210 research outputs found

    New structural model for GeO2/Ge interface: A first-principles study

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    First-principles modeling of a GeO2/Ge(001) interface reveals that sixfold GeO2, which is derived from cristobalite and is different from rutile, dramatically reduces the lattice mismatch at the interface and is much more stable than the conventional fourfold interface. Since the grain boundary between fourfold and sixfold GeO2 is unstable, the sixfold GeO2 forms a large grain at the interface. On the contrary, a comparative study with SiO2 demonstrates that SiO2 maintains a fourfold structure. The sixfold GeO2/Ge interface is shown to be a consequence of the ground-state phase of GeO2. In addition, the electronic structure calculation reveals that sixfold GeO2 at the interface shifts the valence band maximum far from the interface toward the conduction band.Comment: 18 pages, 5 figures, and 2 table

    First-Principles Study on Electron-Conduction Properties of C60_{60} Chains

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    The electron-conduction properties of fullerene chains are examined by first-principles calculations based on the density functional theory. The conductivity of the C60_{60} dimer is low owing to the constraint of the junction of the molecules on electron conduction, whereas the C60_{60} monomer exhibits a conductance of \sim 1 G0_0. One of the three degenerate tu1t_{u1} states of C60_{60} is relevant to conduction and the contributions of the others are small. In addition, we found a more interesting result that the conductance of the fullerene chain is drastically increased by encapsuling metal atoms into cages.Comment: 10pages and 5 figure

    Improvement of accuracy of wave-function-matching method for transport calculation

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    The wave-function-matching (WFM) technique for first-principles transport-property calculations was modified by S\o{}rensen {\it et al.} so as to exclude rapidly decreasing evanescent waves [S\o{}rensen {\it et al.}, Phys. Rev. B {\bf 77}, 155301 (2008)]. However, this method lacks translational invariance of the transmission probability with respect to insertion of matching planes and consistency between the sum of the transmission and reflection probabilities and the number of channels in the transition region. We reformulate the WFM method since the original methods are formulated to include all the generalized Bloch waves. It is found that the translational invariance is destroyed by the overlap of the layers between the electrode and transition regions and by the pseudoinverses used to exclude the rapidly decreasing evanescent waves. We then devise a method that removes the overlap and calculates the transmission probability without the pseudoinverses. As a result, we find that the translational invariance of the transmission probability with respect to insertion of the extra layers is properly retained and the sum of the transmission and reflection probabilities exactly agrees with the number of channels. In addition, we prove that the accuracy in the transmission probability of this WFM technique is comparable with that obtained by the nonequilibrium Green's function method. Furthermore, we carry out the electron transport calculations on two-dimensional graphene sheets embedded with B--N line defects sandwiched between a pair of semi-infinite graphene electrodes and find the dependence of the electron transmission on the transverse momentum perpendicular to the direction of transport

    New structural model for GeO2/Ge interface: A first-principles study

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    First-principles modeling of a GeO2/Ge(001) interface reveals that sixfold GeO2, which is derived from cristobalite and is different from rutile, dramatically reduces the lattice mismatch at the interface and is much more stable than the conventional fourfold interface. Since the grain boundary between fourfold and sixfold GeO2 is unstable, the sixfold GeO2 forms a large grain at the interface. On the contrary, a comparative study with SiO2 demonstrates that SiO2 maintains a fourfold structure. The sixfold GeO2/Ge interface is shown to be a consequence of the ground-state phase of GeO2. In addition, the electronic structure calculation reveals that sixfold GeO2 at the interface shifts the valence band maximum far from the interface toward the conduction band.Comment: 18 pages, 5 figures, and 2 table

    Fully spin-dependent transport of triangular graphene flakes

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    The magnetic moment and spin-polarized electron transport properties of triangular graphene flakes surrounded by boron nitride sheets (BNC structures) are studied by using first-principles calculations based on density functional theory. Their dependence on the BNC structure is discussed, revealing that small isolated graphene flakes have large magnetic moment. When the BNC structure is suspended between graphene electrodes, the spin-polarized charge density distribution accumulates at the edge of the graphene flakes and no spin polarization is observed in the graphene electrodes. We also found that the BNC structure demonstrates perfectly spin-polarized transport properties in the wide energy window around the Fermi level. Our first-principles results indicate that the BNC structure provides new possibilities to electrically control spin
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