Ab initio Calculations for SrTiO_3 (100) Surface Structure

Results of detailed calculations for SrTiO_3 (100) surface relaxation and the electronic structure for the two different terminations (SrO and TiO_2) are discussed. These are based on ab initio Hartree-Fock (HF) method with electron correlation corrections and Density Functional Theory (DFT) with different exchange-correlation functionals, including hybrid (B3PW, B3LYP) exchange techniques. Results are compared with previous ab initio plane wave LDA calculations. All methods agree well on both surface energies and on atomic displacements. Considerable increase of Ti[Single Bond]O chemical bond covalency nearby the surface is predicted, along with a gap reduction, especially for the TiO_2 termination

Computer Modeling of Point Defects, Impurity Self-Ordering Effects and Surfaces in Advanced Perovskite Ferroelectrics

The calculated optical properties of basic point defects - F-type centers and hole polarons - in KNbO_3 perovskite crystals are used for the interpretation of available experimental data. The results of quantum chemical calculations for perovskite KNb_xTa_(1-x)O_3 solid solutions are presented for x=0, 0.125, 0.25, 0.75, and 1. An analysis of the optimized atomic and electronic structure clearly demonstrates that several nearest Nb atoms substituting for Ta in KTaO_3 - unlike Ta impurities in KNbO_3 - reveal a self-ordering effect, which probably triggers the ferroelectricity observed in KNb_xTa_(1-x)O_3. Lastly, the (110) surface relaxations are calculated for SrTiO_3 and BaTiO_3 perovskites. The positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized using the classical shell model. Strong surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces

Simulations of high disruption colliding beams

Recent B-factory proposals that use a linac beam colliding with the beam from a storage ring to achieve high luminosities (L > 10{sup 34} cm{sup {minus}2}sec{sup {minus}1}) result in very high disruption of the linac beam. The effects of such high disruption have been studied using the relativistic, 3-D code SWARM. We discuss the assumptions, parameters, and results of a series of runs that model such collisions. Regimes of high beam loss and methods to avoid them are also discussed. 5 refs., 4 figs

First principles and semi-empirical calculations of atomic and electronic structure for the (100) and (110) perovskite surfaces

We present and discuss results of the calculations for BaTiO_3 and SrTiO_3 surface relaxation with different terminations using a semi-empirical shell model (SM) as well as ab initio methods based on Hartree-Fock (HF) and Density Functional Theory (DFT) formalisms. Using the SM, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized. This permits us determination of surface rumpling and surface-induced dipole moments (polarization) for different terminations of the (100) and (110) surfaces. We also compare results of the ab initio calculations based on both HF with the DFT-type electron correlation corrections, several DFT with different exchange-correlation functionals, and hybrid exchange techniques. Our SM results for the (100) surfaces are in a good agreement with both our ab initio calculations and LEED experiments. For the (110) surfaces O-termination is predicted to be the lowest in energy

Risk of acquired drug resistance during short-course directly observed treatment of tuberculosis in an area with high levels of drug resistance.

BACKGROUND: Data on the performance of standardized short-course directly observed treatment (DOTS) of tuberculosis (TB) in areas with high levels of drug resistance and on the potential impact of DOTS on amplification of resistance are limited. Therefore, we analyzed treatment results from a cross-sectional sample of patients with TB enrolled in a DOTS program in an area with high levels of drug resistance in Uzbekistan and Turkmenistan in Central Asia. METHODS: Sputum samples for testing for susceptibility to 5 first-line drugs and for molecular typing were obtained from patients starting treatment in 8 districts. Patients with sputum smear results positive for TB at the end of the intensive phase of treatment and/or at 2 months into the continuation phase were tested again. RESULTS. Among 382 patients with diagnoses of TB, 62 did not respond well to treatment and were found to be infected with an identical Mycobacterium tuberculosis strain when tested again; 19 of these patients had strains that developed new or additional drug resistance. Amplification occurred in only 1.2% of patients with initially susceptible or monoresistant TB strains, but it occurred in 17% of those with polyresistant strains (but not multidrug-resistant strains, defined as strains with resistance to at least isoniazid and rifampicin) and in 7% of those with multidrug-resistant strains at diagnosis. Overall, 3.5% of the patients not initially infected with multidrug-resistant TB strains developed such strains during treatment. Amplification of resistance, however, was found only in polyresistant Beijing genotype strains. CONCLUSIONS: High levels of amplification of drug resistance demonstrated under well-established DOTS program conditions reinforce the need for implementation of DOTS-Plus for multidrug-resistant TB in areas with high levels of drug resistance. The strong association of Beijing genotype and amplification in situations of preexisting resistance is striking and may underlie the strong association between this genotype and drug resistance

Calculations of Perovskite Polar Surface Structures

Results of calculations for the (110) polar surfaces of three ABO3 perovskites — STO, BTO and LMO — are discussed. These are based on ab initio Hartree-Fock method and classical Shell Model. Both methods agree well on both surface energies and on near-surface atomic displacements. A novel model of the "zig-zag" surface termination is suggested and analyzed. Considerable increase of the Ti[Single Bond]O chemical bond covalency nearby the surface is predicted for STO

Quantum corrected electron holes

The theory of electron holes is extended into the quantum regime. The Wigner--Poisson system is solved perturbatively based in lowest order on a weak, standing electron hole. Quantum corrections are shown to lower the potential amplitude and to increase the number of deeply trapped electrons. They, hence, tend to bring this extreme non--equilibrium state closer to thermodynamic equilibrium, an effect which can be attributed to the tunneling of particles in this mixed state system.Comment: 12 pages, 3 figure

Comparative density-functional LCAO and plane-wave calculations of LaMnO3 surfaces

We compare two approaches to the atomic, electronic, and magnetic structures of LaMnO3 bulk and the (001), (110) surfaces—hybrid B3PW with optimized LCAO basis set (CRYSTAL-2003 code) and GGA-PW91 with plane-wave basis set (VASP 4.6 code). Combining our calculations with those available in the literature, we demonstrate that combination of nonlocal exchange and correlation used in hybrid functionals allows to reproduce the experimental magnetic coupling constants Jab and Jc as well as the optical gap. Surface calculations performed by both methods using slab models show that the antiferromagnetic (AF) and ferromagnetic (FM) (001) surfaces have lower surface energies than the FM (110) surface. Both the (001) and (110) surfaces reveal considerable atomic relaxations, up to the fourth plane from the surface, which reduce the surface energy by about a factor of 2, being typically one order of magnitude larger than the energy difference between different magnetic structures. The calculated (Mulliken and Bader) effective atomic charges and the electron density maps indicate a considerable reduction of the Mn and O atom ionicity on the surface

On the relativistic KMS condition for the P(\phi)_2 model

The relativistic KMS condition introduced by Bros and Buchholz provides a link between quantum statistical mechanics and quantum field theory. We show that for the $P(\phi)_2$ model at positive temperature, the two point function for fields satisfies the relativistic KMS condition