published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

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No description available.

No description available.

Ir L3 edge Resonant Inelastic X-ray Scattering (RIXS) and La2 Resonant X-ray Emission Spectroscopy (RXES) have been used to investigate A2IrO3 (A= Na or Li) and CuIr2S4. First principle calculations show that the low-energy dd-excitations in the RIXS spectra of A2IrO3 originate from a split jeff=3/2 to jeff=1/2 excitation. A magnetic excitation is observed in Na2IrO3, whose dispersion is consistent with the theoretical prediction including a bond-dependent Kitaev interaction between jeff=1/2 magnetic moments. In CuIr2S4, RXES measurements reveal that the unoccupied Ir t2g level shift upward in energy as a result of the metal-insulator transition, confirming density functional theory calculations. In the x-ray irradiation induced phase of CuIr2S4, a mid-infrared peak emerges in the RIXS spectrum around 0.4 eV, suggesting that both the crystal structure and the electronic structure are modified due to x-ray irradiation. The size of the magnetic moments in Fe based superconductors has been investigated using Fe Kb X-ray Emission Spectroscopy (XES). Local magnetic moments are found in Fe pnictides and chalcogenides, independent of temperature or carrier concentration. This result supports the notion of local moments coexisting with metallic bands. Our investigation of Ca{1-x}RExFe2As2 (RE=La, Pr, and Nd) shows results consistent with a spin-state transition. We argue that the gradual change of the spin-state over a wide temperature range reveals the importance of multiorbital physics, and in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

Ir L3 edge Resonant Inelastic X-ray Scattering (RIXS) and La2 Resonant X-ray Emission Spectroscopy (RXES) have been used to investigate A2IrO3 (A= Na or Li) and CuIr2S4. First principle calculations show that the low-energy dd-excitations in the RIXS spectra of A2IrO3 originate from a split jeff=3/2 to jeff=1/2 excitation. A magnetic excitation is observed in Na2IrO3, whose dispersion is consistent with the theoretical prediction including a bond-dependent Kitaev interaction between jeff=1/2 magnetic moments. In CuIr2S4, RXES measurements reveal that the unoccupied Ir t2g level shift upward in energy as a result of the metal-insulator transition, confirming density functional theory calculations. In the x-ray irradiation induced phase of CuIr2S4, a mid-infrared peak emerges in the RIXS spectrum around 0.4 eV, suggesting that both the crystal structure and the electronic structure are modified due to x-ray irradiation. The size of the magnetic moments in Fe based superconductors has been investigated using Fe Kb X-ray Emission Spectroscopy (XES). Local magnetic moments are found in Fe pnictides and chalcogenides, independent of temperature or carrier concentration. This result supports the notion of local moments coexisting with metallic bands. Our investigation of Ca{1-x}RExFe2As2 (RE=La, Pr, and Nd) shows results consistent with a spin-state transition. We argue that the gradual change of the spin-state over a wide temperature range reveals the importance of multiorbital physics, and in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

An important fundamental problem in the understanding of the high-Tc superconducting systems is the determination of their equilibrium magnetization behaviour, in particular their constitutive Brev(H) orMrev(H) behavior. Single crystal specimens of these materials are typically small (order of micron/millimeter), and are generally in the form of platelets. Their superconductivity properties are, moreover, highly anisotropic. The magnetization [M(H0)] curves in these systems also manifest a hysteresis due to vortex pinning, and, at fields below the lower critical field Hc1, due to a “geometry” effect, which results from a non-uniform internal field distribution in the platelet specimen geometry in a perpendicular applied magnetic field H0. In the present work a brief review of the field is given and a treatment (due to Doyle and Labusch) of the problem is described in some detail, and is used in the analysis of magnetization data [M(H0)] on single-crystal platelet specimens of the YBCO and BSCCO high-Tc superconducting systems. The treatment, which is based on a rigorous theoretical analysis of a quasi-static arbitrary distribution of vortices in a specimen of arbitrary shape (Labusch and Doyle ), predicts the quasi-static magnetization behavior M(H) of the specimen, and allows for the inclusion of explicit relations for the equilibrium “constitutive” Brev(H, T), and for the bulk vortex pinning force density Pv(B). An analytical formula for Brev(H, T) in terms of the fundamental characteristic properties ?ij(T) (the anisotropic Ginsburg -Landau parameter) and the critical field Hc(T) (or the lower critical field Hc1(T)) is obtained from an accurate model fit to a numerical solution of the non-linear Ginsburg-Landau equation (Labusch and Doyle ). For the determination of ? and Hc c1, (i.e. the G-L parameter and the lower critical field along the crystalline c- axis of platelet specimens) from M(H0, T) experimental isotherms (where H0 is the magnetic field applied along the c-axis -the thin dimension of the platelet specimens), a computer algorithm, which incorporates the above treatments, was used. In order to obtain a fit between theoretical model results (of the numerical algorithm for equilibrium behavior) and the experimental M(H0, T) data, experimentally obtained hysteresis curves were averaged by taking the mean values of M(H0) for H0 increasing and decreasing over the entire M(H0) loop. This data was then normalized by Hc1(T) for both M and H0, with Hc1(T) and ?(T) being used as fitting parameters. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2007.

Grain boundary weak-link behaviour in superconductors is investigated using a critical state model (CSM), which has been specifically formulated for the applied field range o ~ Ha ~ Hc1g , where Hclg is the intra-granular lower critical field. The CSM includes an expression for the inter-granular critical current density, Jc(H, T), that has been derived from a percolation model for a random 3D network of weak-links in which the current Bowing through the individual grain boundary junctions is assumed to be spatially random. This expression, namely: Jc(H,T) = Jco[exp(-H/Ho)+b], where JCO ) Ho , and b are characteristic weak-link parameters, includes a field-dependent component and a field-independent component both of which are microstructurally sensiti ve. Calculated magnetisation behaviour, obtained from the critical state model, are fitted to experimental isothermal D.C magnetisation data, j\1(Ha) , for 0 ~ Ha ~ Hclg , 71.5K~ T ~ Tc, obtained from the two control specimens, namely: Y IBa2Cu307-x and EUIBa2Cu307-x , x ~ I, and from a third specimen, which is a mixture of the two control compounds. In the CSM fits, the temperature dependent characteristic parameters Jco , Ho, and bare treated as free fitting parameters. These free fitting parameters are subsequently fitted to specific theoretical models for Jca(T), Ho(T), and b(T) and a comparison is made between the control specimens and mixed specimen parameters to establish the effect of mixing on the grain boundary weak-link behaviour. Jca and b are found to be significantly larger in the mixed specimen as compared with either of the two control specimens. / Thesis (M.Sc.)-University of Natal, Durban, 2002.