Mechanism of phase transitions and the electronic density of states in (La,Sm)FeAsO1−x_{1-x}Fx_x from ab initio calculations

The structure and electronic density of states in layered LnFeAsO$_{1-x}$F$_x$ (Ln=La,Sm; $x$=0.0, 0.125, 0.25) are investigated using density functional theory. For the $x$=0.0 system we predict a complex potential energy surface, formed by close-lying single-well and double-well potentials, which gives rise to the tetragonal-to-orthorhombic structural transition, appearance of the magnetic order, and an anomaly in the specific heat capacity observed experimentally at temperatures below $\sim$140--160 K. We propose a mechanism for these transitions and suggest that these phenomena are generic to all compounds containing FeAs layers. For $x>$0.0 we demonstrate that transition temperatures to the superconducting state and their dependence on $x$ correlate well with the calculated magnitude of the electronic density of states at the Fermi energy.Comment: 4 pages, 3 figures, 1 tabl

A microscopic mechanism of dielectric breakdown in SiO2 films: An insight from multi-scale modeling

Despite extensive experimental and theoretical studies, the atomistic mechanisms responsible for dielectric breakdown (BD) in amorphous (a)-SiO2 are still poorly understood. A number of qualitative physical models and mathematical formulations have been proposed over the years to explain experimentally observable statistical trends. However, these models do not provide clear insight into the physical origins of the BD process. Here we investigate the physical mechanisms responsible for dielectric breakdown in a-SiO2 using a multi-scale approach where the energetic parameters derived from a microscopic mechanism are used to predict the macroscopic degradation parameters of BD, i.e. time-dependent dielectric breakdown (TDDB) statistics, and its voltage dependence. Using this modeling framework, we demonstrate that trapping of two electrons at intrinsic structural precursors in a-SiO2 is responsible for a significant reduction of the activation energy for Si-O bond breaking. This results in a lower barrier for the formation of O vacancies and allows us to explain quantitatively the TDDB data reported in the literature for relatively thin (3-9nm) a-SiO2 oxide films

Modelling charge self-trapping in wide-gap dielectrics: Localization problem in local density functionals

We discuss the adiabatic self-trapping of small polarons within the density functional theory (DFT). In particular, we carried out plane-wave pseudo-potential calculations of the triplet exciton in NaCl and found no energy minimum corresponding to the self-trapped exciton (STE) contrary to the experimental evidence and previous calculations. To explore the origin of this problem we modelled the self-trapped hole in NaCl using hybrid density functionals and an embedded cluster method. Calculations show that the stability of the self-trapped state of the hole drastically depends on the amount of the exact exchange in the density functional: at less than 30% of the Hartree-Fock exchange, only delocalized hole is stable, at 50% - both delocalized and self-trapped states are stable, while further increase of exact exchange results in only the self-trapped state being stable. We argue that the main contributions to the self-trapping energy such as the kinetic energy of the localizing charge, the chemical bond formation of the di-halogen quasi molecule, and the lattice polarization, are represented incorrectly within the Kohn-Sham (KS) based approaches.Comment: 6 figures, 1 tabl

Structure and diffusion of intrinsic defects, adsorbed hydrogen, and water molecules at the surface of alkali-earth fluorides calculated using density functional theory

Using periodic density functional theory, we calculate the structure and migration energies of fluorine vacancies and interstitials in the bulk and at the stoichiometric bulk-truncated surface of three alkali-earth fluorides: CaF2, SrF2, and BaF2. We then study the adsorption of water and hydrogen, in both molecular and dissociated form, at the ideal surface, and at neutral and charged vacancies in the surface and subsurface layers. The results demonstrate that in nearly all cases molecular adsorption is strongly favored. For the most probable configurations on the surfaces, we also studied the migration paths and barriers, and found that water is highly mobile on the surface, even when adsorbed at defects. In general, CaF2 and SrF2 show similar behavior with respect to water, while adsorption energies and migration barriers for BaF2 are smaller. Finally, we discuss our results in the context of recent experimental Atomic Force Microscopy studies on CaF2 and compare to calculations on other insulating surfaces.Peer reviewe

Negative oxygen vacancies in HfO2_2 as charge traps in high-k stacks

We calculated the optical excitation and thermal ionization energies of oxygen vacancies in m-HfO$_2$ using atomic basis sets, a non-local density functional and periodic supercell. The thermal ionization energies of negatively charged V$^-$ and V$^{2-}$ centres are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are the likely candidates for intrinsic electron traps in the hafnum-based gate stack devices.Comment: 3 pages, 2 figure

Mechanism of Interstitial Oxygen Diffusion in Hafnia

We have performed density functional calculations of oxygen incorporation and diffusion in monoclinic hafnia (HfO2) for a range of oxygen charge states. The calculations demonstrate that oxygen favors atomic incorporation and that O2− is the most stable species. We find that oxygen interstitials diffuse via exchange with lattice oxygen sites in hafnia, and that O− species have the smallest diffusion barrier.Peer reviewe

Theories of scanning probe microscopes at the atomic scale

Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods—scanning tunneling microscopy and scanning force microscopy—in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface—the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors' own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation.Peer reviewe

Unambiguous Interpretation of Atomically Resolved Force Microscopy Images of an Insulator

The (111) surface of CaF2 was imaged with dynamic mode scanning force microscopy and modeledusing atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy.Peer reviewe

Vacancy and interstitial defects in hafnia

We have performed plane wave density functional theory calculations of atomic and molecular interstitial defects and oxygen vacancies in monoclinic hafnia (HfO2). The atomic structures of singly and doubly positively charged oxygen vacancies, and singly and doubly negatively charged interstitial oxygen atoms and molecules are investigated. We also consider hafnium vacancies, substitutional zirconium, and an oxygen vacancy paired with substitutional zirconium in hafnia. Our results predict that atomic oxygen incorporation is energetically favored over molecular incorporation, and that charged defect species are more stable than neutral species when electrons are available from the hafnia conduction band. The calculated positions of defect levels with respect to the bottom of the silicon conduction band demonstrate that interstitial oxygen atoms and molecules and positively charged oxygen vacancies can trap electrons from silicon.Peer reviewe