Base manifolds for fibrations of projective irreducible symplectic manifolds

Given a projective irreducible symplectic manifold $M$ of dimension $2n$, a projective manifold $X$ and a surjective holomorphic map $f:M \to X$ with connected fibers of positive dimension, we prove that $X$ is biholomorphic to the projective space of dimension $n$. The proof is obtained by exploiting two geometric structures at general points of $X$: the affine structure arising from the action variables of the Lagrangian fibration $f$ and the structure defined by the variety of minimal rational tangents on the Fano manifold $X$

Modeling of intrinsic electron and hole trapping in crystalline and amorphous ZnO

Recent advances in ultrafast liquid quenching and deposition of thin films on cold substrates make growing amorphous (a)‐ZnO films increasingly feasible. The electronic structure and electron and hole trapping properties of amorphous ZnO are predicted using density functional theory (DFT) simulations with a hybrid density functional (h‐DFT). An ensemble of fifty 324‐atom structures is employed to obtain the distribution of structural and electronic properties of a‐ZnO. The results demonstrate that electrons do not localize in a‐ZnO, but holes form deep localized states with average trapping energy of about 0.9 eV. It is also shown that dispersion at the conduction band minimum (CBM) is not affected upon amorphization, suggesting that high electron mobility should be retained. An average value of a‐ZnO band gap of 3.36 eV is calculated with no states splitting into the band gap, which accounts for no substantial detrimental effect on the optical transparency upon amorphization. These findings may have important implications for future applications of a‐ZnO as a transparent conductor and photocatalyst

New Model for Electron-Impact Ionization Cross Sections of Molecules

A theoretical model for electron-impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public-domain molecular structure codes such as GAMESS. The theory is called the binary encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals

Anisotropic strains and magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3}

Thin films of perovskite manganite La_{0.7}Ca_{0.3}MnO_{3} were grown epitaxially on SrTiO_3(100), MgO(100) and LaAlO_3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal for SrTiO_3, MgO, and LaAlO_3 cases, respectively, while the unit cell of the bulk is cubic. It is found that the samples with cubic unit cell show smaller peak magnetoresistance than the noncubic ones do. The present result demonstrates that the magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3} can be controlled by lattice distortion via externally imposed strains.Comment: Revtex, 10 pages, 2 figure

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p\agt 4 \times 10^{9} cm$^{-2}$), the nonmonotonic temperature dependence ($\sim 50-300$ mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of $T$= 50 mK, the conductivity vs. density data indicates an inhomogeneity driven percolation-type transition to an insulating state at a critical density of $3.8\times 10^9$ cm$^{-2}$.Comment: accepted for publication in PR