The Ge(001) (2 × 1) reconstruction: asymmetric dimers and multilayer relaxation observed by grazing incidence X-ray diffraction

Grazing incidence X-ray diffraction has been used to analyze in detail the atomic structure of the (2 × 1) reconstruction of the Ge(001) surface involving far reaching subsurface relaxations. Two kinds of disorder models, a statistical and a dynamical were taken into account for the data analysis, both indicating substantial disorder along the surface normal. This can only be correlated to asymmetric dimers. Considering a statistical disorder model assuming randomly oriented dimers the analysis of 13 symmetrically independent in-plane fractional order reflections and of four fractional order reciprocal lattice rods up to the maximum attainable momentum transfer qz = 3c* (c* = 1.77 × 10−1 Å−1) indicates the formation of asymmetric dimers characterized by R>D = 2.46(5) Å as compared to the bulk bonding length of R = 2.45 Å. The dimer height of Δ Z = 0.74(15) Å corresponds to a dimer buckling angle of 17(4)°. The data refinement using anisotropic thermal parameters leads to a bonding length of RD = 2.44(4) Å and to a large anisotropy of the root mean-square vibration amplitudes of the dimer atoms (u112) 1/2 = 0.25 Å, (u222)1/2 = 0.14 Å, (u332)1/2 = 0.50 Å). We have evidence for lateral and vertical disp tenth layer below the surface

Electron correlation in the Si(100) surface

Motivated by the controversy between quantum chemists and solid-state physicists, and by recent experimental results, spin-polarized density-functional (DFT) calculations are used to probe electron correlation in the Si(100) reconstructed surface. The ground state displays antiferromagnetic spin polarization for low dimer inclinations indicating, not magnetic order, but the importance of Mott-like correlations among dangling bonds. The lowest energy corresponds to a higher dimer inclination with no spin. DFT energies, however, should be taken with caution here. Our results together with quantum-chemical findings suggest dimers with highly correlated electrons that tend to buckle due to interactions with other dimers.Comment: 5 pages, 1 eps figure, 1 table; RevTeX v3.1. To appear in Surface Science (proceedings of the European Conference On Surface Science, ECOSS-19, Madrid, Sept. 5-8, 2000

First-Principles Studies of Hydrogenated Si(111)--7×\times7

The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7$\times$7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental STM and differential reflectance characteristics of the hydrogenated surfaces agree well with the calculated features.Comment: REVTEX manuscript with 3 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.htm

Writing ability in adults with intellectual or communicative disabilities at daily activity centres

Syftet var att undersöka skrivförmåga och vissa underliggande funktioner till skrivförmåga hos vuxna med intellektuella eller kommunikativa funktionsnedsättningar inom daglig verksamhet genom jämförelser med barn och vuxna utan funktionsnedsättning. 66 deltagare ingick i gruppen vuxna med intellektuella eller kommunikativa funktionsnedsättningar, 39 i barngruppen och 14 i den vuxna referensgruppen. Följande tester/uppgifter utfördes: Test for Reception of Grammar-2, grafomotorisk snabbhet, nonordsrepetition, Rapid Automatized Naming samt Skrivning till bilder. Bildeliciterade skriftliga narrativor undersöktes med avseende på längd, stavning, syntaktisk komplexitet, interpunktion, innehåll och helhetsintryck. Resultaten visade att vuxna inom daglig verksamhet presterade på en lägre nivå än den vuxna referensgruppen. Däremot skiljde sig gruppens resultat oftast inte från barnens, förutom att de gjorde en lägre andel fonologiskt acceptabla stavfel och fick lägre resultat på Nonordsrepetition och Rapid Automatized Naming, vilket skulle kunna tyda på begränsning i fonologisk bearbetningsförmåga. Fonologisk intervention och hjälpmedel som stöttar skrivförmåga föreslås.The aim was to examine writing ability and some of its underlying functions in adults with intellectual or communicative disabilities at daily activity centres by comparisons with children and adults without disabilities. 66 adults with intellectual or communicative disabilities, 39 children and 14 adults without disabilities were included. The following tests/tasks were executed: Test for Reception of Grammar-2, graphomotor task, nonword repetition, Rapid Automatized Naming and Writing elicited by pictures. The picture elicited narratives were examined regarding length, spelling, syntactic complexity, interpunctuation, content and general impression. The results demonstrated that adults at daily activity centres performed lower scores than adults without disabilities. The group generally didn’t differ from the children, but they made a smaller proportion of phonologically acceptable misspellings and achieved lower scores on nonword repetition and Rapid Automatized Naming, which could indicate limitations in phonological processing ability. Phonological intervention and tools to support writing are suggested

Photoelectron spectroscopy study of Ag/Si(111)root 3X root 3 and the effect of additional Ag adatoms

High-resolution core-level spectroscopy has been applied to the Ag/Si(111)root3xroot3 surface. The Si 2p line shape is found to depend critically on the presence of additional Ag adatoms on the surface. A significant broadening caused by the surplus of Ag atoms could be eliminated by careful annealing. The resulting Si 2p spectra are significantly sharper than any published data for this or other Si based surface systems. Two major surface components are identified for the root3xroot3 surface, which find a natural explanation in terms of the honeycomb-chained-trimer model. A small but characteristic contribution to the Si 2p spectrum of the Ag/root3xroot3 surface is tentatively assigned to defects