Superlattice Pattern Research Articles

Forthcoming articles

Recent experiments [A. Kudrolli, B. Pier, J.P. Gollub, Physica D 123 (1998) 99–111] on two-frequency parametrically excited surface waves produced an intriguing “superlattice” wave pattern near a codimension-two bifurcation point where both subharmonic and harmonic waves onset simultaneously, but with different spatial wave numbers. The superlattice pattern is synchronous with the forcing, spatially periodic on a large hexagonal lattice, and exhibits small-scale triangular structure. Similar patterns have been shown to exist as primary solution branches of a generic 12-dimensional D6+̇T2-equivariant bifurcation problem, and may be stable if the nonlinear coefficients of the bifurcation problem satisfy certain inequalities [M. Silber, M.R.E. Proctor, Phys. Rev. Lett. 81 (1998) 2450–2453]. Here we use the spatial and temporal symmetries of the problem to argue that weakly damped harmonic waves may be critical to understanding the stabilization of this pattern in the Faraday system. We illustrate this mechanism by considering the equations developed by Zhang and Viñals [J. Fluid Mech. 336 (1997) 301–330] for small amplitude, weakly damped surface waves on a semi-infinite fluid layer. We compute the relevant nonlinear coefficients in the bifurcation equations describing the onset of patterns for excitation frequency ratios of 23 and 67. For the 23 case, we show that there is a fundamental difference in the pattern selection problems for subharmonic and harmonic instabilities near the codimension-two point. Also, we find that the 67 case is significantly different from the 23 case due to the presence of additional weakly damped harmonic modes. These additional harmonic modes can result in a stabilization of the superpatterns.

Chlorine-induced de-reconstruction on Au(001) and Cl-adsorbed layers

Chlorine adsorption on the reconstructed Au(001) surface causes de-reconstruction to a 1*1 structure at room temperature above an absolute coverage of approximately 0.06. The de-reconstruction is considered to be due to the chemical-bonding effect between the adsorbate and the substrate in the same way as that in potassium adsorption on Au(001). Various superlattice patterns in low-energy electron diffraction are observed at low temperature ( approximately 100 K) which form a one-dimensional structure depending on coverage. The coverage dependence of the thermal desorption spectra for the Cl and Cl2 species suggests a frustrated desorption from one-dimensional chains formed by adsorbates.

The crystal structure of eugaAl

A polycrystalline specimen of nominal formula EuGaAl with unknown crystal structure was investigated by various electron microscope techniques; EDS-, SED-, and CBED data were taken on a Philips 400 electron microscope operated at 100kV, HREM data on a Hitachi 9000 microscope operated at 300kV. The EDS data confirmed the composition for the bulk of the material but, in addition, revealed particles with other fractions of the elements. Only the EuGaAl particles were further investigated. The unit cell was determined to be orthorhombic with a ratio: a/b=0.969(2) , a/c=0.234(2) and b/c=0.234(2). The lattice constants are a=4.54(5)Ȧ, b=4.68Ȧ and c=19.97(20)Ȧ. Based on systematic extinctions for hkl reflections with h+l=2n+l, the unit cell was found to be b-centered. CBED patterns of the [001], [100], and [010] zone axes are shown in Fig. 1. The zone axis patterns are in agreement with the above stated data except for diffused (2m+l,2n+l,0)- reflections, not compatible with the above stated systematic absences. But these occurred only occasionally in conjunction with a complicated noncommensurate superlattice pattern.

Epitaxially grown semiconductor surfaces

The atomic distributions on surfaces of Si, GaAs, HgTe and CdTe are studied as functions of temperature and substrate orientation in the [100], [111] and [111] directions. Surface entropy is calculated within the quasichemical approximation and the pair interaction energies are obtained using the tight-binding Green's function method. In most cases considered, the interactions between the atoms are attractive and they tend to congregate into islands for submonolayer coverage at temperatures below the roughening transition temperatures. Free energy curves for double-layer growth are presented. We find that growth in this case is mostly layer by layer as is often observed in atomic layer or molecular epitaxial growth. However, in cases such as Ga terminated [111] GaAs surfaces and most CdTe surfaces, the in-plane interactions are predicted to be repulsive. The calculated order-disorder transition temperatures for these cases are often much larger than usual growth temperatures and, consequently, incompletely filled surfaces are temperatures for these cases are often much larger than usual growth temperatures and, consequently, incompletely filled surfaces are expected to have domains in which atoms and vacancies arrange themselves in superlattice patterns.

Microdensitometer—computer correlation analysis of two distinct, spatially segregated classes of microtubule bridges in Allogromia pseudopodia

Previous video-light microscopic studies have shown that the microtubule bundles in the pseudopodia of foraminiferan protists display several types of movements in vivo, including active bending, zipping/splaying, and axial translocations. To gain insight into the types and arrangement of microtubule-associated proteins ( e.g., mechanoenzymes, crosslinkers) in such a highly dynamic system, we employed microdensitometric-computer correlation methods to analyze, quantitatively, intermicrotubule bridges in thin-section electron micrographs of Allogromia laticollaris and Allogromia sp. (strain NF). Two distinct bridges occupying mutually exclusive zones between adjacent microtubules were identified. Type I bridges displayed a single axial repeat (34 nm for A. laticollaris and 28 nm for Allogromia sp.) and Type II bridges showed a typical 12-dimer helical superlattice pattern. In A. laticollaris, the two types of bridges were morphologically distinct: Type I bridges were aligned perpendicular to the microtubule wall and were 23-nm wide with an electron-lucent core; Type II bridges were irregular filaments projecting from the microtubules at various angles. When compared with the known distribution of microtubule-associated proteins in other systems, our findings indicate that, in viuo, Allogromia pseudopodial microtubules are decorated with MAP2-like bridges interrupted by discrete clusters of a dynein-like component.

Molecular Dynamics Studies of Tweed and ω -Phase Instabilities in B2 NI62.5AL37.5 Alloys

AbstractDiffuse TEM and x-ray data has been reported by several research groups that has been interpreted in terms of both tweed and (ω -phase instabilities. Computer simulations of 1,024 atom arrays were run at various temperatures using the Voter-Chen Embedded Atom Method (EAM) interactions. If the extra Ni atoms were placed at random on the Al sub-lattice of the B2 structure, at low temperatures both <110>* and <112>* diffuse streaking were observed in reciprocal lattice plots, indicative of tweed and ω - phase instabilities, respectively. The atomic displacements causing the <110> diffuse streaking could generally be classified as {110}<1-10> local shears, in agreement with accepted models of tweed. However, the displacements producing the <112>* streaking were best described as coordinated atom row motions in <111> directions, which does not agree with traditional ω -phase models based on (111) interplanar collapse. Both types of diffuse streaking decreased with increasing temperature, but the <110> type was stronger at all temperatures and persisted to higher temperatures. Furthermore, the atomic displacements producing the streaking were static at low temperatures ( persistence times greater than 1000 vibrational periods) but became dynamic at higher temperatures ( persistence times of the order of a few vibrational periods). Another interesting result was that by merely rearranging the extra Ni atoms into an ordered superlattice pattern, virtually no diffuse streaking of any type was observed at any temperature, strongly suggesting that compositional disorder ( or other kinds of point defects) are essential to induce tweed instabilities.*The asterisk represents a quantity in reciprocal space.

Adsorption structure of CO on Pd (111) films and measurement of desorption energy of CO from the films

Pd (111) films were grown on mica by vapor deposition in ultra-high vacuum. Adsorption structure of CO on the surface of the films was investigated by LEED. When the Pd surface was exposed to CO at room temperature, the LEED pattern changed from the 1 × 1 original pattern of the clean Pd surface to a superlattice pattern c (4 × 2) with broad spots. The superlattice structure was completed with an exposure of 10 l of CO. After saturated coverage of CO on the Pd film was obtained, measurement of the isothermal desorption was carried out using Auger electron spectroscopy (AES). AES peak-to-peak heights as a function of time were measured on the surface, kept at constant temperatures of 90–130°C. The data were analyzed using first-order kinetics, and as an activation energy for desorption of CO from the Pd surface, a value of 24.0 ± 0.3 kcal mol −1 was obtained.

Change in oxide epitaxy on Ni(111): Effects of oxidation temperature

Studies of the oxidation of Ni(111) in the temperature range 30–275°C by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and ellipsometry have shown that the epitaxy of the oxide is dependent on the growth temperature. Below 150°C the commonly reported hexagonal p(2×2)-O and NiO(111) patterns are seen. For oxidation temperature above 200°C, LEED observations at low oxygen exposures reveal a superlattice pattern consistent with a lightly expanded NiO(100) lattice on Ni(111) with NiO[0 1 1] rotated 45° with respect to Ni[0 1 1]. This is followed by the development of an additional square pattern with NiO(100)‖Ni(111) and NiO[0 1 1]‖NiO[0 1 1]. NiO(111) covers the Ni substrate (as indicated by the disappearance of the Ni(111) LEED beams) after about 60 L of oxygen exposure. NiO(100), however, does not coalesce on Ni(111) after this exposure but rather thickens as large islands; the substrate beams remain visible after oxygen exposures of at least 500 L. The growth of the NiO(100) face above 200°C may be due to the instability of the (111) face of a crystal such as NiO while the (100) face is one of low surface energy.

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ∼0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The I 4 d 5 2 core electron binding energy in both cases decreases by ∼0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.

Electron Diffraction Study of TaS2 Intercalated with Pyridine

TaS 2 intercalated with pyridine was investigated by electron diffraction. The single-phase crystals showed streaks and diffuse spots. Besides this pattern mixed-phase crystals showed two types of superlattice patterns with rectangular cells \(9a \times 2 \sqrt{3}a\) and \(13a \times 2 \sqrt{3}a\).

Investigation of order in Ag xNbS 2 by electron diffraction

Electron diffraction investigation of single crystals of Ag xNbS 2 (x in the range 0.2 to 0.4) has shown that a variety of diffuse scattering and superlattice patterns is observed. The Ag is mobile in the crystals and some regions were observed where the Ag content had decreased to almost zero. In such regions, strong diffuse scattering was observed above a critical temperature, and below this temperature diffraction patterns corresponding to long-range orthorhombic superlattices were observed. A possible explanation of these observations is presented and discussed.

Intralayer crystal structure and order-disorder transformations of graphite intercalation compounds using electron diffraction techniques

We have used electron diffraction through the c-face to study the intercalate ordering of graphite-alkali metals (K, Rb, Cs) and graphite-halogens (Br 2, IBr, ICl). Order-disorder transformations for the in-plane structure of alkali metal and halogen intercalates have been observed by the vanishing of all superlattice diffraction spots as the sample temperature is raised. Variations of the electron diffraction patterns and of the order-disorder transformation temperatures with the intercalate species are reported. Compounds of various concentrations have been investigated, including the stage 1 graphite-alkali metals. Results obtained here are compared with structural information obtained by other techniques and by previous workers. The patterns for graphite-halogens are much more complicated than those for graphite-alkali metals. Of significance is the observation that the superlattice patterns are relatively insensitive to the intercalate concentration for compounds more dilute than stage 2 and are similar for lamellar and residue compounds. Additional superlattice patterns are observed in graphite-Br 2 below 194 °C and are similar in appearance to charge-density wave superlattice patterns reported for 2H-TaSe 2.

Charge density waves in intercalated 1T-TaS2

Abstract Electron diffraction studies of alkali-metal (Li, Na and K) and rare-earth metal (Eu) intercalates of lT-TaS2 reveal the existence of a number of complex super-lattice patterns which are interpreted in terms of a coupled charge density wave and periodic lattice distortion arising from a Kohn anomaly, rather than in terms of a simple ordering scheme for the intercalate ions. The results presented here are consistent with recent structural studies and band structure calculations for 1T-TaS2.

Ultramicrostructural characteristics of some intercalates of graphite: An electron microscopic study

The ultramicrostructure of certain graphite-intercalate residue compounds known to exhibit ordering has been investigated using high-resolution transmission electron microscopy and electron diffraction. Some novel ternary graphite-intercalates formed as a result of interactions between existing intercalates and other guest species have also been investigated. The superlattice patterns associated with the ordering of the intercalating species are readily interpretable, and it has been possible to obtain direct lattice images of the{101¯0} graphite (0.21 nm) of the {100} intercalated FeCl 3 (0.52 nm) and of{101¯0} intercalated potassium (0.36 nm). It is shown that potassium acts as an effective reducing agent, and, in the graphite-ferric chloride-potassium system, free iron and potassium chloride are formed. The catalytic activity of this ternary intercalate is probably associated with highly dispersed free iron.