We conduct first-principles total-energy density functional calculations to study the band structures in Ge 1Àx Sn x infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge 1Àx Sn x lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect-to direct-gap in Ge 1Àx Sn x alloys is estimated to be as low as x $ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0 < x < 0:375 and the alloys become metallic for x > 0.375, in very good agreement with the theoretical observed behavior [D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994, 1987]. For homonuclear and heteronuclear complexes of Ge 1Àx Sn x alloys, the indirect band gap at L-pointis is found to decrease homonuclear Ge-Ge bonds or increase homonuclear Sn-Sn bonds as a result of the reduced L valley. All findings agree with previously reported experimental and theoretical results. The analysis suggests that the top of valence band exhibits the localization of bond charge and the bottom of the conduction band is composed of the Ge 4s4p and/or Sn 5s5p atomic orbits. V

We have performed high resolution photoemission study of substitutionally disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy spectra and pure metal spectra are found to have dips at the Fermi level when the residual resistivity is high and when rather strong repulsive electronelectron interaction is expected. This is in accordance with Altshuler and Aronov's model which predicts depletion of density of states at the Fermi level when both disorder and electron correlation are present.

We present ab initio calculations to find the migration pathways of the hydrogen atom through Stone-Wales defects into the inside of the double-shell fullerene. We report that the most favorable pathway consists of the tunneling pathway through Stone-Wales defects on the double-shell C60/C240 fullerene. This tunneling pathway gives rise to three barrier heights of 0.54 eV, 0.47 eV, and 0.7 eV. The driving force for the hydrogen atom diffusion through the tunneling pathway towards the inside of the double-shell fullerene is 0.82 eV. Our findings lead to a relatively low energy pathway, which provides a practical route to develop newly inexpensive solid-state hydrogen storages.

We present first-principles calculations on the heterojunction between a wurtzite GaN(1 100) film and a tetragonal LiAlO 2 (100) substrate. The relative barrier heights of different models of the GaNð1 100Þ k LiAlO 2 ð100Þ interface are examined as a function of the valence-band offset. The most favorable interface consists of fourfold-coordinated N and Ga, which has the lowest valence-band offset of 0.534 eV. This interface structure indicates that the nitrided LiAlO 2 (100) surface stabilizes the GaN/LiAlO 2 heteroepitaxial structures. Our findings explain a variety of experiments and may prove useful for improving the internal quantum efficiency of nonpolar GaN-based optoelectronic devices.A mong the alternative substrates for GaN epitaxy, the tetragonal lithium aluminum oxide (space group P4 1 2 1 2, -LiAlO 2 , LAO) was regarded as one of the most closely lattice-matched substrates. The epitaxial formation of a wurtzite m-plane GaN(1 " 100) film on a tetragonal LAO(100) substrate, i.e., GaNð1 "100Þ k LAOð100Þ, is nearly lattice matched and confirms that the lattice mismatches between wurtzite GaN and tetragonal LAO are 0.3% for GaN½0001 k LAO½010 and 1.7% for GaN½11 " 20 k LAO½001. 1-4) These small lattice mismatches suggest that LAO is a promising substrate material for the epitaxial growth of the hexagonal GaN thin film. In addition, the fabrication of m-plane GaN(1 " 100) film, i.e., the growth of GaN films oriented along nonpolar directions, has shown the promising potential for improving the internal quantum efficiency of GaN-based multiple-quantum-well (MQW) light-emitting diodes (LEDs) due to the absence of polarization-related electrostatic fields. 4) However, the nature of the atomic bonding between GaN epilayers and LAO templates has not been elucidated yet. The bonding configurations or electronic properties of heterojunctions can be characterized on the basis of the valence-or conductionband discontinuities. In this paper, we calculate the valenceband offset (VBO) of the optimized GaN(1 " 100)/LAO(100) heterostructure to elucidate the bonding configuration that enables strain relaxation and analyze the electronic charge density. The VBO using the standard bulk-plus-lineup procedure can be split into two terms as follows: 5,6)