4,851 research outputs found

    Point defects and clustering in uranium dioxide by LSDA+U calculations

    Full text link
    A comprehensive investigation on point defects and their clustering behavior in nonstoichiometric uranium dioxide UO2+x is carried out using LSDA+U method based on density functional theory. Accurate energetic information and charge transfers available so far are obtained. With these energies that have improved more than 50% over that of pure GGA and LDA, we show the density functional theory predicts the predominance of oxygen defects over uranium ones at any compositions, which is possible only after treated the localized 5f electrons properly. Calculations also suggest an upper bound of x~0.03 for oxygen clusters to start off. The volume change induced by point uranium defects is monotonic but nonlinear, whereas for oxygen defects, increase x always reduces the system volume linearly, except dimers that require extra space for accommodation, which has been identified as meta-stable ionic molecule. Though oxygen dimers usually occupy Willis O'' sites and mimic a single oxygen in energetics and charge state, they are rare at ambient conditions. Its decomposition process and vibrational properties have been studied carefully. To obtain a general clustering mechanism in anion-excess fluorites systematically, we also analyze the local stabilities of possible basic clustering modes of oxygen defects. The result shows an unified way to understand the structure of Willis type and cuboctahedral clusters in UO2+x and beta-U4O9. Finally we generalize the point defect model to the independent clusters approximation to include clustering effects, the impact on defect populations is discussed.Comment: 20 pages, 12 figure

    Parton energy loss limits and shadowing in Drell-Yan dimuon production

    Get PDF
    A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is reported. The behavior of the Drell-Yan ratios at small target parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross section ratios as a function of the incident parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus

    Measurement of Angular Distributions of Drell-Yan Dimuons in p+pp + p Interactions at 800 GeV/c

    Full text link
    We report a measurement of the angular distributions of Drell-Yan dimuons produced using an 800 GeV/c proton beam on a hydrogen target. The polar and azimuthal angular distribution parameters have been extracted over the kinematic range 4.5<mμμ<154.5 < m_{\mu \mu} < 15 GeV/c2^2 (excluding the Υ\Upsilon resonance region), 0<pT<40 < p_T < 4 GeV/c, and 0<xF<0.80 < x_F < 0.8. The p+pp+p angular distributions are similar to those of p+dp+d, and both data sets are compared with models which attribute the cos2ϕ\cos 2 \phi distribution either to the presence of the transverse-momentum-dependent Boer-Mulders structure function h1h_1^\perp or to QCD effects. The data indicate the presence of both mechanisms. The validity of the Lam-Tung relation in p+pp+p Drell-Yan is also tested.Comment: 4 pages, 3 figure

    Measurement of Angular Distributions of Drell-Yan Dimuons in p + d Interaction at 800 GeV/c

    Get PDF
    We report a measurement of the angular distributions of Drell-Yan dimuons produced using an 800 GeV/c proton beam on a deuterium target. The muon angular distributions in polar angle θ\theta and azimuthal angle ϕ\phi have been measured over the kinematic range 4.5<mμμ<154.5 < m_{\mu \mu} < 15 GeV/c2^2, 0<pT<40 < p_T < 4 GeV/c, and 0<xF<0.80 < x_F < 0.8. No significant cos2ϕ2\phi dependence is found in these proton-induced Drell-Yan data, in contrast to the situation for pion-induced Drell-Yan. The data are compared with expectations from models which attribute the cos2ϕ2\phi distribution to a QCD vacuum effect or to the presence of the transverse-momentum-dependent Boer-Mulders structure function h1h_1^\perp. Constraints on the magnitude of the sea-quark h1h_1^\perp structure functions are obtained.Comment: 4 pages, 3 figure

    Fermion Pairing Dynamics in the Relativistic Scalar Plasma

    Full text link
    Using many-body techniques we obtain the time-dependent Gaussian approximation for interacting fermion-scalar field models. This method is applied to an uniform system of relativistic spin-1/2 fermion field coupled, through a Yukawa term, to a scalar field in 3+1 dimensions, the so-called quantum scalar plasma model. The renormalization for the resulting Gaussian mean-field equations, both static and dynamical, are examined and initial conditions discussed. We also investigate solutions for the gap equation and show that the energy density has a single minimum.Comment: 21 pages, latex, 4 postscript figures, new sections, some literary changes, notation corrections, accepted for publication in Phys. Rev

    dbar/ubar Asymmetry and the Origin of the Nucleon Sea

    Full text link
    The Drell-Yan cross section ratios, σ(p+d)/σ(p+p)\sigma(p+d)/\sigma(p+p), measured in Fermilab E866, have led to the first determination of dˉ(x)/uˉ(x)\bar d(x) / \bar u(x), dˉ(x)uˉ(x)\bar d(x) - \bar u(x), and the integral of dˉ(x)uˉ(x)\bar d(x) - \bar u(x) for the proton over the range 0.02x0.3450.02 \le x \le 0.345. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates these non-perturbative processes play an important role in the origin of the dˉ\bar d, uˉ\bar u asymmetry in the nucleon sea.Comment: 5 pages, 3 figures, ReVTe

    Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound U2_2Zn17_{17}

    Full text link
    We have performed magnetization measurements at high magnetic fields of up to 53 T on single crystals of a uranium heavy-fermion compound U2_2Zn17_{17} grown by the Bridgman method. In the antiferromagnetic state below the N\'{e}el temperature TNT_{\rm N} = 9.7 K, a metamagnetic transition is found at HcH_c \simeq 32 T for the field along the [112ˉ\bar{2}0] direction (aa-axis). The magnetic phase diagram for the field along the [112ˉ\bar{2}0] direction is given. The magnetization curve shows a nonlinear increase at HmH_m \simeq 35 T in the paramagnetic state above TNT_{\rm N} up to a characteristic temperature TχmaxT_{{\chi}{\rm max}} where the magnetic susceptibility or electrical resistivity shows a maximum value. This metamagnetic behavior of the magnetization at HmH_m is discussed in comparison with the metamagnetic magnetism of the heavy-fermion superconductors UPt3_3, URu2_2Si2_2, and UPd2_2Al3_3. We have also carried out high-pressure resistivity measurement on U2_2Zn17_{17} using a diamond anvil cell up to 8.7 GPa. Noble gas argon was used as a pressure-transmitting medium to ensure a good hydrostatic environment. The N\'{e}el temperature TNT_{\rm N} is almost pressure-independent up to 4.7 GPa and starts to increase in the higher-pressure region. The pressure dependences of the coefficient of the T2T^2 term in the electrical resistivity AA, the antiferromagnetic gap Δ\Delta, and the characteristic temperature TρmaxT_{{\rho}{\rm max}} are discussed. It is found that the effect of pressure on the electronic states in U2_2Zn17_{17} is weak compared with those in the other heavy fermion compounds
    corecore