Irreversible structural transformation of Si(1 1 4)-2 × 1 induced by subsurface carbon

Using scanning tunneling microscopy (STM), it has been found that the reconstruction of Si(1 1 4) is transformed irreversibly from a 2 × 1 structure composed of dimer (D), rebonded atom (R), and tetramer (T) rows (phase A) to a different kind of 2 × 1 structure composed of D, T, and T rows (phase B) by C incorporation. It has been confirmed by high-resolution synchrotron core-level photoemission spectroscopy (PES) that such an irreversible structural transformation is due to stable subsurface C atoms. They induce anisotropic compressive stress on the surface, which results in insertion of Si dimers to an R row to form a T row.     

Origin of enhanced Ge interdiffusion at the initial stage of Ge deposition on Si(5 5 12)-2 × 1: Tensile stress induced by substrate chain structures

By combined investigation of STM and synchrotron PES on Ge/Si(5 5 12)-2 × 1 at 530 °C, it has been found that, in addition to the upward-relaxed surface Si atoms, a subsurface Si atom is also readily replaced by an arriving Ge atom at the initial adsorption stage. Such enhanced interdiffusion is due to a unique character of one-dimensional chain structures of the reconstructed substrate, such as π-bonded and honeycomb chains not existing on other low-index Si surfaces such as Si(001)-c(4 × 2) and Si(111)-7 × 7, applying a tensile surface stress to the neighbouring subsurface atoms. Interdiffusion of Ge having lower surface energy induces adsorption of the displaced Si atoms on the surface to form sawtooth-like facets composed of (113)/(335) and (113)/(112) with arriving Ge atoms until the surface is filled with those facets. Such displacive adsorption is the origin of high Si concentration of formed facets.     

Self-limited growth of the CaF nanowire on the Si(5 5 12)-2 × 1 template

The atomic structure and interfacial bonding of the ordered-and-isolated CaF nanowires on Si(5 5 12)-2 × 1 have been disclosed by scanning tunneling microscopy and synchrotron photoemission spectroscopy. Initially, CaF molecules dissociated from thermally deposited CaF₂ molecules are adsorbed preferentially on the chain structures of Si(5 5 12)-2 × 1 held at 500 °C. With increasing CaF₂ deposition amount, one-dimensional (1D) CaF nanowires composed of (113) and (111) facets are formed. The line density of these CaF nanowires increases as a function of deposition amount. Finally, at a submonolayer coverage, the surface is saturated with these 1D nanowires except for the (225) subunit, while the original period of Si(5 5 12)-2 × 1, 5.35 nm, is preserved. It has been deduced by the present studies that, owing to these preferential adsorption of CaF and facet-dependent growth of a CaF layer within a unit periodic length of Si(5 5 12)-2 × 1, such a self-limited growth of the CaF nanowire with a high aspect ratio becomes possible.     

Reducibility of steady-state bifurcations in coupled cell systems

A general theory for coupled cell systems was formulated recently by I. Stewart, M. Golubitsky and their collaborators. In their theory, a coupled cell system is a network of interacting dynamical systems whose coupling architecture is expressed by a directed graph called a coupled cell network. An equivalence relation on cells in a regular network (a coupled cell network with identical nodes and identical edges) determines a new network called quotient network by identifying cells in the same equivalence class and determines a quotient system as well. In this paper we develop an idea of reducibility of bifurcations in coupled cell systems associated with regular networks. A bifurcation of equilibria from subspace where states of all cells are equal is called a synchrony-breaking bifurcation. We say that a synchrony-breaking steady-state bifurcation is reducible in a coupled cell system if any bifurcation branch for the system is lifted from those for some quotient system. First, we give the complete classification of codimension-one synchrony-breaking steady-state bifurcations in 1-input regular networks (where each cell receives only one edge). Second, we show that under a mild condition on the multiplicity of critical eigenvalues, codimension-one synchrony-breaking steady-state bifurcations in generic coupled cell systems associated with an n  -cell coupled cell network with _Dn$D_n$ symmetry, a regular network, is reducible for n>2$n>2$.     

Stress-driven structural transformation of Sb-passivated Si(114)

Combined investigation of STM, high-resolution synchrotron photoemission, and density functional theory calculations allowed us to understand the Sb-induced structural-transformation of Si(114)-2 × 1. When 2 ML of Sb is deposited on Si(114)-2 × 1 at room temperature and postannealed at 500 °C, all of the surface Si atoms with dangling bonds are replaced by Sb atoms. Among one-dimensional (1D) structures consisting of Si(114)-2 × 1, such as a dimer with a 6-membered ring (D₆) row, a rebonded-atom (R) row, and a tetramer (T) row [D₆-R-T], the T row is split into a dimer row with a 7-membered ring (D₇) and an R row [D₆-R-D₇-R]. Since the R-D₇-R unit, a building block of Sb/Si(113)2 × 2, is under stress-balance, the Sb/Si(114)-2 × 1 surface is stressed compressively due to the extra D₆ unit. As a result, with additional postannealing at 600 °C, two periods of this 2 × 1 [(D₆-R-D₇-R)-(D₆-R-D₇-R)] are gradually converted to 2 × 2 [(D₆-R-D₆-R)-(R-D₇-R)], where the D₆-R (115) unit is stress-balanced. The corresponding photoemission data obtained from both of the phases show that all of the surface components of the clean surface have disappeared, instead the single Sb–Si interfacial component has appeared, which indicates that the charge transfers from interfacial Si to surface Sb atoms. Finally, the density functional theory calculations have also confirmed that there are two distinct phases determined by the chemical potential of passivating Sb atoms.