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The atomic and electronic structures and stability of native defects in a single-layer ${\mathrm{MoS}}_{2}$ are investigated, based on density-functional theory calculations. Native defects such as a S vacancy (${V}_{\mathrm{S}}$), a S interstitial (${\mathrm{S}}_{i}$), a Mo vacancy (${V}_{\mathrm{Mo}}$), and a Mo interstitial (${\mathrm{Mo}}_{i}$) are considered. The ${\mathrm{S}}_{i}$ is found to have S-adatom configuration on top of a host S atom, and the ${\mathrm{Mo}}_{i}$ has Mo-${\mathrm{Mo}}_{i}$ split interstitial configuration along the $c$ direction. The formation energies of the native defects in neutral and charged states are calculated. For the charged states, the artificial electrostatic interactions between image charges in supercells are eliminated by a supercell size scaling scheme and a correction scheme that uses a Gaussian model charge. It is found that the ${V}_{\mathrm{S}}$ has a low formation energy of 1.3--1.5 eV in the Mo-rich limit condition, and the ${\mathrm{S}}_{i}$ has 1.0 eV in the S-rich limit condition. The ${V}_{\mathrm{S}}$ is found to be a deep single acceptor with the (0/$\ensuremath{-}$) transition level at 1.7 eV above the valence-band maximum (VBM). The ${\mathrm{S}}_{i}$ is found to be an electrically neutral defect. The Mo-related native defects of ${V}_{\mathrm{Mo}}$ and ${\mathrm{Mo}}_{i}$ are found to be high in formation energy above 4 eV. The ${V}_{\mathrm{Mo}}$ is a deep single acceptor and the ${\mathrm{Mo}}_{i}$ is a deep single donor, of which the (0/$\ensuremath{-}$) acceptor and (+/0) donor transition levels are found at 1.1 and 0.3 eV above the VBM, respectively.

We have determined the structure of AlSb and GaSb (001) surfaces prepared by molecular beam epitaxy under typical Sb-rich device growth conditions. Within the range of flux and temperature where the diffraction pattern is nominally $(1\ifmmode\times\else\texttimes\fi{}3)$, we find that there are actually three distinct, stable $(4\ifmmode\times\else\texttimes\fi{}3)$ surface reconstructions. The three structures differ from any previously proposed for these growth conditions, with two of the reconstructions incorporating mixed III-V dimers within the Sb surface layer. These heterodimers appear to play an important role in island nucleation and growth.

We present an ab initio pseudopotential local density functional calculation for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set. We have overcome well-known difficulties in applying pseudopotential methods to first-row elements, transition metals, and rare-earth materials by carefully generating norm-conserving pseudopotentials with excellent transferability and employing an extremely efficient iterative diagonalization scheme optimized for our purpose. The self-consistent band structures, the total and site-projected densities of states, the partial charges and their symmetry-decompositions, and some characteristic charge densities near E_f are presented. We compare our results with various existing (F)LAPW and (F)LMTO calculations and establish that the ab initio pseudopotential method is competitive with other methods in studying the electronic structure of such complicated materials as high-Tc cuprates. [8 postscript files in uuencoded compressed form]

By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states.

Initial adsorption configurations of $\mathrm{N}{\mathrm{H}}_{3}$ molecules on the Si(001) surface are studied by using scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. In contrast to the previous consensus of single configuration, we demonstrate that there exist two distinct STM features. One is found to be the existing model of on-dimer configuration, that is characterized by the previously unreported ``U-shape'' empty-state feature. The other is identified to be $\mathrm{N}{\mathrm{H}}_{2}$ and H adsorbed at the same side of two adjacent dimers along the dimer row. The identification of all the adsorption configurations would be useful for the quantitative analysis of Si nitridation.

We report the discovery of pressure-induced superconductivity in a semimetallic magnetic material ${\mathrm{CeTe}}_{1.82}.$ The superconducting transition temperature ${T}_{c}=2.7\mathrm{K}$ (well below the magnetic ordering temperatures) under pressure $(g2\mathrm{kbar})$ is remarkably high, considering the relatively low carrier density due to a charge-density-wave transition associated with lattice modulation. The mixed magnetic structure of antiferromagnetism coexisting with ferromagnetism can provide a clue for this high ${T}_{c}.$ We discuss a possible theoretical model for the superconducting pairing mechanism.

We have investigated the Li/GaAs(110) surface by combining the low-energy electron diffraction (LEED) and the ab initio pseudopotential calculations. The potential energy surface for the single atomic adsorption shows the existence of two inequivalent adsorption sites. At 0.5 monolayer coverage of Li, a well defined $1\ifmmode\times\else\texttimes\fi{}2$ LEED pattern is observed. The atomic structure at this coverage is characterized by Li zigzag chains running in the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction. This structure is stabilized by electron transfer from the Li adatoms to the dangling bonds of certain surface Ga atoms. The simulated scanning tunneling microscopy images show linear (asymmetric zigzag) chains along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction for positive (negative) sample biases.