Simulation of reconstructions of the polar ZnO (0001) surfaces

Surface reconstructions on the polar ZnO(0001) surface are investigated using empirical potential models. Several possible reconstructions based around triangular motifs are investigated. The quenching of the dipole moment in the material dominates the energetics of the surface patterns so that no one particular size of surface triangular island or pit is strongly favoured. We employ Monte Carlo simulations to explore which patterns emerge from a high temperature quench and during deposition of additional ZnO monolayers. The simulations show that a range of triangular islands and pits evolve in competition with one another. The surface patterns we discover are qualitatively similar to those observed experimentally

Theory of lattice effects on magnetic interactions in solids

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research.pdf file viewed on (November 13, 2006)Includes bibliographical references.Vita.Thesis (Ph. D.) University of Missouri-Columbia 2005.Dissertations, Academic -- University of Missouri--Columbia -- Physics.This dissertation focuses on studying the effect of lattice distortions on the magnetic properties of nickelates and manganites. These two families of materials have great potential in industrial applications in the fields of magnetic (superdense hard-drives, fast memory) and charge storage (batteries). The introduction and methods sections present the main ideas of the dissertation and discuss the various techniques used. Electron-lattice coupling is first examined in chapter three for a two-site model where we estimate the magnitude of the isotope effect on the critical temperature and show that it decreases magnetic exchange. In the next part we study electronic structure and magnetism of NaNiO2 and show that inter-planar exchange is reduced by lattice coupling. In the fifth chapter we examine the magnetic polaron and discuss the effect of static lattice coupling on its binding energy, and find it to further stabilize the polaron

Jahn-Teller coupling and double exchange in the two-site Van Vleck-Kanamori model

http://link.aip.org/link/JAPIAU/v85/i8/p4346/s1 doi:10.1063/1.369780The effect of the dynamical Jahn-Teller coupling on the Anderson-Hasegawa double exchange (DE) in the manganites is studied in a two-site model taking into account the double degeneracy of the eg orbitals and their coupling to the three MnO6 vibrational modes (Q1 , Q2 , and Q3). Both exact diagonalization and the Lang-Firsov approach are used. We find that coupling to the Q2 and Q3 modes reduces the DE, while the Q1 mode is ineffective. The isotope dependence of the DE interaction is consistent with recent experiments.The authors thank the Research Board of the University of Missouri for partial financial support

Orbital ordering and exchange interaction in the manganites

URL:http://link.aps.org/doi/10.1103/PhysRevB.64.094433 DOI:10.1103/PhysRevB.64.094433The microscopic origin of the exchange interaction in manganites is studied by solving an electronic model Hamiltonian for the Mn-O-Mn triad. It is shown that the magnetic structure of La1-xCaxMnO3 is correctly described within an electronic Hamiltonian model, provided that the appropriate orientation of the Mn(eg) orbitals induced by the Jahn-Teller effect is taken into account. The Jahn-Teller distortions of the MnO6 octahedra control the orientation of the eg orbitals in the crystal, which in turn is shown to determine the sign of the magnetic exchange. Electron hopping involving the Mn(t2g) orbitals is found to be important in certain situations, for instance, it can cause a sign change in the exchange interaction, from ferromagnetic to antiferromagnetic, as a function of the Mn-O-Mn bond angle. All our results are obtained by exact diagonalization of the model Hamiltonian, either by direct diagonalization or by diagonalization using the Lanczos method, if the Hamiltonian is too big, and are rationalized using results of the fourth-order perturbation theory. The exchange interactions (signs and magnitudes) of the end members LaMnO3 and CaMnO3 as well as of the half-doped compound, La1/2Ca1/2MnO3, are all described correctly within the model.This work was supported in part by a grant from the Department of Energy under Contract No. DOE FG02-00E0045818

Erratum: Orbital ordering and exchange interaction in the manganites [Phys. Rev. B 64, 094433 (2001)]

Refers to: http://hdl.handle.net/10355/9317 URL:http://link.aps.org/doi/10.1103/PhysRevB.68.029901 DOI:10.1103/PhysRevB.68.029901Erratum concerning Orbital ordering and exchange interaction in the manganites [Phys. Rev. B 64, 094433 (2001)]

Does the Self-Trapped Magnetic Polaron Exist in Electron-Doped Manganites?

URL:http://link.aps.org/doi/10.1103/PhysRevLett.92.056401 DOI:10.1103/PhysRevLett.92.056401We show from ab initio density-functional calculations and model studies that, in the electron-doped manganite LaxCa1-xMnO3 (x≪1), unbound electrons are introduced into the conduction band, which then trap themselves in the exchange-induced magnetic potential wells forming the self-trapped magnetic polarons (STMP). Hopping beyond the nearest neighbors drastically reduces the binding energy, while the Jahn-Teller coupling increases it somewhat, resulting in a net binding of about 100±20   meV. The electron is self-trapped in a seven-site ferromagnetic region, beyond which the lattice is essentially antiferromagnetic. In light of the recent experiments of Neumeier and Cohn, our results suggest that the STMP may be present in the lightly electron-doped manganites.We acknowledge support of this work by the U.S. Department of Energy (DE-FG02-00ER45818)

Electronic structure and exchange interaction in the layered perovskite Sr3Mn2O7

URL:http://link.aps.org/doi/10.1103/PhysRevB.65.094402 DOI:10.1103/PhysRevB.65.094402The electronic structure of the Ruddlesden-Popper layered perovskite compound Sr3Mn2O7 is studied from density-functional calculations using the linear muffin-tin orbital method. An antiferromagnetic, insulating solution is obtained in agreement with the experiments, with a magnetic moment of about 2.52μB for each Mn atom. The magnetic interactions between the Mn atoms, both within the bilayer and between the bilayers, are shown to arise from superexchange. The intrabilayer interaction involves the three-site Mn-O-Mn superexchange much like the case of the well-known CaMnO3, while the interbilayer exchange, mediated via the longer Mn-O-O-Mn superexchange path, is considerably weaker. Consistent with the layered nature of the compound, we find a strong out-of-plane to in-plane band-mass anisotropy for Sr3Mn2O7(mz*/mx,y*∼10.9 for electrons and ∼4.2 for holes), while for the related compound LaSr3Mn2O7, which is a ferromagnetic metal, we obtain a strong anisotropy in the resistivity ρc/ρab∼40 using kinetic transport theory, in qualitative argument with the experimental value of ∼100.This work was supported by the U. S. Department of Energy under Contract No. DOE FG02-00E0045818

Examination of the concept of degree of rate control by first-principles kinetic Monte Carlo simulations

The conceptual idea of degree of rate control (DRC) approaches is to identify the "rate limiting step" in a complex reaction network by evaluating how the overall rate of product formation changes when a small change is made in one of the kinetic parameters. We examine two definitions of this concept by applying it to first-principles kinetic Monte Carlo simulations of the CO oxidation at RuO2(110). Instead of studying experimental data we examine simulations, because in them we know the surface structure, reaction mechanism, the rate constants, the coverage of the surface and the turn-over frequency at steady state. We can test whether the insights provided by the DRC are in agreement with the results of the simulations thus avoiding the uncertainties inherent in a comparison with experiment. We find that the information provided by using the DRC is non-trivial: It could not have been obtained from the knowledge of the reaction mechanism and of the magnitude of the rate constants alone. For the simulations the DRC provides furthermore guidance as to which aspects of the reaction mechanism should be treated accurately and which can be studied by less accurate and more efficient methods. We therefore conclude that a sensitivity analysis based on the DRC is a useful tool for understanding the propagation of errors from the electronic structure calculations to the statistical simulations in first-principles kinetic Monte Carlo simulations.Comment: 27 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm