Unfolding first-principles band structures

A general method is presented to unfold band structures of first-principles super-cell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angular resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders.Comment: 4 pages, 3 figure

Unified Picture for Magnetic Correlations in Iron-Based Superconductors

The varying metallic antiferromagnetic correlations observed in iron-based superconductors are unified in a model consisting of both itinerant electrons and localized spins. The decisive factor is found to be the sensitive competition between the superexchange antiferromagnetism and the orbital-degenerate double-exchange ferromagnetism. Our results reveal the crucial role of Hund's rule coupling for the strongly correlated nature of the system and suggest that the iron-based superconductors are closer kin to manganites than cuprates in terms of their diverse magnetism and incoherent normal-state electron transport. This unified picture would be instrumental for exploring other exotic properties and the mechanism of superconductivity in this new class of superconductors.Comment: Revised for publication. 3 figure

Dynamical Linear Response of TDDFT with LDA+U Functional: strongly hybridized Frenkel excitons in NiO

Within the framework of time-dependent density-functional theory (TDDFT), we derive the dynamical linear response of LDA+U functional and benchmark it on NiO, a prototypical Mott insulator. Formulated using real-space Wannier functions, our computationally inexpensive framework gives detailed insights into the formation of tightly bound Frenkel excitons with reasonable accuracy. Specifically, a strong hybridization of multiple excitons is found to significantly modify the exciton properties. Furthermore, our study exposes a significant generic limitation of adiabatic approximation in TDDFT with hybrid functionals and in existing Bethe-Salpeter-equation approaches, advocating the necessity of strongly energy-dependent kernels in future development.Comment: 5 pages, 2 figure

An investigation of the response of different materials to blast loading

Includes bibliographical references.This dissertation reports on the results of an experimental and numerical investigation into the response of different materials to air-blast loading. Mild steel, armour steel (Armox 370T and 440T), Aluminium alloy 5083-H116, Twintex and Dyneema square plates were blast loaded on a horizontal pendulum at the Blast Impact and Survivability Research Unit (BISRU), University of Cape Town. The blasts were generated by detonating disc-shaped PE4 explosives of various diameters and standoff distances. The chosen plates are of side length 500mm (4mm thick mild steel and armour steel plates) and side length of 400mm (aluminium, Twintex and Dyneema panels). The charge mass was varied between 9g and 60g for two charge diameters, namely: 50mm and 75mm, and stand-off distances of 25mm, 38mm and 50mm. A polystyrene bridge was used to position the charges at the centre of plates, without any polystyrene between the charge and the plate in order to minimise any effects the polystyrene may have had on the plate deformation. The transient response of the 500mm square plates (mild steel and Armox 370T) was measured with the use of Light Interference Equipment (LIE) and numerical simulations performed in ANSYS AUTODYN, with the aim of gaining greater insight into the response of the two different materials. The details of the experimental setup and method used for the LIE as well as the development of the AUTODYN computational model are presented. The air and explosive were modelled as Arbitrary Langrange-Euler (ALE) elements while the test plates were modelled as Langrangian shell elements. Since the geometry of the plates was square, the simulations had to be performed in 3D quarter-symmetry. The transient response, permanent final displacement and maximum transient displacement of the numerical simulations were compared to the corresponding experimental results. The mild steel plates all exhibited good correlation between experimental and simulated results. However, the Armox 370T simulated results showed an under-prediction of the displacement magnitude and impulse compared to the experimental results. Experimentally, both the mild steel and armour steel exhibited a linear increase in deformation with increasing charge mass. Blast tests were also performed on 3mm thick mild steel, aluminium, Twintex and Dyneema square plates of 400mm side length. The aim was to gain a greater understanding and compare of the response of different material types (ferrous, non-ferrous, Glass Fibre Polypropylene and Ultra High Molecular Weight Polyethylene) under blast loading. The aluminium plates performed better than the mild steel, on an equivalent mass basis, in terms of permanent displacements and failure threshold impulse. The aluminium plates were significantly thicker (10.5mm compared to 3mm) than the mild steel plates, which may have contributed to its response under blast. The Twintex panels mostly exhibited failure in the form of fibre fracture and matrix failure whereas the Dyneema panels only exhibit large inelastic deformation, although the Dyneema were clamped differently to the other panels. Dimensionless analysis was performed on all of the materials except for Dyneema. Initially a scaling factor was used to account for the varying stand-off distances but proved to be unnecessary due to the type of confinement used (unconfined free air-blasts versus partially confined tube). Once the scaling factor was removed, the dimensionless impulse values showed relatively good linear correlation with the predicted trend

Ferro-Orbital Order and Strong Magnetic Anisotropy in the Parent Compounds of Iron-Pnictide Superconductors

The puzzling nature of magnetic and lattice phase transitions of iron pnictides is investigated via a first-principles Wannier function analysis of representative parent compound LaOFeAs. A rare ferro-orbital ordering is found to give rise to the recently observed highly anisotropic magnetic coupling, and drive the phase transitions--without resorting to widely employed frustration or nesting picture. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the magnons advocates active roles of light orbitons in spin dynamics and electron pairing in iron pnictides.Comment: accepted by Physical Review Letter

The Nucleolus of Caenorhabditis elegans

Nucleolar size and appearance correlate with ribosome biogenesis and cellular activity. The mechanisms underlying changes in nucleolar appearance and regulation of nucleolar size that occur during differentiation and cell cycle progression are not well understood. Caenorhabditis elegans provides a good model for studying these processes because of its small size and transparent body, well-characterized cell types and lineages, and because its cells display various sizes of nucleoli. This paper details the advantages of using C. elegans to investigate features of the nucleolus during the organism's development by following dynamic changes in fibrillarin (FIB-1) in the cells of early embryos and aged worms. This paper also illustrates the involvement of the ncl-1 gene and other possible candidate genes in nucleolar-size control. Lastly, we summarize the ribosomal proteins involved in life span and innate immunity, and those homologous genes that correspond to human disorders of ribosomopathy