Table of contents

In our recent study Xu et al (2002 Chem. Phys. Lett. 364 57–63), a phase transformation from the hexagonal to the tetragonal structure in the annealed ZnO films on silicon was studied by atomic force microscopy. Cathodoluminescence (CL) and glancing-angle x-ray diffraction analysis of the ZnO films indicated that such a transformation is due to the generation of a tetragonal zinc silicate. In order to identify the formation mechanism of the zinc silicate and the bottom broadening of the UV band, a depth profile secondary ion mass spectroscopy experiment was carried out. The results show that vast atomic diffusion between the ZnO film and the silicon substrate occurred due to the annealing temperature. Such interdiffusion can create not only a mixed crystal of ZnO and Zn₂SiO₄, but also an amorphous silicon dioxide (a-SiO₂) in a deep range from the surface to the interface of the ZnO/Si system. The a-SiO₂ is most probably the source of the 453 nm blue band hidden in the tail of the 390 nm UV band, since the blue band agrees with the CL spectra of the amorphous quartz glass and the thermally oxidized silicon.

The results of Stern–Gerlach experiments on free clusters of Fe, Co, and Ni are usually interpreted in terms of magnetic moments that show oscillations as a function of cluster size. We demonstrate that the observed behaviour can be more convincingly explained in terms of magnetic anisotropy energies (MAEs) that oscillate with the size of the clusters. The magnitudes of the estimated MAEs are in reasonably good agreement with the experimental results for supported/embedded cluster assemblies. The oscillation of the MAE of a cluster with respect to its size may reveal the geometrical shell structures of free transition metal clusters.

We are the first group to succeed in measuring the dynamic structure factor S(Q,ω) of liquid Si close to melting using high-resolution inelastic x-ray scattering. The spectra clearly demonstrate the existence of propagating short wavelength modes in the melt with a Q–ω relation similar to those in other liquid metal systems. A specific variation of the quasi-elastic line shape with increasing Q is observed close to the structure factor maximum. This observation is related to the onset of atomic correlations on the sub-picosecond timescale in the vicinity of a metal-to-insulator transition. Such observations have been made previously only in computer simulations of metallic systems with increasing covalent character. Our data provide the first experimental evidence for these ultrashort density correlations.

An analytical expression for the contact value of the density of n-component mixtures under gravity has been obtained by means of the inhomogeneous pressure equation within fluids. The gravity-induced liquid–solid transition temperature has been estimated for hard spheres, Lennard-Jones spheres, and binary hard sphere mixtures and has been compared with those of other approximations. The density functional approximation based on the weighted density has also been used to calculate the density profile and the pressure of hard spheres and Lennard-Jones spheres at the liquid–solid interface.

The behaviour of a solution of equilibrium polymers (or living polymers) between two surfaces is studied using a Bethe–Guggenheim lattice model for molecules with orientation-dependent interactions. The average monomer concentration, the average length of the chains and the interaction between the surfaces are calculated as a function of the separation distance between the surfaces. When the gap is in full equilibrium with a homogeneous bulk solution, the equilibrium polymers cause a depletion attraction, which becomes stronger with increasing bulk monomer concentration. The range of the interaction passes through a maximum as a function of the concentration. In dilute solutions the range of the interaction increases and the strength decreases with increasing bonding energy, while above the overlap concentration the bonding energy is irrelevant. For restricted equilibrium between the gap and the bulk, when the amount of polymer in the gap is determined by the flow of fluid out of the gap upon compression, the interaction becomes repulsive. This repulsion becomes stronger with increasing concentration and depends only very weakly on the bonding energy. Two limiting cases for the fluid flow were considered: (i) perfect-slip conditions at the surfaces, resulting in a constant monomer concentration in the gap and (ii) no-slip conditions at the surfaces, resulting in a parabolic flow profile of solution out of the gap.

In this paper, the interaction of Q-switched Y AG:Nd³⁺ laser radiation of moderate intensity (up to 50–100 MW cm⁻²) with an aqueous suspension of submicron light-absorbing particles is investigated. The experiments reveal significant self-induced attenuation of laser radiation which is accompanied by laser-induced incandescence of suspended particles. The transformation of shape is also observed for the laser pulses propagating through the suspension and for the scattered pulses. A model is proposed which describes the experimental data obtained. The model supposes that gaseous shells grow around the laser-heated particles due to the vaporization of surrounding water. With the use of the proposed model, calculations were performed for the optical transmittance of the cell and for the shape of the transmitted and scattered laser pulses. The calculated and experimental data are in satisfactory agreement with each other. It is also concluded that only a small fraction (approximately 0.01–0.5%) of the laser-heated particle energy is spent in evaporation of the surrounding water.

Disc shaped samples of the glass composition: 0.40Bi₂O₃–0.60B₂O₃ were prepared by a melt quenching technique by keeping the quenching rate constant but gradually increasing the melt annealing or ageing time from 15 to 220 min at a temperature of 850 °C. A total of five glass samples were prepared. Density, ultrasonic velocity and glass transition temperature measurements showed significant changes in the final glass properties. X-ray fluorescence studies confirmed that the glass composition does not change with the heat treatment of the melt. Our findings challenge the conventional theories of liquids and glass formation which predict that structural changes in a low viscosity melt occur almost instantaneously and a liquid is in its internal equilibrium state above the melting point. We report some unusually slow structural relaxations in bismuth borate glass melts even at elevated temperatures of 850 °C, due to which the melt transforms from a thermodynamically fragile, high density amorphous state to a low density amorphous state which is kinetically stronger. Our findings indicate that bismuth borate melts are metastable liquids and strong candidates for the phenomenon of liquid state polyamorphism.

Magnetic susceptibility, electron paramagnetic resonance (EPR) and optical absorption have been studied in a glass system 20La₂O₃–22Al₂O₃–23B₂O₃– 35(SiO₂+GeO₂) with a part of La₂O₃ substituted by Gd₂O₃ in different concentrations. Positive Weiss constants have been found in more heavily doped glasses and ascribed to clustering of Gd³⁺ ions. Computer simulations of the EPR spectra show that the short-range ordering in the environment of the Gd³⁺ ions is well preserved. The relative distribution widths of the ligand coordinates are less than 2%. In the more heavily doped glasses the EPR spectra are superpositions of signals arising from isolated ions and ferromagnetic clusters. The increase of Gd³⁺ concentration is shown to change substantially the strong optical absorption edge while only small changes of f–f absorption band characteristics are observed. This difference is associated with the different effect of the Gd ion clustering on the mechanisms of the strong absorption in the ultraviolet region and the f–f absorption.

First three non-vanishing sum rules for the bulk and longitudinal stress auto-correlation functions have been evaluated for liquid Rb at six thermodynamic states along the liquid–vapour coexistence curve. The Mori memory function formalism and the frequency sum rules have been used to calculate bulk and longitudinal viscosities. The results thus obtained for the ratio of bulk viscosity to shear viscosity have been compared with experimental and other theoretical predictions wherever available. The values of the bulk viscosity have been found to be more than the corresponding values of the shear viscosity for all six thermodynamic states investigated here.

We perform a detailed analysis of solutions of the inverse problem applied to experimentally measured two dimensional radial distribution functions for highly charged latex dispersions. The experiments are carried out at high colloidal densities and under low-salt conditions. At the highest densities studied, the extracted effective pair potentials contain a long-range attractive part. At the same time, we find that for the best distribution functions available the range of stability of the solutions is limited by the nearest neighbour distance between the colloidal particles. Moreover, the measured pair distribution functions can be explained by purely repulsive pair potentials contained in the stable part of the solution.

Bloch and Wannier functions of the Kohn type for a quite general one-dimensional Hamiltonian with inversion symmetry are studied. Important clarifications on null minigaps and the symmetry of those functions are given, with emphasis on the Kronig–Penney model. The lack of a general selection rule on the miniband index for optical transitions between edge states in semiconductor superlattices is discussed. A direct method for the calculation of Wannier–Kohn functions is presented.

The effect of host glass composition on the optical absorption and fluorescence spectra of Nd³⁺ has been studied in mixed alkali borate glasses of the type xNa₂O–(30− x)K₂O–69.5B₂O₃–0.5Nd₂O₃ (x = 5, 10, 15, 20 and 25). Various spectroscopic parameters such as Racah (E¹, E² and E³), spin–orbit (ξ_4f) and configuration interaction (α,β) parameters have been calculated. The Judd–Ofelt intensity parameters (Ω_λ) have been calculated and the radiative transition probabilities (A_rad),radiative lifetimes (τ_r), branching ratios (β) and integrated absorption cross sections (Σ) have been obtained for certain excited states of the Nd³⁺ ion and are discussed with respect to x. From the fluorescence spectra, the effective fluorescence line widths (Δλ_eff) and stimulated emission cross sections (σ_p) have been obtained for the three transitions , and of Nd³⁺. The stimulated emission cross section (σ_p) values are found to be in the range (2.0–4.8) × 10⁻²⁰ cm² and they are large enough to indicate that the mixed alkali borate glasses could be potential laser host materials.

We show theoretically that the photonic band structures of photonic crystals (PhCs) with constituent materials containing free carriers can be tuned by an external magnetic field in the frequency range below the far infrared region. We take one-dimensional PhCs consisting of an intrinsic InSb layer and an air layer stacked alternately as an example in studying photonic band structures and transmissive properties within the magnetic field.

The thermal conductivities of stoichiometric CaB₆, vacancy-doped Ca_1−δB₆, and EuB₆ have been measured between 6 and 300 K. All our data may be rather well described across the entire temperature regime covered on the basis of a Debye-type relaxation-time approximation and by assuming the concurring influence of various scattering channels on the mean free path of the phonons. An unusual and strong resonance in the scattering rate of the phonons of all investigated materials is attributed to a strong interaction between acoustic itinerant and localized modes, the latter arising from oscillations of the metal cations around their equilibrium position.

In the present work we report results from neutron and synchrotron radiation diffraction measurements that confirm the presence of a very weak phase transition in Sr₂CuWO₆ at about 600 °C. This phase transition is continuous and changes the symmetry from I4/m at low temperature to another tetragonal phase at high temperature. On the basis of the experimental results we identify I4/mmm as the most probable space group for the high-temperature tetragonal phase.

An elegant formula for coordinates of carbon atoms in a unit cell of a single-walled nanotube (SWNT) is presented and the potential of neon (Ne) inside an infinitely long SWNT is analytically derived under the condition of the Lennard-Jones potential between Ne and carbon atoms. Specific heats of dilute Ne inside a long (20, 20) SWNT are calculated at different temperatures. It is found that Ne exhibits three-dimensional (3D) gas behaviour at high temperature but behaves as a 2D gas at low temperature. Especially, at ultra-low temperature, Ne inside (20, 20) nanotubes behaves as a lattice gas. A rough method to determine the characteristic temperature for low density gas in a potential is put forward. If , we just need to use the classical statistical mechanics without solving the Schrödinger equation to consider the thermal behaviour of gas in the potential. But if , we must solve the Schrödinger equation. For Ne in a (20, 20) nanotube, we obtain  K.

The temperature behaviour of the optical absorption edge in the range of phase transitions for CuCrP₂S₆ crystals is studied. The absorption edge is shown to be described by the Urbach rule with different convergence point coordinates in antiferroelectric and paraelectric phases. In the intermediate phase there is no convergence point and a parallel shift of the exponential absorption edge is observed. The specific features of the temperature behaviour of the absorption edge are treated assuming the intermediate phase in CuCrP₂S₆ crystals to be an incommensurate one.

In this paper we study the twist disclination within the elastoplastic defect theory. Using the stress function method, we found exact analytical solutions for all characteristic fields of a straight twist disclination in an infinitely extended linear isotropic medium. The elastic stress, elastic strain and displacement have no singularities at the disclination line. We found modified stress functions for the twist disclination. In addition, we calculate the disclination density, effective Frank vector, disclination torsion and effective Burgers vector of a straight twist disclination. By means of gauge theory of defects we decompose the elastic distortion into the translational and rotational gauge fields of the straight twist disclination.

The real defect structure of LiB₃O₅ crystal grown by a top seeded solution growth method has been observed using x-ray projection and reflective topography. Space mapping of such defects as growth sector boundaries, striation and dislocations have been produced by the topography analysis. The density of the dislocations is as low as <30 cm⁻². It has been shown that the dominant part of the dislocations detected is generated in the under-seed zone of the crystal.

The effect of Al content on the Zener relaxation peak of annealed Fe–Al alloys has been investigated using a computer-controlled automatic inverted torsion pendulum through the method of free decay and forced vibration. It has been shown that alloys with medium Al contents show strong relaxation while lower or higher Al contents lead to relatively weak Zener relaxation peaks. The Zener relaxation in Fe–Al alloys originates from the next-nearest-neighbour atom pairs and their interaction rather than the nearest-neighbour atom pairs. The latter may not produce Zener relaxation because of the ordered structure, which suppresses Zener relaxation.

Magnetotransport properties of the compound Ca₄Mn₃O₁₀ are interpreted in terms of activated hopping of small magnetic polarons in the non-adiabatic regime. Polarons are most probably formed around Mn³⁺ sites created by oxygen sub-stoichiometry.

The application of an external field reduces the size of the magnetic contribution to the hopping barrier and thus produces an increase in the conductivity. We argue that the change in the effective activation energy around T_N is due to the crossover to variable range hopping conduction as antiferromagnetic order sets in.

Inelastic neutron scattering measurements were made on CePt₂ at pressures up to 5 kbar. At ambient pressure spin wave excitations were observed in an antiferromagnetic phase that transformed into a quasi-elastic response above T_N. At low temperatures (), the magnetically ordered state appears to show some instability at a pressure of around 4 kbar where the dynamic susceptibility determined from the neutron data exhibits distinctive features that resemble those of heavy fermion compounds showing non-Fermi liquid behaviour.

We introduce and discuss a new class of solutions of the Dorokhov–Mello–Pereyra–Kumar (DMPK) equation in which some of the eigenvalues are grouped into clusters which are conserved in the asymptotic large distance limit (i.e. as the length of the wire increases). We give an explicit expression for the asymptotic expansion of these solutions and suggest some possible applications. In particular, these new solutions could be useful for avoiding the quasi-one-dimensional constraint in the DMPK equation and for studying the crossover between the metallic and insulating phases.

We present a model for microwave photoconductivity of two-dimensional electron systems in a magnetic field which describes the effects of strong microwave and steady-state electric fields. Using this model, we derive an analytical formula for the photoconductivity associated with photon- and multi-photon-assisted impurity scattering as a function of the frequency and power of microwave radiation. According to the developed model, the microwave conductivity is an oscillatory function of the frequency of microwave radiation and the cyclotron frequency which becomes zero at the cyclotron resonance and its harmonics. It exhibits maxima and minima (with absolute negative conductivity) at microwave frequencies somewhat different from the resonant frequencies. The calculated power dependence of the amplitude of the microwave photoconductivity oscillations exhibits pronounced sublinear behaviour similar to a logarithmic function. The height of the microwave photoconductivity maxima and the depth of its minima are nonmonotonic functions of the electric field. The possibility of a strong widening of the maxima and minima due to a strong sensitivity of their parameters on the electric field and the presence of strong long-range electric-field fluctuations is pointed to. The obtained dependences are consistent with the results of the experimental observations.

In Be-doped MgO crystals, a new hole centre has been discovered whose physical properties are similar to those of [Li]⁰-type centres (a hole localized at a cation impurity). The centre can be observed best by electron paramagnetic resonance (EPR) at liquid He temperature; thermal destruction of the centre through the loss of a hole takes place at about 160 K. The symmetry of the centre is almost tetragonal, with a slight rhombic distortion, and the hyperfine structure of its EPR spectrum suggests the following model for the centre: Be²⁺–O⁻. However, a detailed investigation of the temperature dependence of the EPR spectrum did not reveal, contrary to our preliminary announcement, any temperature averaging. At higher temperatures the spectrum is complicated by the wealth of spectral lines induced by the ubiquitous Be impurity ions. In particular, this led to our former misinterpretation. The physical properties of the observed centre are discussed and compared with those of analogous centres in MgO.

Single-phase perovskite PbMg_1/3Nb_2/3O₃ (PMN) ceramics were synthesized from stoichiometric oxide mixtures under conditions of high pressures and temperatures. Results of dielectric studies of this material show that the high-pressure PMN ceramics consist of two phases. One of them exhibits the properties of a dipole-ordering phase with a diffuse transition and another is a relaxor–ferroelectric phase. When annealing, the high-pressure synthesized PMN ceramics acquire the features of classical relaxor PMNs. The effect of high-pressure processing on cation ordering/displacements and thereby on dielectric properties is discussed.

An experimental investigation of the thermal behaviour of the dissolution process of the Suzuki phase (SP) by continuous heating (1 °C min⁻¹) of KBr:Eu²⁺ crystals is reported in this work. The thermal profiles were determined by the correlation functions between subsequent photoacoustic (PA) signals registered during the dissolution process. The behaviour of the thermal profile is directly related to the absorption coefficient of the Eu²⁺ ion in precipitated states that are present in the crystal. The PA signal is detected as a consequence of the non-radiative processes that take place after the excitation of the low-energy band of the Eu²⁺ ion by means of a focused laser pulse at 355 nm. The results obtained by this method are compared with those simultaneously obtained by the photoluminescence (PL) technique. The samples were heated from room temperature to 205 °C. The PA signal and PL spectrum were obtained every 6 °C. The temperature range of the SP dissolution process was from 77 to 115 °C. These results are in agreement with those obtained by the PL technique and with the data reported in the literature.

The microdomain–macrodomain transformations and phase transitions in 0.69Pb(Mg_1/3Nb_2/3)O₃–0.31PbTiO₃ single crystals were investigated by measuring the temperature dependence of relative permittivity as a function of crystal orientation and applied dc bias. A rhombohedral–tetragonal phase transition in the original crystal has been confirmed by powder x-ray diffraction at temperatures from 25 to 180 °C. The results show that microdomain relaxor ferroelectric states in the original crystals could be transformed to macrodomain ferroelectric states by applying a dc bias along any of the , and directions. It is also proposed that an electric field applied along the or direction could induce an orthorhombic or a monoclinic ferroelectric phase, respectively, between the rhombohedral and tetragonal ferroelectric phases in an intermediate temperature range.

This paper was published without the consent of E Tronc and M Nogues. They did not take part in this work and do not approve it. The list of authors is therefore corrected as: D Predoi, V Kuncser, U Russo, G Principi and G Filoti.