Showing papers in "Journal of Physics: Condensed Matter in 1997"

First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method

Abstract: The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

First-principles calculations of the electronic structure and spectra of strongly correlated systems: Dynamical mean-field theory

Abstract: A recently developed dynamical mean-field theory, in the iterated perturbation theory approximation, was used as a basis for the construction of a `first-principles' calculation scheme for investigating the electronic structure of strongly correlated electron systems. This scheme is based on the local density approximation (LDA) within the framework of the linearized muffin-tin orbitals (LMTO) method. The classical example of the doped Mott insulator was studied by the new method, and the results showed qualitative improvement when compared with experimental photoemission spectra.

Ultra-thin epitaxial films of graphite and hexagonal boron nitride on solid surfaces

Abstract: In this article, we have reviewed the recent progress of the experimental studies on ultra-thin films of graphite and hexagonal boron nitride (h-BN) by using angle-resolved electron spectroscopy together with other techniques. The fundamental properties of these high-quality films are discussed on the basis of the data on dispersion relations of valence electrons, phonon dispersion etc. The interfacial orbital mixing of the -state of the monolayer graphite (MG) with the d states of the reactive substrates is the origin for the phonon softening, expansion of the nearest-neighbour C - C distance, modification of the -band, low work function, and two-dimensional plasmons with high electron density, etc. In the cases of weak mixing at the interface between the MG and relatively inert substrates, the observed properties of the MG are very close to the bulk ones. In contrast to the case for MG, the interfacial interaction between the h-BN monolayer and the substrate is weak.

Structural, magnetic and electronic properties of perovskites (R = rare earth)

Abstract: perovskites (R = rare earth) provide a remarkable opportunity to study the relationship between structural and physical properties since, by moving along the 4f rare earth series, the evolution of several transport and magnetic properties can be nicely correlated to the steric effects associated with the lanthanide contraction. The most appealing example is probably the metal - insulator transition discovered for the compounds with , whose critical temperature increases with decreasing size of the rare earth ion. On the other hand, several experimental results show that in perovskites the degree of Ni 3d - O 2p hybridization is extremely high. The purely ionic configuration, first suggested by neutron diffraction experiments, has been then substituted by a ground state where the and configurations are strongly mixed. In this paper, a summary of the scientific work performed on the system during the last 6 years is presented. The results of the different experimental techniques are successively reviewed, with special emphasis on the relationship between the structural and the physical properties. The possible mechanisms responsible for the metal - insulator transition, which is the major open question for perovskites, are also discussed.

Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide

Abstract: The electronic structures of the two thermoelectric materials and are studied using density-functional theory with the spin - orbit interaction included. The electron states in the gap region and the chemical bonding can be described in terms of interaction between the atomic p orbitals within the `quintuple' layer. For , we find both the valence-band maximum as well as the conduction-band minimum, each with a nearly isotropic effective mass, to occur at the zone centre in agreement with experimental results. For , we find that the six valleys for the valence-band maximum are located in the mirror planes of the Brillouin zone and they have a highly anisotropic effective mass, leading to an agreement between the de Haas - van Alphen data for the p-doped samples and the calculated Fermi surface. The calculated conduction band, however, has only two minima, instead of the six minima indicated from earlier experiments. The calculated Seebeck coefficients for both p-type and n-type materials are in agreement with the experiments.

Scaling properties of cracks

Abstract: Experiments concerning the morphology of fracture surfaces of various materials are reviewed. The observations are interpreted within the framework of models of lines moving in a random environment. This suggests that fracture of heterogeneous materials could be seen as a dynamic phase transition.

Elastic instabilities in crystals from ab initio stress - strain relations

Abstract: Pressure-induced elastic instabilities are investigated in the prototypic ionic and covalent solids (MgO, CaO, and Si) using generalized elastic stability criteria based on the elastic stiffness coefficients which are determined directly from stress - strain relations. From first-principles computer simulations of the instabilities, we demonstrate the validity and importance of the generalized criteria relative to the conventional criteria in describing the crystal stability under hydrostatic pressure in relation to the real structural transformations. We examine systems for which the two phases can be related by a simple deformation, and in all cases we show that the generalized elastic stiffness coefficient associated with that deformation softens toward the transition. The shear stability criterion bounds the first-order B1 - B2 phase transition pressure from above and below in MgO and CaO, suggesting a wide pressure regime of metastability, whereas the tetragonal shear stability criterion predicts precisely the second-order rutile-to- transition in . The high-pressure elastic behaviour of diamond structure Si is studied in detail. A tetragonal shear instability corresponding to its transformation to the -Sn structure should occur in diamond structure Si at a pressure of 101 GPa, compared to the experimental value of 9 to 13 GPa for the transition pressure.

Contribution of the irreversible displacement of domain walls to the piezoelectric effect in barium titanate and lead zirconate titanate ceramics

Abstract: The contribution from the irreversible displacement of non-180 degrees domain walls to the direct longitudinal piezoelectric d(33) coefficient of BaTiO3 and Pb(Zr,Ti)O-3 ceramics was determined quantitatively by using the Rayleigh law. Effects of the crystal structure and microstructure of the ceramics as well as the external d.c. pressure on the domain wall contribution to d(33) were examined. In barium titanate, this domain wall contribution is large (up to 35% of the total d(33), under the experimental conditions used) and dependent on the external d.c. pressure in coarse grained ceramics, and much smaller and independent of the external d.c. pressure in fine-grained samples. The presence of internal stresses in fine-grained ceramics could account for the observed behaviour. The analysis shows that the domain-wall contribution to the d(33) in lead zirconate titanate ceramics is large in compositions close to the morphotropic phase boundary that contain a mixture of tetragonal and rhombohedral phases, and in rhombohedral compositions (up to 40% of the total d(33)). The domain-wall contribution to the piezoelectric response from the irreversible displacement of domain walls is significantly smaller in tetragonal Pb(Zr,Ti)O-3 samples where it decreases with increasing spontaneous strain.

Phase transitions of fluids in shear flow

Abstract: We review theories and experiments on the effects of shear in fluids undergoing phase transitions. We put emphasis on near-critical fluids and polymer solutions as representative examples, but also discuss related problems in polymer blends, gels, and surfactant systems, etc. In near-critical fluids, convective deformations can drastically alter the critical behaviour, spinodal decomposition, and nucleation. In this case the hydrodynamic interaction suppresses the fluctuations and gives rise to a downward shift of the critical temperature (shear-induced mixing). The rheology in two-phase states, and effects of random stirring are also reviewed. In semidilute polymer solutions near the coexistence curve, on the other hand, the composition fluctuations can be strongly influenced by the viscoelastic stress. In shear flow, this dynamical coupling results in enhancement of the composition fluctuations (shear-induced demixing). They grow, but are eventually disrupted by convective deformations, yielding chaotic dynamical steady states where phase separation is incompletely taking place. Such nonlinear shear regimes are examined using computer simulations based on a viscoelastic Ginzburg - Landau model.

Resonant ultrasound spectroscopy

Abstract: Resonant ultrasound spectroscopy (RUS) involves the study of the mechanical resonances of solids. The resonant response of a particular object depends on its shape, elastic constants, crystallographic orientation, density, and dissipation. It is possible to obtain the complete elastic constant matrix of relatively low-symmetry materials from a RUS spectrum on a single small sample . The measurement and the computation of the RUS spectra of solids are reviewed. Several examples of the use of the technique are discussed.

Shearing the vacuum—quantum friction

Abstract: We consider two perfectly smooth featureless surfaces at T = 0, defined only by their respective dielectric functions, separated by a finite distance, and ask the question of whether they can experience any friction when sheared parallel to their interface. We find large frictional effects comparable to everyday frictional forces provided that the materials have resistivities of the order of and that the surfaces are in close proximity. The friction depends solely on the reflection coefficients of the surfaces for electromagnetic waves and its detailed behaviour with shear velocity and separation is dictated by the dispersion of the reflectivity with frequency.

Theory and simulation of crystal growth

Abstract: Crystal growth phenomena are discussed with special reference to growth from vapour. The basic concepts of crystal growth are recalled, including the different growth modes, the dependence of the growth rate on disequilibrium and temperature, and the atomic processes relevant for growth. The methods used in crystal growth simulations are reviewed, with special reference to kinetic Monte Carlo methods. The roughness of growing surfaces, and the roughness properties of the discrete and continuum growth models (the latter being described via stochastic differential equations) are discussed, together with the special phenomena occurring in the vicinity of the roughening temperature. A number of simulations based on the six-vertex model and on kinetic counterparts of the BCSOS model are reviewed. Finally, the instabilities arising during growth are considered, including a discussion of phenomena such as dendritic growth and ramified cluster growth and reviewing the recent, extensive studies concerning unstable MBE growth.

The composition dependence of the Raman spectrum and new assignment of the phonons in

Abstract: The Raman spectrum of has been measured for various scattering configurations in crystals with different compositions. The comparison between spectra recorded for the congruent and the nearly stoichiometric crystals shows significant differences in the shape and the number of Raman peaks. The analysis of results leads to a new and complete assignment of the long-wavelength optical phonons in .

Muon spin rotation and relaxation in magnetic materials

Abstract: A review of the muon spin rotation and relaxation studies on magnetic materials published from July 1993 is presented. It covers the investigation of magnetic phase diagrams, that of spin dynamics and the analysis of the magnetic properties of superconductors. We have chosen to focus on selected experimental works in these different topics. In addition, a list of published works is provided.

Order and disorder in the relaxor ferroelectric perovskite (PSN): comparison with simple perovskites and

Abstract: We report a structural evolution analysis of the long-range and local order and disorder of lead scandium niobate (PSN) down to 10 K, using a combination of neutron and x-ray diffraction on powder and single crystals. The structure of PSN is discussed and compared with those of simple perovskite compounds and . In PSN the existence of a long-range but still disordered ferroelectric phase, different from those of simple perovskites, is evidenced. The disorder of lead and scandium/niobium atoms on short-, medium- and long-range scales is discussed in connection with the dielectric properties of these materials.

The electronic structure of PdO found by photoemission (UPS and XPS) and inverse photoemission (BIS)

Abstract: The electronic structure of the 4d transition-metal oxide PdO is investigated by photoemission (UPS and XPS), inverse photoemission (BIS; ), and electron energy loss spectroscopy in reflection geometry (REELS; primary energy, ). The valence band spectra are compared to recent theoretical ab initio band-structure calculations. Good agreement between theory and experiment is found in the occupied part of the band structure down to 8 eV below as well as in the unoccupied part up to 6 eV above . This confirms the common view that the electronic structure of the 4d transition-metal oxides, e.g. PdO, can be explained in terms of a single-electron picture. Nevertheless correlation effects among the Pd 4d electrons are clearly visible in the spectra, as e.g. satellites of the Pd core level spectra. In order to explain the origin of these satellites we performed simple cluster model calculations and as a result we can explain one satellite in a screening picture by means of a charge transfer process. In addition radiation damage effects in PdO during the electron bombardment in the BIS experiments are reported. This is explained by the formation of the Pd -like states connected with oxygen loss due to the electron bombardment.

Spin canting due to structural disorder in maghemite

Abstract: The spin-canting effect has been studied in samples of maghemite particles with the same width of about 100 nm, but different length and with different degree of cation disorder. Mossbauer spectra obtained at 5 K with a magnetic field of 4 T applied parallel to the propagation direction of the gamma rays showed that there is a correlation between the degree of structural disorder and the spin-canting effect. The results show that the observed spin canting is not a surface effect, but that atoms in the interior of the particles can be significantly influenced by canting effects.

Practical methods in ab initio lattice dynamics

Abstract: A popular method of extracting phonon frequencies from ab initio calculations is to find the equilibrium structure of a material and then build up the matrix of force constants by calculating forces acting due to small displacements of the atoms. If the range of the force constants is assumed to be short, as it typically is in parametrized force-model calculations, the entire dispersion relation can be evaluated from data taken from small ab initio calculations. In this paper we highlight the practical difficulties introduced by low-symmetry structures with free internal parameters and present practical solutions to them. By way of example, we present a number of calculations where solution of these problems is essential. These include ab initio calculation of phonon dispersion in a non-centrosymmetric structure (silver gallium diselenide) and good agreement between calculations and neutron scattering data for a structure with free internal parameters (germanium sulphide).

Kinetics of polyelectrolyte adsorption.

Abstract: The kinetics of polyelectrolyte adsorption has been investigated theoretically. In analogy with Kramers' rate theory for chemical reactions we present a model which is based on the assumption that a polyelectrolyte encounters a barrier in its motion towards an adsorbing surface. The height of the barrier, which is of electrostatic origin, is calculated with a self-consistent-field (SCF) model. The salt concentration strongly affects the height of the barrier. At moderate salt concentrations () equilibrium in the adsorption is attained; at low salt concentration () equilibrium is not reached on the time scale of experiments. The attachment process shows resemblances to the classical DLVO theory.

Ab initio density functional studies of transition-metal sulphides: I. Crystal structure and cohesive properties

Abstract: The structural and cohesive properties of more than thirty transition-metal sulphides of various stoichiometries and crystal structures have been investigated using density functional theory, with the aim of establishing a correlation between the strength of the metal - sulphur bond and the catalytic activities of these materials. It is shown that the local density approximation has a tendency to overestimate the strength of the bonding. The overbinding manifests itself in the prediction of too small atomic volumes and too large cohesive energies. Non-local corrections to the local exchange - correlation functional in the form of a generalized-gradient approximation correct the overbinding (albeit with a certain tendency to overcorrect, especially for the sulphides of the heavy transition metals) and result in accurate structural prediction and cohesive energies. A correlation between the sulphur - metal bond strength and the catalytic activities is established.

Network models of fluid, hexatic and polymerized membranes

Abstract: The thermal behaviour of membranes - surfaces of nearly vanishing tension - depends strongly on their internal state, which can be either fluid, crystalline (or polymerized), or hexatic. Thermal fluctuations have a dramatic effect on the conformation and elastic properties of membranes. We describe in this review both the continuum models of membranes which are used for theoretical analyses as well as the network models employed in simulations. The fruitful interaction between these two approaches, which has lead to recent progress in this field, is emphasized. We summarize the essential results of recent research on fluctuating membranes; in particular, the effects of bending rigidity, self-avoidance, attractive interactions, disorder, topological defects and external compression forces are discussed in detail.

Ab initio density functional studies of transition-metal sulphides: II. Electronic structure

Abstract: A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).

Wall slip in polymer melts

Abstract: We present a review of the recent characterizations of the flow behaviour of high-molecular-weight polymer melts, with special emphasis on situations in which slip at the wall appears. These characterizations are based on direct measurements of the local velocity of the fluid, in the immediate vicinity of the solid wall, through near-field velocimetry techniques. The results demonstrate the importance of polymer molecules anchored on the solid surface, either by strong adsorption or by chemical grafting, and entangled with the bulk polymer, to produce a strong friction at low shear rates and to lead to a shear rate threshold above which strong slip at the wall and low friction develop. The evolution of the shear rate threshold and of the flow characteristics (the length of the extrapolation of the velocity profile to zero, the critical slip velocity for the onset of strong slip, ...) with the molecular parameters of the system (the molecular weights of the bulk and surface chains, and the surface density of anchored chains) is analysed and compared with the predictions of recent theoretical models.

A structural study from neutron diffraction data and magnetic properties of (R = La, rare earth)

Abstract: The title compounds (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) have been prepared in polycrystalline form by a citrate technique and, excepting the Sm and Eu phases, structurally studied by high-resolution neutron powder diffraction. All the materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of octahedra sharing edges, linked together by and units. The size of the three kinds of coordination polyhedron regularly decreases as R cations become smaller. A bond-valence study allowed us to detect the presence of important tensile and compressive stresses in the crystal structure of , which are progressively released along the series as the rare-earth size decreases. The magnetic properties strongly depend on the nature of R, going from the spin-glass behaviour observed at low temperature in to the field-induced transitions exhibited by . A cusp in the susceptibility curves suggests an antiferromagnetic ordering at low temperatures, which is masked in the compounds containing strongly paramagnetic rare earths (Tb, Ho, Er). At high temperatures the paramagnetic moments are consistent in all cases with the presence of high-spin and cations.

Simulation studies of fluid critical behaviour

Abstract: We review and discuss recent advances in the simulation of bulk critical phenomena in model fluids. In particular we emphasize the extensions to finite-size scaling theory needed to cope with the lack of symmetry between coexisting fluid phases. The consequences of this asymmetry for simulation measurements of quantities such as the particle density and the heat capacity are pointed out and the relationship to experiment is discussed. A general simulation strategy based on the finite-size scaling theory is described and its utility illustrated via Monte Carlo studies of the Lennard-Jones fluid and a two-dimensional spin-fluid model. Recent applications to critical polymer blends and solutions are also briefly reviewed. Finally we consider the outlook for future simulation work in the field.

Brillouin scattering study of gallium nitride: elastic stiffness constants

Abstract: High-resolution Brillouin scattering measurements on a high-quality wurtzite gallium nitride (GaN) single crystal were carried out and elastic stiffness constants were determined. A comparison is given with the results of a recently reported model for calculation of the elastic constants of III-V semiconductors based on the modified version of Keyes's relations. A good agreement is found between the experimental and theoretical elastic constants for GaN.

Anderson impurity in pseudo-gap Fermi systems

Abstract: We use the numerical renormalization group method to study an Anderson impurity in a conduction band with the density of states varying as with r > 0. We find two different fixed points: a local-moment fixed point with the impurity effectively decoupled from the band and a strong-coupling fixed point with a partially screened impurity spin. The specific heat and the spin susceptibility show power-law behaviour with different exponents in the strong-coupling and local-moment regimes. We also calculate the impurity spectral function which diverges (vanishes) with in the strong-coupling (local-moment) regime.

Strong antiferromagnetic spin fluctuations and the quantum spin-liquid state in geometrically frustrated -Mn, and the transition to a spin-glass state caused by non-magnetic impurity

Abstract: The magnetic properties of -Mn and the effect of the substitution of non-magnetic Al were systematically investigated by measuring thermodynamic and transport properties, and the NMR and polarized neutron scattering for alloys with -Mn is paramagnetic down to the lowest temperature (1.4 K) but with strong spin fluctuations. Substitution of Al atoms which preferentially occupy one of two crystallographic sites (site II) results in the formation of local moments on Mn atoms, and makes the ground state into a spin-glass-like state. NMR measurements revealed almost independent magnetic behaviour of the two different sites, and static magnetic ordering of Mn at site II for , although the possibility of very weak magnetism of the site-I atoms cannot be excluded. The temperature dependence of the spin - lattice relaxation rate indicates antiferromagnetic correlations of paramagnetic spin fluctuations, and its saturation at higher temperatures for and a critical slowing down of the fluctuations at low temperatures for . Polarized neutron scattering experiments showed directly strong spin fluctuations with antiferromagnetic correlations for both -Mn and . The characteristic energy of fluctuations for -Mn is fairly large even at 7 K , implying a quantum origin of the fluctuations, while the energy spectrum of becomes very sharp at low temperatures, within a resolution-limited energy width, indicating damping of spin fluctuations into a spin-glass-like state. These results can be interpreted in terms of the transition from a spin liquid to a spin glass caused by the substitution of Al that gives rise to the release of the antiferromagnetic frustration of the characteristic crystal lattice. Considerable similarity of the characteristic features to those of the highly frustrated Laves phase compounds is argued. The possible frustration on site II in the -Mn structure, a three-dimensional network of corner-sharing regular triangles, which is similar to the two-dimensional Kagome lattice, is pointed out.

Numerical study of a mixed Ising ferrimagnetic system

Abstract: We present a study of a classical ferrimagnetic model on a square lattice in which the two interpenetrating square sublattices have spins one-half and one. This model is relevant for understanding bimetallic molecular ferrimagnets that are currently being synthesized by several experimental groups. We perform exact ground-state calculations for the model, and employ Monte Carlo and numerical transfer-matrix techniques to obtain the finite-temperature phase diagram for both the transition and compensation temperatures. When only nearest-neighbour interactions are included, our non-perturbative results indicate no compensation point or tricritical point at finite temperature, which contradicts earlier results obtained with mean-field analysis.

Spectroscopy of ? crystals

Abstract: The polarized absorption spectra, time-resolved emission spectra, and emission lifetimes of doped crystals were measured. An intense emission from the level was observed. The positions of the levels, experimental oscillator strengths, and branching ratios from the and levels were determined. The results are discussed in terms of the Judd - Ofelt parameters.