Table of contents

The coefficient of thermal expansion of the Si-O bond has been obtained from neutron total scattering measurements of five different phases of silica, with value (2.2±0.4)×10^-6 K^-1. This value is smaller than values obtained by conventional x-ray diffraction measurements corrected for rigid-body thermal motion. Three of the datasets used in this study, tridymite and two zeolite structures, are completely new.

In line with our previous study (Eglitis R I et al 1998 J. Phys.: Condens. Matter10 6271) for a single Nb impurity and Nb clusters in KTaO₃ we present here the results of calculations for a series of perovskite KNb_xTa_1-xO₃ (KTN) solid solutions (x = 0, 0.125, 0.25, 0.75, and 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-centre in KTaO₃ already at the lowest studied Nb concentration. Its off-centre displacement is in a good agreement with XAFS measurements. We compare our results with previous FP-LMTO calculations.

Direct evidence of a new phase transition at T_CO = 100.6 K in the organic quasi-one-dimensional (TMTTF)₂AsF₆ salt has been obtained from conductivity and dielectric permittivity measurements. This phase transition is assigned to a charge-ordered state (4k_F charge density wave of Wigner type) due to charge-induced electron-electron interactions.

The infrared to visible luminescence of the Nd³⁺ ion in the Ca₃Ga₂Ge₃O₁₂:Nd garnet crystal at different Nd concentrations has been investigated in the excitation spectral range corresponding to diode pumping (720-840 nm) under continuous wave and pulsed pumping. Three visible bands centred at 541, 601 and 677 nm were observed, these emissions mainly departing from the excited state ⁴G_7/2. By using a simple rate equation model to fit the experimental data, it was possible to infer that this excited state is populated via up-conversion energy transfer between Nd³⁺ ions in the garnet crystal.

It is pointed out that real-space images recovered from LEED I /E data by the current holographic reconstruction algorithm can contain strong artifacts which can be misinterpreted as atomic images (`ghost atoms'), thereby misguiding a subsequent structural refinement through conventional LEED. We show that such ghost atoms can be avoided by using an alternative approximation to the kernel in the reconstruction integral. This is demonstrated for both calculated and experimental intensities of the structure considered, i.e. a (2×2) phase of 6H-SiC(000). A theory is also developed for a practical implementation of a more general kernel which fully takes account of the scattering of an electron by the substrate atoms before its first encounter with the adatom (beam splitter).

The surface stress, which characterizes the state of stress at the surface of a macroscopic crystal, is calculated from first principles by two methods for Mo(001) to be 3.1 mRyd bohr^-2 = 2.4 J m^-2. Both methods use the energy of a fully relaxed seven-layer slab as a function of the in-plane lattice parameter a. One method uses the slope and the other the curvature at particular values of a. Fully relaxed energies give surface stress values 40% smaller than partially relaxed energies which have relaxed just a single common layer spacing. The slab is divided into bulk and surface regions with different parameters. Estimates are made of the surface region parameters including its equilibrium in-plane lattice constant, its epitaxial elastic constant, its Poisson ratio for in-plane strains and its thickness.

A constant-temperature molecular dynamics (MD) simulation was performed to model the soft-landing and adsorption of C₆₀ molecules on a graphite substrate with the C₆₀s treated as soft molecules and released individually towards the substrate. The intra-molecular and intra-planar covalently bonding interactions were modelled by very accurate many-body potentials, and the non-bonding forces were derived from various pairwise potentials. The simulation extended over 1.6 million time steps covering a significant period of 160 picoseconds. The final alignment of the molecules on the surface agrees closely with that observed in an experiment based on scanning tunnelling microscopy (STM) on the same system, performed at room temperature and under ultrahigh-vacuum (UHV) conditions. Using a tungsten tip in a constant-current mode of imaging, we have also computed the STM-like images of one of the adsorbed molecules using a formulation of the STM tunnelling current based on Keldysh's non-equilibrium Green function formalism. Our aim has been to search for tip-induced states, which were speculated, on the basis of another STM-based experiment, performed in air, to form one of the possible origins of the extra features purported to have been observed in that experiment. We have not obtained any such states.

The electronic states of a finite crystal are studied using Goodwin's model of a tight-binding linear chain of N one-level atoms with nearest-neighbour overlap. Using a transfer matrix approach we obtain the explicit form of the secular equation which correctly yields N eigenvalues in the interval (0,π) of wavenumber q, unlike Goodwin's equation which involves spurious solutions at q = 0 and q = π. We present a new general analysis of bulk- and surface-state eigenvalues as a function of the parameter ε₀/γ describing the difference (ε₀) of Coulomb integrals for surface and bulk atoms relative to the overlap integral γ. We identify four distinct domains of values of |ε₀/γ| in three of which one or two surface states of different origins exist, which we determine explicitly. Our discussion is valid for both signs of ε₀/γ and differs considerably in detail from Goodwin's analysis. In particular, it does not require distinct analyses for chains with even and odd numbers of sites.

The interference between boundary and bulk scattering processes is analysed for ultrathin films with random rough walls. The effective collision and transport relaxation times for scattering by random bulk and surface inhomogeneities are calculated, when possible analytically, in quantum size effect conditions. The transport and localization results are expressed via the bulk transport parameters and statistical characteristics of the surface corrugation. The diagrammatic calculation includes second-order effects for boundary scattering and full summation for bulk processes. The interference contribution is large in systems with robust bulk scattering and can be comparable to, or even exceed, the pure wall contribution to the transport coefficients.

We studied the persistence of magnetism in ultrathin nickel films on copper. Layer-dependent magnetic moments in Ni films on the (001), (110) and (111) surfaces of Cu have been calculated using the Korringa-Kohn-Rostoker Green's function method. The results show that, at temperature T = 0, a single nickel monolayer is ferromagnetic on Cu(001) and Cu(110) but magnetically `dead' on the more closely packed Cu(111) surface. Films of two and more layers of Ni are always ferromagnetic, with the magnetic moment enhanced in the surface layer but strongly reduced in the interface layer. Due to the short screening length, both the effect of the interface and that of the surface are confined to only a few atomic layers.

The process of oxidation of the Bi₂Te₃ surface was investigated by x-ray photoelectron spectroscopy (XPS). The oxidized surface layer was found to have a definite thickness, with configurations where O is bonded with Bi and Te, and Bi and Te are bonded with three and four oxygens, respectively. The oxidation time dependence of the oxidized layer thickness d(t) estimated from the XPS behaved as (t-t₀)^1/2 when d(t) was smaller than the thickness of a single oxidized quintuple atomic layer in our oxide model and behaved as t-t₁ when it was larger than that. Experimental data were compared to our oxidation process model for the layered structure with the van der Waals gap and very good agreement was found.

We report a study on the structural and magnetic properties of iron-vanadium thin films grown in multilayer form and mixed by thermal treatment. The multilayer was composed of 30 Å Fe layers alternated by 30 Å of V. This typical bilayer was repeated 15 times. The samples were structurally characterized by x-ray diffraction (XRD) in the (θ-2θ) geometry. The magnetic characterization was made using a conventional alternate gradient magnetometer (AGM) with the magnetic field applied along the plane of the film, and by conversion electron Mössbauer spectroscopy (CEMS). This multilayer was annealed at temperatures between 550 and 670 °C for 60 minutes. The XRD result for the as-deposited multilayer shows a high-degree crystallinity, while CEMS suggests an abrupt interface, since no significant contribution from vanadium in iron is observed. After the thermal treatment, the results from XRD show a phase transformation of the bcc-disordered structure (α phase) into a tetragonal structure (σ phase). CEMS results show a magnetic moment reduction for temperatures above 640 °C and an order-disorder transition. The magnetic measurements suggest also a phase transformation.

The transition between nanometre-scaled diamond and graphite as well as the corresponding thermodynamic functions are discussed based on the temperature-pressure phase diagram of carbon including the contribution due to surface effects. As a result, the equilibrium transition size between diamond and graphite as a function of temperature and the thermodynamic function for the transition are obtained. It is found that as the size and temperature decrease, diamond becomes more stable than graphite. The obtained result is consistent with the reported experimental results on the chemical vapour synthesis of nanometre-sized diamond under ambient pressure.

In this paper we examine the existence of anharmonic localized modes using a full two-body potential to describe all interactions between particles in a zinc-blende-structure material, and we make a comparison with the results obtained with a nearest-neighbour force constant model that includes interactions up to quartic anharmonicity. We show that for amplitude up to a maximum displacement of the order of 0.25 Å there are no appreciable differences between the two approaches, while for the largest amplitudes the force constant model gives unphysical results.

The structural phase transformations under high pressure of ZnO and ZnS have been investigated by using the Vogel-Krüger-Pollmann (VKP) scheme, in which the electronic self-interaction correction to the local density approximation (LDA) is introduced in a non-self-consistent manner within the pseudopotential approach. In these calculations, I have used highly optimized pseudopotentials and a plane-wave expansion of the wavefunctions. Moreover, the electronic structures of the zinc-blende (ZB) and rock-salt (RS) phases of both compounds have been similarly calculated. It has been found that the VKP scheme provides a highly improved description, relative to the LDA results, for the structural and electronic structure properties of the considered systems. However, the so-calculated transition pressures of the ZB-to-RS transition for both ZnO and ZnS are found to be significantly larger than the experimental data. RS-ZnO is predicted to be an indirect-gap semiconductor, with a wide band gap of 4.2 eV.

We have performed an ab initio investigation for a series of boron compounds, BP, BAs, and BSb, and have compared their structural and electronic properties with those of c-BN. The calculations are performed using a plane-wave expansion within the local density approximation and the pseudopotential approximation. Results are given for lattice constants, bulk moduli, band structures, and band-gap pressure coefficients. The electronic properties of these compounds are shown to have features that differ from those of other III-V materials. We found that the direct-band-gap pressure coefficient in boron compounds is nearly independent of the anion substitutions. As a result, this trend is similar to the one resulting from cation substitutions in other zinc-blende compounds. This is another anomalous behaviour which can be characterized by reversing the standard assignments for the anion and cation in these compounds.

Metamagnetism in YCo₂ and LuCo₂ is investigated in the spin-fluctuation theory, considering the contribution from both the zero-point and the thermal spin fluctuation. The metamagnetic transition at low temperature and the temperature dependence of the paramagnetic susceptibility at finite temperature are investigated in a self-consistent numerical calculation based on the spin-fluctuation theory and good agreements with experimental results have been obtained in a systematic way. It is shown that the effect of strong enhancement of the paramagnetic susceptibility and the electronic specific coefficient at low temperature can be explained satisfactorily by taking into consideration the zero-point spin fluctuation.

We show that a Thue-Morse aperiodic structure presents a unique kind of positional correlation between its constituents, leading to an unattenuated transmission of light as well as electrons through it. The reason for this is a resonant tunnelling, whose origin can be traced back to the presence of certain `dimers' which are not explicitly displayed in the structure. It is interesting to observe that under suitable conditions, two apparently uncorrelated constituents in a Thue-Morse sequence combine together to form a dimer and the entire system can be thought of as being composed of nested dimers only. This aspect has been analysed in terms of light propagation through a Thue-Morse multilayered system and its electronic counterpart.

We have extended the fractional-dimensional space approach to study exciton states and diamagnetic shifts in symmetric coupled double GaAs-Ga_1-xAl_xAs quantum wells. In this scheme, the fractional dimension is chosen using an analytical procedure, and the real anisotropic `exciton + double quantum well' semiconductor system is mapped, for each exciton state, into an effective fractional-dimensional isotropic environment. We have performed calculations within the fractional-dimensional space scheme for the binding energies of 1s-like heavy-hole direct excitons and for the energy difference between 1s- and 2s-like direct heavy-hole exciton states in GaAs-Ga_1-xAl_xAs symmetric coupled double quantum wells. Also, theoretical results were obtained for the magnetic-field dependence of the 1s-like heavy-hole exciton energy shift and for the exciton diamagnetic coefficient in quantum wells and symmetric coupled double quantum wells. Fractional-dimensional theoretical results are shown to be in good agreement with available experimental measurements and previous theoretical calculations.

We present a study of electronic behaviours in the k-component Fibonacci (KCF) quantum waveguides, in which k different incommensurate intervals are arranged according to a substitution rule. On the basis of the transfer matrix method, the quantum transmission properties of the KCF stub structures are obtained. It is shown that the transmission coefficient depends on the wavevector of the electron and the number of different incommensurate intervals k. For the KCF waveguides with the same k, on increasing the number of stubs, the minima in transmission extend gradually into the band gap over which the transmission is blocked. Meanwhile more transmission peaks appear. For finite KCF stub structures, on increasing the number of different incommensurate intervals k, the total transmission over the spectral region of interest decreases gradually and the width of the electronic band gap is enlarged. Moreover, when the value of k is large enough, the transmission is basically shut off, except at a few energies where resonant tunnelling takes place. These properties make it possible to use this kind of KCF waveguide as a switching device for digital applications. On the other hand, the charge-density distributions in these structures are singularly continuous. We propose that they can be analysed using a multifractal concept. A dimensional spectrum of singularities associated with the charge density, f (α), demonstrates that the electronic transport in the KCF waveguide presents scaling properties; hence the charge-density distribution shows a genuine multifractality.

The nearest-neighbour-interaction spin-1 Ising spin glass, in the presence of a random crystal field, is considered on diamond hierarchical lattices of fractal dimensions d = 2, 3 and 4. The coupling constants and crystal fields follow Gaussian probability distributions, which are taken as independent, at the beginning of the iteration process. By monitoring simultaneously the evolution of two probability distributions, associated respectively with the renormalized coupling constants and crystal fields, the phase diagrams of the model are obtained. A spin-glass phase, at finite temperatures, is found for hierarchical lattices with d = 3 and 4, but not for d = 2. Two distinct attractors characterized by zero effective coupling constants are detected. Following the usual procedure, i.e. associating an equilibrium phase with each basin of attraction, one obtains two phases with absence of magnetic order, namely, a zero-spin phase (where the spins prefer the 0 state) and a ±1-spin phase (where the spins prefer ±1 states at random).

We study a quantum generalization of the infinite-range Sherrington-Kirkpatrick spin-glass model with biaxial crystal-field effects described by two uniaxial anisotropy parameters D_x and D_y. For spin dimensionality S = 1 we report an analytical and numerical analysis in the (T, D_x, D_y) parameter space (with T being the temperature). For D ≡ D_x = D_y the model effectively becomes classical and identical with the crystal-field-split spin-glass Ising model (introduced by Ghatak and Sherrington) showing a discontinuous phase transition to the spin-glass phase on a portion of the T-D line.

Considering the inter-site electron-electron interaction, we investigate the ground state and spin-wave excitation of a theoretical model proposed for quasi-one-dimensional π-conjugated organic polymer ferromagnets. Within mean-field theory, the ground state of the system is shown to be a high-spin ferromagnetic state due to the topological structure of the system. By employing the random-phase approximation (RPA), the spin-wave excitation spectrum is obtained. It is found that the acoustic spin-wave mode displays the feature of the ferromagnetic magnon, and the inter-site electron-electron interaction has influence on it. With the increase of inter-site electron-electron interaction, the high-spin ferromagnetic ground state of the system will be unstable.

The magnetoresistance oscillation of the charge density wave (CDW) material NbSe₃ whisker has been studied under strain at different temperatures (4.2 K, 30 K, 40 K). With a constant measuring current, the magnetoresistance (MR) curves oscillate at 30 K while they are smooth at 4.2 K and become a series of zigzag curves at 40 K. These oscillation periods are ΔH, rather than Δ(1/H ). It is found that either increasing the temperature to 40 K, or straining the sample to a displacement as large as ε = 3.1%, makes the regular oscillations disappear. By analysing the data, we found our results can be interpreted as the quantum coherence effect of a moving CDW.

Low-field magnetic properties of ceramic La_1-xCa_xMnO₃ (0⩽x⩽0.4) are investigated between T = 5 and 310 K. The paramagnetic-ferromagnetic transition is observed in all the samples. The dependence of the Curie temperature, T_C, on x is described within a model of spin polarons associated with electronic localization. Critical behaviour of the susceptibility χ^-1(T ) ~ (T-T_C)^γ is observed for T>T_C, with the critical exponents γ = 1.20±0.05 and γ* = 1.64±0.06 below and above the composition x≈0.18 corresponding to the Mn⁴⁺ ion concentration c ≈ 0.23, respectively. For the compound with x = 0.3 no temperature hysteresis of the resistivity is observed in magnetic fields between 0 and 8 T. In all the samples the field-cooled and zero-field-cooled magnetizations deviate below T_C, the difference being approximately equal to the thermoremanent magnetization (TRM). Long-time relaxation of TRM in LCMO is observed for time scales up to 10⁴ s. The relaxation rate reaches a maximum near a wait time t_W ~ 10³ s. The time dependence of TRM can be described with a stretched exponential law, as in spin or cluster glasses in conditions where the observation time is comparable with t_W .

Atom-atomic potential calculation of the channel non-stoichiometric thiourea-hexachloroethane inclusion compound shows that the structure of the guest sublattice comprises two types of finite molecular chain, having different structure and separated by domain walls. In the present paper we present results of ³⁵Cl NQR and ¹H NMR measurements of thiourea-hexachloroethane, [2.95(NH₂)₂CS]C₂Cl₆, in the temperature range from 7.5 to 90 K, which confirm this model and show the existence of such a state at least below 60 K. Two resonances in the NQR spectra were assigned to the two nearly commensurate regions, while the third resonance, showing an anomalous behaviour, was attributed to the guest molecules in the domain wall. The observed structure results from the different periodicity of the guest and host substructures and shows a difference from conventional continuum models of the incommensurate state. Propagation motion of the domain wall over the channel is discussed.

Information on both radial and angular atomic coordinations is provided by the quadrupole interaction between the quadrupole moment of the atomic nuclei and the electric field gradients (EFGs) originating from the distribution of electric charges around the nuclei. ⁶⁹Ga and ⁷¹Ga are quadrupolar nuclei active in NMR. Their spectra are recorded in amorphous GaF₃ which may be considered as a model compound for disordered fluorides with fluorine corner sharing octahedra. An unique and continuous quadrupolar parameter Czjzek distribution allows us to simulate the experimental spectra. The measured chemical shifts indicate that the Ga³⁺ ions are at the centre of (GaF₆)^3- octahedra in the amorphous GaF₃ phase.

A polarizable point charge model is used to explain NMR quadrupolar parameters which are related to the electric field gradients at the Ga site. Lattice summations are performed in the direct space over spherical volumes. Atomic position sets generated by molecular dynamics are shown to give quadrupolar parameter distributions which look like Czjzek ones whatever the EFG calculation approximations. Provided the polarizabilities are adjusted, they allow us to reconstruct the experimental NMR spectra which correspond in any case to slightly distorted (GaF₆)^3- octahedra. The present approach may be applied to any ionic disordered compound which contains quadrupolar nuclei and used to quantify short range order around such nuclei.

The dielectric relaxation responses of pelite, which is a porous silicate sediment containing a low content of inherent humidity, were identified and characterized by employing the experimental scheme of the thermally stimulated depolarization current (TSDC) spectroscopy. Comparative experiments were performed on dry samples. The elementary responses that compose the dielectric spectrum were recorded by applying certain sampling techniques. The dielectric relaxation spectrum consists of two low-temperature mechanisms, which are related to different modes of relaxation of water molecules. A third one is probably produced by permanent dipoles consisting of point defects in the calcium participant. Three relaxation mechanisms were sampled within the intermediate temperature region and were strongly affected by the outgassing of the pore network. They correspond to polarization processes occurring in the multi-layer shell of humidity over the surface the solid aggregates. At higher temperatures, two mechanisms were traced: the first is related to the homogeneous polarization of the specimen as charge carriers migrate within conducting territories until they are trapped at internal boundaries and the latter is described as a long-distance conduction mechanism which is enhanced by the presence of humidity. The activation energy profiles of the above-mentioned relaxation mechanisms were obtained from the analysis of the experimental signals of the thermal sampling and the partial heating schemes.

We study the optical quantum-confined Stark effect in the emission spectra of a symmetric quantum well coherently coupled by an infrared laser. It is shown that this effect is dynamically affected by the layer interface fluctuations and other defects, causing a field-dependent quenching of the emission spectra. We discuss how at low carrier densities the quenching process can be attributed to the intraband transitions between the populated exciton states and at high carrier densities to those between the conduction subbands.

Recent electronic structure calculations for the title compounds performed by Wu et al [1] are critically reconsidered, applying high precision full-potential bandstructure methods. It is shown that the bandstructure calculations presented by the authors contain several important inconsistencies, which make their main conclusions highly questionable.

In this reply our previous calculations are checked. Small variances of the calculated results do not change our conclusions.

Figure 10 of this article was reproduced incorrectly, the correct version is given below.

Temperature evolution of the normalized zero-time amplitudes of the two components in the ZF-µ⁺SR spectra of La_1.875Ba_0.025Sr_0.100CuO₄. The amplitudes are associated with the fractions of the muons due to randomly frozen (No 1: quasi-static) and paramagnetic (No 2: superconducting) spin states. The inset shows the depolarization rate, σ of the quasi-static component No 1.