Showing papers in "Journal of Physics C: Solid State Physics in 1994"

Cluster Approach to Order-Disorder Transformations in Alloys

Abstract: Publisher Summary This chapter presents the formalism and applications of cluster expansions to the problem of ab initio calculations of thermodynamic properties of crystalline alloys. The success of the method owes much to the parallel developments of statistical and quantum mechanical methods and, of course, to the availability of powerful computational hardware and software. The technique of orthogonal expansions in cluster functions has revolutionized the treatment of configurationally disordered systems. In addition to offering the most rational and general state-of-order description of alloys, cluster methods provide the rigorous and essential link between quantum and statistical mechanical aspects of first-principles thermodynamic calculations. Cluster functions constitute a complete orthonormal set. This basis set produces a rigorous cluster algebra that is used for systems containing an arbitrary degree of configurational order (or disorder). Cluster methods are, in principle, ideally suited for calculating alloy phase equilibria.

Giant Magnetoresistance in Magnetic Layered and Granular Materials

Abstract: Publisher Summary Magnetoresistance (MR) is the change in electrical resistance of a material in response to a magnetic field. All metals have an inherent, albeit small, MR owing to the Lorentz force that a magnetic field exerts on moving electrons. Although earlier studies reported unusual magnetoresistive effects in layered structures, it was discovered that the application of magnetic fields to atomically engineered materials known as magnetic superlattices greatly reduced their electrical resistance, that is, superlattices had a giant magnetoresistance. Superlattices are a special form of multilayered structures, artificially grown under ultrahigh-vacuum conditions by alternately depositing on a substrate several atomic layers of one element, say, iron, followed by layers of another, such as chromium. The original observation of giant magnetoresistance was made on iron–chromium superlattices with nearly perfect crystallinity, which was grown by molecular beam epitaxy (MBE). Giant magnetoresistance observed in layered and granular structures arises from the dependence of the resistivity on their internal (local) magnetic configuration.

Defects at Surfaces and Interfaces

Abstract: Publisher Summary This chapter discusses the crystallographic features of dislocations, with emphasis on their topological characteristics as constrained by the symmetry of their hosts. Microscopic observations of metallic, ceramic, semiconducting, superconducting, and composite materials have confirmed that dislocations are ubiquitous features in interfaces, occurring both as isolated discrete defects and in organized arrays. The chapter presents several crystallographic tools and methodology. It reviews several important (infinite) crystal structures and their space groups and outlines the assignment of space groups to (1) crystals exhibiting a planar surface and (2) bicrystals. The chapter also discusses the topological properties of defects in single crystals together with surface discontinuities and isolated interfacial defects. Only line defects arise in the class of manifest symmetry, namely, dislocations, disclinations, and dispirations. A broad range of defect types can arise in the broken symmetry class, and it is convenient to subdivide these into surface-like features (steps, demisteps, and facet junctions), and bulk-like defects (dislocations, disclinations, and dispirations). Finally, the chapter discusses several interface structures modeled by dislocation arrays.

Homogeneous Nucleation and the Temperature Dependence of the Crystal-Melt Interfacial Tension

Abstract: Publisher Summary This article presents an analysis of the crystal melt interfacial tension and its temperature dependence which emerged from application of classical nucleation theory to experiments on the kinetics of homogeneous nucleation of crystals in undercooled melts. The analysis shows that the temperature coefficients obtained from homogeneous nucleation experiments can be accounted for the entropy loss in the undercooled melts due to ordering near the crystal surface. Analysis of the equilibrium interface in the hard sphere system confirms that the interfacial entropy losses are sufficiently large to account entirely for the magnitude of the interfacial tension, because the enthalphic contribution in this system is by definition zero. The chapter also suggests that metallic and other simple liquids require remarkably deep undercoolings for the onset of measurable crystal nucleation. This behavior could be interpreted in terms of polytetrahedral models for melt structure.

Preparation of Fullerenes and Fullerene-Based Materials

Abstract: Publisher Summary This chapter discusses the preparation, isolation, and characterization of fullerene clusters, and solids based on these clusters. Several techniques are used to prepare crude mixtures of fullerenes, and isolate and characterize pure C. Simple resistive vaporization of graphite rods could produce fullerenes in substantial yield. This procedure is often termed the Kratschmer–Huffman method and is a straightforward and low cost method for generating large quantities of fullerene-containing carbon soot. There are several experimental factors that can be varied to maximize the yield of fullerene clusters in the soot produced from the vaporization of graphite electrodes. These factors include the vaporization current density and helium partial pressure. A second, very useful and powerful technique for producing fullerene clusters involves laser ablation of graphite in a helium atmosphere. Several other techniques, including hydrocarbon combustion, low-pressure helium sputtering, electron beam evaporation, and inductively coupled RF evaporation of graphite targets have been used to prepare fullerene containing carbon products. The chapter also discusses several approaches for preparing isotopically substituted clusters. Finally, the chapter discusses the status and prospects of several new fullerene building blocks, including endohedral metal clusters and carbon nanotubes.

Solid State Properties of Fullerenes and Fullerene-Based Materials

Abstract: Publisher Summary This chapter provides an introduction of fullerenes by comparing gas phase and solid state properties, examining thin film and bulk crystal growth, and doping with alkali metals and other atoms. The fullerenes are all-carbon molecules with a closed-cage structure and nearly spherical appearance. These cages are derived from 12 pentagons and an appropriate number of hexagons. The chapter also discusses the electronic structure of correlated-electron systems in fullerenes, relating the electronic properties and vibrational properties to superconductivity. Early descriptions of fullerene formation suggested that small carbon chains would grow by the sequential addition of carbon radicals. Finally, the chapter discusses the latest additions to the fullerene family—namely, endofullerenes, tubes, and the buckyonions.

Electrons and Phonons in C60-Based Materials

Abstract: Publisher Summary This chapter discusses the properties of C 60 -based materials. The C 60 molecule is characterized by 12 regular pentagons and 20 hexagons. There are two distinct types of bonds, one that separates two hexagons and one that separates a pentagon and a hexagon. The symmetry of the C 60 molecule is independent of whether these two lengths are equal. The chapter also discusses the interaction energy of two electrons on the actual C 60 molecule. It also discusses the spectral properties of the C 60 molecule and solids in the lowest approximation in terms of the one-electron eigenvalues of Hiickel, Hartree–Fock, or local density approximation (LDA) calculations. The linear electric polarizability and the hyperpolarizability of the C 60 molecule are properties of widespread interest. The C 60 solid is a collection of van der Waals-bonded molecules (a molecular solid), thus it is reasonable to consider the polarizability of the solid to arise from to a lattice of point dipoles and use the Clausius–Mossotti relationship between the crystal dielectric constant and the molecular polarizability. The size and carbon bonding of the C 60 molecule suggest the possibility of unusual nonlinear optical response.

The Physics and Mechanics of Brittle Matrix Composites

Abstract: Publisher Summary A major development in materials technology has been the synthesis of composites consisting of high strength particles, often in filamentary form dispersed in various solid matrices. This chapter discusses several composites in which the matrices, such as ceramics, glasses or intermetallic compounds, are brittle rather than ductile with elastic properties quite similar to those of the embedded fibers. Such fibers impart good tensile strength to the matrices, even in the presence of holes and notches. This characteristic is important because composite components generally need to be attached to other components, usually metals. At these attachments, stress concentrations arise, which dominate the design and reliability. Inelastic deformation at these sites is crucial. It alleviates the elastic stress concentration by locally redistributing stress. Such inelasticity is present in brittle matrix composites. In association with the inelastic deformation, various degradation processes occur that affect the useful life of these composites. The most severe degradation appears to occur subject to out-of-phase thermomechanical fatigue (TMF). In addition, creep and creep rupture also occur at high temperatures. The chapter also addresses the mechanisms of stress redistribution in these composites upon monotonic and cyclic loading, as well as the mechanics needed to characterize the notch. The basic phenomena that give rise to inelastic strains are matrix cracks and fiber failures subject to interfaces that debond and slide.

Structure and Dynamics of Crystalline C60

Abstract: Publisher Summary This chapter discusses the ideas underlying the description of molecular rotations in solids and the cooperative phenomena associated with phase transformations involving molecular rotations. The nature of the phase transition of C 60 and its description via a Landau treatment reveals a complex behavior due to the effects of rotational localization. The chapter also discusses the intermolecular potential in crystalline C 60 deduced through the combination of scattering methods and statistical mechanics. Molecular rotator functions and their thermodynamic averages provide the necessary parameters for describing the orientational dynamics and equations of state of C 60 . It is interesting that the C 60 molecule, while seemingly coupled rather weakly to its neighbors, experiences at room temperature a potential barrier height to free rotation. This activation barrier forbade a linearized treatment of the orientational distribution function.

Physical Properties of Metal-Doped Fullerene Superconductors

Abstract: Publisher Summary This chapter discusses the physical properties of metal-doped fullerene superconductors. The key component of the fullerene superconductors is the molecular cluster C 60 , or Buckminsterfullerene. The chapter provides an overview of the experimental status of the fullerene superconductors emphasizing on (1) the normal state and superconducting state phenomenology and (2) experimental probes of the microscopic mechanism of superconductivity in two new molecular superconductors (K 3 C 60 and Rb 3 C 60 ). The chapter also presents a few models to explain fullerene superconductivity. Theoretical models put forth to explain superconductivity in the fullerenes range from the conventional electron-phonon-mediated pairing model of Bardeen, Copper, and Schreiffer (BCS) to models in which pairing is mediated by electron correlation effects. Finally, within the context of these models, the chapter provides an overview of (1) the dependence of critical transition temperatures on lattice constant of C 60 , (2) the energy gap, (3) phonons, and (4) the isotope effect.