Existence of a phase transition in a spin-1/2 Falicov-Kimball model

In this paper we present a proof for the existence of a phase transition in the symmetric case of the spin-1/2 Falicov-Kimball model in more than one dimension. For a certain regime of the parameters of this model we are able to show, that the superstructure of the localizedf-electrons in the ground-state, which has been investigated and published some time ago, persists in part up to small nonzero temperatures.Supported in part by the Deutsche Forschungsgemeinschaft     

Ground state properties of a spinless Falicov-Kimball model; additional features

We have studied some energetic and structural ground state properties of a spinless Falicov-Kimball model. Using a method based on “Tchebycheff-Markoff inequalities”, we have calculated sharp upper and lower bounds for the ground state energy. These calculations lead to rigorous results for a special range of parameters, where we are able to give the exactf-level occupation pattern for a square lattice. The results for the “symmetric” case, where a superstructure occurs leading to a metal-insulator transition, have already been published. In this paper we present some additional results also for the “unsymmetric case” and study valence-transitions for fixed particle-number.     

Self-assembled MnN superstructure

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.     

用直接法测定晶体的超结构

在许多无机物(包括矿物和一些重要的固体材料)的晶体中,往往形成所谓“超结构”。反映超结构细节的所谓“超结构衍射点”,其强度通常都很弱。因此,在测定这类晶体结构时,一般都先将超结构点略去以求出一个只反映概貌的“赝结构”。然后在此基础上再探寻超结构的细节。但是,用普通的结构修正办法不可能从赝结构直接解出超结构。这就增加了超结构分析的困难。本文提出从赝结构衍射相角推引超结构衍射相角,借以简化超结构的分析过程。试验表明,这一方法是有效的。 关键词：     

垂直非结构模型应用于换热网络优化

节点非结构模型优化换热网络时会出现换热器交叉排列的情况,分析发现在相同传热负荷下交叉结构比垂直结构换热单元总传热面积增大,同时明显降低算法的计算效率.基于此,建立垂直非结构模型.与节点非结构模型相比,垂直非结构模型降低了优化过程中的计算复杂度,并且能够消除交叉结构带来的不利影响,有效提升了优化的效率及求解精度.应用两个算例对基于垂直非结构模型的强制进化随机游走算法（RWCE）进行优化性能分析,取得了优于文献的结果.     

Electronic structure and energetics of tetragonal SrCuO? and its high-pressure superstructure phase

First-principles calculations have been used to investigate the electronic structure and energetics of the simple tetragonal SrCuO? (P4=mmm) and its high-pressure tetragonal superstructure (P4=mmm). Based on the calculations, the high-pressure phase is metastable as compared with the low pressure tetragonal phase, with an energy difference of 0.13 eV per SrCuO? formula unit. The energy barrier to the transition from the superstructure to the simple tetragonal structure is 0.24 eV at 7 GPa; thus, high temperatures are required to synthesize the latter. Among the possible structural configurations resulting from the partially occupied oxygen site in the superstructure phase, the most stable structure has a space group PN4m2, reduced from that of the simple tetragonal structure P4=mmm. The detailed analysis of the electronic band structures of the simple tetragonal and superstructure phases suggests that the out-of-plane buckling of the O atoms in the superstructure leads to significant decrease in the O p-Cu d orbital overlap, allowing the energy of the system to be lowered, which is necessary for the structural stability. An understanding of the electronic structure and energetics of the high-pressure superstructure phase and its relation to the simple tetragonal phase provides a basis for exploring the physical properties of the infinite layer, high-TC superconductor.     

神光Ⅱ升级装置中靶场桁架的结构稳定性

 从提高结构稳定性的基本理论出发,利用有限元方法研究了靶场桁架结构稳定性。结果表明：影响大口径反射镜动态稳定性的主要因素为靶场桁架的第三阶模态振型,其薄弱环节为支撑立柱;通过在支撑立柱间增加横向加强筋、桁架立柱采用钢管混凝土结构等措施可有效提高桁架的结构稳定性;立柱截面最小方形空心型钢的尺寸为300 mm×6 mm。给出的靶场桁架的稳定性设计方案满足大口径反射镜的动态稳定性指标。     

Absolute coverage of C2H2 and C2H4 on Pt(111)

Quantitative XPS measurements have been performed in order to determine the absolute coverage of acetylene and ethylene adsorbed on Pt(111) showing a 2 × 2 LEED pattern. This LEED pattern has so far been attributed to a 2 × 2 superstructure with a coverage of 0.25. A quantitative evaluation of the C(1s) peak intensities for these adsorbed layers in comparison with adsorbed CO shows that the coverage is 0.5 instead of 0.25. Therefore the 2 × 2 LEED pattern should be assigned to a 2 × 1 superstructure in three domains rather than a 2 × 2 superstructure.     

Reduction of structure-borne sound in simple ship structures: Results of model tests

A method for the prediction of the transmission of structure-borne sound in ship structures is presented. Various methods to decrease the noise levels in the accomodation spaces in superstructures are investigated in model tests. The attenuation of structure-borne sound in the propagation path between source and receiver is increased by means of damping layers, resilient mounts and changed boundary conditions between main deck and superstructure. Damping layers are found to have only a local effect. Resilent mounts between superstructure and main deck can reduce the noise levels in the superstructure by the order of 10 dB(A).     

用直接法测定黄河矿晶体的超结构

本文分析了黄河矿晶体超结构中赝对称性的特点,详述了用于测定黄河矿晶体超结构的直接法。 关键词：     

Order-to-order phase transition in alloys with face-centered superstructures

The possibility of a temperature phase transition from one superstructure in a binary alloy to a superstructure of another type is theoretically considered. A possible phase diagram for these transitions is constructed.Translated from Izvestiya Vysshikh Uchebnykh. Zavedenii, Fizika, No. 8, pp. 26–30, August, 1982.     

The nature of distribution of atoms,and its connection with electron structure in a number of ternary alloys on Ni3Mn basis

The influence of alloying the Ni₃Mn alloy with titanium, vanadium, or copper, on the superstructure of the alloy, was studied by the neutron diffraction method. It was shown that Ti, V, and Al stabilize the superstructure of Ni₃Mn, and Cu actively destroys it. The distribution of atoms of the alloying elements along the sites of fcc lattice and its connection with the electron structure of the alloy was analyzed. The different influence of alloying with a third element on the superstructure in alloys Ni₃Mn and Ni₃Fe is explained.     

Graphene on Ru(0001): a 25 x 25 supercell

The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation. Strong intensity oscillations in the superstructure rods demonstrate that the Ru substrate is also significantly corrugated down to several monolayers and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru substrate to the graphene expands and weakens the C-C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the potential application of graphene as a template for nanocluster arrays.     

Superstructure in chemically etched poly(ethylene terephthalate)

The new technique of contact etching has been utilized to study the bulk morphology of polyethylene terephthalate by the echant, n-propylamine. A variety of films and fibers with different mechanical and thermal histories have been subjected to contact etching. The sample surfaces have been studied principally by scanning electron microscopy. A network superstructure with its characteristic dimension (thickness) of from 700 to 3000 Å, depending on sample history, has been observed. In the oriented samples the network superstructure aligns perpendicular to the direction of sample orientation. A simple model is proposed to describe the network superstructure which is believed to be moderately crystalline.     

Growth of Si nanostructures on Ag(0 0 1)

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 × 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 × 3) and of the “complex” superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 × 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the “complex” superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure.     

Superconductivity and structural transformation in HfRuAs

HfRuAs exists in two structural modifications. The high-temperature phase (ZrRuSi-type structure) exhibits superconductivity at 4.93-4.37 K, whereas the low-temperature phase (TiFeSi-type superstructure) remains normal at 1 K. The lattice distortion giving rise to the superstructure indicates zig-zag chains of metal atoms are crucial to superconductivity in this class of compounds.     

Different STM images of the superstructure on a clean Si(133)-6 × 2 surface

New detailed scanning tunneling microscopy images of the superstructure on an atomically clean Si(133)-6 × 2 surface are obtained. Similarities and differences with previously reported images are discussed. On the basis of these data, a new atomic model for the unit cell of the superstructure on the Si(133)-6 × 2 surface is suggested taking into account inconsistencies in the previously proposed models.     

Induced magnetic superstructure in the FeBO3: Mg weak ferromagnet

The domain structure of a FeBO₃: Mg single crystal was studied with a polarizing microscope. It was found that application of a magnetic field along the hard axis in the basal plane of this weak ferromagnet gives rise, within a certain field-strength interval, to a magnetic superstructure observed against the background of the macrodomain structure of the sample. The magnetic superstructure is visually represented as a quasi-periodic system of bands oriented perpendicular to the applied field, with an alternating magneto-optic image contrast along an axis coinciding with the magnetic-field direction. The absence of sharp changes in the contrast of the magnetic superstructure image along this axis is explained as being due to the smooth variation of the sublattice magnetic-moment azimuth with spatial coordinates. The results obtained are discussed within the parameters of the instability of a uniform magnetic state of a system in the random field induced by a magnetic field.     

Surface dynamics and reconstruction of Si and Ge

The dynamical behavior of a Si(III) and a Ge(III) surface is studied. The formation of a superstructure, i.e. the surface reconstruction, is shown to be a consequence of the softening of surface phonons. Within our model we derive a (7×7) reconstruction for Si and an (8×8) reconstruction for Ge. A buckled model for the superstructure on Si(III) surface as well as on the Ge(III) surface is proposed.     

Diffuse scattering in RHEED induced by linear disorders of sulphur segregated on nickel (111) surface

RHEED diffuse scattering intensity is related to the growth of one-dimensional sulphur chains segregated on a nickel (111) surface. These linear disorders lead to wall domains in the reciprocal lattice which give the well-known “arching” phenomenon in RHEED patterns. The diffuse scattering process disappears when the (5√3 × 2)R30° superstructure is obtained. Knowing the absolute coverage of sulphur by calibrated Auger spectroscopy, it is possible to give a new high symmetry model for this superstructure.