Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 116-121). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 116-121). Also available on the Internet.

Thesis (Ph.D.)--University of Wollongong, 2008. / Typescript. Includes bibliographical references.

Thesis (Ph. D.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains xiv, 141 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 135-141).

M.Sc. / Impurity resonance scattering effects are investigated in the Cr-Ga alloy system. This system has a triple point on its magnetic phase diagram where the paramagnetic (P), incommensurate (I) and commensurate (C) spin-density-wave (SDW) states co-exist. Alloying Cr with the nonmagnetic nontransitional element Ga affects the magnetic properties of Cr in a very unique way. In order to investigate the presence of resonant impurity scattering effects in binary Cr-Ga alloys, electrical resistivity measurements were carried out in the temperature range between 6 K and 85 K. The results of the investigation show: • A nonmonotonic increase in the residual resistivity of the Cr-Ga system with an increase in the Ga content, due to the presence of resonant impurity scattering of conduction electrons. • A low-temperature resistivity minimum observed in some of the Cr-Ga alloys, taken as further evidence for the presence of resonant impurity scattering effects on the conduction electrons. The impurity resonance scattering effects on the electrical resistivity of a Cr + 1.2 at.% Ga alloy, doped with V and Mn to tune the Fermi level through the impurity level, are also investigated. The investigation was complemented by thermal expansion and velocity of sound measurements in the temperature range 77 K to 450 K for the Cr + 1.2 at.% Ga alloy only. This specific Ga concentration was chosen to allow for studying resonant scattering effects in both the ISDW and CSDW phases of the system. This is possible because concentration of 1.2 at.% Ga is just above the triple point concentration. Doping with Mn to increase the electron concentration (eA) drives the alloy deeper into the CSDW phase region of the phase diagram, while doping with V, on the other hand, will drive the alloy towards the ISDW phase region. The results of the study are summarized as follows: • Two relatively sharp peaks, attributed to resonant impurity scattering effects, are observed in the curve of the residual resisitivity as a function of dopant concentration in the ISDW phase of the ternary (Cr0.988Ga0.012)1-xVx and (Cr0.988Ga0.012)1-yMny alloy systems. v • At 0 K the (Cr0.988Ga0.012)1-yMny alloy system transforms from the ISDW to the CSDW phase at y ≅ 0.0032, giving a CSDW phase for y > 0.0032. A peak is observed in the residual resistivity at about this Mn content. This peak can then either be ascribed to a jump occurring in the residual resistivity when the CSDW phase is entered from the ISDW phase or to resonant scattering effects. The conclusion is that the peak is rather related to the latter effect. • The resistivity as a function of temperature of the above two ternary alloy series show well-developed or weak minima at low temperatures for some of the samples. This is taken as further evidence of the influence of impurity resonant scattering effects on the resistivity of these alloys. • The resistivity and thermal expansion coefficient of the polycrystalline Cr0.988Ga0.012 alloy of the present study behaves anomalously close to the ISDW-CSDW phase transition temperature and warrant further investigation. The concentration-temperature magnetic phase diagram of the (Cr0.988Ga0.012)(Mn,V) alloy system was constructed from the magnetic transition temperatures obtained from electrical resistivity measurements. Theoretical analysis of the phase diagram was done using the two-band imperfect nesting model of Machida and Fujita. The results show: • A triple point at (0.21 at.% V, 225 K) where the ISDW, CSDW and P phases coexist on the magnetic phase diagram. • The curvature of all three theoretically calculated phase transition lines in the region of the triple point is of the same sign as that observed experimentally. • The theoretical fit is very good for the ISDW-P and ISDW-CSDW phase transition boundaries, while there is some discrepancy for the CSDW-P phase transition line. This may be attributed to the fact that the theory is one dimensional and that it does not include electron-hole pair breaking effects due to impurity scattering and also not effects of changes in the density of states due to alloying. / Dr. A.R.E Prinsloo Prof. H.L. Alberts

Mott insulators are a family of materials in which strong electron-electron interactions induce an unconventional insulating state in the system that would otherwise behave as an electrical conductor according to the non-interacting band theory. In particular, the nature of the Mott metal-insulator transition (MIT) has been the subject of intense research interest because it can involve a complicated interplay between magnetic and electronic properties. In some Mott systems, Mott transitions occur in the one-step process, from an antiferromagnetic insulator (AFI) to a paramagnetic metal phase (PMM), while in other Mott systems a two-step transition with an intermediate antiferromagnetic metal (AFM) phase can be observed. Since 2015, the muon spin relaxation (𝜇SR) group at Columbia University started systematic 𝜇SR studies on a series of Mott systems, including one-step transition Mott systems 𝑅𝐸NiO₃ and V₂O₃, as well as two-step transition Mott systems Ba(Co, Ni)S₂, Ni(S, Se)₂ and (La, Sr)VO₃. This dissertation first introduces the comprehensive 𝜇SR research on multiple families of Mott systems conducted by our 𝜇SR group, including 𝑅𝐸NiO₃, V₂O₃, and BaCoS₂. Then the 𝜇SR experimental findings on the Mott system NiS₂₋ₓSeₓ will be presented, which is the most extensively studied material in this thesis. The NiS₂₋ₓSeₓ system is of particular interest because there is a large region of intermediate AFM state emerging between AFI and PMM states with Se doping, making it an ideal platform to provide information on static magnetism in the AFM state and thus can help us better understand the evolution of magnetic NiS₂₋ₓSeₓ, with our key findings being: (1) The AFM state of the NiS₂₋ₓSeₓ system shows significantly random spin correlations, and the magnetic order is suppressed by a gradual reduction of the ordered moment size, with a nearly full ordered volume fraction until very close to the AFM-PMM boundary. (2) No signature of dynamic critical behavior was observed in the thermal phase transition, indicating a first-order thermal phase transition. The next part of this dissertation presents our computational simulations on the NiS₂₋ₓSeₓ system. Dipolar field simulations have shown that only the combination of easy axis randomization and Ni moment dilution in NiS₂ can lead to the internal field distribution corresponding to the observed 𝜇SR spectrum in the AFM region. Also, this picture could qualitatively explain the neutron and muon results consistently, thus reconciling the seemingly contradicting experimental results by 𝜇SR and the previous neutron scattering studies shown in the AFM region. Furthermore, we propose a percolation model that can capture both the charge and spin connections of the interpenetrating percolating spin and charge networks in the NiS₂₋ₓSeₓsystem, which raises the possibility of "compromising metallicity and magnetic order" in the two-step Mott transition evolving AFI to AFM to PMM states in NiS₂₋ₓSeₓ.

The feasibility of achieving carrier inversion of a properly doped crystal via optical excitation is studied. This process involves a host substrate doped with deep donors for n-type light characteristic and compensated by a shallow acceptor for p-type characteristic in the dark. This substrate is analyzed using well-known semiconductor equations. In addition conditions which must exist for carrier inversion are also specified. The solutions found are applied to a realistic set of dopants for illustrative purposes as well as indication of feasibility range. This inversion technique may possibly be used to generate bipolar junctions and thus devices. Other forms of photoconductivity are also qualitatively considered to supplement and extend the range of the inversion techniques applications. The processing of circuits using the developed concept offers possible interesting and useful advantages over existing techniques. The motivation for further research thus becomes obvious and is indeed the purpose of the thesis. / Master of Science

Thermoelectric materials are playing an increasingly significant role in the global effort to develop sustainable energy technologies. Consequently, the demand for materials with greater thermoelectric efficiency has stimulated the development of state-of-the-art interstitially doped skutterudite-based materials. However, since intermetallics are often embrittled by interstitial substitution, optimal skutterudite-based device design, manufacture, and operation require thorough assessment of the fracture toughness of interstitially doped skutterudites. This research determines whether the fracture toughness of skutterudites is sacrificed upon interstitial doping. Both pure and interstitially doped cobalt antimonide skutterudites were synthesized via a solid-state technique in a reducing atmosphere with antimony vapor. Their crystal structures were analyzed by X-ray diffraction, and then sintered by hot uniaxial pressing into dense pellets. The electronic properties of the sintered samples were characterized. Fracture toughness of the pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ samples was evaluated by the Vicker's indentation technique and by loading beam-shaped singe-edge vee-notched bend specimens (SEVNB) in 4-point flexure. The intrinsic crack-tip toughness of both materials was determined by measuring the crack-tip opening displacements (COD's) of radial cracks introduced from Vicker's indentations. The intrinsic crack-tip toughness of both pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ were found to be similar, 0.523 and 0.494 MPa√m, respectively. The fracture toughness of both pure and interstitially doped skutterudites, derived from SEVNB specimens in 4-point flexure were also found to be statistically identical, 0.509 and 0.574 MPa√m , respectively, and are in agreement with the intrinsic crack-tip toughness values. However, the magnitude of the toughness was found to be much lower than previously reported. Moreover, fracture toughness values derived from Vickers's indentations were found to be misleading when compared to the results obtained from fracture toughness tests carried out on the micronotched (SEVNB) specimens loaded in 4-point flexure. / Graduation date: 2012

Light emitting transistors (LETs) and transistor lasers (TLs) are newly-emerging optoelectronic devices capable of emitting spontaneous or stimulated light while performing transistor actions. This dissertation describes the design, growth, and performances of long wavelength LETs and TLs based on InAlGaAs/InP material system. First, the doping behaviors of zinc (Zn) and carbon (C) in InAlGaAs layers for p-type doping were investigated. Using both dopants, the N-InP/p-In0.52(AlxGa1-x)0.48As/N-In0.52Al0.48As LETs with InGaAs quantum wells (QWs) in the base demonstrate both light emission and current gains (β). The device performances of Zn- and C-doped LETs have been compared, which is explained by a charge control analysis involving the quantum capture and recombination process in the QWs. A TL based on a C-doped double heterostructure (DH-TL) with single QW was designed and fabricated. The device lases at 77 K with a threshold current density (Jth) of 2.25 kA/cm2, emission wavelength (λ) at ~1.55 µm, and β of 0.02. The strong intervalence band absorption (IVBA) is considered as the main intrinsic optical loss that prohibits the device from lasing at room temperature. Based on a threshold condition analysis taking into account the strong IVBA, it is found that room-temperature lasing of a DH-TL is achieved only when the base thickness and doping level are within a specific narrow range and improved performance is expected in a separate confinement heterostructure (SCH) TL.

Thesis (M.S.) -- Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.