Spontaneous SU(2) symmetry violation in the SU(2) L × SU(2) R × SU(4) electroweak model

A new approach to electroweak (EW) composite scalars is developed starting from the fundamental gauge interaction on high scale. The latter is assumed to have the group structure SU(2) _L × SU(2) _R × SU(4), where SU(4) is the Pati-Salam color-lepton group. The topological EW vacuum filled by instantons is explicitly constructed and the resulting equations for fermion masses exhibit spontaneous SU(2) flavor symmetry violation with possibility of very large mass ratios.     

Optical second harmonic generation studies of the nature of the oxide-covered and clean c(4 × 4) and (2 × 4) reconstructed GaAs(001) surfaces

We report the application of the “epioptic” technique of optical second harmonic generation (SHG) to the study of surface chemical processes occurring at the technologically important (001) surfaces of the non-centrosymmetric compound semiconductor GaAs. SHG has been used to monitor the thermal removal of surface contamination from an GaAs(001) substrate mounted in a conventional molecular beam epitaxy (MBE) chamber. Rotational anisotropy plots of the reflected SH signal recorded at near-normal incidence indicate that the oxide layer is not fully disordered and possesses some structural anisotropy, i.e. some preferred net surface orientation. In addition the rotational anisotropy plots were observed to change in both intensity and apparent phase between the oxide-covered surface and the clean reconstructed surfaces. Following heating to 850 K under the background As₂ flux present in the chamber, the SH signal from the clean, reconstructed surface was approximately 5 × weaker than that observed for the contaminated surface and was subsequently found to be insensitive to the reconstruction change between the (2 × 4) and c(4 × 4) surfaces, in agreement with theoretical predictions for surfaces having 2mm and 4mm symmetry, respectively.These data are discussed in terms of the difficulties encountered when attempting to observe a surface SH response from a non-centrosymmetric substrate. The sensitivity of the technique towards the thin oxide overlayer suggests that for overlayers which are likely to possess net oriented in-plane dipoles due to the structural mismatch with the substrate, an SH response may be observed in preference to a bulk response when a normal incidence geometry is employed.     

Order-disorder transition on Si(001): c(4 × 2) to (2 × 1)

A phase transition between c(4 × 2) and (2 × 1) structures on the clean Si(001) surface has been observed at about 200 K by low energy electron diffraction. From the temperature dependence of the width and intensity of the diffraction spots this transition is concluded to be a second order order-disorder transition. The transition proceeds by a single stage, which is in strong contrast with the case of Ge(001) for which a two-stage transition has been reported.     

NH3-induced c(4 × 2) to (2 × 2) transition on the Ge(001) surface by a domino-like dimer flip

Under the conditions of thermodynamic adsorption-desorption equilibrium, the first strongly bound molecular adsorption state of ammonia on Ge(001) saturates at one molecule per Ge reconstruction dimer (1/2 ML). High-resolution electron diffraction studies show that this adsorption is accompanied by a structural transition from c(4 × 2) on the clean surface to a (2 × 2) structure which is already completed for a coverage of about 0.04 ML, far below saturation. We propose a model implying the formation of NH₃ islands locally covered with 1/2 ML and a (2 × 2) periodicity caused by a flip of the dimer tilt direction of every second dimer Beyond the edge of these islands, the dimer flip continues domino-like along the dimer rows over the clean parts of the surface. Elongated (2 × 2) domains about 280–330Å long and 30–65Å wide are formed, depending on coverage.     

A leed crystallographic analysis for the Cu(100)-(2 × 2)-S surface structure

An intensity analysis with low-energy electron diffraction is reported for the (2 × 2) surface structure obtained by the adsorption and presumed dissociation of H₂S on the (100) surface of copper. Intensity-versus-energy curves were measured with a video LEED analyser for five diffracted beams at normal incidence and for eleven beams at an off-normal direction with a polar angle of incidence equal to 14°. Comparisons were made with intensity curves calculated with the renormalised forward scattering method for three types of structural models in which the metal atoms remain in their regular bulk positions. The best correspondence between experimental and calculated intensities occurs with sulphur atoms adsorbed in the “expected” 4-coordinate adsorption sites. The reliability index proposed by Pendry is minimised with S atoms 1.32 Å above the topmost metal layer; this corresponds to nearest-neighbour S-Cu bond distances equal to 2.24 Å. This value appears broadly consistent with a measurement by photoelectron diffraction, as well as from model predictions.     

Low-energy electron-diffraction analysis of the Rh(110)-(2 × 1)-O phase

A quantitative low-energy electron-diffraction (LEED) analysis of the Rh(110)-(2 × 1)pg-O structure revealed a model which consists of O zigzag chains with oxygen located in threefold-coordinated sites on an otherwise undistorted Rh(110) surface. Due to the strong interaction of oxygen with Rh the contraction of the first Rh interlayer spacing (observed for the clean surface) is globally removed. LEED investigations on the (2 × 2)pg-O structure prove the presence of a missing-row reconstruction of the Rh substrate.     

Acetylene adsorption on silicon (100)-(4 × 2) revisited

The aim of this work is to revisit the problem of acetylene adsorption on silicon (100). Extending previous theoretical work and including van der Waals forces explicitly in the simulations we remove existing ambiguities about the adsorption sites. The simulated adsorption energies and scanning tunneling microscopy contours are in good agreement with experimental data, they support the interpretation of a two-dimer feature at the surface as resulting from the adsorption of two individual molecules. It is also found that the simulated apparent heights agree with experimental values, if the actual bandgap of silicon is taken into account.     

STM study of the Ni(110)-(2 × 1)-2CO system: structure and bonding-site determination

The structure of the Ni(110)-(2 × 1)-2CO system has been studied with scanning tunneling microscopy (STM) at room temperature. Contrary to many previous studies, it is found that atomically-resolved STM imaging of CO molecules is attainable. By comparing the apparent CO adlayer registry with coadsorbed low concentrations of O and S, it is found that the CO molecules bond in short-bridge sites along the Ni [11̄0] rows and alternately tilt away from the normal direction within the (001) plane.     

The Rh(100)-(3 × 1)-2O structure

The O adsorption on Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. In addition to the well known (2 × 2), (2 × 2)-pg and c(8 × 2) structures at coverages of 0.25, 0.5 and 1.75 ML respectively, an intermediate (3 × 1) structure with a coverage of 2/3 ML is identified.     

Atomic and electronic properties of tert-butanol on the Si(001)-(2×1) surface

The atomic and electronic properties of the adsorption of tert-butanol [(CH₃)₃OH] molecule on the Si(001)-(2×1) surface have been studied by using the ab-initio density functional theory (DFT) based on pseudopotential approach. We have found that tert-butanol bonded the Si(001) surface by oxygen atom, cleaving a O–H bond and producing a Si-H bond and tert-butoxy surface species. We have also investigated the influence of chemisorption of tert-butanol on the electronic structure of the clean Si(001)-(2×1) surface. Two occupied surface states situated entirely below the bulk valence band maximum have been identified, which means that the clean Si(001)-(2×1)surface was passivated by the chemisorption of tert-butanol. In order to explain the nature of the surface components we have also plotted the total and partial charge densities at the [`(K)]\bar{K} point of the surface Brillouin zone (SBZ).     

Confirmation of the missing-row model with three-layer relaxations for the reconstructed Ir(110)-(1 × 2) surface

The reconstruction of Ir(110)-(1 × 2) has been re-analyzed by low-energy electron diffraction. In this study, the missing-row model with paired rows in the second layer and buckled rows in the third layer, as well as the Bonzel-Ferrer (sawtooth) model have been examined. In addition, two other models, which are obtained by putting the missing-row atoms back onto the surface in other sites, have also been considered. It is found that the missing-row model with paired rows in the second layer and buckled rows in the third layer gives the best R-factor among all the models considered in this study. This missing-row model with a three-layer reconstruction is thus proposed to solve the Ir(110)-(1 × 2) structure.     

Ge(112)-(4×1)-In表面重构的原子结构

用扫描隧道显微镜(STM)研究了亚单层In原子引起的Ge(112)-(4×1)-In表面重构.结合随偏压极性不同而显著不同的STM图象和相应的“原子图象”,为这个重构提出了一个原子结构模型,供进一步研究参考.其中,In原子的吸附位置与它在Si(112)表面的吸附位置一致,但与Al原子和Ga原子在Si(112)表面的吸附位置不同.这个吸附位置的不同主要是由In原子较长的共价键键长引起的 关键词： 表面结构 In Ge 扫描隧道显微镜(STM)     

Asymmetric <Emphasis Type="Italic">c</Emphasis>(4×4) → γ(2×4) reconstruction phase transition on the (001)GaAs surface

The c(4×4) → γ(2×4) reconstruction phase transition on the (001)GaAs surface is studied experimentally. It is shown that it is a first-order phase transition. The phase transition is found to exhibit a highly asymmetric hysteresis. The difference between the direct and inverse runs of the hysteresis is explained in terms of the mean field theory of an adsorption-induced phase transition by the substantial contribution of lateral multiparticle interactions in the adsorbate.     

AN SU(4)×SU(2)×U(1) UNIFIED MODEL

In this paper we propose an SU(4)×SU(2)×U(1) unified model, in which the stratons and leptons of every generation belong to the same Multiplet, and the leptons will couple to the stratons. But this kind of coupling cannot cause the proton decay, and has only some effects in high energy scatterings. This model predicts the existense of two neutral intermediate bosons, instead of one, and the value of Weinberg angle to be sin²θ_w=0.25. The neutrinos have reasonable masses in the model.