Showing papers in "Journal of Physics: Condensed Matter in 1998"

Low frequency plasmons in thin-wire structures

Abstract: A photonic structure consisting of an extended 3D network of thin wires is shown to behave like a low density plasma of very heavy charged particles with a plasma frequency in the GHz range. We show that the analogy with metallic behaviour in the visible is rather complete, and the picture is confirmed by three independent investigations: analytic theory, computer simulation and experiments on a model structure. The fact that the wires are thin is crucial to the validity of the picture. This new composite dielectric, which has the property of negative below the plasma frequency, opens new possibilities for GHz devices.

Microscopic origin of magnetocrystalline anisotropy in transition metal thin films

Abstract: We investigate the relation between the magnetocrystalline anisotropy energy (MAE) and the electronic structure for transition metal thin films and surfaces which can display enhanced orbital magnetic moments. When the spin-orbit interaction is treated in second order, the MAE is proportional to the expectation value of the orbital magnetic moment as given by Bruno's model. However, there are additional terms which are related to the spin-subband orbital moment and to the magnetic dipole operator due to the anisotropy of the field of the spin. The latter term accounts for the spin-flip excitations between the exchange split majority and minority spin bands. A conjecture is proposed which relates the MAE to the expectation values of the orbital moments and the magnetic dipole term. It is shown how the different terms can be obtained experimentally with (transverse) magnetic circular x-ray dichroism. The model explains the experimentally observed perpendicular magnetic anisotropy in Co and Fe based multilayers and thin films.

Universality of phase transitions of frustrated antiferromagnets

Abstract: Recent theoretical and experimental studies on the critical properties of frustrated antiferromagnets with non-collinear spin order, including stacked-triangular antiferromagnets and helimagnets, are reviewed. Particular emphasis is put on the novel critical and multicritical behaviours exhibited by these magnets, together with the important role played by the `chirality'.

Crossover from semiconductor to magnetic metal in semi-Heusler phases as a function of valence electron concentration

Abstract: Experimental and theoretical investigations of intermetallic semi-Heusler compounds (CoTiSn, FeTiSb, CoTiSb, NiTiSn, CoNbSn, CoVSb, NiTiSb) and their solid solutions are presented. The physical properties of these systems are found to be mostly determined by the number of valence electrons. Resistivity experiments show that compounds with 18 valence electrons are either semiconductors (CoTiSb, NiTiSn) or semi-metals (CoNbSn). The electronic structure calculations performed on 18-valence-electron systems by the KKR method show nine valence bands below the Fermi level and a gap of order 0.4-0.9 eV. A decrease or increase of the number of valence electrons in CoTiSb, NiTiSn or CoNbSn leads in either case to a metallic state and either ferromagnetic (CoTiSn, CoVSb) or paramagnetic (FeTiSb, NiTiSb) properties. The KKR results concerning 17- and 19-valence-electron systems correspond well with experimental characteristics, except in the case of CoVSb which KKR calculations predict to be a half-metallic ferromagnet, which conflicts with experimental data. Magnetization and resistivity measurements indicate that semiconductor-metal crossovers occur together with the appearance of ferromagnetism in the and series, for x near 0.4. This behaviour is discussed in the context of the KKR-CPA results.

Numerical renormalization group calculations for the self-energy of the impurity Anderson model

Abstract: We present a new method for calculating directly the one-particle self-energy of an impurity Anderson model with Wilson's numerical renormalization group method by writing this quantity as the ratio of two correlation functions. This way of calculating turns out to be considerably more reliable and accurate than that via the impurity Green's function alone. We give results for the self-energy for the case of a constant coupling between the impurity and the conduction band and the effective arising in the dynamical mean-field theory of the Hubbard model. The implications of the problem of the metal-insulator transition in the Hubbard model are also discussed.

Sheet superconductivity in twin walls: experimental evidence of

Abstract: Single crystals of that contain ferroelastic twin walls have been chemically deoxygenated using a gas transport reaction with Na vapour. The reaction product consists of a non-superconducting matrix of tetragonal and superconducting twin boundaries. The superconducting transition temperature is 3 K. The upper critical field has been measured; its temperature dependence shows BCS type behaviour at T < 2.5 K and a weak tail near .

On the existence of positively charged single-substitutional nitrogen in diamond

Abstract: Infra-red (IR) absorption results on irradiated and annealed synthetic diamond are presented which confirm an earlier proposal that a component found in the defect-induced one-phonon region of some diamonds arises from positively charged single-substitutional nitrogen . The concentration ratio of to neutral substitutional nitrogen centres may be changed by shining light of various energies onto the examined samples. By correlating changes in absorption of the IR component associated with centres with changes in the component, and using a previously determined relation between the concentration of centres and peak absorption coefficient at 1130 , the relationship between peak absorption at 1332 and concentration of centres has been derived, namely 1 of absorption is produced by ppm centres. Other defects may also give rise to absorption at 1332 , but the component is uniquely identified by further peaks at 1046 and 950 . The significance of this component is demonstrated by the fact that some samples can contain in excess of 80 ppm centres, and this must consequently be accounted for when assaying the total nitrogen concentration in such samples. Using the above relationship useful parameters relating the concentration of neutral vacancies, negative vacancies and negatively charged nitrogen-vacancy centres to their respective zero-phonon line integrated absorptions have been derived.

The relationship between field-cooled and zero-field-cooled susceptibilities of some ordered magnetic systems

Abstract: Analysis of the irreversible field-cooled (FC) and the zero-field-cooled (ZFC) magnetic susceptibilities of one ferrimagnetic and three ferromagnetic systems, measured at different applied magnetic fields, shows that the irreversibility indicated by the difference between the FC and the ZFC susceptibilities arises from magnetic anisotropy. The two susceptibilities are related to each other through the coercivity which is a measure of the anisotropy. The ZFC susceptibility can be calculated from the FC susceptibility (or vice versa) and the coercivity.

Self-consistent-field theories for complex fluids

Abstract: Recent developments in off-lattice self-consistent-field theories for inhomogeneous complex fluids are reviewed. Particular emphasis is given to the treatment of intermolecular interactions and compressibility, to the role of fluctuations, and to the discussion of the coarse-graining length which is inherent to the theory. Valuable insight can be gained from the comparison of self-consistent-field calculations with Monte Carlo simulations. Finally, some applications of the theory to orientational properties of polymers and copolymers at interfaces, and to the phase behaviour of amphiphiles at surfaces, are presented.

Preparation of nano-crystalline diamonds using pulsed laser induced reactive quenching

Abstract: Nano-crystalline diamond is prepared with a unique method in which a graphite target is etched by a high-power pulsed laser in water. Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) indicate nano-crystalline diamond is obtained, having a hexagonal lattice or cubic lattice.

Experimental evidence for the existence of absolute acoustic band gaps in two-dimensional periodic composite media

Abstract: Transmission of acoustic waves in two-dimensional binary solid/solid composite media composed of arrays of Duralumin cylindrical inclusions embedded in an epoxy resin matrix is studied. The experimental transmission spectrum and theoretical band structure of two periodic arrays of cylinders organized on a square lattice and on a centred rectangular network are reported. Absolute gaps extending throughout the first two-dimensional Brillouin zone are predicted. The measured transmission is observed to drop to noise level throughout frequency intervals in reasonable agreement with the calculated forbidden frequency bands.

Pre-edge fine structure of the 3d atom K x-ray absorption spectra and quantitative atomic structure determinations for ferroelectric perovskite structure crystals

Abstract: A complete interpretation is proposed for the pre-edge fine structure (PEFS) of the x-ray Ti K-absorption spectra for perovskite structure crystals. The interpretation is based on the results of numerous calculations performed by a modified full multiple scattering method which provides the theoretical spectra for the 3d transition metal oxides in fair agreement with experiment. It is shown that the three main peaks in the PEFS have quite different origin. The first long-wave side peak A is caused mainly by quadrupole transitions. The middle peak B is caused by the p-d mixture effect and the high intensity of it is considered to be a qualitative spectroscopic indication of ferroelectricity in the perovskite structure crystal. A simple formula is obtained which expresses the area under peak B through the lattice constants and mean-square displacement of the absorbing Ti atom from the instantaneous centre of the coordination polyhedron. The peak B area averaged over thermal atomic vibrations is determined by the three-particle atomic distribution function. The short-wave side peak C is caused by the Ti 1s electron transition to the unoccupied 3d states of the neighbouring transition metal atoms. We show that an additional peak on the short-wave side of peak C occurs if there are 4d atoms (for instance Zr atoms in the vicinity of the absorbing Ti atom in the (PZT) solid solution) within the oxygen atom octahedrons surrounding the absorbing 3d atom. The area under peak is directly determined by the average number of 4d atoms in the vicinity of the absorbing Ti one.

Wetting of rough surfaces

Abstract: A theory is presented to describe the wetting phenomena and the contact line depinning as a function of the microstructure of rough surfaces. The noise and fluctuations of the quenched disorder on self-affine rough surfaces play a important role in the analysis of the spreading of liquids on non-planar substrates. By using the long-range noise correlation function, functional relationships that show the influence of surface roughness on the contact angle, the critical surface tension and the depinning of the contact line are derived. Roughness enhances wetting and broadens the three-phase contact line.

Work functions of elements expressed in terms of the Fermi energy and the density of free electrons

Abstract: On the basis of Brodie's definition of the work function and the length of spontaneous polarization of plasma, the following new formula for calculation of the work functions of elements has been derived: , where is the electron density parameter expressed in units of the Bohr radius, is the Fermi energy and is an empirical constant ( for the alkali metals, Ca, Sr, Ba, Ra and Tl, whereas for the remaining elements). The density parameter was calculated from the atomic mass, the bulk density of the element and the assessed number of free electrons per atom which is equal to the nominal valence of the element or, in the case of transition metals, close to this number (within ). The values obtained by using the above formula are in excellent agreement with experimental data for pure-metal polycrystalline surfaces, within 5% in most cases. A table with the work functions and with complete input data for most of the elements is presented.

A neutron diffraction investigation into the rhombohedral phases of the perovskite series

Abstract: Rietveld refinements using neutron powder profiles are reported for a series of samples (commonly known as PZT), with x ranging from to . Cation shifts, octahedral distortion and tilts are determined with varying composition across the ferroelectric rhombohedral regions, and , of the PZT phase diagram. These parameters are then used in conjunction with a simple Landau-Devonshire model to investigate the nature of the phase transition. It is found that the cation shifts, octahedral distortion and tilt angles decrease with increasing Ti content, but, surprisingly, the octahedral strain, as indicated by the rhombohedral angle, increases. This is in contrast to the case for all other known rhombohedral perovskites. Furthermore, the refined anisotropic displacement parameters of the cations are anomalous and cannot be accounted for by the average crystal structure. A model is presented in which a domain-type `local' structure is considered, containing `ordered' additional cation displacements, consistently with the reports of extra reflections observed in electron microscopy studies by Viehland et al, Dai et al and Ricote et al.

Strong hybridization in vanadium oxides: evidence from photoemission and absorption spectroscopy

Abstract: We present x-ray photoemission spectra of the vanadium oxides , and , and their analysis in terms of a simple cluster model based on the Anderson impurity Hamiltonian. The electronic structure of these materials is characterized by a strong V 3d-O 2p hybridization energy which exceeds the energy scales related to on-site Coulomb correlation and metal-ligand charge transfer. This result is at variance with the usual Mott-Hubbard picture, but agrees with recent studies of other early 3d transition metal compounds. The V 3d ground-state occupations obtained by the cluster-model analysis are considerably higher than the values derived from the formal valencies. Covalency also affects the exchange splitting observed in the V 3s core-hole spectra. X-ray absorption measurements and resonant photoemission spectroscopy at the V 2p-3d threshold provide further evidence for a strong V 3d-O 2p coupling.

Frustration in Ising-type spin models on the pyrochlore lattice

Abstract: We compare the behaviour of ferromagnetic and antiferromagnetic Ising-type spin models on the cubic pyrochlore lattice. With simple `up - down' Ising spins, the antiferromagnet is highly frustrated and the ferromagnet is not. However, such spin symmetry cannot be realized on the pyrochlore lattice, since it requires a unique symmetry axis, which is incompatible with the cubic symmetry. The only two-state spin symmetry which is compatible is that with four local anisotropy axes, which direct the spins to point in or out of the tetrahedral plaquettes of the pyrochlore lattice. We show how the local `in - out' magnetic anisotropy reverses the roles of the ferro- and antiferromagnetic exchange couplings with regard to frustration, such that the ferromagnet is highly frustrated and the antiferromagnet is not. The in - out ferromagnet is a magnetic analogue of the ice model, which we have termed the `spin ice model'. It is realized in the material . The up - down antiferromagnet is also an analogue of the ice model, albeit a less direct one, as originally shown by Anderson. Combining these results shows that the up - down spin models map onto the in - out spin models with the opposite sign of the exchange coupling. We present Monte Carlo simulations of the susceptibility for each model, and discuss their relevance to experimental systems.

Spin waves and the critical behaviour of the magnetization in

Abstract: Inelastic neutron scattering from a single crystal of the quasi-two-dimensional antiferromagnet has been used to measure the spin wave dispersion curve at 4 K. The exchange integrals were subsequently calculated from linear spin wave theory. The values meV, meV, meV and meV are within stability conditions calculated from mean-field theory. In addition, the critical behaviour of the gap in the spin wave energy at the Brillouin zone centre has been measured, and compared to the critical behaviour of the magnetization from neutron scattering data of the magnetic (020) Bragg peak. The gap varies with magnetization for , and with the square of the magnetization for . Two possible explanations are proposed: a competition between single-ion and dipolar anisotropies; or a crossover to XY-like excitations.

The Hubbard model - an introduction and selected rigorous results

Abstract: The Hubbard model is a `highly oversimplified model' for electrons in a solid which interact with each other through extremely short-ranged repulsive (Coulomb) interaction. The Hamiltonian of the Hubbard model consists of two parts: which describes quantum mechanical hopping of electrons, and which describes non-linear repulsive interaction. Either or alone is easy to analyse, and does not favour any specific order. But their sum is believed to exhibit various non-trivial phenomena including metal-insulator transition, antiferromagnetism, ferrimagnetism, ferromagnetism, Tomonaga-Luttinger liquid, and superconductivity. It is believed that we can find various interesting `universality classes' of strongly interacting electron systems by studying the idealized Hubbard model. In the present article we review some mathematically rigorous results relating to the Hubbard model which shed light on the `physics' of this fascinating model. We mainly concentrate on the magnetic properties of the model in its ground states. We discuss the Lieb-Mattis theorem on the absence of ferromagnetism in one dimension, Koma-Tasaki bounds on the decay of correlations at finite temperatures in two dimensions, the Yamanaka-Oshikawa-Affleck theorem on low-lying excitations in one dimension, Lieb's important theorem for the half-filled model on a bipartite lattice, Kubo-Kishi bounds on the charge and superconducting susceptibilities of half-filled models at finite temperatures, and three rigorous examples of saturated ferromagnetism due to Nagaoka, Mielke, and Tasaki. We have tried to make the article accessible to non-experts by giving basic definitions and describing elementary materials in detail.

Diffusion controlled grain growth in primary crystallization: Avrami exponents revisited

Abstract: The anomalous behaviour of the Avrami exponents found in the primary crystallization of amorphous alloys leading to nanostructured materials is considered. A kinetic model able to adequately treat such phase transformation has been formulated by means of the implementation of a soft-impingement diffusion mechanism after a transient interface controlled growth. A decrease in the nucleation rate as crystallization proceeds has also been considered. Comparison of the model with experimental data is performed, giving excellent agreement. The soft-impingement diffusion mechanism is demonstrated to be responsible for the anomalous behaviour of the Avrami exponent, the decreasing nucleation rate being a second-order effect.

Dielectric response analysis of a conducting polymer dominated by the hopping charge transport

Abstract: The d.c. conductivity and the electric a.c. response from 100 Hz up to 40 MHz of poly(3n-decylpyrrole) were measured in the 80-330 K interval to characterize the charge transport behaviour of the system. The d.c. conductivity well fitted the variable range hopping model, and the loss factor, after having deducted the d.c. contribution, showed a relaxation peak when the conductivity versus frequency started to rise. The strength of this relaxation increased with temperature and became too large to be related to a dipolar relaxation; moreover, the temperature dependence of the loss peak frequency and d.c. conductivity coincided. The observed relaxation was attributed to the hopping charge transport, as further confirmed by the temperature behaviour of the relaxation strength and by the frequency dependence of the exponents of the power law which locally approximate the conductivity behaviour. As the activation energy of the d.c. conductivity differed from the frequency of the loss peak, the theoretical prediction concerning the selfsimilarity of the a.c. conductivity was roughly verified.

General models for the distributions of electric field gradients in disordered solids

Abstract: Hyperfine studies of disordered materials often yield the distribution of the electric field gradient (EFG) or related quadrupole splitting (QS). The question of the structural information that may be extracted from such distributions has been considered for more than fifteen years. Experimentally most studies have been performed using Mossbauer spectroscopy, especially on . However, NMR, NQR, EPR and PAC methods have also received some attention. The EFG distribution for a random distribution of electric charges was for instance first investigated by Czjzek et al [1] and a general functional form was derived for the joint (bivariate) distribution of the principal EFG tensor component and the asymmetry parameter . The importance of the Gauss distribution for such rotationally invariant structural models was thus evidenced. Extensions of that model which are based on degenerate multivariate Gauss distributions for the elements of the EFG tensor were proposed by Czjzek. The latter extensions have been used since that time, more particularly in Mossbauer spectroscopy, under the name `shell models'. The mathematical foundations of all the previous models are presented and critically discussed as they are evidenced by simple calculations in the case of the EFG tensor. The present article only focuses on those aspects of the EFG distribution in disordered solids which can be discussed without explicitly looking at particular physical mechanisms. We present studies of three different model systems. A reference model directly related to the first model of Czjzek, called the Gaussian isotropic model (GIM), is shown to be the limiting case for many different models with a large number of independent contributions to the EFG tensor and not restricted to a point-charge model. The extended validity of the marginal distribution of in the GIM model is discussed. It is also shown that the second model based on degenerate multivariate normal distributions for the EFG components yields questionable results and has been exaggeratedly used in experimental studies. The latter models are further discussed in the light of new results. The problems raised by these extensions are due to the fact that the consequences of the statistical invariance by rotation of the EFG tensor have not been sufficiently taken into account. Further difficulties arise because the structural degrees of freedom of the disordered solid under consideration have been confused with the degrees of freedom of QS distributions. The relations which are derived and discussed are further illustrated by the case of the EFG tensor distribution created at the centre of a sphere by m charges randomly distributed on its surface. The third model, a simple extension of the GIM, considers the case of an EFG tensor which is the sum of a fixed part and of a random part with variable weights. The bivariate distribution is calculated exactly in the most symmetric case and the effect of the random part is investigated as a function of its weight. The various models are more particularly discussed in connection with short-range order in disordered solids. An ambiguity problem which arises in the evaluation of bivariate distributions of centre lineshift (isomer shift) and quadrupole splitting from Mossbauer spectra is finally quantitatively considered.

Analysis of ferroelectric switching in finite media as a Landau-type phase transition

Abstract: A detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions. Some important characteristics of homogeneous switching have been emphasized and later used in studying the more general case of inhomogeneous switching. The two extremes of switching current correspond to the inflexion points of the dielectric hysteresis loop. Hysteresis loops of poled ferroelectric samples are expected to include negative-susceptibility regions, for high-frequency applied electric fields. The switching current minimum is eliminated by the experimental method used for recording the switching responses. Equivalent Landau coefficients and electric fields have been defined, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter. We correlated the size effects on the critical parameters of the switching (the coercive field) and the ferroelectric-to-paraelectric phase transition (the Curie temperature). Polarization reversal in small-size ferroelectrics can be regarded as a diffuse phase transition, whereas its character is closer to normal for large-size samples. The size dependencies of the reversal speed and maximum current result from the size dependencies of the equivalent Landau coefficients and electric field inducing reversal.

A local moment approach to the Anderson model

Abstract: A theory is developed for the single-particle spectra of the symmetric Anderson model, in which local moments are introduced explicitly from the outset. Dynamical coupling of single-particle processes to low-energy spin-flip excitations leads, within the framework of a two-self-energy description, to a theory in which both low- and high-energy spectral features are simultaneously captured, while correctly preserving Fermi liquid behaviour at low energies. The atomic limit, non-interacting limit and strong-coupling behaviour of the spectrum are each recovered. For strong coupling in particular, both the exponential asymptotics of the Kondo resonance and concomitant many-body broadening of the Hubbard satellite bands are shown to arise naturally within the present approach.

Excitons and optical nonlinearities in hybrid organic-inorganic nanostructures

Abstract: We present a theoretical review of the properties of electronic excitations in nanostructures based on combinations of organic materials with inorganic semiconductors, having respectively Frenkel excitons and Wannier-Mott excitons with nearly equal energies. We show that in this case the resonant coupling between organic and inorganic quantum wells (or wires or dots) may lead to several interesting effects, such as splitting of the excitonic spectrum and enhancement of the resonant optical nonlinearities. First, we discuss the properties of hybrid Frenkel-Wannier-Mott excitons, which appear when the energy splitting of the excitonic spectrum is large compared to the width of the exciton resonances (the case of strong resonant coupling). Such peculiar excitations share at the same time both the properties of the Wannier excitons (e.g., the large radius) and those of the Frenkel excitons (e.g., the large oscillator strength). We discuss mainly two-dimensional configurations (interfaces or coupled quantum wells) which are the most extensively studied. In particular, we show that hybrid excitons are expected to have resonant optical nonlinearities significantly enhanced with respect to those of traditional inorganic or organic systems. We also consider analogous phenomena in microcavities where the exciton resonances are close to the cavity photon mode resonance. Next, we consider the case of weak resonant coupling and show the relevance of the Forster mechanism of energy transfer from an inorganic quantum well to an organic overlayer. Such an effect may be especially interesting for applications: the electrical pumping of excitons in the semiconductor quantum well can be used to efficiently turn on the organic material luminescence.

The cubic-tetragonal phase transition in strontium titanate: excess specific heat measurements and evidence for a near-tricritical, mean field type transition mechanism

Abstract: The transition between the cubic and tetragonal phase in shows an excess specific heat of . Comparison between the temperature evolution of the excess entropy and the structural order parameter Q shows within experimental errors . The apparent order parameter exponent was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy with A = 0.70 J , B = 31.22 J , C = 42.17 J , K and K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around and a small tail of excess entropy at .

Charge ordering in the rare-earth manganates: the origin of the extraordinary sensitivity to the average radius of the A-site cations,

Abstract: The charge ordering in $Nd_{0.5}Sr_{0.5}MnO_3 (\langle r_A\rangle = 1.24 \AA),$ which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For $Y{0.5}Ca_{0.5}MnO_3 (\langle r_A\rangle = 1.13 \AA),$ for which the ground state is charge ordered, on the other hand, magnetic fields have no effect on the charge ordering. In order to understand such a marked difference in charge-ordering behaviour of the manganates, we have investigated the structure as well as the electrical and magnetic properties of $Ln_{0.5}Ca_{0.5}MnO_3$ compositions (Ln D Nd, Sm, Gd and Dy) wherein $\langle r_A\rangle$ varies over the range $1.17–1.13 \AA$. The lattice distortion index, D, and charge-ordering transition temperature, $T_{CO}$, for the manganates increase with the decreasing $\langle r_A\rangle$. The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of $Nd_{0.5}Ca_{0.5}MnO_3$ $(\langle r_A\rangle = 1.17 \AA),$ but this is not the case with the analogous Sm, Gd and Dy manganates with $\langle r_A\rangle$ less than $1.17 \AA$. In order to explain this behaviour, we have examined the $\langle r_A\rangle$ -dependence of the Mn–O–Mn bond angle, the average Mn–O distance and the apparent one-electron bandwidth, obtained from these structural parameters. It is suggested that the extraordinary sensitivity of the charge ordering to $\langle r_A\rangle$ arises from factors other than those based on the Mn–O–Mn bond angle and average Mn–O distances alone. It is possible that the competition between the covalent mixing of the oxygen $O:2p\sigma$ orbital with the A-site and B-site cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in $T_{CO}$.

Molecular-dynamics simulation of structural and thermodynamic properties of boron nitride

Abstract: Structural and thermodynamic properties of cubic boron nitride (c-BN) under pressure and for varying temperature are studied by molecular-dynamics (MD) simulation with the use of a well-tested Tersoff potential. Various physical quantities including the thermal expansion coefficient and heat capacity are predicted. Our simulation is extended to study liquid boron nitride at various densities.

Mössbauer spectroscopic examination of a single crystal of

Abstract: Mossbauer spectra have been recorded in fields of up to 13 T from a single crystal of cut perpendicular to the [111] axis. The spectra were best fitted with five magnetic components complementing studies on a crystal cut normal to a [100] axis. One component corresponds to situated on the tetrahedral A sites of the inverse spinel-related structure whilst the other four correspond to and on two non-equivalent octahedral sites.