Alignment of Ge nanoislands on Si(111) by Ga-induced substrate self-patterning

A novel mechanism is described which enables the selective formation of three-dimensional Ge islands. Submonolayer adsorption of Ga on Si(111) at high temperature leads to a self-organized two-dimensional pattern formation by separation of the 7 x 7 substrate and Ga/Si(111)-(square root[3] x square root[3])-R30 degrees domains. The latter evolve at step edges and domain boundaries of the initial substrate reconstruction. Subsequent Ge deposition results in the growth of 3D islands which are aligned at the boundaries between bare and Ga-covered domains. This result is explained in terms of preferential nucleation conditions due to a modulation of the surface chemical potential.     

Kinetics of H→P cross-polarization in human trabecular bone

Bloch-decay and cross-polarization (CP) ³¹P nuclear magnetic resonance (NMR) spectra of healthy human trabecular bone were acquired under magic-angle spinning (MAS) at 3 kHz. A single peak at 3.1 ppm was detected. Variable-contact time ¹H → ³¹P CP experiments revealed three signal components growing at various rates. The fast, moderate and slow components were assigned and assessed in P atom % to proton-rich (24%), hydroxyapatite (58%) and proton-deficient (18%) phosphate domains, respectively. Examination of CP kinetics is useful for the chemical characterization of bone tissue.     

Surface morphology, chemical contrast, and ferroelectric domains in TGS bulk single crystals differentiated with UHV non-contact force microscopy

Ferroelectric bulk single crystals of tri-glycine sulphate (TGS) have been investigated in ultra-high vacuum (UHV) using dynamic force microscopy (DFM) in the non-contact (nc) mode. Both chemical contrast from different sub-unit cell cleavage steps, and ferroelectric domains were differentiated by recording the variation in interaction force affecting the excitation amplitude A_exc applied to the piezo shaker in constant amplitude DFM. No chemical difference was found for steps measuring half the unit cell height b, in contrast to b/4—steps where sulphate ions change the local short-range chemical forces. By varying the bias voltage applied to the TGS counter electrode, the sign of bound surface charge within each ferroelectric domain was determined. Domain walls separating regions with antiparallel polarisation vectors are resolved down to a 9 nm domain wall width. Furthermore, we achieved atomic resolution with nc-DFM on cleaved TGS samples indicating the monoclinic unit cell at the ferroelectric sample surface with a=1.0±0.05 nm, c=0.55±0.05 nm, and β=107±3°.     

Coherent X-ray diffraction investigation of twinned microcrystals

Coherent X-ray diffraction has been used to study pseudo-merohedrally twinned manganite microcrystals. The analyzed compositions were Pr(5/8)Ca(3/8)MnO(3) and La(0.275)Pr(0.35)Ca(3/8)MnO(3). The prepared loose powder was thermally attached to glass (and quartz) capillary walls by gentle heating to ensure positional stability during data collection. Many diffraction data sets were recorded and some of them were split as expected from the main observed twin law: 180° rotation around [101]. The peak splitting was measured with very high precision owing to the high-resolution nature of the diffraction data, with a resolution (Δd/d) better than 2.0 × 10(-4). Furthermore, when these microcrystals are illuminated coherently, the different crystallographic phases of the structure factors induce interference in the form of a speckle pattern. The three-dimensional speckled Bragg peak intensity distribution has been measured providing information about the twin domains within the microcrystals. Research is ongoing to invert the measured patterns. Successful phase retrieval will allow mapping out the twin domains and twin boundaries which play a key role in the physical properties.     

Projection structure of photosystem II In vivo determined by cryo-electron crystallography

Photosystem II (PSII) is a protein-pigment complex situated in the thylakoid membranes of plants and cyanobacteria where it catalyses the conversion of light into chemical energy. This energy is used to extract electrons from water, during which process oxygen is evolved. Owing to its extreme fragility and the large number of polypeptides (>20) it is composed of, the complex has so far proven recalcitrant to high-resolution structural studies. Cryo-electron crystallography of 2-D crystals (a = 15.4 nm, b = 23.1 nm, γ = 97.2°, p1) comprising in situ PSII revealed the first projection structure of the native complex. The unit cell contain one monomeric complex in which three central domains straddle an elongated intramolecular cavity. In conjunction with earlier data, these central domains were assigned to the reaction centre core subunits of PSII consisting of CP43, CP47, the reaction centre heterodimer D1/D2 and cytochrome b-559. The data are discussed in view of the evolution of reaction centres from anoxygenic to oxygenic photosynthesis.     

Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

2 bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces – including the inner surfaces of tubes – are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour–liquid–solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube. Received: 26 November 1998 / Accepted: 14 January 1999     

Thermal and electronic transport properties and two-phase mixtures in La(5/8-x)Pr(x)Ca(3/8)MnO3

We measured thermal conductivity kappa, thermoelectric power S, and electric conductivity sigma of La(5/8-x)Pr(x)Ca(3/8)MnO3, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of kappa, S, and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.     

Ni deposition on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal

We have evaporated Ni on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal kept at room temperature. At the initial stage of growth, Ni intermixes with the substrate surface. Subsequently, Al from the quasicrystal matrix migrates to growing layers. The modified chemical composition in an initially icosahedral region near the surface induces a structural transformation. An Al-Pd-Mn alloy is formed which consists of five cubic domains with dimensions in the nm-range exposing their (1 1 0) faces parallel to the surface. These domains are azimuthally rotated by 2π/5 with respect to each other and aligned with symmetry directions of the icosahedral substrate. Al-Mn-Ni, Al-Ni, and Ni overlayers adopt both structure and orientation of these domains which stabilises Ni in a novel body-centred cubic phase. Ni-rich overlayers exhibit out-of-plane magnetic ordering.     

Dynamic aspect of bacteriorhodopsin as a typical membrane protein as revealed by site-directed solid-state 13C NMR

We demonstrate here a general feature of dynamic aspect of membrane proteins as revealed by site-directed 13C NMR studies on bacteriorhodopsin (bR) as a typical membrane protein and a variety of mutants at ambient temperature. 13C NMR signals of [3-13C]Ala- or [1-13C]Val-labeled proteins were assigned regio-specifically with reference to the data of the conformation-dependent 13C chemical shifts from model polypeptides, followed by site-specific assignment based on site-directed mutants. Revealed picture of membrane protein at ambient temperature is not static in contrast to anticipation from crystalline structures but flexible enough to undergo a variety of local fluctuations with frequencies from 10(2) to 10(8)Hz, as pointed out already. This picture was further refined by taking into account of residue-specific dynamics of interfacial domains between the surface and inner part of the transmembrane helices and conformational fluctuation induced by the presence of a kinked structure. The residue-specific dynamics of the former was revealed by observation of broadened or suppressed peaks from the interfacial domains caused by acquisition of internal fluctuation motions interfered with frequencies of proton decoupling or magic angle spinning. The presence of such suppressed peaks due to molecular fluctuations in the interfacial domains was further confirmed by insensitivity of the peak-intensities from the interfacial domains in spite of the presence of accelerated relaxation rate to nearby residues from surface bound Mn2+ ion. Further, conformational change of the transmembrane alpha-helix F due to a plausible kinked structure at Pro 186 was confirmed in view of specific displacements of Ala 184 and Val 187 13C NMR peaks from chemically synthesized [3-13C]Ala(184)-, [1-13C]Val(187)-labeled wild type and P186L mutant of transmembrane fragment F(164-194) incorporated into lipid bilayer. It is emphasized that the observed displacement of [3-13C]-labeled Ala 184 peak at 17.4 ppm in the presence of kinked structure in this model peptide is consistent with that of intact protein at 17.27 ppm.     

EM, XPS and LEED study of deposition of Ag on hydrogenated Si substrate prepared by wet chemical treatments

The deposition of Ag on a hydrogenated Si(111) substrate, prepared by wet chemical treatment, was carried out at room temperature (RT), 250°C and above 350°C. The samples were examined in situ by LEED, XPS and then by SEM and TEM. Our results show that flat Ag crystallite domains are growing on the H/Si(111) surface at all three substrate temperatures. The h/w ratio of the islands increases but the density decreases with increasing substrate temperature which is attributed to the differences between hydrogenated and clean Si(111) surface. The so-called A-type and B-type domains resulting from analyses of TOF ICISS are attributed to twinning in the deposited layer.     

Electric field induced avalanche breakdown and non-volatile resistive switching in the Mott Insulators AM4Q8

The Mott insulator compounds AM₄Q₈ exhibit a new type of volatile and non volatile resistive switchings that are of interest for RRAM application. We found that above a threshold electric field E _TH of the order of a few kV/cm these compounds undergo a volatile resistive switching based on an avalanche process. For electric field much higher than the threshold avalanche breakdown field, the resistive switching turns non volatile. Our EDXS and STEM analyses show that the non volatile resistive switching originating from the avalanche breakdown can neither be ascribed to local chemical modifications nor to a local phase change with symmetry breaking at a resolution better than a few nanometer. This is in strong contrast with non volatile resistive switching reported so far that are all based on chemical or structural changes. Conversely, our results suggest that the avalanche breakdown induce the collapse of the Mott insulating state at the local scale and the formation of a granular conductive filament formed by compressed metallic domains and expanded “superinsulating” domains.     

Structure refinement of large domain relaxors in the Pb(Mg1/3Ta2/3)O3-PbZrO3 system

The relaxor ferroelectric compound Pb(Mg_0.3Ta_0.6Zr_0.1)O₃ (PMT-PZ) was studied by X-ray, neutron and electron diffraction and transmission electron microscopy in the as-sintered and annealed states. The as-sintered sample was comprised of nanometer-sized 1:1 chemically ordered domains dispersed in a disordered matrix. After annealing at 1325°C the domain size increased to ∼30 nm and the degree of order exceeded 95% in terms of the volume fraction of the ordered domains, yet the sample retained its diffuse, frequency dependent relaxor characteristics. Refinements of the chemically ordered structure using the Rietveld analysis revealed that the octahedral (B) site occupancies were in excellent agreement with a “random site” model for the chemical ordering. In this charge-balanced model for the 1:1 ordered Pb(β′_1/2β″_1/2)O₃ structure the Ta cations predominantly occupy the β″ site, while the β′ site is populated by a random distribution of the Mg, Zr and remaining Ta cations. Large temperature factors for Pb and O atoms are observed in both as-sintered and annealed samples, indicating localized displacements of the Pb and O atoms. The mixed occupancy of the β′ position appears to be responsible for the relaxor characteristics in the dielectric response in spite of the growth of the chemical domains.     

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO₃ substrates and Ti:LiNbO₃ waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO₃ crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO₃ waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.     

STM investigations of CO coadsorption with O and with S on Ni(110)

The interaction between CO coadsorbed with oxygen and sulfur on Ni (110) has been studied with room temperature STM and LEED. In the case of CO/O/Ni(110)−(θ_o0.3 ML), it is found that due to a large local repulsion between the differing species, the coadsorbed species phase separate into large domains of O−(3 × 1) and CO−p2mg(2 × 1) structure. Similarly in the case of CO/S/Ni(110)−(θ_s0.4 ML), at low local coverages of coadsorbed CO, island segregation of CO and S-c(2 × 2) is observed. At locally saturated CO coverage, the S-c(2 × 2) structure transforms into long -S-S- chains running predominantly along the [ ] direction and separated by a local p2mg(2 × 1)-CO structure; this transformation is attributed to the large CO-CO repulsion in the condensed overlayer structure.     

快速冷凝法制备纳米复合Nd3:8Dy0:7Pr3:5Fe86Nb1B5永磁材料的织构和磁畴

" 通过快速冷凝的方法制备了高性能、强织构纳米复合Nd3:8Dy0:7Pr3:5Fe86Nb1B5永磁材料．X射线衍射和磁测量分析显示薄带有优先取向的特征．随着快淬速度的提高,易磁化方向由垂直与薄带面的方向转向平行于薄带面的方向．通过SPM分析了样品中磁畴结构的形成以及对交换耦合作用强弱的影响．同时,Henkel曲线也表明了在快淬速度为30 m/s下制备的薄带晶粒有较强的交换耦合作用,从而使其剩磁提高,磁性能增强．在快淬速度为30 m/s下制备的样品平均晶粒尺寸为16 nm,样品含有高织构的硬磁相(Nd,     

LEED studies of the adsorption of CO2 on thin epitaxial NaCl(100) films

The adsorption of CO₂ on the NaCl(100) surface was studied with a high-resolution LEED-system. Measurements without charging up at low electron energies and without damage by the e-beam could be performed by using ultrathin epitaxial films on a conducting Ge(100) substrate. The adsorption behavior was recorded as a function of time and pressure at constant substrate temperatures of 78 and 83 K and CO₂ partial pressures from 4 × 10⁻⁸−2 × 10⁻³ Pa. The adsorption system shows a first-order two-dimensional phase transition to a (2 × 1) superstructure including glide planes (herringbone-like structure) at p = 7.2 × 10⁻⁸Pa (T = 78 K). The condensation of the CO₂ solid is starting at p = 1.5 × 10⁻⁴ Pa (T = 78 K). The LEED-pattern shows in this c(2 × 2) superstructure, which corresponds to the pyrite-like structure of the CO₂ solid. Both observed superstructures are commensurable with the NaCl(100) surface. Observation of island growth shows that the domains of the (2 × 1) superstructures have already at coverage of 5% of a monolayer an average lateral size of at least 200 A.     

Motion of Chern-Simons number at high temperatures under a chemical potential

I investigate the evolution of finite temperature, classical Yang-Mills field equations under the influence of a chemical potential for Chern-Simons number N_cs. The rate of N_cs diffusion,, Γ_d, and the linear response of N_cs to a chemical potential, Γ_μ, are both computed; the relation Γ_d = 2Γ_μ is satisfied numerically and the results agree with the recent measurement of Γ_d by Ambjørn and Krasnitz. The response of N_cs under chemical potential remains linear at least to μ = 6T, which is impossible if there is a free energy barrier to the motion of N_cs. The possibility that the result depends on lattice artefacts via hard thermal loops is investigated by changing the lattice action and by examining elongated rectangular lattices; provided that the lattice is fine enough, the result is weakly if at all dependent on the specifics of the cutoff. I also compare SU(2) with SU(3) and find Γ_SU(3) 7(_{s/w)⁴ΓSU(2)}.     

A Complete 2D Stability Analysis of Fast MHD Shocks in an Ideal Gas

 An algorithm of numerical testing of the uniform Lopatinski condition for linearized stability problems for 1-shocks is suggested. The algorithm is used for finding the domains of uniform stability, neutral stability, and instability of planar fast MHD shocks. A complete stability analysis of fast MHD shock waves is first carried out in two space dimensions for the case of an ideal gas. Main results are given for the adiabatic constant γ=5/3 (mono-atomic gas), that is most natural for the MHD model. The cases γ=7/5 (two-atomic gas) and γ>5/3 are briefly discussed. Not only the domains of instability and linear (in the usual sense) stability, but also the domains of uniform stability, for which a corresponding linearized stability problem satisfies the uniform Lopatinski condition, are numerically found for different given angles of inclination of the magnetic field behind the shock to the planar shock front. As is known, uniform linearized stability implies the nonlinear stability, that is local existence of discontinuous shock front solutions of a quasilinear system of hyperbolic conservation laws. Received: 12 March 2002 / Accepted: 8 November 2002 Published online: 10 February 2003 Communicated by P. Constantin     

3-氧,11α-羟基、Δ12-齐墩果烯及其类似化合物的13C NMR分析

化合物1-8属于近似结构的齐墩果烷型三萜,它们分别为:β-amyrin(1);3-氧,Δ^11,13(18)-齐墩果二烯(2);3β-羟基,Δ^11,13(18)-齐墩果二烯(3);3β,24-二羟基,Δ11,13(18)-齐墩果二烯(4);3,11-二氧,Δ¹²-齐墩果烯(5);3-氧,Δ9^(11),12-齐墩果二烯(6);3-氧,11α-羟基,Δ¹²-齐墩果烯(7);3β,11α-二羟基,Δ¹²-齐墩果烯(8).它们的¹³C NMR化学位移归属已确定,并通过了DEPT谱的验证.按结构特点分为三组:2~4为第一组,1,5,7,8为第二组,6为第三组.本文重点讨论了它们的¹³C NMR化学位移由其结构特点引起的变化,得到了化学位移与结构相关的有意义的结果.     

Formation of the ferroelectric domains in epitaxial PbTiO3 thin films grown by metalorganic chemical vapor deposition

3 thin films have been prepared by metalorganic chemical vapor deposition under reduced pressure. The formation of ferroelectric domains in films grown on SrTiO₃ and LaAlO₃ substrates was investigated by synchrotron radiation and Rutherford backscattering spectroscopy. Single-domain (3000-Å thick) and multi-domain (4500-Å thick) PbTiO₃ films were produced on SrTiO₃. For multi-domain PbTiO₃ film, the c-domain presented epitaxial structure with its c-axis perpendicular to the substrate surface, while a-domains aligned four-fold symmetrically with c-domains by 2.79^° off the c-axis of c-domains. In the film, the measured lattice constants (a, b and c) of the a- and c-domains were different from each other, indicating that the films suffered a modulated strain during domain formation. In contrast, both the a and c domains of films on LaAlO₃ were alternatively aligned on substrate with the a-axis of the a-domain and the c-axis of c-domains perpendicular to the substrate surface. Two-dimensional distribution of these domains is proposed and the formation of these kinds of domains is discussed. The surface morphology and phase transition process of single and multi domain PbTiO₃ film on SrTiO₃ were studied by atomic force microscope (AFM) and high temperature X-ray diffraction, respectively. Received: 15 August 1996/Accepted: 21 January 1997