Crystalline Configuration Research Articles

Growth and characterization of Fe nanostructures on GaN

We have investigated the growth of Fe nanostructures on GaN(0 0 0 1) substrates at room temperature using reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and superconducting quantum interference device magnetometer. Initially, a ring RHEED pattern appears, indicating the growth of polycrystalline α-Fe. At around 0.5 nm deposition, the surface displays a transmission pattern from α-Fe films with the epitaxial relationship of Fe(1 1 0)//GaN(0 0 0 1) and Fe[1 −1 1]//GaN[1 1 −2 0] (Kurdjumov–Sachs (KS) orientational relationship). Further deposition to 1 nm results in the appearance of a new spot pattern together with the pattern from domains with the KS orientation relationship. The newly observed pattern shows that Fe layers are formed with the epitaxial relationship of Fe(1 1 0)//GaN(0 0 0 1) and Fe[0 0 1]//GaN[1 1 −2 0] (Nishiyama–Wasserman (NW) orientational relationship). From STM images for Fe layers with the KS and NW orientational relationships, it can be seen that Fe layers with the KS relationship consist of round-shaped Fe nanodots with below 7 nm in average diameter. These nanodots coalesce to form nanodots elongating along the Fe[1 0 0] direction, and they have the KS orientational relationship. Elongated Fe nanodots with the NW relationship show ferromagnetism while round-shaped Fe nanodots with the KS relationship show super-paramagnetic behavior. We will discuss their magnetic properties in connection with the change in crystalline configurations of nanodots.

Sculpted Amphiphilic Liposomal Particles for Modifiable Medicinal Applications

The surface coverage and leaflet anchoring of polymeric surfactants and other amphiphiles can be used to provide a particle with extensive modifiable 'stealth' properties. This is not a new conceptual tool but one which has significant strategic advantage and recent proven clinical application in a "drug-delivery-package". This has been established and widely reported as a fundamental basis for optimal utility with long-circulation drug nanoparticles. The indirect "disguising" of the medicinal payload presents itself as a form of "pro-drug" system that can be used to extend circulation lifetime and that is key to a number of essential current and future risk-bearing therapies. Appropriate particle sizing and leaflet chemical structuring, in terms of number of strata, complexity, extent and curvature can also be used as the means to "drug anchoring" for the incorporation of "hydrophobic" drugs. Actives that are chemo- or solvent labile can also be housed, often routinely, within the inner-protected chamber of such capsular systems. This has frequently formed the basis of a number of recently developed commercial preparations that are used ubiquitously in now standard chemotherapy. Additionally, the use of complementary and non-complementary phospholipid or other vesicle ingredient mixtures below, at or above the lipid "gel transition" temperature or within a "glassy" crystalline packing configuration can provide a particle of varied physical and mechanical composition that provides a possible route to the production of fusogenic nano-drugs or control over the drug release profile. Successive layering of the nanoparticle can in principle provide the means to a step-wise or timed release of the particle contents and one that can be adapted to the site of intended use.

Biological observations on the common guitarfish Rhinobatos rhinobatos from İskenderun Bay (Turkey, Eastern Mediterranean)

Abstract Histological examination of the gonads showed that spermatogenesis occurs in every male R. rhinobatos collected in autumn and winter months of the year. The testis was composed of lobes having numerous spermatocysts in a dorsoventral zonated arrangement. Present results suggest that since R. rhinobatos have a cystic mode of spermatogenesis it can be used as a model for biomedical and toxicological studies. Eight stages of maturation for females and seven stages of maturation for males were identified. Moreover, vitellogenic oocytes were consistently found in every female collected during the summer months. In vitellogenic oocytes, yolk is present as pleomorphic droplets and very often; small yolk droplets have a crystalline configuration. These data suggest that both sexes are reproductively active at different times of the year. Both, the right ovary/testis and the left ovary/testis were functional. Ripe oocytes in the ovaries were usually symmetrically distributed. The population consisted of aplacental viviparous individuals with lecithotrophic embryo development. Although, sex ratios of R. rhinobatos were not significantly different from the expected 1:1 ratio (p>0.05), slightly female biased sex ration were recorded.

Frustrated self-assembly of dendron and dendrimer-based supramolecular liquid crystals

A new “inverted” topological configuration is demonstrated both experimentally and theoretically for self-assembled dendron and dendrimer-based supramolecular liquid crystals in which the dendrons/dendrimers occupy the continuous domain and the ionically attached pendant chains are confined in discrete domains. All previous studies on dendrimer and dendron-based liquid crystals have reported “normal” liquid crystalline configurations in which the dendritic templates occupy discrete domains (in spherical or columnar phases) or continuous struts (in bicontinuous cubic phases), while the pendant chains occupy the continuous space-filling domain. These surprising results mandate a re-examination of the packing mechanisms for this important class of materials and open new routes to unique nanostructures of possible use in existing and emerging technologies.

Structure and melting behavior of classical bilayer crystals of dipoles

We study the structure and melting of a classical bilayer system of dipoles in a setup where the dipoles are oriented perpendicular to the planes of the layers and the density of dipoles is the same in each layer. Due to the anisotropic character of the dipole-dipole interactions, we find that the ground-state configuration is given by two hexagonal crystals positioned on top of each other, independent of the interlayer spacing and dipolar density. For large interlayer distances these crystals are independent, while in the opposite limit of small interlayer distances the system behaves as a two-dimensional crystal of paired dipoles. Within the harmonic approximation for the phonon excitations, the melting temperature of these crystalline configurations displays a nonmonotonic dependence on the interlayer distance, which is associated with a re-entrant melting behavior in the form of solid-liquid-solid-liquid transitions at fixed temperature.

Bubble-raft model for a paraboloidal crystal

We investigate crystalline order on a two-dimensional paraboloid of revolution by assembling a single layer of millimeter-sized soap bubbles on the surface of a rotating liquid, thus extending the classic work of Bragg and Nye on planar soap bubble rafts. Topological constraints require crystalline configurations to contain a certain minimum number of topological defects such as disclinations or grain boundary scars whose structure is analyzed as a function of the aspect ratio of the paraboloid. We find the defect structure to agree with theoretical predictions and propose a mechanism for scar nucleation in the presence of large Gaussian curvature.

Structural study of molten Ag halides and molten AgCl–AgI mixture

The structures of molten Ag halides and pseudo-binary mixtures between AgCl and AgI have been investigated by neutron diffraction measurements. For AgI, a crystalline local configuration persists even in the molten state while, for AgCl and AgBr, the first neighbor coordination number decreases to 3.4 compared with 6 in the crystalline rock salt structure and the second neighbor distribution is modified according to the shortening of Ag–halogen distance. This difference between AgI and AgCl (AgBr) may be originated from the nature of unlike-pair bonding. The former has much covalent character. The eutectic mixture between AgCl and AgI, AgCl0.43I0.57, exhibits large temperature dependence in the total structure from the AgCl like feature at a lower temperature to the AgI like feature at a high temperature. As increasing temperature, Cl ions become more diffusive and relatively strong interaction between Ag and I rules physical property such as sound absorption.

R. Kerner: Models of Agglomeration and Glass Transition

A significant blurring of boundaries between the dynamic behavior of deterministic systems and those of random systems is one of the major achievements of modern statistical physics. Indeed, the phenomenon of deterministic chaos demonstrates stochastic behavior in nonlinear deterministic systems, while stochastic resonance implies its converse; deterministic phenomena in random systems. In a similar manner, the book here being reviewed softens the difference between crystalline and amorphous configurations through its discussion of “intermediate” self-similar structures. The latter of these are characterized by the property that at different scales similar structures are found which differ from respective atomic structures in crystals, as well as from entirely different structures in amorphous materials. In addition to known glass systems, a new experimental impetus for studying these materials was stimulated by the recent discovery of quasicrystalline lattices and fullerene C60 molecules. Due to the many possible local possible arrangements of these systems one has necessarily to use a statistical rather than a deterministic description to fully describe their properties. As the author states in a lively Introduction reviewing the history and basic features of the large field of condensed matter physics being discussed the main goal of the book is to present a unified and coherent approach to explaining general features of agglomeration and growth of local atomic structures. The novelty of the proposed approach to modeling various atomic structures appears to be in using mathematical models applied in very different fields of physics, exemplified by the theory of dynamical systems and field theory. The approach is sufficiently general to give rise to mathematical models of agglomeration and growth of different structures, including those of quasicrystals, fullerenes and glasses, and for the latter, to discuss phenomenological models of glass transitions. The general properties of atomic and molecular networks are considered in Chapters 2–5, including their topological and stochastic properties, stochastic agglomeration and

Nonlinear modulated dust lattice wave packets in two-dimensional hexagonal dust crystals

The amplitude modulation of dust lattice waves (DLWs) propagating in a two-dimensional hexagonal dust crystal is investigated in a continuum approximation, accounting for the effect of dust charge polarization (dressed interactions). A dusty plasma crystalline configuration with constant dust grain charge and mass is considered. The dispersion relation and the group velocity for DLWs are determined for wave propagation in both longitudinal and transverse directions. The reductive perturbation method is used to derive a (2+1)-dimensional nonlinear Schrödinger equation (NLSE). New expressions for the coefficients of the NLSE are derived and compared, for a Yukawa-type potential energy and for a “dressed” potential energy, taking into account interaction and geometric nonlinearities.

Derivation of a rod theory for multiphase materials

A rigorous derivation is given of a rod theory for a multiphase material, starting from three-dimensional nonlinear elasticity. The stored energy density is supposed to be nonnegative and to vanish exactly on a set consisting of two copies of the group of rotations SO(3). The two potential wells correspond to the two crystalline configurations preferred by the material. We find the optimal scaling of the energy in terms of the diameter of the rod and we identify the limit, as the diameter goes to zero, in the sense of Γ-convergence.

Crystallization of Random Trigonometric Polynomials

We give a precise measure of the rate at which repeated differentiation of a random trigonometric polynomial causes the roots of the function to approach equal spacing. This can be viewed as a toy model of crystallization in one dimension. In particular we determine the asymptotics of the distribution of the roots around the crystalline configuration and find that the distribution is not Gaussian.

A comparison between EAM interatomic potentials for Al and Ni: from bulk systems to nanowires

AbstractTwo different kinds of interatomic potentials within the Embedded Atom Method (EAM) have been used to study several properties of selected crystalline structures and nanowire configurations (ordered and helical) for Al and Ni based systems. Reliability of these potentials has been explored when describing cohesive energy and geometrical properties of the systems under consideration as the atomic coordination number decreases. Results provide a criteria for stablishing the limits of validity of EAM potentials when applied to such systems as metallic ultra‐narrow or single atom nanowires. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dust lattice wave dispersion relations in two-dimensional hexagonal crystals including the effect of dust charge polarization

A dusty plasma crystalline configuration with equal charge dust grains and mass is considered. Both charge and mass of each dust species are taken to be constant. Two differential equations for a two-dimensional hexagonal crystal on the basis of a Yukawa-type potential energy and a “dressed” potential energy, accounting for dust charge polarization, are derived and compared. The dispersion relation for both longitudinal and transverse wave propagation in an arbitrary direction is derived. A comparison to analytical and experimental results reported previously is carried out.

Linear and nonlinear dynamics of a dust bicrystal consisting of positive and negative dust particles

A dusty plasma crystalline configuration consisting of charged dust grains of alternating charge sign (⋯∕+∕−∕+∕−∕+∕⋯) and mass is considered. Both charge and mass of each dust species are taken to be constant. Considering the equations of longitudinal motion, a dispersion relation for linear longitudinal vibrations is derived from first principles and then analyzed. Two harmonic modes are obtained, namely, an acoustic mode and an inverse-dispersive optic-like one. The nonlinear aspects of acoustic longitudinal dust grain motion are addressed via a generalized Boussinesq (and, alternatively, a generalized Korteweg–de Vries) description.

Effects of the type and structure of modified clays on adsorption performance

The structure and adsorption performance of various clays, bentonite, montmorillonite K10 and montmorillonite KSF, were investigated in this study. The raw clays were modified by polymeric Fe/Al species, hexadecyl‐trimethylammonium (HDTMA) surfactant and a complex of Fe/Al‐HDTMA. The X‐ray diffraction spectra (XRD) and X‐ray fluorescence spectra (XRF) were applied to analysethe structure of the raw and modified clays. Cu and phenol were selected as adsorbates for evaluating the adsorption performance of the raw and modified clays. The results indicated that the basal spacing of the bentonite and montmorillonite K10 reduced to a small extent after modification, and the crystalline configuration may be changed due to the modification process. The greater basal spacing could improve the Cu adsorption but it will be compromised by the physicochemical surface properties of the modified clays when the modifiers posses the specific complexing capacity to the inorganic/organic contaminants. When the basal spacing was smaller, however, the predominant adsorption mechanism could be chemical complexing between clay adsorbents and contaminants. In general the inorganic contaminant (e.g. Cu) tends to be adsorbed by the polymeric Fe/Al species while the organic impurity (e.g. phenol) will be preferably captured by the HDTMA modifier.

Formation of large NAT particles and denitrification in polar stratosphere: possible role of cosmic rays and effect of solar activity

Abstract. The formation of large nitric acid trihydrate (NAT) particles has important implications for denitrification and ozone depletion. Existing theories have difficulty in explaining the formation of large NAT particles at temperatures above the ice frost point, which has been observed recently over wide Arctic regions. Our analyses reveal that high-energy comic ray particles might induce the freezing of supercooled HNO3-H2O-H2SO4 droplets when they penetrate these thermodynamically unstable droplets. The cosmic ray-induced freezing (CRIF) appears to be consistent with the observed, highly selective formation of NAT particles. We suggest a possible physical process behind the CRIF mechanism: the reorientation of polar solution molecules into the crystalline configuration in the strong electrical fields of moving secondary ions generated by passing cosmic rays. A simple formula connecting the CRIF rate to cosmic ray flux is derived with an undefined parameter constrained by observed NAT formation rates. Our simulations indicate that strong solar proton events (SPEs) may significantly enhance the formation of large NAT particles and denitrification. The CRIF mechanism offers a possible explanation for the observed high correlations between the thin nitrate-rich layers in polar ice cores and major SPEs, and the observed enhancement in the aerosol backscattering ratio at PSC layers shortly after an SPE and the significant precipitation velocity of the enhanced PSC layers. The key uncertainty in the CRIF mechanism is the probability (P) of freezing when a CR particle hits a thermodynamically, unstable STS droplet. Further studies are needed to either confirm or reject the CRIF hypothesis.

Stability of thermally-induced martensitic transformations in Bi-atomic lattices

A well-known feature of some metallic alloys, such as NiTi, is their thermally induced, stress-dependent shape memory behavior. These alloys' remarkable properties are due to one or more martensitic transformations near room temperature, in which the crystalline configuration changes from a higher symmetry austenite (cubic lattice), to a lower symmetry martensite (rhombohedral, orthorhombic, tetragonal or monoclinic lattice) with decreasing temperature. In contrast to existing phenomenological approaches, the present work constructs a continuum energy density function W(F;θ) (as a function of a uniform deformation gradient and temperature) of a perfect periodic bi-atomic lattice from temperature dependent atomic potentials. Of interest in this work are the equilibrium solutions and their stability as functions of temperature for crystals under an applied pressure. Although the full problem is solved numerically, a post-bifurcation asymptotic analysis is necessary to guide the numerical solution near multiple bifurcation points. For the particular choice of a Morse-type pair potential, two stable cubic phases are predicted, one that corresponds to austentite (CsCl structure), which is stable at higher temperatures, and one that has a denser packing (NaCI structure), which is stable at lower temperatures. Theses stable portions overlap at intermediate temperatures, which is suggestive of a hysteretic temperature-induced martensitic transformation. Lower symmetry crystals, such as orthorhombic, monoclinic, and rhombohedral structures, are also predicted.

Competition between crystallization and gelation: A local description

We develop a model to describe the competition between gelation and crystallization in colloidal suspensions where particle interactions are represented by square well attractions. The competition is discussed locally in terms of the tendencies of individual particles to attain amorphous or crystalline configurations on cluster surfaces. These tendencies are dictated by three independent processes, the aggregation of particles onto, the dissociation of particles from, and the rearrangement of particles on the cluster surfaces. Models are developed to determine the rates of each of these processes. The relative magnitudes of these rates determine the probability that a particle arriving onto a cluster surface reaches a crystalline configuration, remains arrested in an amorphous configuration, or dissociates back into the suspension. These probabilities are employed to determine whether stable crystalline or amorphous clusters nucleate, resulting in predictions of the occurrence of crystallization or gelation as a function of solution conditions. Comparisons of model predictions with recent experiments on globular protein suspensions show excellent agreement, suggesting that the model captures much of the underlying physics of the competition between gelation and crystallization in attractive colloidal suspensions.

Cooling rate dependence of the glass transition temperature of polymer melts: Molecular dynamics study

A coarse-grained bead spring model of short polymer chains is studied by constant pressure molecular dynamics (MD) simulations. Due to two competing length scales for the length of effective bonds and the energetically preferred distance between nonbonded beads, one observes a glass transition when dense melts are cooled down (as shown in previous work, at a pressure p=1 the mode coupling critical temperature is at Tc≈0.45 and the Vogel–Fulcher temperature is T0≈0.33, in Lennard-Jones units). The present work extends these studies, estimating a cooling-rate-dependent glass transition temperature Tg(Γ) by cooling the model system from T=0.6 down to T=0.3, applying cooling rates from Γ≈10−3 to Γ≈10−6 (in MD time units), and attempting to identify Tg(Γ) from a kink in the volume versus temperature or potential energy versus temperature curves. It is found that Tg(Γ) lies in the range 0.43⩽Tg(Γ)⩽0.47, for the cooling rates quoted, and the variation of Tg(Γ) for Γ is compatible with the expected logarithmic variations. We will show why a detailed distinction between competing theories on these cooling rate effects would need an excessive amount of computer time. To estimate also the melting transition temperature Tm of this model, the sytem was prepared in a crystalline configuration as an initial state and heated up. The onset of diffusion, accompanied by an isotropization of the pressure tensor was observed for Tm≈0.77. This implies that the model is suitable for studying deeply supercooled melts.

Atomistic simulations of jog migration on extended screw dislocations

We have performed large-scale atomistic simulations of the migration of elementary jogs on dissociated screw dislocations in Cu. The local crystalline configurations, transition paths, effective masses, and migration barriers for the jogs are determined using an interatomic potential based on the Effective Medium Theory. The minimum energy path through configuration space and the corresponding transition state energy are obtained using the Nudged Elastic Band path technique. We find very similar migration properties for elementary jogs on the (110){111} octahedral slip systems and the (110){110} non-octahedral slip systems, with energy barriers in the 15–19 meV range.