Strong-correlation effects in Born effective charges

Large values of Born effective charges are generally considered as reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, these quantities can be very much influenced by strong electron correlation and metallic behavior, which are not exclusive properties of ferroelectric materials. In this paper we compare the Born effective charges of some prototypical ferroelectrics with those of magnetic, non-ferroelectric compounds using a novel, self-interaction free methodology that improves on the local-density approximation description of the electronic properties. We show that the inclusion of strong-correlation effects systermatically reduces the size of the Born effective charges and the electron localization lengths. Furthermore we give an interpretation of the Born effective charges in terms of band energy structure and orbital occupations which can be used as a guideline to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure

Characterization of a Mixed Methanotrophic Culture Capable of Chloroethylene Degradation

A consortium of methanotrophs cultured from the St. Joseph's aquifer in Schoolcraft, MI, was found to exhibit similar methane consumption rates as pure cultures of methanotrophs. The methanotrophic consortium resides within a portion of the aquifer contaminated with a mixed waste plume of perchloroethylene (PCE) and its reductive dechlorination products from natural attenuation, trichloroethylene (TCE), cis-dichloroethylene (c-DCE), and vinyl chloride (VC). Oxidation kinetics for TCE, c-DCE, and VC were measured for the mixed methanotroph consortium and compared to reported rate parameters for degradation of these chloroethylene compounds by pure methanotrophic cultures. The results demonstrate that the kinetics of chloroethylene oxidation by the Schoolcraft methanotroph population mimic the degradation rates of pure methanotrophic cultures that primarily express particulate methane monooxygenase (pMMO). Molecular and biochemical analyses confirmed that sMMO was not being expressed by these cells. Rather, using competitive reverse transcriptionpolymerase chain reaction, pmoA, a gene encoding one of the polypeptides of the pMMO was found at a level of (1.57 ± 0.10) × 10–17 mol pmoA mRNA/g wet soil in soil slurries and (2.65 ± 0.43) × 10–17 mol pmoA mRNA/μl in groundwater. No expression of mmoX, a gene encoding one of the polypeptides of the sMMO, was detected.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63398/1/ees.2005.22.177.pd

A self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systems

Local-spin-density functional calculations may be affected by severe errors when applied to the study of magnetic and strongly-correlated materials. Some of these faults can be traced back to the presence of the spurious self-interaction in the density functional. Since the application of a fully self-consistent self-interaction correction is highly demanding even for moderately large systems, we pursue a strategy of approximating the self-interaction corrected potential with a non-local, pseudopotential-like projector, first generated within the isolated atom and then updated during the self-consistent cycle in the crystal. This scheme, whose implementation is totally uncomplicated and particularly suited for the pseudopotental formalism, dramatically improves the LSDA results for a variety of compounds with a minimal increase of computing cost.Comment: 18 pages, 14 figure

Measurement of W Polarisation at LEP

The three different helicity states of W bosons produced in the reaction e+ e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation

Search for Anomalous Couplings in the Higgs Sector at LEP

Anomalous couplings of the Higgs boson are searched for through the processes e^+ e^- -> H gamma, e^+ e^- -> e^+ e^- H and e^+ e^- -> HZ. The mass range 70 GeV < m_H < 190 GeV is explored using 602 pb^-1 of integrated luminosity collected with the L3 detector at LEP at centre-of-mass energies sqrt(s)=189-209 GeV. The Higgs decay channels H -> ffbar, H -> gamma gamma, H -> Z\gamma and H -> WW^(*) are considered and no evidence is found for anomalous Higgs production or decay. Limits on the anomalous couplings d, db, Delta(g1z), Delta(kappa_gamma) and xi^2 are derived as well as limits on the H -> gamma gamma and H -> Z gamma decay rates

Measurement of W Polarisation at LEP

The three different helicity states of W bosons produced in the reaction e+ e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation

Neutral-Current Four-Fermion Production in e+e- Interactions at LEP

Neutral-current four-fermion production, e+e- -> ffff is studied in 0.7/fb of data collected with the L3 detector at LEP at centre-of-mass energies root(s)=183-209GeV. Four final states are considered: qqvv, qqll, llll and llvv, where l denotes either an electron or a muon. Their cross sections are measured and found to agree with the Standard Model predictions. In addition, the e+e- -> Zgamma* -> ffff process is studied and its total cross section at the average centre-of-mass energy 196.6GeV is found to be 0.29 +/- 0.05 +/- 0.03 pb, where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model prediction of 0.22 pb. Finally, the mass spectra of the qqll final states are analysed to search for the possible production of a new neutral heavy particle, for which no evidence is found

Measurement of Exclusive rho+rho- Production in Mid-Virtuality Two-Photon Interactions and Study of the gamma gamma* -> rho rho Process at LEP

Exclusive rho+rho- production in two-photon collisions between a quasi-real photon, gamma, and a mid-virtuality photon, gamma*, is studied with data collected at LEP at centre-of-mass energies root(s)=183-209GeV with a total integrated luminosity of 684.8pb^-1. The cross section of the gamma gamma* -> rho+ rho- process is determined as a function of the photon virtuality, Q^2, and the two-photon centre-of-mass energy, W_gg, in the kinematic region: 0.2GeV^2 < Q^2 <0.85GeV^2 and 1.1GeV < W_gg < 3GeV. These results, together with previous L3 measurements of rho0 rho0 and rho+ rho- production, allow a study of the gamma gamma* -> rho rho process over the Q^2-region 0.2GeV^2 < Q^2 < 30 GeV^2

Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays

Bose-Einstein correlations of both neutral and like-sign charged pion pairs are measured in a sample of 2 million hadronic Z decays collected with the L3 detector at LEP. The analysis is performed in the four-momentum difference range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be smaller than that of charged pions. This result is in qualitative agreement with the string fragmentation model

Z Boson Pair-Production at LEP

Events stemming from the pair-production of Z bosons in e^+e^- collisions are studied using 217.4 pb^-1 of data collected with the L3 detector at centre-of-mass energies from 200 GeV up to 209 GeV. The special case of events with b quarks is also investigated. Combining these events with those collected at lower centre-of-mass energies, the Standard Model predictions for the production mechanism are verified. In addition, limits are set on anomalous couplings of neutral gauge bosons and on effects of extra space dimensions