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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.

The C amount and the structure of the $\mathrm{S}\mathrm{i}(001)\mathrm{\text{\ensuremath{-}}}c(4\ifmmode\times\else\texttimes\fi{}4)$ surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase is found to contain $1/8$ monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase constitutes the subsurface ${\mathrm{S}\mathrm{i}}_{0.875}{\mathrm{C}}_{0.125}$ $\ensuremath{\delta}$ layer with two-dimensionally ordered C atoms.

The C amount and the structure of the Si(001)-c(4 x 4) surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The c(4 x 4) phase is found to contain 1/8 monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The c(4 x 4) phase constitutes the subsurface Si(0.875)C(0.125) delta layer with two-dimensionally ordered C atoms.

We present a dynamic pathway model for the formation of C(60) using the action-derived molecular dynamics simulations. We propose candidate precursors for dynamic pathway models in which carbons spontaneously aggregate due to favorable energetics and kinetics. Various planar polycyclic models are in a disadvantageous state where they cannot be trapped in the forward reaction due to their high excess internal energies. Our simulation results show that precursors either in the shape of tangled polycyclics or in the shape of open cages are kinetically favored over precursors in the shape of planar hexagonal graphite fragments. Calculated activation energies for the probable precursor models are in good agreement with experiment. Existence of chains in the models of tangled polycyclics and open cages is beneficially for the formation of C(60) molecule. Chains attached to the precursor model are energetically favorable and display lithe movements along the dynamic pathway.

By carrying out ab initio total-energy calculations on incorporation of ${\mathrm{O}}_{2}$ into $\mathrm{Si}(001)$, we have reinvestigated possible initial oxidation structures and processes at the $\mathrm{Si}(001)$ surfaces. We found a new oxidation structure that is more stable than previously proposed backbond-oxidation models. The new structure is achieved by the oxidation process of ejecting $\mathrm{Si}$ atoms onto surfaces as observed in experiments. The atomic structure after the $\mathrm{Si}$ ejection is characterized by the ${\mathrm{O}}_{2}$-incorporated single dimer-vacancy $(\mathrm{O}2∕1\text{\ensuremath{-}}\mathrm{DV})$ structure. In particular, our finding indicates that the $\mathrm{Si}$ ejection also occurs at surfaces during initial stages of oxidation, in contrast to hitherto proposed $\mathrm{Si}$ ejection at ${\mathrm{SiO}}_{2}∕\mathrm{Si}$ interfaces.

We study the atomic structure of the $\mathrm{Si}(111)3\ifmmode\times\else\texttimes\fi{}2\ensuremath{-}\mathrm{Ba}$ surface using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and ab initio calculations. A $3\ifmmode\times\else\texttimes\fi{}2$ periodicity is observed not only in STM but also in LEED. From dual-bias STM imaging and total-energy calculations, the recently proposed honeycomb chain-channel (HCC) structure [G. Lee et al., Phys. Rev. Lett. 87, 056104 (2001)] is confirmed to be the most favorable among various candidate structures. Calculations varying Ba-Ba distance in the channel show that the Ba atoms interact repulsively. Diffusion of Ba atoms along the channel, occurring in a concerted manner, is found to be unlikely at room temperature. This is consistent with the STM observation that the Ba atoms occupy specific sites. However, some static disorder in site occupation is observed and shown to be energetically probable. We suggest that the Si(111) surface has a tendency towards the $3\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{HCC}$ reconstruction when one electron per $3\ifmmode\times\else\texttimes\fi{}1$ unit are supplied and fully saturate the Si dangling bonds.

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.

The initial formation of the samarium/silicon interface is studied by combining scanning tunneling microscopy (STM), low-energy electron diffraction, and ab initio pseudopotential calculations. A $\mathrm{Si}(111)3\ifmmode\times\else\texttimes\fi{}2$-Sm reconstruction is formed at a Sm coverage of 1/6 monolayer. High-resolution STM images reveal a strong bias-voltage dependence for the $3\ifmmode\times\else\texttimes\fi{}2$ reconstruction. In the empty-state STM images, the rows with a $\ifmmode\times\else\texttimes\fi{}2$ periodicity are shown at high bias voltages, and attributed to Sm atoms. At low bias voltage, an additional double row feature with a $\ifmmode\times\else\texttimes\fi{}1$ periodicity appears and dominates the empty-state image as the bias voltage decreases. The paired protrusions with the $\ifmmode\times\else\texttimes\fi{}1$ periodicity in the double rows in the empty-state image are attributed to Si atoms. In the filled-state STM images, double rows of protrusions forming zigzag chains with a $3\ifmmode\times\else\texttimes\fi{}1$ structure are observed and assigned to Si atoms. We propose a honeycomb chain-channel model for the $3\ifmmode\times\else\texttimes\fi{}2$ phase, common to alkali metals and alkaline-earth metals on Si(111). Simulated STM images based on this model are in excellent agreement with experiment.

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.

We report on the absolute measurement of In coverage on the Si(111)4\ifmmode\times\else\texttimes\fi{}1-In surface using medium-energy ion scattering, and present bias-dependent scanning tunneling microscopy (STM) images. The In coverage was determined to be 1 monolayer (ML). This result is in contrast to previous results of 3/4-ML In determined by semidirect measurements using Auger electron spectroscopy and STM. Our result supports a 4\ifmmode\times\else\texttimes\fi{}1-In structural model consisting of four surface In rows, which was constructed from the recent surface x-ray diffraction study and tested by subsequent theoretical calculations. The bias-dependent STM images are well reproduced by ab initio simulations based on this 1-ML-In model.

We investigate the initial stage of the C incorporation into Si(001) using thermal dissociation of ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{2}}$. The scanning tunneling microscopy shows that C-induced dimer vacancies (DVs) with depressed adjacent dimers are generated on the surface and aligned in the dimer direction, forming the $2\ifmmode\times\else\texttimes\fi{}n$ structure. The ab initio pseudopotential calculations reveal that, with the presence of a DV in the surface, the $\ensuremath{\alpha}$ site in the fourth subsurface layer directly below the DV is the most favorable for the incorporated C atoms. The embedded C atoms align one dimensionally due to the interaction which is attractive in neighboring dimer rows but repulsive in the same dimer row.

The recently developed spin-dependent pseudopotentials markedly improve the description of the energetics of isolated transition metal atoms [S. C. Watson and E. A. Carter, Phys. Rev. B 58, R13 309 (1998)]. Spin-dependent pseudopotentials are obtained from a combination of spin-neutral and fully polarized atomic pseudopotentials, employing the self-consistent local spin polarization to adapt to different environments. Their use is extended to bulk materials in the current work, where we have implemented the spin-dependent pseudopotential formalism within a real-space density functional theory code. Calculations on bulk Fe, Co, and Ni yield lattice constants and bulk moduli within the accuracy expected of a method employing the local density approximation of density functional theory, except for an overestimated Fe bulk modulus. However, the magnetic moment is in dramatically better agreement with experiment and published all-electron calculations when the spin-dependent pseudopotentials are employed.

Initial adsorption configurations of acetylene molecules on the Si(001) surface are investigated by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. Three distinctive features are observed in the STM images at low coverage. The comparison of the atomic resolution STM images with the simulation reveals that there are basically two kinds of di-$\ensuremath{\sigma}$ bonding configurations: (i) on-top over a single dimer and (ii) end-bridge between two successive dimers in the same dimer row. The third feature is identified to be a pair of end-bridge di-$\ensuremath{\sigma}$ configurations occupying the same two dimers. The tetra-$\ensuremath{\sigma}$ bonding configurations are not found. The existence of the two kinds of di-$\ensuremath{\sigma}$ bonding configurations can resolve the apparently conflicting results of the existing studies.

The second nearest-neighbor modified embedded atom method (MEAM) [Phys. Rev. B 62, 8564 (2000)], developed in order to solve problems of the original first nearest-neighbor MEAM on bcc metals, has now been applied to all bcc transition metals, Fe, Cr, Mo, W, V, Nb, and Ta. The potential parameters could be determined empirically by fitting to $(\ensuremath{\partial}B/\ensuremath{\partial}P),$ elastic constants, structural energy differences among bcc, fcc and hcp structures, vacancy-formation energy, and surface energy. Various physical properties of individual elements, including elastic constants, structural properties, point-defect properties, surface properties, and thermal properties were calculated and compared with experiments or high level calculations so that the reliability of the present empirical atomic-potential formalism can be evaluated. It is shown that the present potentials reasonably reproduce nonfitted properties of the bcc transition metals, as well as the fitted properties. The effect of the size of radial cutoff distance on the calculation and the compatibility with the original first nearest-neighbor MEAM that has been successful for fcc, hcp, and other structures are also discussed.

The $\mathrm{Ba}/\mathrm{Si}(111)$ surface, previously known as a $3\ifmmode\times\else\texttimes\fi{}1$ phase, is found to have a $3\ifmmode\times\else\texttimes\fi{}2$ periodicity and a semiconducting band gap. The substrate reconstructs into the honeycomb chain-channel (HCC) structure with Ba atoms in the channel, as in the alkali-metal-induced $\mathrm{Si}(111)\ensuremath{-}(3\ifmmode\times\else\texttimes\fi{}1)$. However, the metal coverage is determined to be 1/6 monolayers, half the alkali-metal coverage. We propose that the structure and the metal coverage determined for the Ba adsorbate is universal for other alkaline-earth-metal adsorbates. With the alkali-metal-induced $3\ifmmode\times\else\texttimes\fi{}1$ case, our results lead to a rule that one donated electron per $3\ifmmode\times\else\texttimes\fi{}1$ surface unit is necessary to stabilize the HCC reconstruction of Si.

We discuss a recent investigation of adatom behavior on the AlSb(001) surface using first-principles electronic structure methods based on the density functional theory. For Al and Sb adatoms, we find a number of novel adatom structures that differ dramatically from previous results for the superficially similar group-III arsenides. In particular, we conclude that it is energetically favorable for an Al adatom to incorporate substitutionally into the outermost layer of the AlSb surface. This observation helps motivate a proposed new reconstruction for the AlSb(001) surface. Finally, we argue that the unusual adatom behavior identified for this surface probably results from the presence of a dimer row composed of a double layer of group-V atoms in the reconstruction, and therefore, it should be generic to all of the antimonides, as well as, the c(4 × 4) reconstruction of the arsenides and phosphides.

Scanning tunneling microscopy (STM) is one of the most successful experimental tools for probing the structure of semiconductor surfaces. However, care must be taken in interpreting the images at the atomistic limit. Often a “naive” interpretation of the STM image can yield an incorrect surface structure. We illustrate this situation via ab initio pseudopotential calculations for the STM image of the (110) GaAs surface. We will compare theoretical STM images to experimental images for the relaxed surface and for a surface with an As vacancy.

Based on density-functional theory calculations, the effects of cation compositions on electronic structures of In-Sn-Zn-O amorphous semiconductors were investigated. We considered various composition ratios of In, Sn, and Zn in O stoichiometric condition, and found that the conduction band minimum (CBM) energy level decreases and the valence band tail (VBT) energy level extent increases as the sum of In and Sn ratios (RIn+RSn) increases. The CBM lowering is attributed to the increased overlap of the In-5s and Sn-5s orbitals as the RIn+RSn increases, and correspondingly the electron effective masses (me*) are found to be reduced. The VBT increase is found to be due to the increased density of the In and Sn atoms, near which the O-2p inter-site ppσ* coupling is larger than that near the Zn atoms. The acute O-(In,Sn)-O angles are suggested to be structurally important, giving the stronger O-O ppσ* coupling.

We investigate the maximum available free electron carrier density in Al-doped n-type ZnO, based on density-functional theory calculations. The Coulomb interactions between the Al dopants and the Zn-vacancy native acceptors are found to limit the carrier density. In typical growth conditions, the n-type doping limit is found to be in the range of 1019 ∼ 1021 cm−3.

The atomic and electronic structure of an As vacancy on the GaAs(110) surface is examined using ab initio pseudopotentials. The relaxed atomic structure reveals an inward movement of the neighboring surface Ga atoms which is in disagreement with recent interpretations of the scanning tunneling microscopy (STM) images for this system. However, a careful analysis of the wave-function character of the vacancy states, and the theoretical STM image, for this geometry yields excellent agreement with the experimental STM images.

We studied the vacancy-induced ferromagnetism in bulk and at the (1010) surface of wurtzite ZnO using density functional theory calculations. It is found out that surface vacancies are energetically favored to bulk vacancies and that the ferromagnetism is induced by Zn vacancies but not by O vacancies. Moreover, local magnetic moments induced at O atoms adjacent to the Zn vacancy are calculated to be larger on the surface than in bulk. These findings indicate that Zn vacancies at surfaces are responsible for the experimentally observed room-temperature ferromagnetism in undoped ZnO thin films and nanoparticles.

The structural properties and magnetic phase stability of Fe(x)Pt(1−x) alloys in L12 crystal structure in bulk as well as thin film on Pt (110) substrate are studied by means of the highly precise full-potential linearized augmented plane-wave method within generalized gradient approximation. The antiferromagnetic (AFM) phase is found to be preferred over the ferromagnetic (FM) phase for FePt3 bulk alloy in agreement with experiment while FexPt(1−x), where x=0.25 and 0.5 with a film thickness smaller than 0.5nm, favor the FM phase. The total energy calculation assuming pseudomorphic strain reveals that the AFM preference for bulk is maintained in wide range of tetragonal distortion up to the value of 22% reached by the surface relaxation of thin film, implying that the magnetic configuration change from AFM to FM at the thin film surface is originated in rather complex surface effects beyond the structural relaxation. The FM preference of thin film is predicted to fast suppress as the film thickness incre...

Microscopic mechanism of autocatalytic structural transformations of fullerenes is investigated by the action-derived molecular dynamics. Dynamic pathways and the corresponding activation energies are obtained for the Stone-Wales transformation in fullerene and the fullerene coalescence, under the presence of extra carbon atoms. The adatom-assisted Stone-Wales transformation is proved to be a highly probable process unit for the structural transformations and annealing treatments of carbon-based graphitic networks. The complex processes of adatom-assisted fullerene coalescence, yielding very low activation energies, are presented.

Action-derived molecular dynamics for the simulation of rare event and slow mode systems is reviewed. Theoretical background, implementation details, and comparison with other methods are presented. Numerical examples demonstrated include the structural formation and transformation of carbon fullerenes, and molecular reconfiguration of alanine dipeptide and valine dipeptide. It is shown that the action-derived molecular dynamics is efficient to explore the dynamic pathways of various chemical reactions.

We measured the flux flow noise in a type-II superconductor by a direct flux detection method. The fluctuating component of flux changes in the central part of niobium thin film sample is monitored by a Nb rf-SQUID as a function of the transport current and magnetic field. The flux noise power spectrum showed either l /f or a Lorentzian behavior depending on the size of transport current. The overall magnetude of the flux noise showed a peak near the critical current, whereas the voltage increases monotonically. Based on a modified shot-noise model, we simulated both Lorentzian and l /f noise spectra with appropriate assumptions and found that the characteristic frequency of noise spectrum is related to the vortex transit time.

In order to improve the material properties of the protective layer for alternating current plasma display panels, a small amount of ZnO was added to the MgO protective layer. The electrical properties and the surface characteristics of the Mg1−xZnxO films, deposited by a radio frequency magnetron sputtering method, were investigated. As the concentration of ZnO increases, the crystallinity of Mg1−xZnxO thin films improves and the grain size becomes larger. The firing and the sustaining voltages of panels with the Mg1−xZnxO protective layers, when the concentration of ZnO was 0.5at.%, was reduced by 20V, compared with the conventional panels with the MgO protective layers. It was also found that the panels with Mg1−xZnxO protective layers show the higher discharge intensity as the ZnO content increases at the same applied voltages, compared with panels with the conventional MgO protective layers.

We considered the diffusion behavior which occurred in the initial stages of thin film deposition for the fabrication of a tunneling magneto-resistance (TMR) junction with the use of the density functional theory (DFT). An Al adatom deposited on the Co (111) substrate had the most stable configuration when it was adsorbed on the hcp site and the energy barrier for the diffusion to the neighboring fcc or hcp site was sufficiently low for the Al adatom to move freely. The energy barrier for the Co on the Co substrate was approximately 1.6 times higher than that for the case of Al on a Co substrate. The energies for the step down (step energy barrier) of an adatom from the top of the adatom trimer were estimated as 0.54 eV and 1.09 eV for Al/Al trimer/Co and Co/Co trimer/Co, respectively.

A simple procedure for the fabrication of sub-10 nm scale Si nanopillars in a 2-D array using reactive ion etching with 8 nm Co nanoparticles as etch masks is demonstrated. The obtained Si nanopillars are single crystalline tapered pillar structures of 5 nm (top) x 8 nm (bottom) with a density of approximately 4 x 10(10) pillars cm(-2) on the substrate, similar to the density of Co nanoparticles distributed before the ion etching process. The uniform spatial distribution of the Si nanopillars can also be patterned into desired positions. Our fabrication method is straightforward and requires mild process conditions, which can be extended to patterned 2-D arrays of various Si nanostructures.

We present ab initio pseudopotential calculations, within the local density approximation, for stoichiometric high-Tc copper oxide YBA2Cu3O7 and newly synthesized intermentallic compounds Lu(NiB)2C.

We studied the effect of Ni incorporation on the magnetic properties of ${\mathrm{CeSbNi}}_{x}$ $(0l~xl~0.35)$ using resistivity, susceptibility, and magnetization measurements under high pressure and magnetic field. With increasing pressure antiferromagnetic transition temperature ${(T}_{N})$ of CeSb increases continuously leading to a high pressure phase. Similarly, ${T}_{N}$ of ${\mathrm{CeSbNi}}_{x}$ increases with pressure for $xl~0.15$ while increasing pressure weakens a Cr-like anomaly found in the resistivity of ${\mathrm{CeSbNi}}_{0.08}.$ Interestingly, resistivity of ${\mathrm{CeSbNi}}_{0.08}$ under higher pressure becomes very similar to that of CeSb at ambient pressure. Magnetic hysteresis, metamagnetic transition, and saturation behavior, which are absent in the ambient-pressure magnetization of ${\mathrm{CeSbNi}}_{0.05}$ and ${\mathrm{CeSbNi}}_{0.08},$ begin to appear with pressure too. On the other hand, pressure effects in both resistivity and magnetization of ${\mathrm{CeSbNi}}_{0.15}$ and ${\mathrm{CeSbNi}}_{0.35}$ are different from those of ${\mathrm{CeSbNi}}_{0.05}$ and ${\mathrm{CeSbNi}}_{0.08}.$ We discuss these findings with regard to possible Ni-induced changes in the nature of the magnetic interactions of CeSb.

ZnSe/GaAs heteroepitaxial structures were prepared by metalorganic chemical vapor deposition with the use of thermal cleaning treatments at varying hydrogen flow rates in preparing the substrate. The photoreflectance spectra for the samples prepared revealed Franz–Keldysh oscillation (FKO) signals with the superimposition of free exciton transition features. The FKO signals changed depending on the hydrogen flow rate while the exciton transition features remained unchanged. Fitting through Aspnes’ and Franz–Keldysh’s model showed that the built-in electric field at the interface increased as the hydrogen flow rate decreased. These results imply that the hydrogen flow rate in the course of thermal etching plays a crucial role in the change of the energy-band profile at the interface.

We show that the Berry's phase arises when the phenomenological Hamiltonian for the fast variable in the system consisting of fast and slow variables violates the time-reversal symmetry. This type of Berry's phase is distinguished from the usual monopole-type Berry's phase which is created by the degeneracy of states acting as a topological singularity in the parameter space. Effects of this phase on the transverse phonon modes of solids are studied and the results are applied to the anyon superconductors. Using the semion model for the highz . sbnd ; T c superconductors, we predict that, under specified conditions, degenerate TO phonons are split in energy by ≲0.001 me V. Rotation of the polarization of the TA and TO phonons is also predicted.