87,694 research outputs found

    LaFeAsO1βˆ’x_{1-x}Fx_x: A low carrier density superconductor near itinerant magnetism

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    Density functional studies of 26K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, N(EF)N(E_F) and modest phonon frequencies relative to TcT_c. The high N(EF)N(E_F) leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by doping level. Thus LaFeAs(O,F) is in a unique class of high TcT_c superconductors: high N(EF)N(E_F) ionic metals near magnetism.Comment: Shortened published form. Typos correcte

    Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys

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    Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off-centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling.Comment: 4 pages, 5 figure

    Properties of KCo2_2As2_2 and Alloys with Fe and Ru: Density Functional Calculations

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    Electronic structure calculations are presented for KCo2_2As2_2 and alloys with KFe2_2As2_2 and KRu2_2As2_2. These materials show electronic structures characteristic of coherent alloys, with a similar Fermi surface structure to that of the Fe-based superconductors, when the dd electron count is near six per transition metal. However, they are less magnetic than the corresponding Fe compounds. These results are discussed in relation to superconductivity.Comment: 5 page

    Electronic structure of Ba(Fe,Ru)2As2 and Sr(Fe,Ir)2As2 alloys

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    The electronic structures of Ba(Fe,Ru)2_2As2_2 and Sr(Fe,Ir)2_2As2_2 are investigated using density functional calculations. We find that these systems behave as coherent alloys from the electronic structure point of view. In particular, the isoelectronic substitution of Fe by Ru does not provide doping, but rather suppresses the spin density wave characteristic of the pure Fe compound by a reduction in the Stoner enhancement and an increase in the band width due hybridization involving Ru. The electronic structure near the Fermi level otherwise remains quite similar to that of BaFe2_{2}As2_{2}. The behavior of the Ir alloy is similar, except that in this case there is additional electron doping

    Electronic Structure and Bulk Spin Valve Behavior in Ca3_3Ru2_2O7_7

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    We report density functional calculations of the magnetic properties and Fermiology of Ca3_3Ru2_2O7_7. The ground state consists of ferromagnetic bilayers, stacked antiferromagnetically. The bilayers are almost but not exactly half-metallic. In the ferromagnetic state opposite spin polarizations are found for in-plane and out-of-plane transport. Relatively high out of plane conductivity is found for the majority spin, which is relatively weakly conductive in-plane. In the ground state in-plane quantities are essentially the same, but the out of plane transport is strongly reduced.Comment: 5 page

    Comment on Structural Stability and Electronic Structure for Li3_3AlH6_6

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    Density functional calculations of the electronic structure are used to elucidate the bonding of Li3_3AlH6_6. It is found that this material is best described as ionic, and in particular that the [AlH6_6]3βˆ’^{3-} units are not reasonably viewed as substantially covalent
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