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The In/Si(111)4$\ifmmode\times\else\texttimes\fi{}1$ surface with In wires is a prototypical one-dimensional electron system showing a temperature-induced structural phase transition. While most impurities are known to decrease the transition temperature $({T}_{c})$, oxygen was reported to increase the ${T}_{c}$. In order to understand the exceptional influence of oxygen, we have examined the initial stages of oxygen adsorption on the surface by scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Oxygen molecules were found to dissociate and adsorb as atoms on the surface. STM revealed three distinct O adsorption features as well as occasional transformations from one to another. The high-resolution images showed that all three adsorbed O features were located at the center of the In trimer at both edges of the In wire. This registry is in contradiction with the previous theoretical predictions [S. Wippermann et al., Phys. Rev. Lett. 100, 106802 (2008)]. The present DFT calculations identified two of the features as O atoms incorporated in the interstitial region between the In wire and subsurface Si layer.

A phenomenological model including interlayer pairing interactions is proposed for high-${\mathit{T}}_{\mathit{c}}$ cuprate superconductors. Modified Eliashberg equations corresponding to the model are derived and numerically solved. The results show that the trends of ${\mathit{T}}_{\mathit{c}}$'s for known high-${\mathit{T}}_{\mathit{c}}$ materials can be described reasonably well by mechanisms including (in addition to the phonon interactions) nonphonon, intralayer- and interlayer-pairing interactions. The calculated gap values (3.6\ensuremath{\lesssim}2${\mathrm{\ensuremath{\Delta}}}_{0}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$\ensuremath{\lesssim}4.2) do not deviate significantly from the canonical BCS value of 3.5. This model can be applicable to layer-structured superconductors in general, whether they are high-${\mathit{T}}_{\mathit{c}}$ or low-${\mathit{T}}_{\mathit{c}}$ superconductors.

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 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.

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.

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.