B\sb{S1}\approx 2.5.$ The model also makes detailed predictions regarding the structure and location of valleys for S(x) versus B(x) descending in magnitude which were not reported in the above article. We demonstrate that a "staircase" type of dependence of $j\sb{C}$ on B proposed by Radzihovsky (Phys. Rev. Letters 74, 4923 (1995)) and exploited by Beauchamp et al, (Phys. Rev. B52, 13025 (1995)) can account for the peak in $\vert M(x)\vert$ adjacent to B = 0 but cannot generate the two peaks encountered when B(x) is ascending in magnitude nor reproduce the two peaks seen at higher fields when B(x) is descending in magnitude. (Abstract shortened by UMI.)

Chemical beam epitaxy was used to grow GaAs and InP based heterostructures on selectively masked substrates. The natural tendency of crystals to form facets was studied as a function of growth conditions. By exploiting this tendency, patterns were reduced from ${\sim}1$ micron in width to ${\sim}20$ nm in width, and inverted-V crescent quantum wire structures were grown entirely in situ. Photoluminescence spectra were obtained, showing broad multiple peaks which are related to the structure and migration between facets. Redshifts of over 100 meV were obtained for narrow structures. Mesa roughness and PL line width are identified as difficulties for the fabrication technique. A simple analytic model--the only separable model in two dimensions showing geometrical quantum confinement--is developed in the spirit of the Ben-Daniel Duke approximation for a square well. The model is applied to variational estimates for the exciton binding energy in crescent quantum wire structures and is compared to the experimental data.

A SQUID magnetometer has been used to study the N.M.R. signal and the relaxation time of normal aluminum in low magnetic fields as a function of temperature for the purpose of doing thermometry below 1K. The variation of signal amplitude with temperatures and r.f. power is found to agree with the predictions from the Bloch equations. A description is given of the cryostat that used a circulating helium gas heat exchanger to cool the experimental stage to 1.3K and adiabatic demagnetization of a paramagnetic salt to further cool the stage to 240 mK. The N.M.R. spectrum of lithium in a crystal of lithium niobate was also studied at 4.2K. The quadrupole coupling constant e2qQ/h was measured to be 52 +/- 1 KHz and is compared to previous results. The design of a new cryostat to operate below 100 mK to replace the current one in use is discussed. The construction and operation of a liquid helium level indicator is discussed and commented on.

Room temperature optical energy gap values for GaxIn (1-x)As and GaAsxSb(1-x) alloys have been determined from infrared transmission measurements. For GaxIn(1-x)As alloys Burstein effect was observed and the necessary correction to the energy gap was made by finding the Fermi energy from thermoelectric power data. For both the alloy systems value of C the bowing parameter has been obtained. These values show good agreement with the theoretical values of Van Vechten and Bergstresser. Room temperature infrared reflectivity measurements near the plasma edge and high field Hall effect measurements have been made on polycrystalline n-type samples of GaxIn(1-x)Sb, GaxIn (1-x)As and InAsxSb(1-x) alloys. The Spitzer and Fan expressions for dielectric constant (57S1) applicable for degenerate case have been extended to the case of general degeneracy in a Kane type band, the effects of lattice contributions to the dielectric constant being included. This analysis has been applied to the experimental data to give values of effective mass at the bottom of the (000) conduction band minimum of the three alloy systems. These effective mass values show very good agreement with the values obtained by Faraday rotation and magnetothermoelectric power measurements.

Ingots of GaAsySb1-y alloys have been prepared by various directional freezing techniques, the rate of freezing being kept sufficiently small (e.g. ∼ 0.5cm/day) that equilibrium conditions were obtained in cross-sectional slices. Samples from these slices have then been investigated by powder X-ray photographs and an X-ray fluorescence technique to determine the variation of lattice parameter with composition and the range of single phase solid solution. The results show that this system has a solid miscibility gap of maximum range 0.38 < y < 0.61 and that this corresponds to a peritectic reaction. In the range 0 < y < 0.38 the lattice parameter ao follows the Vegard line, but for 0.61 < y < 1.00 a o lies below this line. By annealing samples in the liquid-solid two-phase fields, data have also been obtained on the solidus curve and the peritectic temperature has been shown to be 745 +/- 5C. Solidus isotherms and isoconcentration lines have been experimentally determined for the Ga-In-Sb ternary diagram by means of the technique of annealing samples in the two-phase liquid-solid field and quenching, a method shown previously to give good results for the pseudobinary section. The results are compared with data predicted by the simple solution model. In this analysis, the linear temperature dependence of the interaction parameters alpha Ga-Sb, alphaIn-Sb, alphaGa-In and alpha GaSb-InSb has been examined and also the effect on the predicted data of variation in the values of these parameters. It is shown that while the model can give values of liquidus isotherms in good agreement with the available experimental data, the predicted solidus isotherms and isoconcentration lines are very different from the experimental data. No reasonable variation of the alpha parameters can eliminate this latter discrepancy and it is seen that in the case of this particular ternary system the simple solution model is of no value for prediction of solidus data. Tie-lines, boundaries of three-phase fields, range of immiscibility of the solid solutions, details of the liquidus sheet have been experimentally determined for the Ga-As-Sb ternary diagram again by means of the technique of annealing and quenching. Samples were annealed to equilibrium in either two-phase liquid-solid fields or three-phase liquid-solid-solid fields then quenched in order to freeze in the solid phase(s) in equilibrium in the solution. The results are compared with the predictions of the simple solution model which is successful in predicting a peritectic behaviour for the pseudobinary section of the diagram. The interaction parameters are treated as linearly dependent on temperature except for alphaGaAs-GaSb found to be constant. The model shows reasonable agreement with the few liquidus data available but fails to reproduce the measured range of the miscibility gap except at the peritectic temperature. No variation of the interaction parameters could bring the theoretical predictions in line with the experimental solidus data. The solid-solid interaction parameters were then tested for a dependence on solidus composition but this latter analysis showed that if any success is to be obtained here, those interaction parameters would have to depend on both the temperature and the solidus composition and in no simple manner.

The dilute Nitride materials, InGaAsN, are studied as promising candidates for optical devices in the telecommunications wavelength range between 1.3 and 1.55mum. Such materials can be grown lattice matched to a GaAs substrate and could remove the need to work with structures based on the InGaAsP/InP materials, which are expensive and have poor temperature performance. There are three main growth techniques for InGaAsN materials, chemical beam epitaxy (CBE), molecular beam epitaxy (MBE), and metal organic chemical vapour deposition (MOCVD). Of these three, MBE and MOCVD are used to grow the structures discussed in this thesis. X-ray diffraction and photoluminescence measurements are used for characterization. X-ray diffraction measurements provide the nitrogen concentration in the material, while photoluminescence measurements give the bandgap energy. The combination of these two results are then compared to the results from a band anti-crossing model that predicts the bandgap energy as a function of Nitrogen concentration. In addition, absorption and photoluminescence excitation measurements are used to investigate the Stokes shift in GaAsN material likely to have large composition non-uniformities. To illustrate the progress made with InGaAsN material growth, processing and characterisation, a broad area laser structure is demonstrated that emits around 1.3mum.

Experimental results acquired with various methods used to optically probe an excitonic Bose-Einstein condensate are presented. The condensate is initially created by a high-intensity pulsed laser illumination (lambda = 532 nm) incident on a high-quality natural single crystal of Cu2O (at T = 1.8 K), having (100) symmetry. The travelling condensate is laterally probed by a laser pulse tuned at the 1S orthoexciton resonance (lambda = 609.51 nm), where significant condensate amplification is observed. Correspondingly, the resonant probing beam is additionally attenuated upon being transmitted through the excitonic packet. In an attempt to measure the condensate's lateral and longitudinal dimensions, the additional attenuation (NDA) is determined at various probing beam positions relative to the perpendicularly propagating packet. A three-dimensional representation of the exciton packet was constructed with spatially dependent NDA measurements. Highly detailed continuous spectra of the 1S line were taken with the use of a tunable dye laser, permitting the observation of never before seen features in the 1S line. The wavelength dependance of both the condensate amplification and the lateral pulse's additional attenuation were studied using this technique. The onset of a secondary exciton packet observed in various excitation geometries, further contributed to the amplification model proposed in previous work. Moreover, a strong correlation between electrical and all-optical measurements was found, providing reassurance on the validity of past interpretations based on electrical measurements.

Original experimental data for the temperature dependence of the Raman shift in silicon and gallium sulfide is presented. An all optical thermal conductivity method, using an equation presented by Perichon et a1.2 and Nonnenmacher et al. 23, is applied to silicon and gallium sulfide. Theoretical calculations by Moody et al.20 and Nissam et al. 22 are compared to the experimental results of Perichon et al. for the first time. The thermal conductivity of silicon is measured at 23°C, 200°C, and 300°C, and of gallium sulfide at 23°C. We investigate the possibility of measuring thermal conductivity at high pressures.

Ferrihydrite is a natural iron hydroxide occurring mostly in surface waters. In the last 40 years, six different structures have been proposed for ferrihydrite, without reaching consensus in the scientific community. The contradictory debate around the nanostructure of ferrihydrite represents the principal motivation of the present thesis, i.e., to verify systematically all structure proposals instead of just the most accepted ones. We compare powder x-ray diffraction patterns recorded from synthetic samples of 2-lines and 6-lines ferrihydrite with the calculated ones (Debye sum) from the proposed structures. The comparisons show that all structure propositions are incorrect. Although, three of the six propositions suggest jointly that oxygen atoms form a double-hexagonal close packing structure in ferrihydrite and that the exact structure determination problem could be solved by finding the correct positions of the iron atoms inside the oxygen layers.

Tunnel junctions for use in solar cells and monolithic multi junction solar cells are studied experimentally. The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction having a mesa resistance of 0.11 mO·cm2 is determined using time-averaged measurements. A tunneling peak higher than the operating point of a solar cell is recorded by this method, with a value of ∼950 A/cm2. Due to the unstable nature of the negative differential resistance region of the current density-voltage curve, measurements of the tunneling peak and valley current densities are obscured. A time-dependent analysis is performed on this sample, from which a tunneling peak of a value larger than 1100 A/cm 2 is determined. An A1GaAs/InGaP tunnel junction having a tunneling peak of 80 A/cm2 is presented. Multi junction solar cells fabricated using indium tin-oxide as transparent top electrodes are measured. These cells have a maximal efficiency of 25.1% at 3 suns illumination and 26.1% at 20 suns, ∼40% lower efficiency than the standard multi junction solar cell.