Showing papers in "Journal of Physics: Condensed Matter in 1996"

The theory of metal - ceramic interfaces

Abstract: The theory of metal - ceramic interfaces is a collection of approaches which are complementary. They range from thermodynamic modelling based on empirical correlations, through the image model of adhesion, semi-empirical tight-binding calculations, to first-principles calculations based on applying the density functional theory or Hartree - Fock theory. This article reviews the present state of theoretical calculations, with particular reference to electronic structure and adhesion. A section on the thermodynamic background clarifies the concept of work of adhesion which is the goal of many calculations. Cluster models and periodic slabs have been considered, both self-consistent and non-self-consistent. The most sophisticated and complete calculations have been made for metals on MgO and alumina. There a consistent picture of the nature of the bonding has emerged, although there are still significant unexplained discrepancies in numerical values.

Pulsed laser deposition of thin films of (

Abstract: Manganites of the series , with x = 0, 0.1, 0.3, 0.5, 0.7 and 1.0, have been characterized in ceramic form and thin films have been prepared by pulsed laser deposition. Characterization techniques included x-ray diffraction, conductivity and magnetoresistance, magnetization and susceptibility, optical spectroscopy and the Faraday effect. Both the films and ceramics exhibit a maximum low-temperature conductivity at which is coexistent with ferromagnetic order. The negative magnetoresistance effect is qualitatively different for the x = 0.3 and x = 0.5 compositions. For x = 0.3 the magnetoresistance peak occurs around the Curie point, whereas for x = 0.5 the onset of magnetoresistance is somewhat below and increases monotonically as . The applied field appears to modify the magnetic order (on the scale of the spin diffusion length) down to the lowest temperatures for x = 0.5, but for x = 0.3 the ferromagnetic order is essentially complete and collinear below the Curie point.

Extracting convergent surface energies from slab calculations

Abstract: The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.

Structural studies of aqueous solutions of PEO - PPO - PEO triblock copolymers, their micellar aggregates and mesophases; a small-angle neutron scattering study

Abstract: The structural characteristics of aqueous solutions of the Pluronic triblock copolymers of poly(ethylene oxide) - poly(propylene oxide) - poly(ethylene oxide), PEO - PPO - PEO, and their self-associated assemblies are reviewed. It is shown by small-angle neutron scattering that at low temperatures and/or concentration the individual copolymers exist in solution as individual unimers. Thermodynamically stable micelles are formed with increasing copolymer concentration and/or temperature. The unimer-to-micelle transition is not sharp, however. Micelles of well defined spherical shape and size coexist with unimers over a relatively wide temperature/concentration range. The micellar volume fraction increases accordingly with increasing temperature, increasing copolymer concentration and decreasing hydrostatic pressure. The copolymer suspension undergoes as a result a transition from a Newtonian liquid to a soft solid material when the micellar volume fraction crosses the critical value for hard-sphere crystallization. Crystallographic investigations on shear-aligned monodomain samples prove that the micelles in the solid phase are organized on a body-centred cubic lattice. As a result of an increasing micellar size upon increasing the temperature, the micelles themselves undergo a sphere-to-rod transition at elevated temperatures. In a shear field these rod-like micelles form a macroscopic nematic phase for low copolymer concentration, and a hexagonal solid phase for higher concentrations. For even higher concentrations, lamellar phases are observed: one lamellar type which is still governed by the hydrophobic interactions, and one type which appears as a result of crystallization of the PEO blocks.

The foam drainage equation

Abstract: The drainage of liquid in a foam may be described in terms of a nonlinear partial differential equation for the foam density as a function of time and vertical position. We review the history and recent development of this theory, analysing various exact and approximate solutions and relating them to each other.

The pseudogap state in high- superconductors: an infrared study

Abstract: We report on a study of the electromagnetic response of three different families of high- superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussion is focused on the ab-plane charge dynamics in the pseudogap state which is realized in underdoped materials below a characteristic temperature , a temperature that can significantly exceed the superconducting transition temperature . We explore the evolution of the pseudogap response by changing the doping level, by varying the temperature from above to below , or by introducing impurities in the underdoped compounds. We employ a memory function analysis of the ab-plane optical data that allows us to observe the effect of the pseudogap most clearly. We compare the infrared data with other experimental results, including the c-axis optical response, dc transport, and angle-resolved photoemission.

High-frequency single-electron transport in a quasi-one-dimensional GaAs channel induced by surface acoustic waves.

Abstract: We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.

Origin of the negative thermal expansion in and

Abstract: The negative thermal expansion recently observed over a wide range of temperatures in may be attributed to the existence of low-frequency phonon modes which can propagate with no distortions of the tetrahedra and octahedra, the so-called `rigid unit modes' Using methods developed for the study of similar modes in silicates we have located the surfaces of these modes in wave-vector space The rigid-unit mode interpretation accounts for the weak effect of the 430 K structural phase transition on the negative thermal expansion, provided that the disordered phase does not involve formation of and complexes On the other hand, the crystal structure of the related material is cross braced by pairs of linked tetrahedra and is therefore significantly less flexible In this case a qualitatively different mechanism may be responsible for the negative thermal expansion observed in

Electronic properties of three- and low-dimensional semiconducting materials with Pb halide and Sn halide units

Abstract: Band-structure calculations, semiempirical as well as ab initio, have been applied to study the electronic band gap of the new exotic natural low-dimensional MX systems (where M = Pb or Sn and X = I, Br or Cl). Moreover, variational calculations are employed to calculate the excitonic binding energies, whose amplification is due not only to the quantum confinement of the excitons but also to a dielectric enhancement effect. A single set of semiempirical parameters is sought to describe the materials; comparison of the calculations with experimental data shows this to be successful in the case of the PbI- and PbBr-containing compounds. .

Non-Fermi-liquid behaviour in the heavy-fermion system

Abstract: In heavy-fermion systems with 4f or 5f atoms (such as Ce or U) the competition between the on-site moment compensation by the Kondo effect and the long-range RKKY interaction between localized magnetic moments leads to the possibility of either a non-magnetic or a magnetically ordered ground state. However, even in the case of no long-range magnetic order as exemplified by , short-range dynamic intersite correlations are observed. Yet, the thermodynamic and transport properties of this alloy at very low temperatures T resemble those of a Fermi liquid (FL). Upon alloying with Au, long-range incommensurate antiferromagnetism is observed in for x > 0.1. For x = 0.1 where , the specific heat C depends on T as , the magnetic susceptibility as , and the T-dependent part of the electrical resistivity as . This is in marked contrast to the FL behaviour . It is suggested that low-energy spin excitations are at the origin of these non-Fermi-liquid (NFL) anomalies which occur at a zero-temperature quantum phase transition. Large magnetic fields B restore FL behaviour. The low-T range of FL behaviour in C and extends towards higher T with increasing B, with the crossover temperature varying roughly linearly with B. Apart from changing the Au concentration x, the magnetic - non-magnetic transition can be tuned by applying pressure p to antiferromagnetic samples with x > 0.1. For x = 0.3, at and NFL behaviour is observed in the specific heat for this critical pressure. For x = 0.2, where we likewise observe a logarithmic divergence of C/T and for p = 6.9 kbar we recover FL behaviour. Finally, we report on a remarkable `universality' of C/T in the system with M = Au, Pd, Pt: regardless of how is reached in this system (alloying with different elements M, varying concentration, or applying pressure), the C/T versus ln T curves are practically identical. Possible origins of NFL behaviour are discussed.

Depletion effects in binary hard-sphere fluids

Abstract: We report a molecular dynamics computation of the entropic depletion force induced between two large spheres (colloidal particles) immersed in a fluid of small spheres. The effective pair potential obtained by numerical integration of the force is used in a Monte Carlo study of the phase behaviour of the binary mixture. The simulation results are compared with the relevant theoretical predictions that follow from various integral equations for liquid mixtures. The simulations provide evidence for a spinodal instability in a liquid mixture of hard spheres with a size ratio of 0.1.

Calculating photonic band structure

Abstract: Photonic materials structured on the scale of the wavelength of light have become the subject of an active field of research fed by the hope of creating novel properties. Theory plays a central role reinforced by the difficulty of manufacturing photonic materials: unusually we are better able to design a photonic material than to build it. In this review we explore the application of scattering theory to Maxwell's equations that has enabled theory to make such a central contribution: implementation of Maxwell's equations on a discrete mesh, development of the electromagnetic transfer matrix, order-N methods, and adaptive meshes. At the same time we present applications to a few key problems by way of illustration, and discuss the special circumstances of metallic photonic structures and their unique properties.

Analysis of atomic orbital basis sets from the projection of plane-wave results

Abstract: The projection of the eigenfunctions obtained in standard plane-wave first-principles calculations is used for analysing atomic orbital basis sets. The `spillage' defining the error in such a projection allows the evaluation of the quality of an atomic orbital basis set for a given system and its systematic variational optimization. The spillage is shown to correlate with the mean square error in the energy bands obtained from the projected Hamiltonian matrix. The method is applied to the characterization of finite-range pseudo-atomic orbitals (Sankey O F and Niklewski D J 1989 Phys. Rev. B 40 3979) in comparison to infinite-range pseudo-atomic and Slater-type orbitals. The bases are evaluated and optimized for several zinc-blende semiconductors and for aluminium; the finite-range orbitals display high quality in spite of the limited range. A simple scheme is proposed to systematically enlarge the basis without increasing its range.

Transport and magnetization in the badly metallic itinerant ferromagnet

Abstract: SrRuO3 is an itinerant ferromagnet with Tc 160 K and a 'bad metal' in the limit of kF l D O.1/. While the magnetic properties of SrRuO3 in the paramagnetic phase, near the ferromagnetic phase transition and at low temperatures are normal and consistent with its being a strong itinerant ferromagnet, the transport properties (resistivity and magnetoresistance) sharply deviate from that of good metallic ferromagnets. We conjecture that the distinct transport behaviour of SrRuO3 is related to its being a 'bad metal' in the kF l D O.1/ limit, and discuss the possible relevance of our results to the unusual transport properties of other 'bad metals' such as high-temperature superconductors.

High-temperature structural phase transitions in perovskite

Abstract: High-temperature powder x-ray diffraction data are presented for perovskite between 293 and 1523 K. The temperature-dependence of superlattice intensities and cell parameters suggests a sequence of phase transitions from the room temperature orthorhombic (Pbnm) structure to a tetragonal (I4/mcm) polymorph at temperatures in the range 1373 - 1423 K, followed by transformation to the cubic aristotype at . The intensity of the diffuse background increases on transformation to the cubic structure and is associated with disorder (and anionic mobility) of the oxygen sub-lattice. The I4/mcm - Pbnm transition induces a large spontaneous strain, but the tetragonal spontaneous strain in the I4/mcm phase due to the - I4/mcm transition is small, below the resolution of this experiment. These results add weight to suggestions from recent computer simulations that orthorhombic may transform to a tetragonal (rather than a cubic) polymorph under the conditions of the Earth's mantle, in which case the effects on electrical conductivity would not be expected to be as great as for a transition to a cubic polymorph, although the consequences for elastic properties may be more significant.

Neutron Compton scattering

Abstract: Neutron Compton scattering measurements have the potential to provide direct information about atomic momentum distributions and adiabatic energy surfaces in condensed matter. First applied to measuring the condensate fraction in superfluid helium, the technique has recently been extended to study a variety of classical and quantum liquids and solids. This article reviews the theoretical background for the interpretation of neutron Compton scattering, with emphasis on studies of solids.

The LO phonon lifetime in GaN

Abstract: The disparity in the masses of the atomic constituents of GaN means that the usually-assumed mode of decay of a long-wavelength LO mode into two LA modes is forbidden. However, given the available information about the phonon spectrum, it appears that a possible decay route is provided by the emission of a TO and an LA phonon.

The physics of foam

Abstract: Some recent progress in the study of liquid foams is reviewed in outline. Calculations of foam conductivity are presented, which further improve upon the approximation recently proposed by us, in accounting for the nonlinearity in the dependence of conductivity on liquid fraction.

A random field theory based model for ferroelectric relaxors

Abstract: A model for ferroelectric relaxors such as PMN, PSN and PLZT giving a quantitative description of their properties and phase diagrams is proposed within the framework of the random field theory. In this model, the relaxors are considered as systems with random sites and orientations of electric dipoles, lattice vacancies, antisite ions and other defects as well as impurities embedded into the paraelectric phase, which is proposed to be the `host' lattice for these materials. The calculations of the temperature which corresponds to the transition from the paraelectric to the ferroelectric phase is carried out as a function of the concentration of lattice defects (point charges and dilatational centres). On the basis of these calculations, the peculiarities of the ferroelectric relaxor phase diagram are discussed. The main features of the phase transition sequence when decreasing the temperature in relaxors with constant dipole and defect concentrations are described. The Cross superparaelectric model and Burns temperature have been shown to appear in a natural way in the proposed model. A comparison between calculated and experimental data has been made for the model ferroelectric relaxor PLZT x/65/35. Fairly good agreements between calculated and measured and critical concentrations of lanthanum have been obtained from the model.

The search for a universal equation of state correct up to very high pressures

Abstract: The universal equations of state of solids recently proposed by several authors have been examined by comparing them with the theoretical results calculated by the augmented-plane-wave method and the quantum-statistical model proposed by Kalitkin and Kuz'mina from low to ultra-high pressures. It has been shown that the Vinet equation is in good agreement with the theoretical results both for the P - V relation and for the pressure dependence of the isothermal bulk modulus up to for monatomic solids and up to for diatomic solids. The Kumari - Dass and the Dodson equations become less successful below if the zero-pressure values for and are used. For monatomic solids the Holzapfel equation has a very similar structure to that of the Vinet equation at low and medium compressions and it is in good agreement with the theoretical values up to ultra-high pressures. For the application to polyatomic solids a remedy for the shortcomings of the Vinet equation at very high pressures is given on the basis of the quantum-statistical model. The resulting equation is in good agreement with the theoretical values from low to ultra-high pressures both for monatomic and for diatomic solids.

The density matrix renormalization group for a quantum spin chain at non-zero temperature

Abstract: We apply a recent adaptation of White's density matrix renormalization group (DMRG) method to a simple quantum spin model, the dimerized XY chain, in order to assess the applicability of the DMRG to quantum systems at non-zero temperature. We find that very reasonable results can be obtained for the thermodynamic functions down to low temperatures using a very small basis set. Low-temperature results are found to be most accurate in the case when there is a substantial energy gap.

Kondo disorder: a possible route towards non-Fermi-liquid behaviour

Abstract: We present a general model of disorder in Kondo alloys that, under certain conditions, leads to non-Fermi-liquid behaviour. The central underlying idea is the presence of a distribution of local Kondo temperature scales. If this distribution is broad enough, such that there are sites with arbitrarily low Kondo temperatures, a non-Fermi-liquid phase is formed. We analyse thermodynamics and transport in this approach and show it is consistent with a number of Kondo alloys. We also compare the predictions of this model with the measured dynamical magnetic response of these systems.

Ground states with cluster structures in a frustrated Heisenberg chain

Abstract: We examine the ground state of a Heisenberg model with arbitrary spin S on a one-dimensional lattice composed of diamond-shaped units. A unit includes two types of antiferromagnetic exchange interaction which frustrate each other. The system undergoes phase changes when the ratio between the exchange parameters varies. In some phases, strong frustration leads to larger local structures or clusters of spins than a dimer. We prove for arbitrary S that there exists a phase with four-spin cluster states, which was previously found numerically for a special value of in the S = 1/2 case. For S = 1/2 we show that there are three ground-state phases, and determine their boundaries.

hydration in concentrated aqueous solution

Abstract: Neutron diffraction experiments were carried out on aqueous solutions of lithium chloride in heavy water at three concentrations - 14, 3.6 and 1 molal. The first-order isotopic difference method was applied to the lithium ions, and structural results were obtained for the hydration: the Li - O and Li - D near-neighbour distances in the first hydration shell remain the same at 1.95(3) and 2.5(5) A over the concentration range studied. However, there is an increase in coordination number from 3.2 at 14 molal to at 1 molal. There is also evidence for a relatively weak second hydration shell, which becomes displaced to larger distances at lower concentrations.

Quantum critical phenomena in undoped heavy-fermion metals

Abstract: We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds and . `Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticle - quasiparticle scattering cross section. For , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at .

Geometrically frustrated magnetic structures of the heavy-fermion compound CePdAl studied by powder neutron diffraction

Abstract: The heavy-fermion compound CePdAl with ZrNiAl-type crystal structure (hexagonal space group ) was investigated by powder neutron diffraction. The triangular coordination symmetry of magnetic Ce atoms on site 3f gives rise to geometrical frustration. CePdAl orders below with an incommensurate antiferromagnetic propagation vector , and a longitudinal sine-wave (LSW) modulated spin arrangement. Magnetically ordered moments at Ce(1) and Ce(3) coexist with frustrated disordered moments at Ce(2). The experimentally determined magnetic structure is in agreement with group theoretical symmetry analysis considerations, calculated by the program MODY, which confirm that for Ce(2) an ordered magnetic moment parallel to the magnetically easy c-axis is forbidden by symmetry. Further low-temperature experiments give evidence for a second magnetic phase transition in CePdAl between 0.6 and 1.3 K. Magnetic structures of CePdAl are compared with those of the isostructural compound TbNiAl, where a non-zero ordered magnetic moment for the geometrically frustrated Tb(2) atoms is allowed by symmetry.

Conductivity studies of polyaniline doped with CSA

Abstract: Polyaniline films prepared via the camphorsulphonic acid (CSA)m-cresol solution processing route have been synthesized with doping levels in the range of 10 - 90%. The electrical conductivity of these films has been measured as a function of temperature between 10 and 300 K. At a doping level of 30% the onset of metallic transport is observed, and at 60% the films are found to exhibit metallic transport down to 135 K, and have a maximum room-temperature conductivity of . The results are modelled in terms of a heterogeneous model of fluctuation induced tunnelling (FIT) and metallic transport.

Quantum paraelectric and induced ferroelectric states in

Abstract: Nominally pure has been studied by dielectric spectroscopy using small (linear regime) as well as large electrical fields (non-linear regime) up to . In addition measurements of the specific heat and its field-dependent contribution have been carried out. The field dependence of the dielectric constant and the specific heat can be well described by the transverse Ising Hamiltonian including tunnelling and external field terms. It gives evidence for the existence of polar clusters at low temperatures which are supposed to be associated with the quantum paraelectric state below in accord with recent free-energy calculations. The low-field third-harmonic susceptibility which measures the polar correlations exhibits anomalies near . At high fields an aligned domain state is induced. These results as well as those on the remanent polarization and the dielectric loss have allowed us to deduce a complex E, T-phase diagram. The assignment of the various phases is discussed in connection with the recent proposal of the appearance of a macroscopic quantum state.

Forces on a colloidal particle in a polymer solution: a study using optical tweezers

Abstract: We report a study of the dynamical behaviour of a polystyrene latex sphere in a telechelic poly(ethylene oxide) solution using optical tweezers. With this new technique, we use a position-sensing detector and a lock-in amplifier to measure the displacement magnitude and phase responses of one latex sphere driven sinusoidally by optical tweezers. For a single particle in solution, the equation of motion of the particle is simply that of the forced oscillation problem with damping from viscous drag and the restoring force from the elasticity of the solution medium and that of the optical tweezers. Because the system is overdamped, it is not feasible to probe the high-frequency regime. Thus we cannot measure the viscosity and elastic moduli separately from frequency-dependent measurements alone. At low polymer concentration, measurements of the viscosity have been achieved. We compared the measured viscosity with that obtained with other measurements. Key issues for further development of this technique, such as measuring the elastic modulus, are briefly discussed.