Showing papers in "Journal of Physics: Condensed Matter in 2001"

Reverse Monte Carlo modelling

Abstract: Reverse Monte Carlo (RMC) modelling is a general method of structural modelling based on experimental data. RMC modelling can be applied to many different sorts of data, simultaneously if wished. Powder and single-crystal neutron diffraction (including isotopic substitution), x-ray diffraction (including anomalous scattering) and electron diffraction, extended x-ray absorption fine structure and nuclear magnetic resonance (magic angle spinning and second moment) have already been used to provide data. RMC modelling can also be applied to many different types of system - liquids, glasses, polymers, crystals and magnetic materials. This article outlines the RMC method and discusses some of the common misconceptions about it. It is stressed that RMC models are neither unique nor `correct'. However, they are often useful for aiding our understanding either of the structure itself, or of the relationships between local structure and other physical properties. Examples are given and the possibilities for further development of the RMC method are discussed.

Heavy-fermion superconductivity in CeCoIn5 at 2.3 K

Abstract: We report the observation of heavy-fermion superconductivity in CeCoIn5 at Tc = 2.3 K. When compared to the pressure-induced Tc of its cubic relative CeIn3 (Tc~200 mK), the Tc of CeCoIn5 is remarkably high. We suggest that this difference may arise from magnetically mediated superconductivity in the layered crystal structure of CeCoIn5.

Confinement effects on freezing and melting

Abstract: A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.

Magnetism of FE, CO and NI nanowires in self-assembled arrays

Abstract: Recent work on magnetic properties of transition-metal nanowire arrays produced by electro-deposition is reviewed. The wires, which are electro-deposited into self-assembled porous anodic alumina, form nearly hexagonal arrays characterized by wire diameters down to less than 10 nm, wire lengths up to about 1 µm, and variable centre-to-centre spacings of the order of 50 nm. The fabrication and structural characterization of the arrays is summarized, magnetic data are presented and theoretical explanations of the behaviour of the wires are given. Emphasis is on extrinsic phenomena such as coercivity, magnetization reversal and interactions of the magnetic nanowires. In particular, we analyse how wire imperfections give rise to magnetic localization and dominate the hysteresis behaviour of the wires. Potential applications are outlined in the last section.

How do Fermi liquids get heavy and die

Abstract: We discuss non-Fermi liquid and quantum critical behaviour in heavy-fermion materials, focusing on the mechanism by which the electron mass appears to diverge at the quantum critical point. We ask whether the basic mechanism for the transformation involves electron diffraction off a quantum critical spin-density wave, or whether a breakdown in the composite nature of the heavy electron takes place at the quantum critical point. We show that the Hall constant changes continuously in the first scenario, but may `jump' discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.

Encapsulation of biomolecules in silica gels

Abstract: A wide variety of biomolecules, ranging over proteins, enzymes, antibodies and even whole cells, have been embedded within sol-gel glasses They retain their bioactivity and remain accessible to external reagents by diffusion through the porous silica Sol-gel glasses can be cast into desired shapes and are optically transparent, so it is possible to couple optics and bioactivity to make photonic devices and biosensors The high specificity and sensitivity of enzymes and antibodies allows the detection of traces of chemicals Entrapped living cells can be used for the production of metabolites, the realization of immunoassays and even for cell transplantation

Elasticity of hexagonal BeO

Abstract: We study the elastic properties, electronic structure, and equation of state of BeO using a first-principles pseudopotential method within the gradient-corrected approximation of the density functional theory. Comparison of the calculated and experimental properties of BeO shows good agreement for all the properties studied here: ground-state structure, linear and bulk compressibilities, and elastic moduli. Calculations are also performed with the local density approximation and the differences in elastic properties are interpreted in terms of a uniform compression. Analysis of the pressure effect on the lattice parameters and on the atomic coordinates shows that the structure changes are close to isotropic from zero to 100 GPa.

Gaining control of pattern formation of dewetting liquid films

Abstract: Thin liquid films on non-wettable solid surfaces are not stable; rather, they are transformed by a symmetry-breaking process termed `dewetting' into their equilibrium state, a set of droplets. The morphologies observed upon dewetting contain information about the kind of symmetry-breaking process. In this study, we report on experiments on a model system, thin (2-80 nm) polystyrene films dewetting solid substrates, the wettability of which can be varied. We characterize and classify the emerging dewetting patterns. With the help of the effective interface potential, which we determined for our experimental system, we discuss the interplay of short- and long-range forces and the possibilities for influencing the stability of the liquid. Our experimental findings are also in accordance with recent three-dimensional numerical simulations of other groups.

III nitrides and UV detection

Abstract: III nitrides have become the most exciting challenge in optoelectronic materials in the last decade. Their intrinsic properties and an intense technological effort have made possible the fabrication of reliable and versatile detectors for short wavelengths. In this work, materials and devices issues are considered to provide a full picture of the advances in nitride UV photodetection. First, basic structures like photoconductors, Schottky, p-i-n and metal-semiconductor-metal photodiodes and phototransistors are compared, with emphasis on their specific properties and performance limitations. The efforts in the design and fabrication of more advanced detectors, in the search for higher quantum efficiency, contrast, signal-to-noise or speed operation, are reviewed afterwards. Metal-insulator-semiconductor diodes, avalanche photodetectors and GaN array detectors for UV imaging are also described. Further device optimization is linked with present materials issues, mainly due to the nitride quality, which is a direct result of the substrate used. The influence of substrates and dislocations on detector behaviour is discussed in detail. As an example of AlGaN photodetector applications, monitoring of the solar UV-B radiation to prevent erythema and skin cancer is presented.

Colossal magnetoresistive manganite thin films

Abstract: Mixed-valence perovskite manganites (Re1-xAxMnO3 where Re = rare earth, A = alkaline earth) provide a unique opportunity to study the relationships between the structure and the magnetotransport properties due to an interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion. This makes these compounds very exciting from both the basic research and from the technological viewpoint. As the technology pursued with these materials requires film growth, extensive studies have been made on materials synthesis, structural and physical characterization and device fabrication. In this article, the results from the different experimental techniques and the effects of the deposition procedure of the manganite thin films are first reviewed. Second, the relation between the structural and the physical properties is mentioned, and the influence of strains discussed. Finally, possible applications of manganite thin films in spin electronics are presented.

Spectroscopic characterization of thiol-derived self-assembling monolayers

Abstract: This article reviews recent progress in the spectroscopic characterization of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on noble metal substrates. Several complementary techniques such as near edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy were applied to study the balance between intermolecular and adsorbate-substrate interactions, chemical identity of the headgroup, and absorption site homogeneity at the sulphur-metal interface. Whereas in the thioaliphatic SAMs the headgroup-substrate interaction was found to be a decisive factor for the structure and packing in these films, these parameters are mainly determined by the intermolecular interactions in the thioaromatic films. Only one sulphur species could be detected in the S 2p HRXPS spectra of both aliphatic and aromatic SAMs suggesting binding of individual molecules as thiolates. Conclusions on the heterogeneity of the adsorption sites are derived and evidence that the investigated films represent highly correlated molecular assemblies are presented.

Colloidal soft matter under external control

Abstract: The physics of soft-matter systems controlled by external fields is reviewed and previewed. Particular emphasis is placed on statistical properties of well-characterized colloidal dispersions in different confining situations, in laser-optical, magnetic and electric fields as well as under shear. These are very active research areas where different complementary methods such as experiments, computer simulations and theory have been applied in parallel. Recently discovered novel phase transitions, generated and triggered by an external field, are described and the perspectives in this field for the next decade are discussed.

Surface polaritons of a left-handed material slab

Abstract: The dispersion relations of the surface polaritons of a slab made of a material which has dispersive permittivity and permeability, and is left-handed over a frequency band in the microwave range of several GHz, are investigated. Four branches of p polarized surface polaritons and two branches of s polarized ones are found to exist. The possibility of experimentally observing the surface polaritons by the attenuated total reflection method is demonstrated.

Physical properties of RCo2 Laves phases

Abstract: The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

Friction experiments on the nanometre scale

Abstract: In this review, we present various results obtained by friction force microscopy in the last decade. Starting with material-specific contrast, commonly observed in friction force maps, we discuss how the load dependence of friction and the area of contact have been estimated and compared to elasticity theories. The features observed in a sliding process on the atomic scale can be interpreted within the Tomlinson model. An extension of the model, including thermal effects, predicts a smooth velocity dependence of friction, which recent experiments have confirmed. Other subjects like anisotropy of friction, role of environment, topographical effects, electronic friction and tip modifications are also discussed. The growing importance of molecular dynamics simulations in the study of tribological processes on the atomic scale is outlined.

Quasi-two-dimensional Fermi surfaces and the de Haas-van Alphen oscillation in both the normal and superconducting mixed states of CeCoIn5

Abstract: We observed de Haas-van Alphen (dHvA) oscillation in both the normal and superconducting mixed states of a heavy-fermion superconductor CeCoIn5. The Fermi surfaces are found to consist of nearly cylindrical Fermi surfaces and small ellipsoidal ones, reflecting the unique tetragonal crystal structure. The detected cyclotron masses of 5-87 m0 for these Fermi surfaces are extremely large, and correspond to a large electronic specific heat coefficient of about 1000 mJ K-2 mol-1. The cyclotron masses are also found to be field dependent in both the normal and mixed states.

Influence of electron-phonon interaction on the optical properties of III nitride semiconductors

Abstract: The electronic band structures of III nitride semiconductors calculated within the adiabatic approximation give essential information about the optical properties of materials. However, atoms of the lattice are not at rest; their displacement away from the equilibrium positions perturbs the periodic potential acting on the electrons in the crystal, leading to an electron-phonon interaction energy. Due to different ways that the lattice vibration perturbs the motions of electrons, there are various types of interaction, such as Frohlich interaction with longitudinal optical phonons, deformation-potential interactions with optical and acoustic phonons and piezoelectric interaction with acoustic phonons. These interactions, especially the Frohlich interaction, which is very strong due to the ionic nature of III nitrides, have a great influence on the optical properties of the III nitride semiconductors. As a result of electron-phonon interaction, several phenomena, such as phonon replicas in the emission spectra, homogeneous broadening of the excitonic line width and the relaxation of hot carriers to the fundamental band edge, which have been observed in GaN and its low dimensional heterostructures, are reviewed.

Full potential linearized augmented plane wave calculations of structural and electronic properties of BN, BP, BAs and BSb

Abstract: A theoretical study of structural and electronic properties of boron compounds BN, BP, BAs and BSb is presented, using the full potential linearized augmented plane wave method. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. Ground state properties such as lattice parameter, bulk modulus and its pressure derivative are calculated as well as structural transition pressure. The band structure is obtained for both zincblende and rocksalt structures. We also give the valence charge density at equilibrium lattice constant and at transition pressure. We show from the latter quantity the inverse role between cation and anion for BP, BAs and BSb. Results are discussed and compared with experimental and other theoretical data with reasonable agreement.

Analytic modified embedded atom potentials for HCP metals

Abstract: Analytic modified embedded atom method (AMEAM) type many-body potentials have been constructed for ten hcp metals: Be, Co, Hf, Mg, Re, Ru, Sc, Ti, Y and Zr. The potentials are parametrized using analytic functions and fitted to the cohesive energy, unrelaxed vacancy formation energy, five independent second-order elastic constants and two equilibrium conditions. Hence, each of the constructed potentials represents a stable hexagonal close-packed lattice with a particular non-ideal c/a ratio. In order to treat the metals with negative Cauchy pressure, a modified term has been added to the total energy. For all the metals considered, the hcp lattice is shown to be energetically most stable when compared with the fcc and bcc structure and the hcp lattice with ideal c/a. The activation energy for vacancy diffusion in these metals has been calculated. They agree well with experimental data available and those calculated by other authors for both monovacancy and divacancy mechanisms and the most possible diffusion paths are predicted. Stacking fault and surface energy have also been calculated and their values are lower than typical experimental data. Finally, the self-interstitial atom (SIA) formation energy and volume have been evaluated for eight possible sites. This calculation suggests that the basal split or crowdion is the most stable configuration for metals with a rather large deviation from the ideal c/a value and the non-basal dumbbell (C or S) is the most stable configuration for metals with c/a near ideal. The relationship between SIA formation energy and melting temperature roughly obeys a linear relation for most metals except Ru and Re.

Phenomenological description of the diffuse phase transition in ferroelectrics

Abstract: Features of the diffuse phase transition in lead magnesium niobate and strontium barium niobate, typical relaxor ferroelectric materials, were studied as a function of temperature and frequency. A new empirical equation for a phenomenological description of the temperature dependence of the dielectric permittivity (e') peak is proposed. In fact, the proposed equation provides an excellent fitting of the experimental curves at temperatures into and above the dielectric dispersion region, enabling us to calculate some characteristic parameters of the phase transitions in ferroelectric materials.

Super-strong magnetorheological fluids

Abstract: Magnetorheological (MR) fluids, which can rapidly be changed from a liquid state to a solid state and vice versa by a magnetic field, have the potential to revolutionize several industrial sectors The key issue is to enhance their yield shear stress This paper reviews the physical mechanism and microstructure of MR fluids It finds that the weak points of the MR microstructure under a shear force are at the chains' ends Hence, a general technique, a compression-assisted-aggregation process, is developed to change the induced MR structure to a structure that consists of robust thick columns with strong ends The scanning electronic micrographic (SEM) images confirm such a structure change With this approach, MR fluids become super-strong The enhanced yield stress of MR fluids reaches 800 kPa at a moderate magnetic field

Partial Coherence Effects on the Imaging of Small Crystals using Coherent X-ray Diffraction

Abstract: Recent achievements in experimental and computational methods have opened up the possibility of measuring and inverting the diffraction pattern from a single-crystalline particle on the nanometre scale. In this paper, a theoretical approach to the scattering of purely coherent and partially coherent x-ray radiation by such particles is discussed in detail. Test calculations based on the iterative algorithms proposed initially by Gerchberg and Saxton and generalized by Fienup are applied to reconstruct the shape of the scattering crystals. It is demonstrated that partially coherent radiation produces a small area of high intensity in the reconstructed image of the particle.

The London-van der Waals interaction energy between objects of various geometries

Abstract: Mathematical expressions for the London-van der Waals (vdW) interaction energies between macroscopic objects of a few common geometrical shapes are derived. The derivation is subjected to the assumption of additivity. The expressions are approximated for some limiting cases. These expressions may find uses for example in complex fluids in calculations of vdW interactions between vesicular (single wall liposomes) and liposomal (onion structures) particles or in colloidal suspensions for the calculation of vdW interaction between colloids that are coated with a stabilizing layer such as adsorbed polymers, polymer brushes or surfactants.

Comprehensive ab initio study of properties of monovacancies and antisites in 4H-SiC

Abstract: We present results of ab initio calculations for the electronic and atomic structures of monovacancies and antisite defects in 4H-SiC in all possible charge states. The calculations make use of a plane-wave pseudopotential method based on density-functional theory and the local spin-density approximation. Formation energies, ionization levels, and local geometries of the relaxed structures are reported for defects at all possible cubic and hexagonal lattice sites. To correct for the electrostatic interaction between charged supercells, we use a Madelung-type correction for the formation energies, leading to good agreement with experimentally observed ionization levels. Our calculations indicate no negative-U behaviour for carbon vacancies. Hence, the singly positive charge state of the carbon vacancy VC+ is stable, as recently found in experiments. The silicon antisite SiC+ is found to be stable at low values of electron chemical potential—again in agreement with experiment.

Spectroscopic and crystal-field analysis of new Yb-doped laser materials

Abstract: Crystal-field effects are very important as far as laser performances of Yb-doped materials are concerned. In order to simplify the interpretation of low-temperature spectra, two tools derived from a careful examination of crystal-field interaction are presented. Both approaches are successfully applied in the case of new Yb-doped materials, namely Ca3Y2(BO3)4 (CYB), Ca3Gd2(BO3)4 (CaGB), Sr3Y(BO3)3 (SrYBO), Ba3Lu(BO3)3 (BLuB), Y2SiO5 (YSO), Ca2Al2SiO7 (CAS) and SrY4(SiO4)3O (SYS). The 2F7/2 splitting is particularly large in these materials and favourable to a quasi-three-level laser operating scheme. Calculations performed using the point charge electrostatic model for these compounds and using a consistent set of effective atomic charges confirm the experimental results. This should permit to use this model in a predictive approach.

Kondo insulating behaviour in the filled skutterudite compound CeOs4Sb12

Abstract: The filled skutterudite CeOs4Sb12 and its nonmagnetic analogue LaOs4Sb12 have been synthesized in single-crystal form using the molten-metal-flux growth technique with Sb flux. Electrical resistivity measurements on CeOs4Sb12 indicate that it is a Kondo insulator with an energy gap of ΔE/kB~10 K. Lattice parameter and magnetic susceptibility measurements suggest that the Ce ions are trivalent in CeOs4Sb12. Specific heat and magnetization measurements reveal an enhanced value of the electronic specific heat coefficient γ~92 mJ mol-1 K-2 and Pauli susceptibility, reminiscent of a moderately heavy-fermion material.

Multiarm Star Polymers Dynamics

Abstract: Multiarm star polymers, consisting of a high number of linear homopolymer arms joined covalently to a central core, represent model soft 'hybrid' spheres encompassing both polymeric (arm) and colloidal (core) character. Due to this topology, the single star has a nonuniform monomer density distribution. In nondilute solutions, a liquid-like ordering occurs as a consequence of the enhanced osmotic pressure that outbalances the entropic stretching of the arms; this type of order persists in the melt as well, due to 'macromolecular excluded volume' effects. The resulting rich dynamic response, which is presented in this review, exhibits signatures of both polymeric and colloidal behaviour. In solution, concentration and number density fluctuations relax via cooperative diffusion, self-diffusion and structural relaxation. In the melt, the viscoelastic terminal relaxation involves arm relaxation (independent of arm number) and structural rearrangements of the stars (strongly dependent on arm number and size). The identification of the relaxation mechanisms in such complex soft spheres provides the necessary ingredients for the molecular design and control of novel composite materials combining properties of polymers and colloids.

Interplay of structure and transport properties of sodium-doped lanthanum manganite

Abstract: The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La1-xNaxMnO3 (0.07≤x≤0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07≤x≤0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La1-xNaxMnO3 compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO6 octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT2 behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

High pressure growth of bulk GaN from solutions in gallium

Abstract: In this paper, the growth of GaN single crystals from solutions of atomic nitrogen in liquid gallium under high N2 pressure is described. GaN single crystals obtained by the high nitrogen pressure solution method, without an intentional seeding, show strong growth anisotropy, which results in their platelet shape. The attempts to enhance the growth into (0001) directions by the increase of the integral supercooling in the solution often lead to growth instabilities on both N-polar and Ga-polar (0001) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface. The results of the seeded growth in directions parallel and perpendicular to the c-axis of GaN are discussed. In particular, it is shown that dominating mechanisms of the unstable growth on (0001) polar surfaces such as the cellular growth or edge nucleation can be suppressed and the crystal can be grown in a much more stable way. Physical properties most relevant for understanding the growth of GaN crystals are reviewed. The most important feature of GaN crystals grown by the HNPS method is that they are almost free of dislocations and therefore used as substrates give a possibility to grow perfect epitaxial structures.

Structural studies of ambient temperature plastic crystal ion conductors

Abstract: A number of novel organic ionic compounds based on the pyrrolidinium cation are described which have been found to be ion conductors in their solid states around room temperature. The properties of the compounds are consistent with their exhibiting plastic crystal phases. In order to understand some of the molecular origins of the plastic crystal behaviour and the ion conductivity that it promotes, a number of related compounds based on the imidazolium and ammonium cations are also described which have structural elements in common with the pyrrolidinium cation, but which do not show the plastic behaviour. It is found therefore that the nature of the cation is quite critical to the development of this behaviour. The alkyl methyl pyrrolidinium cation is found to produce plastic crystal phases when the alkyl chains are short, thereby preserving the ability of the cation to rotate with minimal steric hindrance. The ammonium and imidazolium cations of comparable size and structure are less able to produce these plastic phases, in many cases because the low temperature phase proceeds to melt rather than forming a stable rotator phase.