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Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as $M{X}_{2}$ with $M=(\mathrm{Mo},\mathrm{W})$ and $X=(\mathrm{S},\mathrm{Se},\mathrm{Te})$, the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the nontrivial topology of electron bands in TMDs is caused by the band inversion between metal $d$- and chalcogen $p$-orbital bands where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, $M{\mathrm{S}}_{2}$ and $M\mathrm{S}{\mathrm{e}}_{2}$ exhibits the opposite behavior of TPT such that the chalcogen $p$-orbital band moves down below the metal $d$-orbital band. More interestingly, in $M\mathrm{T}{\mathrm{e}}_{2}$, the band inversion occurs between the metal $d$-orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.

Due to its abundance, silicon is the preferred solar-cell material despite the fact that current silicon materials have indirect band gaps. Although the band gap properties of silicon have been studied intensively, until now, no direct band gap silicon-based material has been found or suggested. We report here the discovery of direct band gap silicon crystals. By using conformational space annealing, we optimize various crystal structures containing multiple (10 to 20) silicon atoms per unit cell so that their electronic structures become direct band gap. Through first-principles calculations, we identify many direct and quasidirect band gap crystal structures, which exhibit excellent photovoltaic efficiency.

Calculations of the electronic structure demonstrate that band gap modification such as opening and closure is easily achieved by radial deformations perpendicular to the tube axis. Deformations open the gap in metallic armchair tubes only if the mirror symmetries are broken, and further distortions greatly enhance the gap due to new bonds formed between facing layers. Similar deformations increase the gap in small-gap zigzag tubes, but gap closure occurs due to enhanced ${\ensuremath{\sigma}}^{*}\ensuremath{-}{\ensuremath{\pi}}^{*}$ hybridization effects. This band gap modification is crucial at metal contacts and has important implication in fabricating a quantum dot or multiple quantum dots.

This work was in part supported by KRISS, by the Korea Ministry of Education, Grant No. BSRI 97-2435, and the CMS at KAIST.

We investigate the energy-loss spectrum of bcc Li through first-principles pseudopotential calculations. Using an N-point Pad\'e approximant, we calculate the dielectric response function and produce the fine structure of the dynamic structure factor, in good agreement with available experimental data. The local-field and exchange-correlation effects are found to be significant in the particle-hole excitation region. The local-field effect gives rise to peaks in the energy-loss spectrum, which are associated with a plasmon-Fano resonance and zone-boundary collective states.

We investigate the Hugoniot melting point of Al using a molecular-dynamics approach based on an embedded-atom method. We calculate the Gibbs free energies of the crystalline and liquid phases as a function of pressure and temperature using the coupling-constant integration method and obtain the melting curve up to 1.6 Mbar, in good agreement with experiments. We also examine the melting properties near zero pressure. The Hugoniot equation of states (EOS) up to 1.8 Mbar is obtained from the Rankine-Hugoniot relation and the isotherms determined for different temperatures. Comparing the melting curve with the Hugoniot EOS's for the solid and liquid phases, the Hugoniot melting is found to begin at 1.2 Mbar and to end at 1.4 Mbar, consistent with shock-wave data.

The static dielectric response functions are calculated for both the room-temperature bcc and low temperature $9R$ phases of Li, and used for estimating the Coulomb repulsion parameters $({\ensuremath{\mu}}^{*})$. With a full dielectric matrix approach within the local-density-functional approximation, the values of ${\ensuremath{\mu}}^{*}$ for bcc and $9R$ Li are estimated to be 0.157 and 0.160, respectively, much higher than most estimates for $\mathrm{sp}$ metals. The enhancement of ${\ensuremath{\mu}}^{*}$ is attributed to the weak dielectric screening caused by the exchange-correlation effect and the peculiar conduction-band structure with a large dip in the density of states.

We investigate the atomic geometry and energetics of Cl-related defects in ZnTe through first-principles pseudopotential calculations. We find a type of defect that results from large lattice relaxations, with ${C}_{3v}$ symmetry and triple broken bonds around the Cl impurity at a Zn sublattice site. The triple-broken-bond structure of ${\mathrm{Cl}}_{\mathrm{Zn}}$ behaves as an acceptor and this defect is more stable than the $\mathrm{DX}$-like broken-bond state of ${\mathrm{Cl}}_{\mathrm{Te}}.$ Thus, we suggest that the triple-broken-bond centers are very effective in donor compensation, particularly in heavily Cl-doped ZnTe as well as in samples grown under Te-rich conditions.

We investigate the atomic and electronic structure of Li-adsorbed Si(100) surfaces through first-principles pseudopotential calculations. We find that Li adatoms interact mainly with the dangling-bond orbitals of Si dimers, however, no hydrogenlike directional bonds exist between the Li and substrate Si atoms, based on calculated scanning tunneling microscopy images. The analysis of charge densities demonstrates a large charge transfer from the Li adatom to a dangling-bond orbital of a Si dimer, which is responsible for a large decrease of work function at submonolayer coverages. The Li atoms form linear chains either by occupying the interdimer bridge sites along the dimer row direction or the dimer bridge and cave sites perpendicular to the dimer row. As Li coverage (\ensuremath{\Theta}) increases, repulsive interactions between the adatoms in the chains are also found to increase. Thus, at 1-ML Li coverage, the stable adsorptions sites are changed to either the cave and pedestal sites or the valley-bridge and pedestal sites, with all the Si dimers symmetric, resulting in a $2\ifmmode\times\else\texttimes\fi{}1$ structure. From the calculated formation energies, we find a $1\ifmmode\times\else\texttimes\fi{}1$ phase to be thermodynamically stable at $\ensuremath{\Theta}=2,$ in contrast to other alkali-metal adsorptions.

We study the diamond-to-\ensuremath{\mathrm{B}}-Sn structural phase transition in Si under hydrostatic and uniaxial compressions using an invariant-molecular-dynamics approach based on an empirical potential model. Isobaric molecular-dynamics simulations show that the diamond-cubic lattice under hydrostatic compression becomes unstable against tetragonal shear deformation into the \ensuremath{\mathrm{B}}-Sn phase at 60 GPa, which is much higher than experimental values. This very high pressure is attributed to the fact that a perfect single crystal is superpressured due to the activation barrier, well above the transition pressure where the two structures coexist, in analogy to isobaric superheating in molecular-dynamics simulations. Under uniaxial compression, the enthalpy barrier for the diamond-to-\ensuremath{\mathrm{B}}-Sn transition is found to be effectively reduced.

We investigate the Schottky barrier and effective work function (EWF) at TiN/HfO${}_{2}$ interface through density functional calculations. For different interfaces that consist of either Ti-O or N-Hf interface bonds, the intrinsic metal-induced gap states are nearly independent of the interface structure, with similar decay lengths into the oxide. Due to the weak Fermi-level pinning, the EWF is more sensitive to the extrinsic effect of interface bonding. As N-rich interface bonds are replaced by O-rich bonds, the EWF decreases by up to 0.36 eV, which is attributed to the formation of opposing interface dipoles. To improve the band gap and EWF, we perform both hybrid functional and quasiparticle (QP) calculations. In the $G{W}_{0}$ approximation, in which the Green's function is self-consistently calculated by updating only QP energies and the full frequency-dependent dielectric function is used, the agreement of the EWF with experiment is greatly improved, while QP calculations at the ${G}_{0}{W}_{0}$ level or using the plasmon-pole dielectric function tend to overestimate the EWF. In the self-consistent $GW$ approach, in which both QP energies and wave functions are updated in iterations, the band gap is overestimated, resulting in the lower EWF. On the other hand, the EWF is severely underestimated with the hybrid functional because of the larger shift of the valence band edge level of HfO${}_{2}$.

We report the results of first-principles calculations for the atomic, electronic, and magnetic properties of the Co atoms embedded in divacancy defects in graphene nanoribbons with zigzag-shaped edges. We find that the edges play an important role in the stabilization of a Co-divacancy complex near the edge, where the edge C atoms undergo large relaxations, resulting in a tilted-edge structure. When Co is positioned in the middle of the ribbon, the edge C atoms generally remain on the sheet, forming a flat-edge structure due to the small edge effect. The spin polarization of the edges is determined by two bipartite lattices separated by the Co atom. When a Co-divacancy pair is formed near the same edge, the edge structure is significantly modified. The magnetic interaction between the Co atoms is either ferromagnetic or antiferromagnetic, depending on the relative positions of the Co atoms, and its magnetic ordering can be described by the combined effect of the bipartite lattices formed around individual Co atoms.

We study the hydrogen passivation of carbon acceptors and the stability of defect complexes containing carbon and hydrogen in GaAs through first-principles pseudopotential calculations. A carbon-hydrogen ${\mathrm{C}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$-(${\mathrm{HC}}_{\mathrm{As}}$${)}^{0}$ complex with two ${\mathrm{C}}_{\mathrm{As}}$ atoms at second-neighbor As sites is found to be energetically more favorable than the isolated configuration of ${\mathrm{C}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$ and H-${\mathrm{C}}_{\mathrm{As}}$. We also find that a (H-${\mathrm{C}}_{\mathrm{As}}$${)}_{2}$ complex containing two H atoms is more stable than two isolated H-${\mathrm{C}}_{\mathrm{As}}$ pairs. Hydrogen dissociation from the ${\mathrm{C}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$-(${\mathrm{HC}}_{\mathrm{As}}$${)}^{0}$ center upon annealing leads to the formation of a ${\mathrm{C}}_{\mathrm{As}}$-${\mathrm{C}}_{\mathrm{As}}$ pair, and the ${\mathrm{C}}_{\mathrm{As}}$-${\mathrm{C}}_{\mathrm{As}}$ complex subsequently dissociates into two isolated ${\mathrm{C}}_{\mathrm{As}}$ acceptors. Here we propose a dissociation process that involves a C-C split-interstitial complex at an As site and a second-neighbor As vacancy, with an energy barrier of about 1.7 eV, which is similar to that for Zn diffusion. \textcopyright{} 1996 The American Physical Society.

We perform first-principles theoretical calculations of the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$ for the d-band metal Nb using a dielectric matrix approach. Although localized d electrons give rise to a large local-field effect that increases the dielectric screening and reduces the strength of ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, the enhancement of ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$ due to the exchange-correlation effect is more significant, resulting in a value of 0.14 for ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, in good agreement with what is usually assumed. \textcopyright{} 1996 The American Physical Society.

We calculate the electronic structure of quantum dots coupled along the growth direction with one or two electrons in magnetic fields. We examine the spin transitions of the ground states and the optical transitions between the energy levels, which are associated with far-infrared absorption. Because of the dot-dot and electron-electron interactions, the coupled quantum dots exhibit rich electronic structures. We suggest that the effects of these interactions on the energy spectra are observable by optical measurements because the transition energies exhibit discontinuous changes for a vertically polarized light as the magnetic field increases.

We perform ab initio pseudopotential calculations for studying the stability of carbon-related defects and the atomic model for the hydrogen passivation of substitutional carbons in InAs. Among various C-related defects, the most stable one is found to be a substitutional C acceptor occupying an As site. As compared to GaAs, substitutional C impurities are found to have higher formation energies, due to large lattice distortions surrounding the C atom, thus, C incorporation into bulk InAs is more difficult. Because of the small atomic radius and deep atomic energy levels of C, when C occupies an In site, its defect energy level lies below the valence band maximum (VBM), and it behaves as an acceptor, however, the formation energy is much higher than for ${\mathrm{C}}_{\mathrm{As}}$. We note that an inversion between the VBM and the ${\mathrm{C}}_{\mathrm{In}}$ energy level takes place as pressure increases. For both the substitutional ${\mathrm{C}}_{\mathrm{As}}$ and ${\mathrm{C}}_{\mathrm{In}}$, we find that hydrogen neutralizes the electrical activity of acceptors by occupying a bond-centered site between the C atom and one of its neighbors. In an ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As alloy, the C acceptor is found to favor an As site with In neighbors, with the formation energy lying between InAs and GaAs. Thus, the calculated acceptor concentration of ${10}^{17}$ \char21{} ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ is much lower than the maximum carrier density achievable in GaAs. \textcopyright{} 1996 The American Physical Society.

We study the structural stability and the optical properties of ordered ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P alloys in the CuPt-, CuAu-I-, and Y2-type structures through first-principles pseudopotential calculations. The Y2-type structure is found to be stabilized against phase segregation into binary constituents at $T=0$ when epitaxially grown, with the smaller formation enthalpy than for the CuPt- and CuAu-I-type phases. The CuPt- and Y2-type structures show an asymmetry feature in ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ for two perpendicular polarizations along the [110] and [$1\overline{1}0$] directions. We find that in the CuPt-type structure the zone folding effect of the $L$ point into the center of the Brillouin zone leads to the enhancement of ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$, giving rise to an anomalous peak at 2.35 eV in electroreflectance spectra, while the lower energy peak observed at 2.2 eV is caused by the folded-$X$ point in the Y2-type structure.

We present first-principles pseudopotential calculations for the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$ using a dielectric matrix approach. An application to Al is made and ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$ is estimated to be 0.091. A comparison of the crystal potential, local field, and exchange-correlation effects on the dielectric screening demonstrates that the exchange-correlation effect contributes the largest reduction in ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$ compared to that obtained for a free electron gas.

We present the explicit symmetric formulas for the nonlocal pseudopotential contribution to the stress tensor. The symmetric stress tensor can be derived by applying the scaling procedure of wave vector K, which satisfies the symmetry property of \ensuremath{\partial}${\mathit{K}}_{\ensuremath{\gamma}}$/\ensuremath{\partial}${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\alpha}}\mathrm{\ensuremath{\beta}}}$=\ensuremath{\partial}${\mathit{K}}_{\ensuremath{\gamma}}$/\ensuremath{\partial}${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\beta}}\mathrm{\ensuremath{\alpha}}}$ for the symmetric strain ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\alpha}}\mathrm{\ensuremath{\beta}}}$. We note that this scaling procedure gives rise to the same expression for the stress tensor as that derived using a semilocal or a separable nonlocal pseudopotential.

Graphene magnetism has been proposed but based on thermodynamically unstable zigzag edges and dangling electrons with broken sublattice symmetry. From results of first-principles calculations, we propose a way to realize thermodynamically stable graphene ferromagnetism by seamlessly incorporating transition metals into the graphene honeycomb network. An Fe atom substituting a carbon-carbon dimer of graphene can result in nearly square-planar covalent bonding between the spin-polarized Fe 3$d$ orbitals and graphene dangling bond states. Dangling bond passivation of the divacancy pore with N and O strongly affects the Fe incorporation into the graphene network in terms of energetics and electronic structure. The Fe-N${}_{4}$ or Fe-C${}_{4}$ incorporated graphene is predicted to show long-range ferromagnetism particularly due to carrier mediation when electron doped.

We perform first-principles density functional calculations to investigate the atomic and electronic properties of various O-vacancy (${V}_{\mathrm{O}}$) defects in amorphous indium gallium zinc oxides ($a$-IGZO). The formation energies of ${V}_{\mathrm{O}}$ have a tendency to increase with increasing number of neighboring Ga atoms, whereas they are generally low in the environment surrounded with In atoms. Thus, adding Ga atoms suppresses the formation of O-deficiency defects, which are considered as the origin of device instability in $a$-IGZO-based thin film transistors. The conduction band edge state is characterized by the In $s$ orbital and insensitive to disorder, in good agreement with the experimental finding that increasing the In content enhances the carrier density and mobility. In $a$-IGZO, while most ${V}_{\mathrm{O}}$ defects are deep donors, some of the defects act as shallow donors due to local environments different from those in crystalline oxides. As ionized O vacancies can capture electrons, it is suggested that these defects are responsible for positive shifts of the threshold voltage observed under positive gate bias stress. Under light illumination stress, ${V}_{\mathrm{O}}$ defects can be ionized, becoming ${V}_{\mathrm{O}}^{2+}$ defects due to the negative-$U$ behavior. When electrons are captured by applying a negative bias voltage, ionized ${V}_{\mathrm{O}}^{2+}$ defects return to the original neutral charge state. Through molecular dynamics simulations, we find that the initial neutral state is restored by annealing, in good agreement with experiments, although the annealing temperature depends on the local environment. Our calculations show that ${V}_{\mathrm{O}}$ defects play an important role in the instability of $a$-IGZO-based devices.

The electronic eigenergies and the magnetization of multiple-quantum-well structures under high magnetic fields have been calculated numerically. The magnetization shows a characteristic oscillating behavior as a function of the chemical potential with much enlarged magnitude compared to the single-well case, suggesting that the present system can be a suitable candidate for the experimental verification of current theories on the magnetization of quantum structures

We present the results of ab initio pseudopotential calculations for studying the structural stability and the electronic structure of short-period (GaP) m (AIP) m superlattices with m ranging from 1 to 6, composed of lattice-matched indirect-gap semiconducton. Both the bulk and epitaxial superlattices ordered in the CuAu-I, CuPt, and chalcopyrite structures are found to be unstable against phase segregation into their binary constituents at T=0. The bulk formation enthalpies are found to be similar to those for epitaxial superlattices grown on (001) GaP. The band gap of superlattices tends to decrease as the superlattice period increases

We report a metal-insulator transition in disordered graphene with low coverages of hydrogen atoms. Hydrogen interacting with graphene creates short-range disorder and localizes states near the neutrality point. The energy range of localization grows with increasing of H concentration. Calculations show that the conductances through low-energy propagating channels decay exponentially with sample size and are well fitted by one-parameter scaling function, similar to a disorder-driven metal-insulator transition in 2-dimensional disordered systems., 5 pages, 4 figures

The stability of bulk and epitaxial ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P alloys is examined for three ordered structures with use of self-consistent ab initio pseudopotential calculations. In both bulk and epitaxial forms, a chalcopyritelike structure is the lowest-energy state, more stable than the (001) and (111) monolayer superlattices (MLS). The [001]-oriented MLS is generally found to be more stable than the (111) MLS. Our calculations indicate that the ordered structures in bulk form are unstable against phase segregation into binary constituents at T=0. This instability mainly results from the incomplete release of elastic energy, a general feature in superlattices of lattice-mismatched constituents. For epitaxial growth, the formation enthalpy is significantly reduced. We find that the epitaxial formation enthalpy decreases as the substrate lattice constant increases. The chalcopyrite-structure phase is found to be stabilized while the (111) superlattice is most unstable when grown epitaxially. Thus, the observed [1\ifmmode\bar\else\textasciimacron\fi{}11]- or [11\ifmmode\bar\else\textasciimacron\fi{}1]-ordered ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P should be related to chemical-bonding effects at the growing surface.

Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tight-binding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters ${V}_{n}$ $(n\ensuremath{\le}36)$, where $n$ is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.

The high-pressure behavior of the direct and indirect band gaps in zinc-blende-structure SiC is examined with use of self-consistent ab initio pseudopotential calculations. The fundamental band gap from ${\mathrm{\ensuremath{\Gamma}}}_{15}^{\mathit{v}}$ to ${\mathit{X}}_{1}^{\mathit{c}}$ is found to decrease linearly with pressure up to 200 kbar. This result disagrees with a recent experimental finding of strong sublinear behavior at pressures of 10 to 15 kbar. The linear pressure coefficients of the fundamental band gap and the transverse-optical-phonon frequency at the \ensuremath{\Gamma} point in the Brillouin zone are in good agreement with measured values.

First-principles calculations show that the electronic structure of graphene on ${\text{SiO}}_{2}$ strongly depends on the surface polarity and interface geometry. Surface dangling bonds mediate the coupling to graphene and can induce hole or electron doping via charge transfer even in the absence of extrinsic impurities in substrate. In an interface geometry where graphene is weakly bonded to an O-polar surface, graphene is $p$ doped, whereas $n$ doping takes place on a Si-polar surface with active dangling bonds. We suggest that electron and hole doping domains observed on ${\text{SiO}}_{2}$ are related to different surface polarities.

We perform first-principles matrix Green's function calculations to study the coherent charge tunneling through ultrathin ${\text{SiO}}_{2}$ layers in metal-oxide-semiconductor devices. The tunneling behavior is analyzed within the atomistic picture based on the overlap of Si-induced gap states in the oxide region. We find that, while interface roughness defects such as suboxide bonds and protruded O atoms only weakly affect the tunneling current, a network of oxygen vacancies composed of Si-Si bonds across the oxide layer drastically increases the gate leakage current due to the defect-assisted tunneling. We show that the formation of such percolation paths is energetically favorable in the nonequilibrium situation, and even the oxygen divacancy is enough to result in the dielectric breakdown for ultrathin oxide layers.

Based on perturbation theory and local-density-functional calculations, we study the effect of atomic-scale defects, whose potentials vary on the scale of interatomic distance, on the electronic structure of graphene in the region of low energies. If defects are identical, for example, vacancies at the same sublattice sites or the Stone-Wales defects with the same orientations, the degeneracy at the Dirac point of graphene is removed with an energy splitting proportional to $\ensuremath{\lambda}$ for low disorder densities $(\ensuremath{\lambda})$, which is attributed to the breaking of the intrinsic symmetry of the honeycomb lattice by the presence of atomic-scale defects. However, the degeneracy at the Dirac point is nearly restored if physically equivalent disorders, which are generated by the symmetry operations of graphene, such as reflection and rotation, coexist with similar concentrations.

L'atome d'hydrogene occupant la position centree par la liaison du complexe possede une frequence d'etirement superieure a celle de l'atome d'hydrogene centre par antiliaison. Ce deplacement de frequence resulte des hybridations d'orbitales differentes des 2 atomes Si au centre du complexe

We perform a comparative study for the quantum transport of telescoping carbon nanotubes, where the (5,5) and (10,10) nanotubes are coaxially aligned, using first-principles local-density-functional and tight-binding calculations. In both calculations, the intertube conductance initially increases as the hybridized length in the contact region increases, and then decreases, exhibiting a maximum conductance. However, the calculated conductances from first principles are generally smaller than those from the single $\ensuremath{\pi}$-orbital tight-binding model. In the first-principles calculations, we obtain the maximum intertube conductance that does not exceed ${G}_{0}\phantom{\rule{0.3em}{0ex}}(=2{e}^{2}∕h)$, while individual tubes have two conducting channels, giving the conductance of $2{G}_{0}$. On the other hand, the single $\ensuremath{\pi}$-orbital tight-binding model gives the maximum conductance close to $2{G}_{0}$, similar to previous calculations. Using a double-wall nanotube, we examine the effect of interwall interactions on conductance and find that the ${\ensuremath{\pi}}^{*}$ states of the inner and outer tubes are strongly coupled in the tight-binding model, allowing for an extra conducting channel, while the ${\ensuremath{\pi}}^{*}$ channel is closed in the first-principles calculations.

We perform first-principles density-functional calculations to study the chemical bonding effect of Ge atoms on the diffusion pathway and migration barrier of a B dopant in Si. The binding energy of a B-Ge pair is extremely small, thus, it is ruled out that the pairing of the B and Ge atoms immobilizes the B atom. When a Ge atom is located in the first neighborhood of a substitutional B, the B impurity is still likely to diffuse via an interstitialcy mechanism by forming a pair with a self-interstitial $({I}_{s})$ without pair dissociation, similar to that previously suggested in pure Si. We find that the presence of the Ge atom increases the migration barrier by a sizable amount, which can affect the B diffusivity, while the formation energy of the stable ${I}_{s}\text{\ensuremath{-}}\mathrm{B}\text{\ensuremath{-}}\mathrm{Ge}$ complex is little affected. The effect of the Ge atom is most significant in the first neighborhood of the B interstitial, thus, the Ge chemical bonding effect plays a role in the retardation of B diffusion observed in SiGe alloys.

We perform first-principles theoretical calculations to study the bonding and electronic characteristics of AlO2 nanotube bundles. We find that the nanotubes are tightly linked by O-O peroxy or Al-O-Al bridge bonds, forming crystalline bundles, where tubes are arranged in a triangular or a square lattice in the two-dimensional plane perpendicular to their common axes. Intertube interactions mostly occur between the outer O shells of individual tubes, and the metallicity of bundles is strongly affected by intertube distance and doping. The bundles with only the peroxy bonds are semiconducting, while those having purely the bridge bonds or a mixture of two types of intertube bonds are metallic. A semiconductor-metal transition can occur by applying compressive strains perpendicular to the tube axes or by intercalating the Li ions, accompanied with the change of intertube bonds. We also examine the formation of bundles in the form of Al2O3, where tubes are connected by bridge-type bonds. We find that these bundles are energetically more favorable than the AlO2 nanotube bundles and exhibit the semiconducting behavior.

Based on first-principles theoretical calculations, we propose a tubular structure of aluminates, which exhibit metallic conduction and are energetically stable in the form of ${\mathrm{AlO}}_{2}$, with fewer strain energies compared with ${\mathrm{MoS}}_{2}$ nanotubes with similar diameters. The stability of ${\mathrm{AlO}}_{2}$ nanotubes is also maintained with Li doping inside the tube cavity. For zigzag nanotubes with small diameters, more electron conduction occurs through the outer O shell with longer Al-O bonds, while the whole tube wall contributes to electron conduction for large diameter tubes or armchair tubes, which have similar inner and outer Al-O bond lengths. We suggest that conducting aluminate nanotubes can be promising materials for nanoscale electronic devices.

We study the trend of structural stability and magnetic moment for $\mathrm{Mn}X\phantom{\rule{0.3em}{0ex}}(X=\mathrm{N}$, P, As, and Sb) binary compounds in the NiAs and zinc-blende structures through first-principles spin-density-functional calculations. The exchange splitting and magnetic moment are generally lower in the stable structure, which corresponds to the NiAs structure for MnP, MnAs, and MnSb whereas the zinc-blende structure for MnN. With the exception of MnN, we find the increasing trend of the stability, exchange splitting, and magnetic moment of the ferromanetic state with the anion size along the series MnP, MnAs, and MnSb. Since the N $p$ level is much lower than other anion $p$ levels, the Mn-N bond is more ionic, and thus MnN favors the zinc-blende structure with a lower coordination number. For MnN and MnP with small and light anions, the ground state is an antiferromagnetic state, while a ferromagnetic state is more favorable for MnAs and MnSb, which have larger equilibrium volumes and thereby reduced $p\text{\ensuremath{-}}d$ and $d\text{\ensuremath{-}}d$ couplings for the majority spin channel. Due to the volume and different bonding effects, MnAs and MnSb show a large exchange splitting for the $d$ states and exhibit a nearly half-metallic behavior. The large volume increases the anion $p$-type character near the Fermi level, and thus the ferromagnetic state is stabilized through double exchange interactions.

We study the electronic and magnetic properties of single-wall carbon nanotubes filled with Fe nanowires through local-spin-density-functional calculations. We find that the magnetic moments of Fe-filled carbon nanotubes for the ferromagnetic state are greatly enhanced due to the reduced coordination number of the Fe atoms on the nanowire surface, compared with bulk Fe. The increase of magnetic moments is more effective for thin nanowires, where the Fe atoms interact very weakly with the nanotube and thus their magnetic properties inside the tube are similar to those for the free-standing nanowires. For thick Fe nanowires, undercoordinated Fe atoms interact more strongly with the carbon nanotube, and thereby the magnetic moments are reduced. The analysis of the densities of states near the Fermi level shows that electron conduction mostly occurs along the Fe wires protected from oxidation by carbon coating. Our calculations suggest that for applications to spin transport devices, it is desirable to form thin Fe wires inside single-wall nanotubes with large diameters.

We perform first-principles pseudopotential calculations to investigate the dielectric response for ${\mathrm{MgB}}_{2}$, and calculate the Coulomb repulsion parameter ${\ensuremath{\mu}}^{*}$ using the full dielectric matrix approach. The calculated value for ${\ensuremath{\mu}}^{*}$ is 0.118 when the Debye energy ${\ensuremath{\omega}}_{D}$ of ${\mathrm{MgB}}_{2}$ is used for the phononic cutoff energy ${\ensuremath{\omega}}_{c}$, while ${\ensuremath{\mu}}^{*}=0.151$ for ${\ensuremath{\omega}}_{c}=0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find that the local-field effect is significant due to the covalent nature of chemical bonds, enhancing the dielectric screening and thereby reducing the Coulomb repulsion parameter. In addition, due to the small density of states at the Fermi level, the value of ${\ensuremath{\mu}}^{*}$ is smaller than those obtained from first principles for $\mathrm{Nb}$ and $\mathrm{Li}$. We also examine the anisotropy of the Coulomb repulsion $\ensuremath{\mu}(\mathbf{k},{\mathbf{k}}^{\ensuremath{'}})$ on the Fermi surface, and find that the repulsions between the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands are much smaller than those within the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands.

Based on first-principles calculations, we suggest a method for fabricating $p$-type ZnO with group-I elements such as Li and Na. With group-I dopants alone, substitutional acceptors are mostly self-compensated by interstitial donors. In ZnO codoped with H impurities, the formation of compensating interstitials is severely suppressed, and the acceptor solubility is greatly enhanced by forming H-acceptor complexes. The H atoms can be easily dissociated from these defect complexes at relatively low annealing temperatures, and thus low-resistivity $p$-type ZnO is achievable with dopants different from group-V elements.

We perform first-principles pseudopotential calculations to investigate the atomic structure and energetics of various defects consisting of B and P impurities and the effect of P on B diffusion in Si. In the equilibrium case, we find that a B-P pair is energetically most stable when P is positioned at a second-neighbor site of B. When excess Si interstitials are generated by implantation, B and P impurities tend to form I s -B-P complexes with self-interstitials (I s ), where P still prefers to a second-neighbor site of B, particularly for high donor concentrations. Using the nudged elastic band method, we examine the diffusion of B and its energy barrier in the presence of P. We find that the diffusion pathways of B are similar to those reported for the I s -B pair without P; however, the migration energy for B diffusion from the I s -B-P complex increases by about 0.2 eV. The increase of the activation energy and the formation of the B-P pair acting as a trap for B diffusion provide a clue for understanding the suppression of B diffusion observed in Si predoped heavily with donor impurities.

Based on first-principles spin-density functional calculations, we find that Co-doped ZnO energetically favors a spin-glass-like state due to antiferromagnetic interactions between transition metal atoms, while ferromagnetic ordering is stabilized by electron doping. We find a short range nature in both antiferromagnetic and ferromagnetic interactions, and suggest that a very high doping level of Co ions is required to achieve ferromagnetism, together with a sufficient supply of electron carriers. Our results explain experimental features such as the low reproducibility of ferromagnetic samples and the very low saturation magnetization per Co ion.

Using ab initio spin-density-functional calculations, we investigate the electronic and magnetic structures of a ${\mathrm{C}}_{60}$ fullerene during a structural transition to a nanotube segment by a series of Stone-Wales transformations. We find that partly opened intermediate cage structures may acquire a magnetic moment of several Bohr magnetons. Our results offer a possible explanation for the ferromagnetic behavior observed in polymerized ${\mathrm{C}}_{60}$ following exposure to high temperatures and pressures, and suggest the use of carbon nanostructures for magnetic measurements.

When monatomic contacts are stretched, their conductance behaves in qualitatively different ways depending on their constituent atomic elements. Under a single assumption of resonance formation, we show that various conductance behavior can be understood in a unified way in terms of the response of the resonance to stretching. This analysis clarifies the crucial roles played by the number of valence electrons, charge neutrality, and orbital shapes., 2 figures

We investigate the phase transformation of ${\mathrm{HfO}}_{2}$ under hydrostatic pressure through first-principles pseudopotential calculations within the local-density-functional approximation (LDA) and the generalized gradient approximation (GGA). We find that with increasing of pressure, ${\mathrm{HfO}}_{2}$ undergoes a series of structural transformations from monoclinic to orthorhombic I and then to orthorhombic II, consistent with experiments. The calculated transition pressures within the GGA are in good agreement with the measured values, while they are severely underestimated by the LDA. Analyzing the distribution of electron densities for the high-pressure phases, we find that the electron densities of the orthorhombic-II phase are more homogeneous than for the orthorhombic-I phase. Due to this distinct difference in the homogeneity of electron densities, the energy difference between the orthorhombic-I and orthorhombic-II phases is enhanced in the GGA; thus, the transition pressure between the two phases increases significantly.