Table of contents

We report an unusual insulating state in one-dimensional quantum wires with a non-uniform confinement potential. The wires consist of a series of closely spaced split gates in high mobility GaAs/AlGaAs heterostructures. At certain combinations of wire widths, the conductance abruptly drops over three orders of magnitude, to zero on a linear scale. Two types of collapse are observed, one occurring in multi-subband wires in zero magnetic field and one in single-subband wires in an in-plane field. The conductance of the wire in the collapse region is thermally activated with an energy of the order of 1 K. At low temperatures, the conductance shows a steep rise beyond a threshold DC source–drain voltage of order 1 mV, indicative of a gap in the density of states. Magnetic depopulation measurements show a decrease in the carrier density with lowering temperature. We discuss these results in the context of many-body effects such as charge density waves and Wigner crystallization in quantum wires.

We have carried out a de Hass–van Alphen (dHvA) experiment for the recently discovered CePt₃Si, which is the first heavy fermion superconductor without inversion symmetry in the tetragonal crystal structure, together with a dHvA experiment for a non-4f reference compound LaPt₃Si. As for LaPt₃Si, several dHvA branches were observed. Among them, the two main dHvA branches with the dHvA frequency (the cyclotron effective mass) of 1.10 × 10⁸ Oe (1.4 m₀) and 8.41 × 10⁷ Oe (1.5 m₀) were found to be well explained from the FLAPW energy band calculations, corresponding to bands 64- and 63-multiply-connected hole Fermi surfaces, respectively. On the other hand, the dHvA frequencies of 10⁷ Oe in CePt₃Si are small in magnitude, although the corresponding cyclotron masses of 10–20 m₀ are extremely large.

We show that a simple soft-core binary fluid mixture with purely repulsive interactions exhibits a λ-line, i.e., a line of continuous transitions to a state characterized by undamped periodic concentration fluctuations (microphase separation). For states in the disordered fluid phase the bulk pair correlation functions exhibit strong 1–2 ordering, similar to that found in ionic fluids, which is also reflected in the density profiles of confined fluids. The latter display fluid layers alternating from rich in species 1 to rich in species 2. We argue that the λ-instability drives a freezing transition to a highly delocalized crystal, with Lindemann ratios that can exceed 90% near melting.

Polydiacetylenes (PDAs) form a unique class of polymeric materials that couple highly aligned and conjugated backbones with tailorable pendant side-groups and terminal functionalities. They can be structured in the form of bulk materials, multilayer and monolayer films, polymerized vesicles, and even incorporated into inorganic host matrices to form nanocomposites. The resulting materials exhibit an array of spectacular properties, beginning most notably with dramatic chromogenic transitions that can be activated optically, thermally, chemically, and mechanically. Recent studies have shown that these transitions can even be controlled and observed at the nanometre scale. These transitions have been harnessed for the purpose of chemical and biomolecular sensors, and on a more fundamental level have led to new insights regarding chromogenic phenomena in polymers. Other recent studies have explored how the strong structural anisotropy that these materials possess leads to anisotropic nanomechanical behaviour. These recent advances suggest that PDAs could be considered as a potential component in nanostructured devices due to the large number of tunable properties. In this paper, we provide a succinct review of the latest insights and applications involving PDA. We then focus in more detail on our work concerning ultrathin films, specifically structural properties, mechanochromism, thermochromism, and in-plane mechanical anisotropy of PDA monolayers. Atomic force microscopy (AFM) and fluorescence microscopy confirm that films 1–3 monolayers thick can be organized into highly ordered domains, with the conjugated backbones parallel to the substrate. The number of stable layers is controlled by the head-group functionality. Local mechanical stress applied by AFM and near-field optical probes induces the chromogenic transition in the film at the nanometre scale. The transition involves substantial optical and structural changes in a highly compressed form. Thermochromism is also studied using spectroscopic ellipsometry and fluorescence intensity measurements, and reveals that ultrathin films can reversibly attain an intermediate phase before irreversibly transforming to a final stable state. Further AFM studies also reveal the relation between the highly anisotropic film structure and its nanomechanical properties. In particular, friction at the nanometre scale depends dramatically upon the angle between the polymer backbone and the sliding direction, with the maximum found when sliding perpendicular to the backbones. The observed threefold anisotropy in mechanical dissipation also leads to contrast in the phase response of intermittent-contact AFM, indicating for the first time that in-plane anisotropy of polymeric systems in general can be investigated using this technique.

The symmetry break of the particle interaction field at surfaces causes a variety of interfacial effects including the variation of the relative stability of different phases of a given compound. In many materials surface melting is observed, i.e. a surface near a portion of the system is in a less ordered state than the bulk. Over the last two decades, surface ordering, i.e. the stabilization of the higher ordered phase by the surface, has been observed for an increasing number of soft condensed matter systems. For melts of low molar mass compounds, surface freezing has been reported so far only for chain molecules, while in systems of colloidal length scale, surface ordering has been observed for spherical particles as well. In this contribution we will review experimental studies, some theoretical approaches as well as computer simulations of surface ordering, both in the field of molecular melts and of colloidal suspensions.

Conformational properties of dipolar chains, and spatial and orientational intra-chain correlations in zero and infinitely strong external fields are investigated theoretically. A striking similarity and essential distinctions between the chains and polymer molecules are revealed and discussed. The main attention is given to the chain flexibility. The coil–globule phase transition in dipolar chains is predicted.

We report a novel-superheating phenomenon in YBa₂Cu₃O_z (Y123) oxide films (over 45 K), which were prepared by vapour deposition on MgO substrates. The origin of this superheating could be mainly attributed to the positive change of surface energy in peritectic melting, during which Y123 melts into the Y₂BaCuO₅ (Y211) solid and the Ba–Cu–O liquid. It was found that the heterogeneous melting initiates at the Y123/MgO interface and the molten liquid does not wet MgO and Y123. These melting features of Y123 films explain the delaying of the melting process.

Steady-state photoconductivity measurements are carried out for bulk and thin-film amorphous selenium (a-Se) samples in the temperature range between 190 and 340 K. The temperature and light-intensity dependences of the photoconductivity reveal the presence of both mono- and bimolecular recombination regimes. The current activation energies measured in the two regions point to energy levels in the gap for the recombination centres at 0.36 ± 0.06 and 1.35 ± 0.10 eV above the valence band mobility edge. These values put a-Se in line with the other chalcogenide semiconducting glasses that exhibit negative-U behaviour.

We studied the electrophoretic mobility μ of highly charged colloidal spheres in very dilute low salt aqueous suspension. We combined experiments on individual particles and ensemble averaged measurements. In both cases μ was observed to be independent of particle size and surface chemistry. Corresponding effective charges Z_μ^*, however, scaled with the ratio of particle size to Bjerrum length λ_B: Z_μ^* = Aa/λ_B with a coefficient . Our results are discussed in comparison to other charge determination experiments and charge renormalization theory and with respect to the issue of charge polydispersity.

We demonstrate that the energy of a system of atoms can be uniquely evaluated using a series of structure independent, perfectly transferable many-body potentials. This allows one to compare empirical energy parametrizations on the basis of the behaviours of their potential expansions. It is shown how the representations of the energy using many-body potentials, which focuses on the positional degrees of freedom, and the conventional cluster expansion method, which focuses on the ordering degrees of freedom in a multicomponent lattice system, can be merged into a generalized cluster expansion.

Confined magnetic Ising films in a L × D geometry (), with short-range competing magnetic fields (h) acting at opposite walls along the D-direction, exhibit a slightly rounded localization–delocalization transition of the interface between domains of different orientations that runs parallel to the walls. This transition is the precursor of a wetting transition that occurs in the limit of infinite film thickness () at the critical curve T_w(h). For T<T_w(h) (T>T_w(h)) such an interface is bounded (unbounded) to the walls, while right at T_w(h) the interface is freely fluctuating around the centre of the film.

Starting from disordered configurations, corresponding to , we quench to the wetting critical temperature and study the dynamics of the approach to the stationary regime by means of extensive Monte Carlo simulations. It is found that for all layers parallel to the wall (rows), the row magnetizations exhibit a peak at a time and subsequently relax to the stationary, equilibrium behaviour. The characteristic time for such a relaxation scales as , as expected from theoretical arguments, that are discussed in detail.

An asymmetric shift in the position of the magnetic Bragg peak with respect to the fiducial lattice has been observed by resonant x-ray scattering in a diverse series of antiferromagnetic compounds. A possible explanation is given in terms of a generalized Berry phase correction.

The crystal structure of the ordered double perovskite Sr₂MnTeO₆ has been refined at ambient temperature from high resolution neutron and x-ray powder diffraction data in the monoclinic space group P 12₁/n 1 with a = 5.7009(1) Å, b = 5.6770(1) Å, c = 8.0334(1) Å and β = 90.085(1)°. This represents a combination of in-phase (+) and out-of-phase (−) rotations of virtually undistorted MnO₆ and TeO₆ octahedra in the (−−+) sense about the axes of the ideal cubic perovskite. High temperature x-ray powder diffraction shows three structural phase transitions at approximately 250, 550 and 675 °C, each corresponding to the disappearance of rotations about one of these axes. The first transition was analysed by differential scanning calorimetry and showed a thermal hysteresis with an enthalpy of 0.55 J g⁻¹. We propose the () sequence of structural transitions which has not been previously reported for a double perovskite oxide.

The structure of Rb_1−x(NH₄)_xI (x = 0.29,0.77) mixed crystals was studied by means of powder neutron diffraction in the temperature range 20–300 K and at ambient pressure. Measurements of spin–lattice relaxation time T₁ have been performed for polycrystalline samples of Rb_1−x(NH₄)_xI (x = 0.29,0.77) in the temperature range 95–300 K and pressure range 0–800 MPa by the proton NMR technique using the saturation method. Activation parameters were obtained for a model of complex ammonium ion reorientations around two-fold C₂ and three-fold C₃ axes. The activation volume for different phases of the investigated compounds was determined. The experimental data were used for a construction of P–T phase diagram of Rb_1−x(NH₄)_xI (x = 0.29,0.77) mixed crystals. The phase diagram of Rb_0.23(NH₄)_0.77I has similar features to that of NH₄I. Four different phases were found to exist in the temperature and pressure range studied: a disordered α phase with an NaCl-type cubic structure, a disordered β phase with a CsCl-type cubic structure, a γ phase with a tetragonal structure and antiparallel ordering of ammonium ions, and a δ phase with a CsCl-type cubic structure and parallel order of ammonium ions. The phase diagram of Rb_0.71(NH₄)_0.29I contains only α and β phases, and is similar to that of RbI.

This work, by means of molecular dynamics simulations, shows that the features of C₆₀ encapsulation into boron nitride nanotubes (BNNTs) are similar to the features of that into carbon nanotubes (CNTs), whereas the encapsulating and the internal dynamics of the C₆₀@BNNT are different from those of the C₆₀@CNT. Since the C₆₀ encapsulation into the BNNTs is energetically more stable than that into the CNTs and the suction force on the C₆₀ molecule induced by the BNNTs is higher than that by the CNTs, the C₆₀ encapsulation into the BNNT is achieved faster than that into the CNT. The internal dynamics of the C₆₀ molecule inside the BNNT is also different from that inside the CNT, because the C₆₀@CNT system includes only one long range interaction of C–C whereas the C₆₀@BNNT system includes both C–B and C–N long range interactions. Because of the difference of the binding energies and the equilibrium distances between C–B and C–N, the C₆₀ molecule frequently collided against the BNNT wall in molecular dynamics simulations. At low temperature, the energy dissipation of the C₆₀@CNT system mainly occurred at both end edges of the CNT, where the C₆₀ molecule is under restoring (or sucking-in) forces. Energy dissipation of the C₆₀@BNNT resulted from collisions against the BNNT wall as well as at both end edges of the BNNT.

The monoclinic P 2₁/m and orthorhombic Pmmn (Watanabe et al' s-type) mechanisms of the high-pressure phase transition of NaCl between the B1 (rocksalt, ) and B2 (CsCl-like, ) cubic phases were investigated by ab initio DFT techniques with all-electron localized basis sets. Enthalpy profiles versus the order parameter were computed at constant pressures of 15, 26.3 (equilibrium) and 35 GPa for each pathway. The monoclinic path shows a lower activation enthalpy at the equilibrium pressure, but at different p values (hysteresis effects) the other mechanism becomes competitive. In the P 2₁/m case, sharp jumps of structural parameters are observed along the transformation coordinate, which can be explained by a mechanism based on discontinuous sliding of alternating pairs of (100) atomic layers. This accounts also for the predicted formation of a metastable intermediate Cmcm phase with TlI-like structure, similar to that observed experimentally at high pressure in AgCl, and the relations with the KOH structure are discussed, too. On the other hand, along the Pmmn pathway the structural parameters vary quite smoothly, indicating a continuous motion of neighbouring atomic planes within the constraint of the additional mirror symmetry.

Molecular dynamics with analytical potentials is commonly used to obtain the distribution of defects produced by energetic particles in elemental and compound semiconductors. Collision cascades simulated by model-potential molecular dynamics are used to collect statistical data on the defect distribution but the local structure in such materials as GaAs is commonly recognized to be unreliable in comparison to tight-binding or ab initio total energy calculations. These two methods, however, are not practical in simulations of collision cascades because of their large computational workload. In this paper, we analyse the properties of the basic point defects in GaAs as obtained by using different model potentials and compare them to recent ab initio calculations based on the density-functional theory (DFT) in the local-density approximation (LDA). The aim of this work is to evaluate how close the model potential molecular dynamics predictions are to the benchmark DFT results and which model potential most accurately predicts realistic local structures of point defects.

A series of non-transition elements bound to the Ni Σ5 (012) symmetrical tilt grain boundary (GB) and the (012) free surface (FS) systems has been studied by first-principles calculation using WIEN2k code, which is based on the full-potential linearized augmented plane wave method with the generalized gradient approximation. The multilayer relaxations in the presence and absence of solutes are determined by the force minimization procedure. The binding energies at some GB/FS/bulk sites including both interstitial and substitutional sites are calculated for all the non-transition elements between H and Rn (from the first-row to the sixth-row elements). The GB/FS segregation energy is obtained by calculating the binding energy difference between the GB/FS site and the inner bulk site. The embrittling potency energy is obtained by calculating the difference between the GB and FS segregation energies on the basis of the Rice–Wang model. The calculated results show that most of the non-transition elements have negative GB/FS segregation energies. In our definition, this means that there exists a segregation site in the GB/FS that is more stable for the solute atom than in the bulk. The embrittling potency energies are positive for most of the solutes. However, some exceptions such as Be, B, C, and Si having negative and large embrittling potency can enhance the GB cohesion. The calculated results are found to be consistent with the various experimental findings within the discussion based on the simple site competition model neglecting the interactions between different solutes.

We have studied the colour centre production by swift electron and heavy ion irradiations of yttria-stabilized zirconia (YSZ), i.e. ZrO₂:Y with 9.5 mol% Y₂O₃. For this purpose, we performed irradiations of - or -oriented YSZ single crystals with 2.5 MeV electrons, 145 MeV ¹³C, 180 MeV ³²S, 200 MeV ⁵⁸Ni, 230 MeV ⁷⁹Br, 120 MeV ¹²⁷I, 200 MeV ¹²⁷I, 200 MeV ¹⁹⁷Au, and 2.6 GeV ²³⁸U ions. X-band electron paramagnetic resonance (EPR) and UV–visible optical absorption measurements were used to monitor the point defect formation. The EPR line saturations were measured between 6 and 150 K, in order to obtain the spin–lattice relaxation time (T₁). Electron and ion beams produce the same two colour centres: (i) the first one is identified as an F⁺-type centre (singly ionized oxygen vacancy) with an axial symmetry, a small g-factor anisotropy ( and ) and long T₁ values, (ii) the second one is similar to the well known T-centre (Zr³⁺ in a trigonal oxygen environment) with an axial symmetry and a large g-factor anisotropy ( and ), which is also produced by photon irradiations. A broad optical absorption band centred at a wavelength near 500 nm is observed with an absorption coefficient proportional to the volume density of the F⁺-type centre deduced from the room temperature EPR spectra. Since no change of this band occurs between 10 and 300 K, it indicates that the electron–phonon coupling of this colour centre must be strong, in agreement with an F⁺-type centre. Owing to the axial symmetry and lack of hyperfine structure of the EPR lines of this defect, it is suggested that the first coordination shell must contain one native oxygen vacancy. The plots of the volume density of this centre versus fluence are on the whole rescaled as functions of the number of displacements per atom induced by elastic collisions.

Drift velocity correlation functions are investigated analytically in a phenomenological approach and numerically through Monte Carlo simulation. The simulation takes into account electron–phonon and electron–electron scattering. The thermodynamic equilibrium state is investigated. Analytical results are in good agreement with those obtained by the Monte Carlo method.

We derive explicit analytical expressions for the ground state excitonic polaron (energy, effective mass) using the fractional-dimensional space formalism. Our results are shown to provide a wider overview of the character of the excitonic polaron than the analysis performed using integral dimensions only. We include a derivation of the energy difference between the 1s and 2s states as well as the phonon Lamb shift (energy difference between the 2p and 2s states) in simple analytical forms. Our results compare well with experimental data and earlier theoretical estimates for a variety of different material systems. We extend the fractional-dimensional model to analyse excitonic polarons in coupled double quantum wells and quantum disc systems.

Using a non-equilibrium Green functions technique and the Hartree–Fock approximation, electronic transport calculations were performed for nanodevices consisting of bi-partite type molecules connected to para- and ferromagnetic leads. For the proper description of the system, the voltage drop effect on the molecule was included. A negative differential resistance in the current–voltage characteristics was found. For the system with ferromagnetic leads, bistable behaviour and significant magnetoresistance were shown.

Thin films of amorphous vanadium oxide have been prepared by electrochemical anodic oxidation. The phase composition of anodic films on vanadium has been shown to depend on the oxidation conditions (electrolyte composition, oxidation current, and time), and the stoichiometry can be controlled from V O₂ to V₂O₅. Physical properties of the oxide films, including the metal–insulator transition in amorphous V O₂, are studied. In addition, it is shown that non-equilibrium electrochemical oxidation leads to the formation of metastable vanadium oxides with extremely high sensitivity to laser () and electron-beam () irradiation. Such films are of considerable technical interest, particularly because of potential applications as an efficient resist material for both photonanolithography and electron-beam nanolithography.

The physical properties of single crystals of the Kondo lattice material Yb₃Ni₅Al₁₉ have been investigated by means of magnetic susceptibility, specific heat, and electrical resistivity measurements. Single-crystal x-ray diffraction measurements indicate that Yb₃Ni₅Al₁₉ adopts the Gd₃Ni₅Al₁₉ orthorhombic crystal structure. This compound exhibits intermediate valence behaviour with a characteristic energy scale  K. Calculations of χ(T) based on the Anderson impurity model within the non-crossing approximation including crystalline electric field effects are also presented.

Energy levels and optical transition intensities of direct and indirect excitons in diluted magnetic semiconductor double quantum wells were calculated as a function of the structure parameters and magnetic field. The effects arising from magnetic field induced crossing and repulsion of the exciton levels were investigated. Two structures were studied: (Zn, Mn)Se-based and (Cd, Mn)Te-based double quantum wells. In the (Zn, Mn)Se-based system, magnetic field induced level crossing of direct and indirect excitons was found. Above some magnetic field the indirect exciton becomes the lowest excited state of the system, which leads to an increase of the exciton lifetime by several orders of magnitude. In the (Cd, Mn)Te-based system, energy level crossing of excitons localized in the different wells of the structure was found. In this case magnetic field rise leads to transfer of the lowest exciton state from one well to another well of the system.

We present a detailed theoretical investigation of the radiation induced giant magnetoresistance oscillations recently discovered in high-mobility two-dimensional electron gas. Electron interactions with impurities, and transverse and longitudinal acoustic phonons in GaAs-based heterosystems are considered simultaneously. Multiphoton-assisted impurity scatterings are shown to be the primary origin of the resistance oscillation. Based on the balance-equation theory developed for magnetotransport in Faraday geometry, we are able not only to reproduce the observed period, phase and the negative resistivity of the main oscillations, but also to predict the secondary peak/valley structures relating to two-photon and three-photon processes. The dependence of the magnetoresistance oscillation on microwave intensity, the role of dc bias current and the effect of elevated electron temperature are discussed. Furthermore, we propose that the temperature dependence of the resistance oscillation stems from the growth of the Landau level broadening due to the enhancement of acoustic phonon scattering with increasing lattice temperature. The calculated temperature variation of the oscillation agrees well with experimental observations.

The effect of Sr chemical pressure on superconductivity was investigated in Hg₂(Ba_1−ySr_y)₂YCu₂O_8−δ. The samples were synthesized at high pressure and high temperature from y = 0.0 to full substitution, y = 1.0. These Sr substituted compounds are superconducting, without Ca doping on the Y site, and show a T_c increasing with Sr, reaching 42 K for y = 1.0. A detailed neutron powder diffraction study compares the structural changes induced by this chemical Sr/Ba substitution and the mechanical pressure effects in the Hg-2212 system. It shows the strong decrease of the three Ba/Sr–O distances and consequently the shrinkage of Cu–O 1_in−plane and Cu–O 2_apical bonds. These structural changes, by affecting the charge transfer which occurs between the charge reservoir and the superconducting block, are responsible of the T_c enhancement with Sr content.

The effects of Sr substitution on superconductivity, and more particularly the changes induced in the hole doping mechanism, were investigated in Hg₂(Ba_1−ySr_y)₂Y Cu₂O_8−δ by a 'bond valence sum' analysis, with Sr content going from y = 0.0 to 1.0. A comparison with CuBa₂Y Cu₂O_7−δ and Cu₂Ba₂Y Cu₂O₈ systems suggests a possible explanation of the T_c enhancement from 0 K for y = 0.0 to 42 K for y = 1.0. The charge distribution among atoms of the unit cell was determined from the refined structure, for y = 0.0–1.0. It shows a charge transfer to the superconducting CuO₂ plane via two doping channels π(1) and π(2), i.e. through O2_apical–Cu and Ba/Sr–O1 bonds respectively.

We have synthesized La_0.67Ca_0.33MnO₃ (LCMO):xZnO () composites through a citrate gel route and have characterized them for magnetic and magnetotransport properties. In lower concentrations (), ZnO mostly goes into the perovskite lattice substituting Mn in LCMO and segregates less in the grain boundary region, but at higher concentration (x>0.13) it segregates mostly at the grain boundaries of LCMO and influences the transport properties significantly. A model is proposed which describes the overall resistivity of the system as a parallel combination of a low resistive intragrain conducting path and a high resistive intergrain insulating path. Using this approach, the grain and grain boundary contributions to the overall resistivity are separated for all the composites. The field dependent resistivity shows that all the composites have higher values of MR at the transition temperatures (T_MI) compared to that in pure LCMO (x = 0). The highest value of MR is obtained for x = 0.10 and is 76.6% at 80 kOe field near T_MI.

The crystal structures and magnetic properties of rare earth tantalates RE₃TaO₇ (RE = rare earths) are reported. Their crystal structures were grouped into three types: La₃NbO₇-type, Y₃TaO₇-type, and defect-fluorite-type. For the Ho₃TaO₇ compound, two different phases (Y₃TaO₇-type and defect-fluorite-type) were prepared. At around room temperature, Nd₃TaO₇ was found to transform from the Y₃TaO₇-type phase to La₃NbO₇-type phase with increasing temperature.

Temperature dependences of the magnetic susceptibilities and specific heats indicated that the Nd and Tb compounds undergo a 'two-step' antiferromagnetic transition with separate ordering of ions on different crystallographic sites below 2.6 and 3.6 K, respectively. The Dy³⁺ ion was found to be in an antiferromagnetic state below 2.3 K. The Ho₃TaO₇ with the Y₃TaO₇-type structure showed an antiferromagnetic-like transition, while no magnetic ordering was observed down to 1.8 K for the defect-fluorite-type Ho₃TaO₇.

The transverse magnetization and longitudinal magnetization hysteresis loops of sputtered thin films of Fe/Ge and Fe/Si multilayers on GaAs(001) substrates have been studied. The dependence of the film magnetic anisotropy on the bilayer period and semiconductor composition was investigated using a MOKE magnetometer. The hysteresis loops were measured as a function of the angle between the applied magnetic field and the hard–hard axis of the film. For the longitudinal magnetization, the Fe/Si film loops had lower remanent magnetization compared to the Fe/Ge film with the same spacer thickness. Thus the Fe/Si film had stronger exchange coupling across the spacer layer compared to the Fe/Ge film. For the thicker Ge spacer layer film, no exchange coupling was measured. For the transverse magnetization, the Fe/Ge multilayer films loops had only one Barkhausen jump. For the Fe/Si multilayer films loops, some contained one Barkhausen jump while others had two jumps, due to the cubic anisotropy contribution. These results are interpreted in terms of anisotropy and exchange energies.

Radiation induced defects in polycrystalline pure and Ce³⁺ doped Sr₂B₅O₉Br storage phosphors have been investigated using EPR and optical absorption methods. The EPR of irradiated pure Sr₂B₅O₉Br represents two overlapping spectra in the 335–340 mT range from paramagnetic electron and hole trapping centres, which are stable at room temperature. These centres are attributed to F(Br⁻) and O_Br⁻ centres, i.e. an electron trapped in a bromine vacancy and a hole trapped on an oxygen ion at a bromine site. The transitions of F(Br⁻) centres cause the optical absorption band at 560 nm. Another observed absorption band at 365 nm was attributed to the transitions of O⁻ centres. Thus electron and hole trapping in pure Sr₂B₅O₉Br occurs in V_Br and O_Br²⁻ aggregates, which are created during the synthesis.

EPR and optical absorption measurements on Ce³⁺ doped Sr₂B₅O₉Br reveal similar defects as in pure material. It is very likely that the same defects take part in the processes of thermally and photo-stimulated luminescence.

The necessity to produce materials with better performances than those observed in ferroelectric perovskites has generated the creation of new oxides, especially those belonging to the Aurivillius family. In the last few years much attention has been paid to the study of these materials, and this has opened up new fields due to their basic and applied properties. In this contribution a Raman analysis and a hyperfine study by perturbed angular correlations spectroscopy of Bi_1.75Te_0.25SrNb_1.75Hf_0.25O₉ were carried out to reveal information about the lattice and the electronic structure. By the use of these techniques, it was observed that the ferroelectric to paraelectric phase transition at about 570 K is driven by a soft mode, and the broadening of the dielectric constant as a function of temperature previously observed at T_C is connected to disorder in the Bi/Te–O layer.

The dependence of the phonon frequencies of the quaternary Cu(In_1−xGa_x)₅Se₈ films with tetragonal structure on the Ga content have been systematically investigated by means of Raman scattering. The dominant A₁ mode shifts from 151 cm⁻¹ for CuIn₅Se₈ to 160 cm⁻¹ for CuGa₅Se₈ in an approximately polynomial but not linear curve, because different types of local tetrahedral cationic clusters around Se distribute in the defect crystal structures. The vibrational modes in the low-frequency region below 125 cm⁻¹ clearly show one-mode behaviour, whereas significant changes in the frequency range from 160 to 235 cm⁻¹ indicate defect-induced structural disorder and show two-mode behaviour due to Ga addition. Additionally, the quenching of the Raman band at 174 cm⁻¹ for the Ga-rich films reveals that this mode should most probably originate from other similar localized structure phases in chalcopyrite-related CuIn₅Se₈ but not CuGa₅Se₈, due to the Ga inhibition effect. Such results should be ascribed to asymmetric distribution of Ga and In on a microscopic scale, resulting in the different properties of bonds between local clusters.

α-Al₂O₃ nanowires have been synthesized in bulk quantity by using simple physical evaporation of mixture of pure Al powders and nanometre-sized TiO₂ powders at 1150 °C. Scanning electron microscopy and transmission electron microscopy observations show that α-Al₂O₃ nanowires have diameters ranging from 20 to 60 nm and lengths up to several tens of micrometres. The growth of α-Al₂O₃ nanowires is controlled by the vapour–liquid–solid (VLS) mechanism. Photoluminescence measurements revealed a blue luminescence band in the wavelength range of 400–500 nm with two peaks at 415 and 440 nm, which could be attributed to the F⁺ centres in the α-Al₂O₃ nanowires.

We consider two-dimensional foam clusters consisting of small collections of N bubbles with two different areas. Different arrangements of these bidisperse clusters were found experimentally for each N. Calculation of their perimeter allowed us to compare the energy of each cluster, giving candidates to the minimal energy arrangement. The number of possible clusters is discussed and the calculated energies are compared with existing approximations.

We present an analytic model for the velocity field within a tubeless siphon (Fano flow), based on a simple differential equation in which extensional, shear and gravitational pressure gradient forces are balanced. The role of surface tension in determining boundary conditions for the flow is considered. The analysis is applied to nuclear magnetic resonance velocimetry data (Xia and Callaghan 2003 J. Magn. Reson. 16 365) on a 1.2 mm diameter column of 0.5%, 8 × 10⁶ Da polyethylene oxide in water.