Near band edge optical properties of MBE GaAs and related layered structures

The research reported in this thesis involves two main strands, both concerned with optical properties near the band edge. One involves studies of reflectance from shallow quantum wells, while the other is concerned with hydrogen passivation studies of MBE bulk GaAs and AlxGa1-xAs and GaAs related layered structures. For the reflectance work the structures employed were very high quality undoped MBE grown multi-quantum wells consisting of 25 periods of a 20 nm GaAs well and a 50 nm or 20 nm AlxGa1-xAs barrier, with x=0.01, 0.02 or 0.045. Low temperature reflectance spectra of such samples are extremely sharp and detailed, ideal for the study of transition parameters such as oscillator strength per unit area and transition linewidth. Spectra were consistent with full quantum confinement of excitons for all samples. Parameters were extracted by simulation of the reflectance spectra until a very close fit was achieved using both a local, semiclassical model and the full nonlocal transfer matrix of Andreani [146,147]. Both methods were in agreement on oscillator strength per unit area, but analysis by means of the nonlocal method was necessary to achieve accurate fits of excitonic transition linewidth where the intrinsic broadening associated with the radiative lifetime of photocreated excitons was comparable in magnitude to that due to all other sources of broadening. Such an outcome has been theoretically predicted. Transition linewidths were as sharp as 53 meV FWHM, as sharp as any reported. Oscillator strengths per unit area were consistently larger than theoretically predicted, ranging for the Ellh transition from 51±7 to 80±4 x 10-5Å-2, depending on x. The acoustic coefficient of broadening was found to be 3.0±0.4 meV/K for the E11h transition and 4.5±0.4 meV/K for E11h One aspect of the hydrogenation work involved the study of MBE -type GaAs samples with strong \u27KP\u27 lines passivated by hydrogen and donor excitonic PL lines that were undetectable before hydrogenation, but strong afterwards. Selective excitation and other experiments revealed that isolated shallow acceptors were not being passivated, and a model was developed to explain the findings. Another aspect involved the passivation of short period superlattices. It was discovered that hydrogen only improves PL efficiency when free carriers are prevented from migrating out of the superlattice b y confinement in a double heterostructure or by disorder. The exp(T/T0) temperature dependence results give evidence that quenching of PL intensity as the temperature is raised is caused by phonon assisted tunneling or hopping of localized excitons or carriers to sites where nonradiative recombination is likely to occur

Developments in high temperature superconductivity

The past four years (1997–2001) have seen many exciting developments in high temperature superconductivity, most notably the discovery of the superconducting nature of magnesium boride in 2001 and the amazing critical temperatures of 52 K, then 117 K, achieved in 2000 by hole doping C60 fullerenes through incorporation into a field effect transistor (FET). Steady progress has also been made in understanding the recently discovered rutheno-cuprate superconducting ferromagnets. Theoretical work over this period has been focused on understanding the pseudogap in high temperature superconductors and on determining the mechanism behind superconductivity in MgB2

Microstructures and enhancement of critical current density in YBa/sub 2/Cu/sub 3/O/sub 7/ thin films grown by pulsed laser deposition on various single crystal substrates modified by Ag nano-dots

YBa2Cu3O7 (Y123) thin films were grown by pulsed laser deposition (PLD) on YSZ (100), SrTiO3 (100), and LaAlO3 (100) single crystal substrates. Prior to the film deposition, a discontinuous layer of Ag nano-dots was deposited on the substrates. The Y123 films grown on such surfaces modified with Ag nano-dots were characterized by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), AC susceptibility and DC magnetization. The effects of the density of Ag nano-dots, which was controlled by the numbers of PLD shots, on the microstructures and resultant critical current density Jc have been studied systematically. Results showed that at fixed physical deposition conditions Jc increased monotonically with number of Ag shots, n, for films grown on both STO and LAO substrates. At 77 K, the Jc increased from 106 to 3.2×106 A/cm2 for LAO and from 8×105 to 3.5×106 A/cm2 for STO as n increased from 0 to 150. At 5 K, the enhancement of Jc was approximately four times at both low and high fields. However, for films grown on YSZ substrate, Jc increased from 2×105 to 2×106 A/cm2 as Ag shots increased from 0 to 30, and decreased to 9×105 for n≥60. Detailed microstructure investigations indicated that the crystallinity and ab alignment gradually improved as the number of Ag-nano-dots increased

Uranium Doping and Thermal Neutron Irradiation Flux Pinning Effects in MgB2

The U/n method is a well-established means of improving flux pinning and critical current performance in cuprate superconductors. The method involves the doping of the superconductor with 235U followed by irradiation with thermal neutrons to promote fission. The resultant columnar damage tracks produced by the energetic fission products pin flux vortices and improve critical current performance in magnetic fields. No such improvement could be observed when the U/n method was applied to the MgB2 superconductor. No fission tracks could be observed in TEM, even for samples that were irradiated at the highest fluence. Gamma-ray spectroscopy indicated that fission had occurred in the expected way. The likely resistance of MgB2 to the formation of fission tracks is highly relevant to attempts to improve flux pinning and superconducting performance in this material through the introduction of columnar defects

Zinc doping effects on the structure, transport and magnetic properties of La0.7Sr0.3Mn1-xZnxO3 manganite oxide

The processing, microstructures and properties of La0.7Sr0.3Mn1−xZnxO3 perovskite manganite, with x=0, 0.1, 0.2, and 0.3 have been investigated. X-ray diffraction, scanning electron microscopy (SEM) and other characterisation methods were used to study the relations of microstructure and properties. The X-ray powder diffraction results show a single phase for the 0\u3cx\u3c0.3 region that confirms the zinc incorporation into the Mn site. The SEM images with magnification 1500× indicate that the studied system is a single phase. The transport measurements show decreasing of insulator to metal transition temperature, Tt, with increasing the zinc concentration on the Mn site, while the resistivity increases with increasing the zinc doping level for 0\u3cx\u3c0.1. The maximum resistivities are found at 380 and 160 K, respectively. The PPMS measurements show that the magnetisation value is greatest for x=0.1 and drastically decrease for the parent material, as well as with increasing the zinc doping concentration above 20%, when the compound changes from ferromagnetic to paramagnetic. The paramagnetic to ferromagnetic transition temperature variations, Tc, agrees with the insulator to metal variations, Tt, from the transport measurements for the parent and 10% doped material. For zinc doping levels above 20%, the behavior of the studied system changes from ferromagnetic to paramagnetic

Magnetoresistive effects in Bi-2212 melt textured bulk with MgO additions

The magnetoresistive (MR) effect in Bi-2212+MgO composites with proportions 6, 15 and 20 wt% of MgO was observed and compared with the MR effect of pure Bi-2212 and previously reported Bi-2212+USr2CaO6. The resistivity of melt textured Bi-2212+MgO composites is characterized by a high sensitivity to an applied magnetic field over an increasing temperature range with increasing MgO addition. X-ray results show that the MgO phase is compatible with the Bi-2212 matrix, stable during the melt texturing process and shows little adverse effect on the superconducting transition temperature. A cryogenic sensor was built using Bi-2212+20 wt% MgO as an active component and tested at 77 K in cycling fields from 0 to 1 T. It showed a high sensitivity and no hysteresis

Enhancement of the in-field Jc of MgB2 via SiCl4 doping

We present the following results. (1) We introduce a doping source for MgB2, liquid SiCl4, which is free of C, to significantly enhance the irreversibility field (Hirr), the upper critical field (Hc2), and the critical current density (Jc) with a little reduction in the critical temperature (Tc). (2) Although Si can not be incorporated into the crystal lattice, a significant reduction in the a-axis lattice parameter was found, to the same extent as for carbon doping. (3) Based on the first-principles calculation, it is found that it is reliable to estimate the C concentration just from the reduction in the a-lattice parameter for C-doped MgB2 polycrystalline samples that are prepared at high sintering temperatures, but not for those prepared at low sintering temperatures. Strain effects and magnesium deficiency might be reasons for the a-lattice reduction in non-C or some of the C-added MgB2 samples. (4) The SiCl4-doped MgB2 shows much higher Jc with superior field dependence above 20 K compared to undoped MgB2 and MgB2 doped with various carbon sources. (5) We introduce a parameter, RHH (Hc2 /Hirr), which can clearly reflect the degree of flux-pinning enhancement, providing us with guidance for further enhancing Jc. (6) It was found that spatial variation in the charge-carrier mean free path is responsible for the flux-pinning mechanism in the SiCl4 treated MgB2 with large in-field Jc