Elastic theory of icosahedral quasicrystals - application to straight dislocations

In quasicrystals, there are not only conventional, but also phason displacement fields and associated Burgers vectors. We have calculated approximate solutions for the elastic fields induced by two-, three- and fivefold straight screw- and edge-dislocations in infinite icosahedral quasicrystals by means of a generalized perturbation method. Starting from the solution for elastic isotropy in phonon and phason spaces, corrections of higher order reflect the two-, three- and fivefold symmetry of the elastic fields surrounding screw dislocations. The fields of special edge dislocations display characteristic symmetries also, which can be seen from the contributions of all orders. Received 21 February 2001 and Received in final form 27 June 2001     

Stabilisation of fcc cobalt layers by 0.4 nm thick manganese layers in Co/Mn superlattices

Epitaxial Co/Mn multilayers (0.75 to 6 nm Co, 0.4 nm Mn layer thickness) have been grown on mica substrates covered by a (0002) Ru buffer layer. The structural properties of these layers have been studied using X-ray diffraction, nuclear magnetic resonance (NMR), and high resolution transmission electron microscopy (HRTEM). The Co layers, grown as face centred cubic (fcc), were found to be stabilised by the very thin Mn layers. Data obtained using X-ray diffraction and NMR were analysed and found to be in good agreement, while Monte-Carlo simulations were used to interpret the data and calculate the expected diffracted intensity and NMR spectra. The HRTEM data show that the Mn layers give rise to a large strain contrast extending, in the growth direction, over a distance which exceeds the thickness of the Mn layers. The superlattices could be described as having an fcc structure containing randomly located stacking faults with varying densities. The results verify the presence of a dominant, almost perfect phase of fcc stacking, and of a faulted hcp phase, while the number of defects increases with the Co layer thickness. Received 27 October 1999 and Received in final form 29 May 2000     

Special interaction between quasiperiodic grain boundaries and lattice dislocations in crystalline solids

A theoretical model is suggested which describes phason imperfections (specific excitations) in a quasiperiodic grain boundary in a polycrystalline solid as dilatation flexes. In the framework of the model, an elastic stress field of the quasiperiodic grain boundary is calculated as a stress field created by an ensemble of dilatation flexes (phason imperfections) located in the boundary. It is shown that there is a special elastic interaction between crystal lattice defects and quasiperiodic grain boundaries comprising phason imperfections. The strengthening effect in plastically deformed polycrystalline solids is quantitatively described which is related to the special elastic interaction between lattice dislocations and quasiperiodic grain boundaries. Received: 29 October 1996 / Revised: 22 August 1997 / Accepted: 13 November 1997     

Unsupported lead clusters and electron diffraction

A beam of Pb clusters is produced with the inert gas aggregation method and probed by electron diffraction. Analysis of the diffraction patterns indicates that average cluster size can vary between 3 and 7 nm, according to nucleation conditions. The diffraction patterns from beams with larger average cluster size are very similar to patterns calculated from model decahedron clusters, while those for smaller cluster size do not appear to have simple geometrical face-centred cubic, decahedral, or icosahedral structure. Received 30 November 2000     

Magnetic behavior and role of the antiphase boundaries in Fe3O4 epitaxial films sputtered on MgO (001)

Magnetite Fe₃O₄ films were grown on single crystal MgO (001) substrates using facing target sputtering technique. Conversion Electron M?ssbauer Spectroscopy and magneto optical polar Kerr spectra have confirmed the stoichiometric repartition of Fe cations corresponding to the inverse spinel structure and the electronic structure characteristic of bulk Fe₃O₄. Hysteresis loops carried out at room temperature show that, in a 1 T applied magnetic field, only 60% of the saturation magnetization is detected. This behavior is discussed in correlation to the antiphase boundaries (APBs) observed by electron microscopy. Magnetic force microscopy studies show that magnetic domains are larger than the mean distance between APBs. Received 2 July 2001     

Structural characterisation of Ni clusters in AlN via X-ray absorption, X-ray diffraction and transmission electron microscopy

Ni ions were implanted in bulk AlN with the goal to form embedded metallic clusters. Combining several characterisation techniques such as X-ray absorption spectroscopy, X-ray diffraction and high resolution transmission electron microscopy, we determined the lattice parameter of the Ni clusters that display a fcc crystalline structure. The average size increases when the ion fluence is increased or after a thermal treatment. Thanks to moiré fringes observed by high resolution transmission electron microscopy and to satellite peaks seen on the diffraction patterns, we concluded that the annealed Ni clusters orientate their (002) planes on the (101) of AlN. Moreover, the satellite positions allowed us to calculate Ni cluster average diameters, that are in agreement with average sizes deduced by X-ray absorption spectroscopy. Received 25 August 1999 and Received in final form 8 February 2000     

Phasons, sliding modes and friction

Aperiodic crystals may have additional low frequency modes related to the possibility to describe them in a higher-dimensional space. Dynamics associated with these degrees of freedom is called phasonic, but there are very different phenomena of this type. A discussion is given of the use of the term. The relation between phason modes, the crystal structure, and the modulation and sliding modes is discussed. Finally a relation with frictionless motion is studied. Received 4 April 2002 / Received in final form 22 July 2002 Published online 17 September 2002     

Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PB_h-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45% PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization, is presented. Received 19 March 1999 and Received in final form 14 December 1999     

Phase transitions in Co nanoclusters grown by pulsed laser deposition

The crystalline structure of Co clusters embedded in an amorphous Al₂O₃ matrix was studied by transmission electron microscopy (TEM) and electron diffraction (TED). In the first stage of the growth a metastable structure (body-centred-cubic) is observed. A face-centred-cubic phase (fcc) is found when the size of the clusters increases ( diameter > 4 nm). The hexagonal-close-packed phase arises in the fcc phase by a succession of stacking faults at the largest sizes. The mechanisms of phase transformation have been determined by using high resolution electron microscopy (HREM). The chemical nature of the clusters, in particular the existence of Co-O bonds, was investigated by using electron energy loss spectroscopy (EELS). Received 03 July 2000 and Received in final form 22 December 2000     

Electron diffraction and microscopy of single-wall carbon nanotube bundles produced by different methods

The atomic structure of single-wall carbon nanotube bundles produced by three different techniques (laser ablation, electric arc discharge and catalytic chemical vapor deposition (CCVD)) has been characterized by electron diffraction and microscopy. Information on the helicity and the lattice packing has been obtained. Concerning the helicity, small bundles produced by CCVD exhibit only one or two tube helicities within a single bundle. The diffraction patterns of laser-ablation produced bundles also present well-defined but more diversified chiralities within a single bundle. By contrast the data acquired on bundles formed by arc discharge show a more diffuse pattern, characteristic of a random chirality dispersion within a single bundle. Concerning the lattice packing, informations are obtained via a detailed study of the equatorial line of the diffraction pattern for bundles produced by the three techniques. This electron diffraction study is completed by high-resolution electron microscopy. Received 8 August 2001 and Received in final form 14 March 2002     

Novel twist grain boundary smectic-C phases

Several new kinds of smectic-C twist grain boundary phases (TGBC) have been observed during the last few years. These pure compounds or mixtures exhibit unusual textures with polygonal lattices (square or hexagonal grids) in the plane normal to the TGB helix. The structure of these new phases seems to be complex and different from reported and predicted TGBC phases. In this article, we review the main results obtained on these different new phases, and we propose new TGBC structures based on the well-known splayed polarization --twisted director structures adopted by chiral smectic-C's in planar aligned (bookshelf) cells. The observed square or hexagonal lattices are made of superimposed pairs of unwinding lines due to the suppression of the helix within smectic-C blocks by the grain boundaries (unwinding walls). A lattice-free TGBC occurs if the helix within smectic-C blocks is suppressed completely. Received 15 March 2001     

Influence of vacancies on the electronic structure of Co ZrSn Heusler alloys

The electronic structure of the Co_2-xZrSn Heusler alloys has been studied by X-ray photoelectron spectroscopy (XPS). XPS valence band spectra can be compared with ab initio electronic structure calculations using the linearized muffin-tin orbital (LMTO) method. The calculated magnetic moments per Co atom agree well with the moments obtained from experiment. The LMTO calculations also show the energy shifts of the Co, Zr and Sn valence electron states towards the Fermi level when the concentration of vacancies increases in these alloys. Received 9 March 1999 and Received in final form 6 May 1999     

Degree of structural perfection of icosahedral quasicrystalline grains investigated by synchrotron X-ray diffractometry and imaging techniques

A study of the structural perfection of icosahedral quasicrystalline grains of various alloys (Al-Pd-Mn, Zn-Mg-RE (RE L rare earth) and Al-Cu-Fe), grown by different slow solidification techniques (Czochralski, Bridgman, flux and annealing) was performed using high-resolution diffraction, including recording rocking curves combined with X-ray topography and phase contrast radiography, at a third-generation synchrotron radiation source (European Synchrotron Radiation Facility, Grenoble, France). For Al-Pd-Mn, additional coherent diffraction and diffuse scattering measurements were also carried out. After evaluating the potentialities of the techniques used, in the light of the criteria defined for crystals, it is shown that the structural perfection of icosahedral quasicrystals is quite comparable with that of metallic crystals but is considerably influenced by either uniform phason strains which can destroy the quasiperiodic long-range order, or by long-wavelength phason fluctuations leading to diffuse scattering. The structural perfection was also found to be extremely variable across the as-grown quasicrystalline grains and to be dependent on the presence and characteristics of inhomogeneities (pores and precipitates) often included in the quasicrystalline matrix. Regarding the grains that we used, it has been impossible to distinguish a clear influence of either the type of alloy or the growth method. It has, however, been noticed that Al-Pd-Mn and Al-Cu-Fe grains appeared less defective than Zn-Mg-RE grains and that the microstructure of these latter grains looks like that of crystals grown by the same technique. Annealing and mechanical polishing effects have also been analysed in the case of Al-Pd-Mn grains. It appeared that annealing improves the quasicrystalline lattice perfection by lowering phason strains insofar as no precipitates are nucleated. Mechanical polishing can introduce defects, located at the external surfaces, having the shape of bands.     

Formation and manipulation of regular metallic nanoparticle arrays on bacterial surface layers: an advanced TEM study

The template-directed formation of regular nanoparticle arrays on two-dimensional crystalline protein layers after their treatment with metal salt complexes was studied by transmission electron microscopy. For these investigations, bacterial surface layers (S layers), recrystallized in vitro into sheets and tube-shaped protein crystals with typical dimensions in the micrometer range, were used as the template. As identified by electron holography and scanning force microscopy, the S-layer tubes form alternating double layers when deposited onto a solid substrate surface. Two distinct pathways for the metal particle formation at the templates have been found: the site-specific growth of metal clusters by chemical reduction of the metal salt complexes, and the electron-beam induced growth of nanoparticles in the transmission electron microscope. Both mechanisms lead to regular arrays with particle densities > 6×10¹¹ cm ^-2. Nanoparticle formation by electron exposure takes exclusively place in the flat-lying double-layered protein tubes, where a sufficient amount of metal complexes can be accumulated during sample preparation. Received 6 December 2000     

On the spectroscopy of atomic electrons (positrons)in the energy range of 10 to 50 MeV from heavy-ion collisions at intermediate energies

The feasibility of the spectroscopy of dynamically ionized electrons (positrons) from heavy-ion collisions at intermediate energies, e.g. Pb+Pb at 60 AMeV has been studied. We propose a magnetic toroid spectrometer for lepton spectroscopy in an energy range between 5 and 50 MeV. Special emphasis was laid on large solid angles, on broad-band characteristics and on a good suppression of secondary events. The device is a versatile compact-size instrument for lepton detection in in-beam experiments at a moderate energy resolution of %. Received: 28 December 1999 / Accepted: 29 March 2000     

Intense electron emission from carbon fiber cathodes

We studied the emission properties of carbon fiber cathodes. These cathodes were made either of a single carbon fiber or of carbon fabric, or of an array of carbon fiber bundles. It was found that an intense emission of electrons occurs from a plasma which is formed on the carbon fiber surface as a result of a flashover process. In addition, the time delay in the appearance of the electron emission with respect to the start of the accelerating voltage pulse was found to depend strongly on the voltage growth rate. A simple model of the plasma formation is suggested. Received 12 March 2001     

Structure and frustration in liquid crystalline polyacrylates II. Thin-film properties

We report on the thin-film behaviour of a polyacrylate with phenyl benzoate mesogenic side groups and a narrow polydispersity. Depending on the degree of polymerisation, these polymers show a nematic, a smectic-A _d, a re-entrant nematic and a C phase with a two-dimensional monoclinic lattice. X-ray reflectivity and atomic-force microscopy have been used to characterize the structure and surface morphology. The system exhibits two stable side-chain packing configurations with incommensurate spacings that can be both stabilized at a free surface. Thin films in the nematic phase show a structural dewetting induced by the growth of surface domains of the C phase. Additionally, surface-induced ripples with a nanoscale lateral period form at the air-film interface. We attribute these patterns to a coupling between the local liquid crystalline ordering of the mesogenic side groups and the surface curvature energy. Received 28 February 2001 and Received in final form 6 August 2001     

Mechanism of grain-boundary magnetoresistance in Fe O films

The magnetotransport properties of magnetite films with different microstructures were investigated in order to identify prerequisites for the attainment of a large tunnelling magnetoresistance in polycrystalline samples. Epitaxial films on MgAl₂O₄, polycrystalline films on Al₂O₃ and rough MgAl₂O₄ substrates and a polycrystalline La_0.7Ca_0.3MnO₃ film on MgO were compared. Although grain boundaries induce a large high-field magnetoresistance in magnetite films, the low-field magnetoresistance characteristic for spin-polarized tunnelling was virtually absent in these samples. Two factors might be responsible for this behaviour: (1) grain boundaries in magnetite are conducting and do not form tunnelling barriers and (2) the spin-polarization near grain boundaries is suppressed due to non-stoichiometry. Received 15 April 2002 Published online 13 August 2002     

Stress fluctuations and macroscopic stick-slip in granular materials

This paper deals with the quasi-static regime of deformation of granular matter. It investigates the size of the Representative Elementary Volume (REV), which is the minimum packing size above which the macroscopic mechanical behaviour of granular materials can be defined from averaging. The first part uses typical results from recent literature and finds that the minimum REV contains in general 10 grains; this result holds true either for most experiments or for Discrete Element Method (DEM) simulation. This appears to be quite small. However, the second part gives a counterexample, which has been found when investigating uniaxial compression of glass spheres which exhibit stick-slip; we show in this case that the minimum REV becomes 10⁷ grains. This makes the system not computable by DEM. Moreover, similarity between the Richter law of seism and the exponential statistics of stick-slip is stressed. Received 8 March 2002 and Received in final form 12 July 2002