Showing papers in "Journal of Physics: Condensed Matter in 1999"
TL;DR: In this paper, an overview of recent tests of the mode-coupling theory for the evolution of structural relaxation in glass-forming liquids is given, focusing on comparisons between the leading-order asymptotic formulae derived for the dynamics near glass transition singularities and the results of neutron scattering, depolarized light scattering, impulsive stimulated light scattering and dielectric-loss spectroscopy for conventional liquids.
Abstract: An overview is given of recent tests of the mode-coupling theory for the evolution of structural relaxation in glass-forming liquids. Emphasis is put on comparisons between the leading-order asymptotic formulae derived for the dynamics near glass transition singularities and the results of neutron scattering, depolarized light scattering, impulsive stimulated light scattering and dielectric-loss spectroscopy for conventional liquids. The tests based on photon-correlation spectroscopy results for the glassy dynamics of colloids and the findings of molecular dynamics simulations for model systems are also considered.
553 citations
TL;DR: In this paper, the authors calculate simple expressions for the photon tunnelling and find that there are drastic effects in many nanostructured systems, for example in the scanning tunneling microscope.
Abstract: Surfaces in close proximity exchange heat through evanescent photon tunnelling modes as well as by freely propagating modes. These additional near field contributions to radiation scale with separation, d, between surfaces as d-2 and are dominant at spacings d<
436 citations
TL;DR: In this paper, it was shown analytically, by appealing to Enskog's original results for the inverse-power potentials, that the quasi-universal entropy scaling can be extended also to dilute gases.
Abstract: A semi-empirical `universal' corresponding-states relationship, for the dimensionless transport coefficients of dense fluids as functions of the reduced configurational entropy, was proposed more than twenty years ago and established by many simulations. Here it is shown analytically, by appealing to Enskog's original results for the inverse-power potentials, that the quasi-universal entropy scaling can be extended also to dilute gases. The analytic form and the possible origin for the entropy scaling for dense fluids are discussed in view of this unexpected result. On the basis of the entropy scaling we predict a minimum in the shear viscosity as a function of temperature for all soft inverse-power potentials, in quantitative agreement with the available simulations.
392 citations
TL;DR: In this article, four n-type dopants have been found for ZrNiSn-based thermoelectric materials, which are Nb or Ta at the zirconium sites, and Sb or Bi at the tin sites.
Abstract: Four efficient n-type dopants have been found for ZrNiSn-based thermoelectric materials. These are Nb or Ta at the zirconium sites, and Sb or Bi at the tin sites. No suitable dopant was found for the nickel sites. In a alloy, a power factor of and a thermal conductivity of were measured at 300 K, resulting in a dimensionless figure of merit ZT = 0.12. These values are increased to and at 700 K.
341 citations
TL;DR: In this paper, the authors derived a formal expression for the effective one-component Hamiltonian of the colloids of a model colloid-polymer mixtures using simulation and the Percus-Yevick approximation.
Abstract: We study the phase behaviour and structure of model colloid-polymer mixtures. By integrating out the degrees of freedom of the non-adsorbing ideal polymer coils, we derive a formal expression for the effective one-component Hamiltonian of the colloids. Using the two-body (Asakura-Oosawa pair potential) approximation to this effective Hamiltonian in computer simulations, we determine the phase behaviour for size ratios q = p/c = 0.1, 0.4, 0.6, and 0.8, where c and p denote the diameters of the colloids and the polymer coils, respectively. For large q, we find both a fluid-solid and a stable fluid-fluid transition. However, the latter becomes metastable with respect to a broad fluid-solid transition for q0.4. For q = 0.1 there is a metastable isostructural solid-solid transition which is likely to become stable for smaller values of q. We compare the phase diagrams obtained from simulation with those of perturbation theory using the same effective one-component Hamiltonian and with the results of the free-volume approach. Although both theories capture the main features of the topologies of the phase diagrams, neither provides an accurate description of the simulation results. Using simulation and the Percus-Yevick approximation we determine the radial distribution function g(r) and the structure factor S(k) of the effective one-component system along the fluid-solid and fluid-fluid phase boundaries. At state-points on the fluid-solid boundary corresponding to high colloid packing fractions (packing fractions equal to or larger than that at the triple point), the value of S(k) at its first maximum is close to the value 2.85 given by the Hansen-Verlet freezing criterion. However, at lower colloid packing fractions freezing occurs when the maximum value is much lower than 2.85. Close to the critical point of the fluid-fluid transition we find Ornstein-Zernike behaviour and at very dilute colloid concentrations S(k) exhibits pronounced small-angle scattering which reflects the growth of clusters of the colloids. We compare the phase behaviour of this model with that found in studies of additive binary hard-sphere mixtures.
293 citations
TL;DR: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods as mentioned in this paper, and two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K, which is a remarkable feature of the results.
Abstract: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods. From inspection of the diffraction data two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K. A remarkable feature of the results is the regular variation in the out-of-phase octahedral tilt angle over the entire temperature range.
277 citations
TL;DR: In this paper, the geometrical and electronic structures for the magnetic structure of the Heusler alloys with composition were calculated and the structural trends with varying X and Y were explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.
Abstract: Magnetically driven actuator materials, such as the ternary and intermetallic Heusler alloys with composition , are studied within the density-functional theory (DFT) with the generalized gradient approximation (GGA) for the electronic exchange and correlation. The geometrical and electronic structures for the magnetic structure are calculated. The structures and magnetic moments at equilibrium are in good agreement with the experimental values. The structural trends with varying X and Y are explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.
241 citations
TL;DR: In this article, the Hartree-Fock approximation (HFA) is replaced by the dynamically screened potential W, e.g. iGW, which is a sum of a statically screened exchange potential plus a Coulomb hole (equal to the energy associated with the charge pushed away around a given electron).
Abstract: The GW approximation (GWA) extends the well-known Hartree-Fock approximation (HFA) for the self-energy (exchange potential), by replacing the bare Coulomb potential v by the dynamically screened potential W, e.g. Vex = iGv is replaced by GW = iGW. Here G is the one-electron Green's function. The GWA like the HFA is self-consistent, which allows for solutions beyond perturbation theory, like say spin-density waves. In a first approximation, iGW is a sum of a statically screened exchange potential plus a Coulomb hole (equal to the electrostatic energy associated with the charge pushed away around a given electron). The Coulomb hole part is larger in magnitude, but the two parts give comparable contributions to the dispersion of the quasi-particle energy. The GWA can be said to describe an electronic polaron (an electron surrounded by an electronic polarization cloud), which has great similarities to the ordinary polaron (an electron surrounded by a cloud of phonons). The dynamical screening adds new crucial features beyond the HFA. With the GWA not only bandstructures but also spectral functions can be calculated, as well as charge densities, momentum distributions, and total energies. We will discuss the ideas behind the GWA, and generalizations which are necessary to improve on the rather poor GWA satellite structures in the spectral functions. We will further extend the GWA approach to fully describe spectroscopies like photoemission, x-ray absorption, and electron scattering. Finally we will comment on the relation between the GWA and theories for strongly correlated electronic systems. In collecting the material for this review, a number of new results and perspectives became apparent, which have not been published elsewhere.
240 citations
TL;DR: In this article, the authors introduce the notion of glass as a solid material that has no translational or orientational order on the scale beyond O(10) diameters of the constituent particles (atoms, colloids, etc.).
Abstract: Glasses are materials that are ubiquitous in our daily life. We find them in such diverse items as window pans, optical fibers, computer chips, ceramics, all of which are oxide glasses, as well as in food, foams, polymers, gels, which are mainly of organic nature. Roughly speaking glasses are solid materials that have no translational or orientational order on the scale beyond O(10) diameters of the constituent particles (atoms, colloids, …) [1]. Note that these materials are not necessarily homogeneous since, e.g., alkali-glasses such as Na2O-SiO2 show (disordered!) structural features on the length scale of 6–10 A (compare to the interatomic distance of 1–2 A) and gels can have structural inhomogeneities that extend up to macroscopic length scales.
213 citations
TL;DR: In this article, the authors used the self-consistent tight-binding linearized muffin-tin orbital method with the atomic sphere approximation to predict zinc-based spinel oxides to be direct-gap materials.
Abstract: The electronic structure of zinc aluminate (ZnAl2O4) and that of zinc gallate (ZnGa2O4) were studied by the self-consistent tight-binding linearized muffin-tin orbital method with the atomic sphere approximation. The calculated results predict these zinc-based spinel oxides to be direct-gap materials. The direct gap at is found to be 4.11 eV for ZnAl2O4 and 2.79 eV for ZnGa2O4. With reference to the calculated band gap of 5.36 eV for MgAl2O4, the systematic decrease in the gap is attributed to the presence of 3d orbitals of Zn and Ga and the associated p-d hybridization in the upper valence band of zinc aluminate and gallate. Comparison of the contour maps of the electron localization function of ZnAl2O4 and ZnGa2O4 with that of MgAl2O4 clearly shows the bonding to be less ionic in the zinc-based spinel oxides. Finally, the calculations yield a smaller electron effective mass for zinc gallate as compared to that for zinc aluminate, suggesting a higher mobility of electrons in gallate.
206 citations
TL;DR: In this article, the structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements.
Abstract: The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mossbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.
TL;DR: The crystal structures of a large number of silica polytypes have been studied using density functional theory, both in the local density approximation and including generalized-gradient corrections to the exchange-correlation functional as mentioned in this paper.
Abstract: The crystal structures of a large number of silica polytypes (- and -quartz, - and -cristobalite, -tridymite, keatite, coesite and stishovite) have been studied using density functional theory, both in the local density approximation and including generalized-gradient corrections to the exchange-correlation functional. All crystal structures have been optimized by minimizing the total energy with respect to all lattice parameters and to the atomic coordinates within the unit cell (up to 40 structural parameters in the case of coesite). The transitions in quartz and cristobalite have been studied in detail, including different variants proposed for the structure of -cristobalite. The tetragonal (I2d) and simple cubic (P213) structures are found to be energetically almost degenerate near the equilibrium volume. On volume expansion both structures converge towards the idealized highly symmetric Fd3m structure. A similar continuous transition from a more compact orthorhombic (C2221) to a highly symmetric hexagonal (P63/mmc) variant is also proposed for -tridymite. For coesite two monoclinic variants (with C2/c and P21/c space-group symmetries, respectively) have been examined and found to be energetically degenerate to within 1 meV per SiO2 unit. It is shown that within the local density approximation (LDA) the equilibrium atomic volume of all polytypes is predicted with an accuracy better than one per cent. The LDA also leads to excellent structural predictions and to accurate values of the bulk modulus. Corrections in the framework of the generalized-gradient approximation (GGA) lead to substantially larger equilibrium volumes, although at fixed volume the LDA and GGA lead to identical crystal structures. The increased volume also leads to less accurate structural parameters. However, we find that gradient corrections are essential for achieving accurate structural energy differences between the tetrahedrally coordinated phases found at larger atomic volumes (all polytypes except stishovite) and the octahedrally coordinated high-pressure polymorphs (stishovite and post-stishovite phases).
TL;DR: In this paper, it has been suggested that the structural phase transition is driven by a band Jahn-Teller distortion involving redistribution of electrons between 3d sub-bands of different symmetries.
Abstract: Polarized neutron scattering has been used to determine the changes in the distribution of unpaired electrons which take place in the martensitic transition in Ni2MnGa. Ni2MnGa is a ferromagnetic Heusler alloy which undergoes a reversible transition at about 220 K from a high temperature cubic phase to a low temperature tetragonal one. It has been suggested, on the basis of band structure calculations, that the structural phase transition is driven by a band Jahn-Teller distortion involving redistribution of electrons between 3d sub-bands of different symmetries. The results of the neutron scattering experiments show that the transition from the cubic to the tetragonal phase is accompanied by a transfer of magnetic moment from Mn to Ni. The unpaired electrons in the cubic phase have overall eg symmetry. In the tetragonal phase, the degeneracy of the eg and t2g bands is raised and the unpaired electrons are redistributed in such a way that the sub-bands based on orbitals extending towards the c-axis are preferentially occupied. Although the experimental moments differ in detail from those expected from band structure calculations, the change in symmetry of the magnetization distribution is consistent with a band Jahn-Teller origin for the phase transition.
TL;DR: In this paper, the photoemission core-level spectra are analyzed in terms of a simple cluster model leading to estimates for the charge-transfer energy, the Coulomb correlation energy, and the hybridization strength V. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization.
Abstract: We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from to . The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy , the Coulomb correlation energy , and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.
TL;DR: The spin density wave (SDW) magnetism of thin epitaxial Cr films has recently become the focus of interest because of its mediating role in exchange coupled superlattices.
Abstract: The spin density wave (SDW) magnetism of thin epitaxial Cr films has recently become the focus of interest because of its mediating role in exchange coupled superlattices. While the incommensurate SDW magnetism and the Neel temperature are well established for bulk Cr, the question arises of how these properties are altered in thin films and superlattices either due to dimensionality effects or due to proximity with the ferromagnetic or paramagnetic boundary layers. After a brief introduction to the basic properties of bulk Cr, this review provides an overview of the SDW magnetism in thin Cr films, starting with surface properties and continuing with the discussion of Cr films of various thickness. The emphasis is more on SDW order in different confined environments than on exchange coupling. The scaling of the Neel temperature with thickness, the critical thickness for the onset of SDW order, the orientation of the SDW wave vector in different environments and the enhancement of SDW order due to proximity effects are extensively discussed. Most important is the role of the interface roughness in case of contact with a ferromagnetic layer. Conflicting results obtained with different experimental techniques are critically reviewed and an interpretation of the SDW order depending on interface quality is proposed.
TL;DR: In this paper, a technique based on macroscopic averages was used to reduce the effect of quantum size on thin-slab calculations and determine more precisely the work functions of metals from ab initio thin-film calculations.
Abstract: Quantum-size effects have been shown to influence significantly the determination of work functions from thin-slab calculations. We show here that a technique based on macroscopic averages can be used to reduce such effects and determine more precisely the work functions of metals from ab initio thin-film calculations. The technique combines the mean electrostatic potential step across the slab surface with the Fermi energy of a bulk crystal. The method is applied to Al(100) slabs containing 1-14 atomic layers.
TL;DR: The phase boundaries between the paramagnetic and ordered states were determined for and. The phase boundaries for these field directions coincide when normalized by the g-factor as discussed by the authors, and the excitation gap at zero temperature was re-evaluated as.
Abstract: The temperature and field variations of the magnetization have been measured for which has a singlet ground state with an excitation gap. It is found that undergoes three-dimensional ordering in magnetic fields. The phase boundaries between the paramagnetic and ordered states are determined for and . The phase boundaries for these field directions coincide when normalized by the g-factor. The excitation gap at zero temperature is re-evaluated as .
TL;DR: In this paper, the fragility parameter, m, in the general classification scheme of glass-forming liquids has been estimated from data provided in the literature for viscosity measurements close to and below the calorimetric glass transition temperature for 21 metallic alloys consisting of ternary, quaternary and quinary alloys.
Abstract: The fragility parameter, m, in the general classification scheme of glass-forming liquids has been estimated from data provided in the literature for viscosity measurements close to and below the calorimetric glass transition temperature for 21 metallic alloys consisting of ternary, quaternary and quinary alloys. These alloys are found to display an intermediate fragility strength with 32 m 66 and an average of m 50. Of this group of alloys, the bulk metallic glass formers (those with critical cooling rates less than 100 K s-1) are stronger liquids with m lying between 30 and 40. There appears to be a general positive correlation between exceptionally good glass-forming ability and stronger dynamical behaviour of supercooled metallic liquids.
TL;DR: In this paper, a review of the forces and structures in thin liquid soap films is presented, including electrostatic double-layer and dispersion forces, together with new and emerging areas of research that cover so-called ''supramolecular'' forces.
Abstract: This review is a topical survey of the forces and structures in thin liquid soap films. Included is a description of both the more classical forces, such as electrostatic double-layer and dispersion forces, together with new and emerging areas of research that cover so-called `supramolecular' forces. This latter category covers self-assembly of macromolecular structures confined in a thin-film region together with surface-induced adsorption of macromolecular complexes. In addition, recent extensions of the relation between thin-film forces and film stability are reviewed.
TL;DR: In this paper, a non-linear signal was observed for antiferromagnetic materials and the shift from linearity was attributed to the tilt in the octahedra of octahedral structures.
Abstract: and are members of the Aurivillius family of compounds having simultaneous electrical and magnetic ordering. Magnetoelectric measurements carried out in a linear time-varying magnetic field with an alternating-current field superimposed yielded a non-linear signal. The shift from linearity, which is not usually observed for antiferromagnetic materials, may be due the tilt in the octahedra. The variation of the magnetoelectric output with temperature for and indicated magnetic anomalies with enhanced sensitivity, corresponding to those in magnetization data.
TL;DR: In this article, the authors investigated nucleation and growth of hard sphere crystals using combined Bragg and small angle light scattering and of charged spheres using Bragg microscopy, and discussed the range of validity of classical nucleation theory and of Wilson Frenkel growth and a discussion of kinetic prefactors.
Abstract: Colloidal spheres interacting via a purely repulsive potential provide an excellent model system to study the kinetics of solidification from the melt. Such systems are readily accessible by comparably simple yet powerful optical methods like time resolved static light scattering and microscopy, reviewed in the first part of this paper. We then present results from our own recent studies within a framework of data available from literature. In particular we investigated nucleation and growth of hard sphere crystals using combined Bragg and small angle light scattering and of charged sphere crystals using Bragg microscopy. Special attention is given to the range of validity of classical nucleation theory and of Wilson Frenkel growth and a discussion of kinetic prefactors for both processes.
TL;DR: The double perovskite oxides Ca2FeMoO6, Sr2FeO6 and Ba2FeMO6 showed evidence of ferromagnetic metallic behavior, with Curie temperatures of 345-426 K and saturation magnetization of 3.5-3.9 µB/formula unit as mentioned in this paper.
Abstract: The double-perovskite oxides Ca2FeMoO6, Sr2FeMoO6 and Ba2FeMoO6 show evidence of ferromagnetic metallic behaviour, with Curie temperatures of 345-426 K and saturation magnetization of 3.5-3.9 µB/formula unit. Ceramics show little anisotropic magnetoresistance, but the low-field isotropic negative magnetoresistance of Sr2FeMoO6 is 7% in 1 T at room temperature and reaches 15% at 77 K. An effect of about 1% is found in 20 mT at room temperature in both ceramics and pressed powders of this compound.
TL;DR: In this article, the authors examine the unusual mechanical properties of nematic and smectic rubbers, their randomly disordered equilibrium textures, some aspects of dynamics and mechanical relaxation and the effect of uniform chiral piezoelectricity in amorphous polymer networks.
Abstract: Recent experimental and theoretical work shows that liquid-crystalline elastomers and gels have a highly mobile axis of anisotropy. Despite being nominally elastic solids, they also show features of fluids, such as the effect of soft elasticity. Work over the last few years is reviewed and some of the most important discoveries, as well as the outstanding problems in this field, are highlighted. We examine the unusual mechanical properties of nematic and smectic rubbers, their randomly disordered equilibrium textures, some aspects of dynamics and mechanical relaxation and the effect of uniform chiral piezoelectricity in amorphous polymer networks.
TL;DR: In this paper, the effects of photo-induced molecular reorientation are fully interpreted within the framework of classical electrodynamics and standard continuum theory of liquid crystals, and a detailed description of the mechanisms of direct optical torque, photoisomerization and photorefractivity and their effect on the macroscopic order of liquid crystal is reported.
Abstract: A review of basic physical phenomena underlying the light-induced molecular reorientation in nematic liquid crystals is presented. A detailed description of the mechanisms of direct optical torque, photoisomerization and photorefractivity and of their effect on the macroscopic order of liquid crystals is reported. The first part of the article deals with the study of reorientation effects in transparent liquid crystalline materials. Here, the effects of photo-induced molecular reorientation are fully interpreted within the framework of classical electrodynamics and standard continuum theory of liquid crystals. We investigate the peculiar properties related to the macroscopic anisotropy and the collective behaviour of liquid crystals that result in extraordinarily large nonlinear optical response. Afterwards, the behaviour of liquid crystals in the presence of light absorption is considered and the related reorientation effects are discussed. We give a review of the wide phenomenology which is met in liquid crystals when doped with absorbing azo-dye molecules. The photoisomerization process that in this case drives the evolution of the dye-liquid crystal mixture consequent to the interaction with the light is discussed in detail. Finally, the relatively new field of photorefractivity in liquid crystals as a source of molecular reorientation is considered. We describe the different mechanisms contributing to the creation of a space-charge field such as conductivity anisotropy, dielectric anisotropy and photocharge production. A theoretical discussion of the fundamental mechanisms regulating the dc-field-assisted optically induced space-charge fields and the optical molecular reorientation in nematic liquid crystal films is also given.
TL;DR: In this paper, a shell model is used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3 and the phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations.
Abstract: Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO3, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.
TL;DR: In this paper, the parameters and directions of the 13C hyperfine structure tensors for the NE1 center were found to confirm the previously proposed model for this defect, with a nickel ion at the centre of a double semivacancy as the basic structural unit.
Abstract: Electron paramagnetic resonance (EPR) and optical spectroscopy have been used to determine the structure and electronic state of nickel-nitrogen centres in natural diamonds and in synthetic diamonds enriched in 13C. The latter were grown in an Fe-Ni-C solvent/catalyst system at 1750 K, under stabilizing pressure, by the temperature gradient method and afterwards treated at high temperature and pressure. The parameters and directions of the 13C hyperfine structure (HFS) tensors for the NE1 centre were found to confirm the previously proposed model for this defect, with a nickel ion at the centre of a double semivacancy as the basic structural unit. In this unit the nickel atom has six atoms in its coordination shell. The NE1 centre has C2h symmetry, and the two equivalent nitrogen atoms in the coordination shell lie in the symmetry plane. New data on the HFS of 14N and 13C for the NE5 centre, also with C2h symmetry, indicated the same structural unit, but the two equivalent nitrogen atoms (and two equivalent carbon atoms) lie out of the symmetry plane and are related to one another by reflection in it. A new paramagnetic centre was found, labelled NE8, also with C2h symmetry, with four equivalent nitrogen atoms in the coordination shell all lying out of the symmetry plane. This centre is responsible for the 793.6 nm vibronic system in absorption and luminescence spectra. The new data have allowed a reinterpretation of the HFS tensors for the NE2 centre, which has C1 symmetry, suggesting that it has the same structure as NE1 but with one additional nitrogen atom in the coordination shell. The electronic states of these nickel-containing centres are discussed using the approach of Ludwig and Woodbury to transition metal ions in covalent crystals.
TL;DR: In this paper, the structural phase transitions in perovskite-like NaTaO3, from room temperature to 933 K, have been examined, and the structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes.
Abstract: Neutron powder diffraction has been used to examine the structural phase transitions in the perovskite-like NaTaO3, from room temperature to 933 K. The room temperature orthorhombic structure (Pbnm, a = 5.4768(1), b = 5.5212(1), c = 7.7890(2)) transforms to orthorhombic Cmcm at around 700 K, then to tetragonal P4/mbm at 835 K, and finally to cubic Pmm above 890 K. The structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes, the tilting of successive octahedra being out of phase along the b-axis, and in phase along the c-axis. The two tilt angles are comparable just above 700 K, but the out-of-phase tilt angle falls smoothly to zero as the transition to tetragonal is approached, in the manner suggestive of a tricritical transition. This results in an unusual variation of lattice parameters with temperature in the orthorhombic Cmcm phase. In the tetragonal phase the lattice parameters vary smoothly; however near the transition to cubic the in-phase tilt angle changes more rapidly with temperature than might be expected in a continuous phase transition.
TL;DR: In this paper, the fundamental magnetic observables, including magnetization, Curie temperature, magnetic moment per atom, susceptibility and magnetic anisotropy, for idealized prototype thin films like Fe, Co, Ni on metal substrates such as Cu, W, Re.
Abstract: In the last decade enormous effort has been made in research and investment to study magnetic properties of thin films because of their obvious practical applications. Coincidentally, it happens that theory has made enormous progress. Ab initio calculations and microscopic theories allow us for the first time in the history of magnetism to study and manipulate the magnetism on an atomic scale. In contrast to bulk magnetic materials, ultrathin films allow us to manipulate magnetism via the thickness and, by use of artificial structure growth, to produce structures which do not appear in nature. Here we discuss the fundamental magnetic observables, i.e. magnetization, Curie temperature, magnetic moment per atom, susceptibility and magnetic anisotropy, for idealized prototype thin films like Fe, Co, Ni on metal substrates such as Cu, W, Re. Finally, we present studies on trilayers, i.e. magnetic thin films separated by a spacer, like Cu. These trilayers present prototypes of interlayer coupling relevant for practical use of multilayer structures.
TL;DR: In this article, the authors investigated the complex ac conductivity of the Anderson-localized charge carriers for, temperatures and frequencies, and showed that the Anderson localization is responsible for the high resistivity regions observed at low doping levels and low temperatures.
Abstract: We have investigated the complex ac conductivity of for , temperatures and frequencies . In addition, results from dc measurements are presented. From the frequency dependence of the complex conductivity we find hopping of Anderson-localized charge carriers as the dominant transport process in certain temperature and composition ranges. We deduce that, while Anderson localization is not the driving mechanism for the metal-insulator transition observed in this compound, it is responsible for the high-resistivity regions observed at low doping levels and low temperatures. The results indicate a polaronic nature of the charge carriers. From the temperature dependence of the ac conductivity and the magnetic permeability, deduced in the skin-effect-dominated regime, various phase transition temperatures have been determined.
TL;DR: In this article, the author's exact envelope function representation method is clarified and contrasted with that of the conventional method, and a simple example showing how to obtain correct operator ordering in electronic valence band Hamiltonians is worked out in detailed tutorial style.
Abstract: The increasing sophistication used in the fabrication of semiconductor nanostructures and in the experiments performed on them requires more sophisticated theoretical techniques than previously employed. The philosophy behind the author's exact envelope function representation method is clarified and contrasted with that of the conventional method. The significance of globally slowly varying envelope functions is explained. The difference between the envelope functions that appear in the author's envelope function representation and conventional envelope functions is highlighted and some erroneous statements made in the literature on the scope of envelope function methods are corrected. A perceived conflict between the standard effective mass Hamiltonian and the uncertainty principle is resolved demonstrating the limited usefulness of this principle in determining effective Hamiltonians. A simple example showing how to obtain correct operator ordering in electronic valence band Hamiltonians is worked out in detailed tutorial style. It is shown how the use of out of zone solutions to the author's approximate envelope function equations plays an essential role in their mathematically rigorous solution. In particular, a demonstration is given of the calculation of an approximate wavefunction for an electronic state in a one dimensional nanostructure with abrupt interfaces and disparate crystals using out of zone solutions alone. The author's work on the interband dipole matrix element for slowly varying envelope functions is extended to envelope functions without restriction. Exact envelope function equations are derived for multicomponent fields to emphasize that the author's method is a general one for converting a microscopic description to a mesoscopic one, applicable to linear partial differential equations with piecewise or approximately piecewise periodic coefficients. As an example, the method is applied to the derivation of approximate envelope function equations from the Maxwell equations for photonic nanostructures.