Browsing by Subject "Electronic structure"
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Item Electronic transitions and multiferroicity in transition metal oxides(2005) Zhou, Haidong; Goodenough, John B.Item Investigation of electronic transport and magnetic properties in Ca₂-₂xLa₂xRu₂-xCoxO₆(2007-08) Kim, Sangwon, 1978-; Goodenough, John B.Ca2-2xLa2xRu2-xCoxO6 ( 0.0 ≤ x ≤ 1.0 ) is prepared by a "Polymerizable-Complex Method." Electronic transport and magnetic properties are measured to show that the system becomes semiconducting at x ≈ 0.1 and goes through long-range antiferromagnetic ordering at x ≈ 0.4. The proximity in Néel temperature TN of La2RuCoO6 to that of La2TiCoO6 previously reported suggests that the magnetic moment of Ru4+ becomes quenched as temperature decreases. This suppression is explained by a theory that due to (1) the strong spin-orbit coupling ( large spin-orbit coupling constant [Greek small letter lambda] ) present in Ru4+ ions and (2) the sign inversion of [Greek small letter lambda] suggested by Kanamori, the Ruions favors the antiparallel alignment of L and S leading to J = 0. Based on (1) the similarity in the inverse magnetic susceptibility 1/[Greek small letter chi] (T) between CaRuO3 and La2RuZnO6 and (2) the presence of features in CaRuO3 commonly associated with phase fluctuations, the suppression of magnetic moment in Ru4+ is proposed to be responsible for the absence of long-range magnetic ordering in CaRuO3 as well. Finally, the appearance of long-range ferromagnetic ordering in CaRu1-xMxO3 is explained by (1) the degree of hybridization of the Ru4+/Ru5+ redox couple with empty d orbitals and (2) the formation of ferrimagnetic clusters from t2g 3-O- t2g 3 interactions between dopants and localized Ru5+: t2g 3eg 0 configurations.Item Metal-to-insulator transitions in transition metal oxides : a first principles study(2015-08) O'Hara, Andrew; Demkov, Alexander A.; Chelikowsky, James R; MacDonald, Allan H; Tsoi, Maxim; Henkelman, GraemeTransition metal oxides have received significant attention in recent decades due to their ability to display a wide range of novel functional properties. In particular, many oxides are able to undergo metal-to-insulator transitions as a function of external stimuli such as temperature, pressure, and electric field or through doping and defect formation. In the present dissertation, density functional theory is used to explore these phenomena in three systems: (1) the Peierls transition in NbO2, (2) defect formation necessary for HfO2’s resistive switching, and (3) La-doping of SrTiO3 and trap states that may limit conductivity. For NbO2, we use successive improvements to the exchange-correlation energy combined with experiment to improve understanding of the material’s band gap in the insulating phase and show it to be close to 1.2 eV for the direct gap with an indirect gap just below 1.0 eV. Furthermore, we are able to explain the orbital contributions to the dielectric function. Using a combination of transition state theory and phonon dispersion, we demonstrate that the phase transition is driven by a second-order structural transition of the Peierls type. For HfO2, we explore the nature of the metallic gettering layer used to create substoichiometric HfO2-x for resistive switching via an atomistic model of the hafnia-hafnium interface and use transition state theory to study the ability for oxygen to diffuse across the interface. Our investigation shows that the presence of hafnium lowers the formation energy of oxygen vacancies in hafnia, but more importantly the oxidation of hafnium through oxygen migration is energetically favored. In La-doped SrTiO3, the calculations are first used to corroborate optical and electrical measurements by giving values for the density of states effective mass as well as understanding the effect of La-doping on the conductivity and DC relaxation time. Motivated by the experimental observation that even after annealing in oxygen rich environments, heavily n-type doped SrTiO3 shows carrier concentrations inconsistent with dopant concentration, we explore the role that interstitial oxygen may play as a trapping state in SrTiO3. We find three meta-stable sites and that for n-type SrTiO3, interstitials with mid-gap states are favored.Item Non-orthogonal spin-adaptation and application to coupled cluster up to quadruple excitations(2014-08) Matthews, Devin Alexander; Stanton, John (John F.)The theory of non-orthogonal spin-adaptation for closed-shell molecular systems is presented, with an emphasis on application to the coupled cluster family of electronic structure methods. To aid in the derivation of efficient and compact working equations, a new diagrammatic interpretation of the Goldstone diagrams is derived which only requires a small number of the many distinct diagrams and which directly produces equations in a factored form in terms of “spin-summed” tensor elements. This diagrammatic interpretation is applied to coupled cluster methods with quadruple excitations (CCSDTQ), including coupled cluster with a perturbative correction for quadruple excitations (CCSDT(Q)) and to CCSDTQ gradients and properties. The advantages of the non-orthogonal spin-adaption with respect to simplification and factorization of the working equations and to efficient implementation are presented and discussed. Additionally, specific optimizations of the implementation for often-overlooked issues such as tensor transposition, disk access, and removal of redundant and/or unnecessary operations are detailed. The resulting algorithm is implemented for the CCSDTQ and CCSDT(Q) methods and compared to existing codes, where a one to two order-of-magnitude improvement in efficiency is observed. The new implementation is also used for calculations on several larger molecular systems to illustrate the scalability of the method.Item Quantum mechanical studies of weakly bound molecular ions(Texas Tech University, 2002-12) Chang, Daniel Ta-JenThe study of weakly bound complexes constitutes an active field of research. High-resolution experimental spectra are necessary to provide precise measurements of the intermolecular forces of these systems. Analogously, highly accurate quantum calculations are required to correctly describe weak intermolecular forces such as dispersion and induction. In this work, theoretical studies of three molecular ions, He3, Li-(H2), and LiH~, are described where each ion is weakly bound to their respective He^-|- He, Li~ +- H2, and LiH + -e - limits. For He3+, an analytical global ground state potential energy surface is developed from high quality ab initio calculations. The linear symmetric global minimum is found to be consistent with previous ab initio results (r^ = 2.340 a.u, and Dg = 0.175 eV). Numerical determination of the bound rovibrational levels reveals that (1) the ZPE is highly anharmonic, (2) a large number of banding states (v2 < 6) is supported, and (3) two quanta of pure asymmetric stretch (f 3 = 2) is not seen in our calculations implying that this state may be unbound. In the second study, an analytical potential energy surface is developed from high quality ab initio calculations for the electrostatic region of the Li~ -f H2 interaction. The Li~(H2) electrostatic complex is found to have a linear minimum energy structure with a De of 64.44 cm~-^ and numerical determinations of the bound levels indicate a Do of only ~7 cm~-^ for Li~(para-H2) and a considerably larger D0 of ~22 cm^-1 for Li-(ortho-B.2). Altogether, the Li-(para-H2) interaction is predicted to support 11 bound levels, whereas the Li~(ortho-H2) interaction is predicted to support 28 bound levels. Analogous results for the D2 and HD isotopomers are also reported. Finally, the photoelectron spectra of LiH" and LiD~ are determined from a first principles theoretical treatment. Satisfactory simulation of the experimental photoelectron spectra is accomplished by assuming a non-Boltzmann distribution of the anion vibrational levels and the discrepancy between the experimental (920 ± 80 cm-1) and theoretical (1176.1 cm~^) values of we of LiH- is resolved by a reassignment of the hot band transition region of the photoelectron spectrum.Item Theory of biomineral hydroxyapatite(2013-05) Slepko, Alexander; Demkov, Alexander A.Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is one of the most abundant materials in mammal bone. It crystallizes in an aqueous environment within spaces between tropocollagen protein chains. However, despite its abundance and possible usefulness in the medical field this complex physical system remains poorly understood to date. We present a theoretical study of the energetics of hydroxyapatite, its electronic, mechanical and thermodynamic properties. Our mechanical and thermodynamic properties from first principles are in excellent agreement with the rare available experimental data. The monoclinic and hexagonal phases are lowest in energy. A comparison of the phonon dispersions of these two phases reveals that a phase transition occurs due to a difference in vibrational free energy. The transition is of order-disorder type. Our calculated phase transition temperature is 680 K, in decent agreement with the experimentally determined 470 K. An alternative theoretical model yields 882 K. The phase transition is mediated by OH libration modes. We also report for the first time on a peculiarity in the phonon spectrum of hexagonal and monoclinic HA. When studying the Lyddane-Sachs-Teller shifts in the spectrum close to the [Gamma]-point we identify two vibration modes showing a systematically increasing Lyddane-Sachs-Teller shift in frequency with decreasing dielectric constant. In experiment, the dielectric constant varies between 5 and 20 depending on the Ca/P ratio in the sample. The frequency shifts in the affected modes are as large as 20 cm⁻¹ as one spans the range of the dielectric constant. Thus, a simple spectroscopic analysis of a sample of bone may determine the quality of the sample in a physiological sense. We also identify the chemically stable low energy surface configurations as function of the OH, PO₄ and Ca concentration. In the experimentally relevant OH-rich regime we find only two surfaces competing for lowest energy. The surface most stable over almost the entire OH-rich regime is OH-terminated, and is currently being investigated in the presence of water and atomic substitutions on the HA surface.