Browsing by Subject "Density functionals"
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Item Density evolution in systems with slow approach to equilibrium(2004) Nelson, Kevin Taylor; Driebe, Dean J.This dissertation investigates the evolution of probability densities under the Frobenius-Perron operator U in chaotic iterated-map systems that are slow to reach equilibrium. It first concentrates on one-dimensional maps that are slow to reach equilibrium because they feature intermittent chaos due to the presence of a marginal fixed point. Using the method of shift states and coherent states under U, certain results are obtained concerning the spectrum of U in various functional spaces, using as the main example the cusp map f (x) = 1 - √ |1 - 2x |. Those results are applied to obtain corrections to the well-known leading 1/t form of the x-x auto correlation function C(t). The symbolic dynamics of one-dimensional maps are then investigated, with particular emphasis on the implications of the existence of intermittent chaos and with applications to topological conjugation. Next, the statistics of extreme values in one dimensional maps are investigated. Fn(x) is defined as the probability that a point chosen from an initial probability distribution and its first n- 1 iterates under a particular map are all less than x; the properties of Fn(x) are derived analytically for a wide variety of one-dimensional maps, and the on conclusions are confirmed numerically. Finally, higher-dimensional area-preserving maps are investigated. The technique of local spectral decomposition for U, in which approximate right and left eigenstates for U are constructed localized on unstable periodic points, is used to study density evolution and correlation of observables over time.Item Density functional studies of magnetic semiconductors and multiferroics(2007-12) Ciucivara, Adrian Ioan, 1976-; Kleinman, LeonardThe present work is mostly focused on theoretical study of multiferroics and magnetic semiconductors within the framework of density functional theory (DFT). We studied Bi₂NiMnO₆ which was recently synthesized as a heavily distorted double perovskite with four formula units in a monoclinic unit cell. The calculated GGA and GGA+U magnetizations per formula unit were 4.92 and 4.99 [mu]B and the calculated ferroelectric polarizations were 16.83 and 16.63 [mu]C cm⁻². We also present the results of completely relaxed electronic structure calculations for multiferroic LaMnO₃/BaTiO₃ structure which was built by joining a slab of magnetic LaMnO₃ with a slab of ferroelectric BaTiO₃. The study of Mn doped GeTe, which is a diluted magnetic semiconductor (DMS), shows the net magnetization as a function of doping and holes concentration. In addition, we present a study of ferroelectric GeTe as a function of applied pressure and the interesting properties exhibited by Si(001)/Si(110) junction which was experimentally built using hybrid orientation technology (HOT).Item First principles modeling of arsenic and fluorine behavior in crystalline silicon during ultrashallow junction formation(2006) Harrison, Scott Anthony; Hwang, Gyeong S.; Edgar, Thomas F.Item Theoretical investigation of contact materials for emerging electronic and spintronic devices(2007-12) Niranjan, Manish Kumar, 1977-; Kleinman, Leonard; Demkov, Alexander A.We present a theoretical study of the electronic structure, surface energies and work functions of orthorhombic Pt monosilicide and germanides of Pt, Ni, Y and Hf within the framework of density functional theory (DFT). Calculated work functions for the (001) surfaces of PtSi, NiGe and PtGe suggest that these metals and their alloys can be used as self-aligned contacts to p-type silicon and germanium. In addition, we also study electronic structure and calculate the Schottky-barrier height at Si(001)/PtSi(001) interface and GaAs(001)/NiPtGe(001) interfaces with different GaAs(001) and NiPtGe (001) terminations. The p-type Schottky barrier height of 0.28 eV at Si/PtSi interface is found in good agreement with predictions of a simple metal induced gap states (MIGS) theory and available experiment. This low barrier suggests PtSi as a low contact resistance junction metal for silicon CMOS technology. We identify the growth conditions necessary to stabilize this orientation. The calculated p-type Schottky barrier heights (SBH) at different GaAs/NiPtGe interfaces vary by as much as 0.18 eV around the average value of 0.5 eV. We further identify and discuss factors responsible for strong Fermi level pinning resulting in small variation in the p-SBH. We also present a theoretical study of magnetic state of [beta]-MaAs and show that it is antiferromagnetic and explain the lack of observed long-range order.