Table of contents

This letter presents experimental results on unlubricated friction, which suggests that stick–slip is described by self-organized criticality (SOC). The data, obtained with a pin-on-disc tribometer examines the variation of the friction force as a function of time—or sliding distance. This is the first time that standard tribological equipment has been used to examine the possibility of SOC. The materials were matching pins and discs of aluminium loaded with 250, 500 and 1000 g masses, and matching M50 steel couples loaded with a 1000 g mass. An analysis of the data shows that the probability distribution of slip sizes follows a power law. In addition, the frequency power spectrum follows a 1/f^α pattern with α in the range 1.1–1.3. We perform a careful analysis of all the properties, beyond the two just mentioned, which are required to imply the presence of SOC. Our data strongly support the existence of SOC for stick–slip in dry sliding friction.

Crystalline C, Si and Ge in a lonsdaleite (hexagonal diamond) structure are studied by plane-wave pseudopotential calculations in the scheme of density-functional theory and the local density approximation. The same calculations with generalized gradient corrections and also for the cubic diamond phases of these elements are also performed for comparison. Our results show that the bulk moduli are quite similar between the diamond and lonsdaleite polytypes of these elements. The theoretical bulk modulus of lonsdaleite C is 0.2–0.3% higher than diamond. It is expected to replace diamond as the hardest material in the world. The LDA result shows lonsdaleite Ge as a semimetal for its zero band gap at its Γ point. Considering the exchange–correlation energy correction, it is estimated that lonsdaleite Ge is a semiconductor with a small direct band gap.

The apparent quasiparticle scattering rates in high-quality crystals of the quasi-two-dimensional superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ are studied using the Shubnikov–de Haas effect and megahertz penetration-depth experiments. The width of the superconducting transition observed in the megahertz experiments, taken in conjunction with the field dependence of the Shubnikov–de Haas oscillations, gives evidence that the broadening of the Landau levels is primarily caused by spatial inhomogeneities. This indicates a quasiparticle lifetime for the Landau states ≫ 3 ps. The megahertz data can also be used to derive an apparent scattering time (0.14– 0.56 ps) from the skin depth. This is much shorter than the Landau-state lifetime, in strong contrast to the expectations of Landau Fermi-liquid theory. The simplest explanation for the data is that only a fraction of the crystal contributes to the metallic conductivity, an observation which may be related to the recently observed 'glassy' transition in κ-(BEDT-TTF)₂Cu(NCS)₂.

Half-metallic CrO₂ powder compact with rod-shaped nanoparticles was studied by micro-Raman scattering in the presence of an external magnetic field at room temperature (300 K). In the low-field region (H ≤ 250 mT), the frequency and intensity of the E_g mode, an internal phonon mode of CrO₂, increase dramatically with increase in the magnetic field, while the corresponding linewidth decreases. The above parameters become constant when the CrO₂ powder enters the saturation state at higher magnetic field. The pronounced anomalies of the Raman phonon parameters under a low magnetic field are attributed to the spin–phonon coupling enhanced by the magnetic ordering, which is induced by the external magnetic field.

Properties of nanocomposite materials are affected by a coupling between mechanical deformations of the materials and electronic charge transport. In one of the simplest systems of this kind, a single electron transistor (SET) with deformable tunnel barriers (nanoelectromechanical SET), mechanically assisted charge transfer becomes possible. This can be viewed as 'shuttling of single electrons' between metallic leads by a movable small-sized cluster. In this article we review some recent theoretical and experimental achievements concerning shuttle transport, in normal as well as superconducting systems.

Defining the stability criteria for biaxially strained Ni, we show that the body-centred tetragonal (bct) Ni structure is not stable on an Au(001) substrate and changes to a face centred cubic (fcc) (110) structure with many stacking faults. Nevertheless, for a thin film, the bct Ni structure can be stabilized by the interface stresses. Using the stress at atomic level, the profile of the internal stresses is given as a function of Ni film thickness.

It has been shown by recent experiments that bulk immiscible metals (e.g. Ag/Cu, Ag/Co and Au/Ni) can form binary alloys on certain surfaces where the substrate mediates the elastic misfits between the two components, thus relieving the elastic strain in the overlayer. These novel surface alloys exhibit a rich phase structure. We formulate a two-component Frenkel–Kontorova model in one dimension to study surface alloy formation. This model can naturally incorporate dislocation formation that plays a crucial role in determining the actual structure of the system. Using energy minimization calculations we provide a phase diagram in terms of average alloy composition and the energy of mixing. Monte Carlo simulations were also performed to study the structure and interaction of the emerging dislocations.

A new method of initialization-free phase-change (PC) optical recording was proposed based on the crystallization kinetics of PC media. It was suggested that two special additional layers could be added to the disk to form a new disk structure for the initialization-free function. The physical mechanism of initialization-free PC optical recording was discussed. A model was proposed to explain the initialization-free method which combined the surface crystallization induced by the additional layer and the temperature change during the sputtering process. The simulation results of the initialization-free disk showed only a slight influence of the initialization-free structure on the writing/erasing properties of the PC recording layer. Experimental results verified the feasibility of the initialization-free method for PC optical recording.

We present results for the local density of states in the superconducting (S) and normal metallic (N) layers of an SN multilayer, and the supercurrent, based on a Green function formalism, as an extension of previous calculations on NS, SNS and SNSNS systems. The gap function is determined self-consistently. Our systems are chosen to have a finite transverse width. We focus on phenomena which occur at so-called critical transverse widths, at which a new transverse mode is starting to contribute. It appears that for an arbitrary width the Andreev approximation (AA), which takes into account only Andreev reflection at the SN interfaces, works well. We show that at a critical width the AA breaks down. An exact treatment is required, which also considers ordinary reflections. In addition, we study the influence of an interface barrier on the coupling between the S layers.

The room-temperature deposition of 0.5 monolayer (ML) Pt on Cu{100} followed by annealing to 525 K results in a sharp c(2 × 2) low-energy electron diffraction (LEED) pattern. The structure of this surface alloy is investigated by means of symmetrized automated tensor low-energy electron diffraction (SATLEED) analysis and ab initio plane wave density functional calculations. The results are then compared with those for the similar system 0.5 ML Pd/Cu{100}. SATLEED results for the Pt/Cu{100} show that it consists of an ordered c(2 × 2) Cu–Pt second layer alloy capped with a pure Cu first layer. The first and second interlayer spacings are found to be expanded by +5.1 ± 1.7 and +3.5 ± 1.7% respectively (relative to the bulk Cu interlayer spacing of 1.807 Å) due to the insertion of the 8% larger Pt atoms into the second layer. The ordered mixed layer is found to be rippled by 0.08 ± 0.06 Å with Pt atoms rippled outwards towards the solid–vacuum interface. A smaller rippling of 0.03 ± 0.11 Å in the fourth pure Cu layer was also detected with Cu atoms directly underneath Pt atoms rippled towards the second layer Pt resulting in a Pt–Cu bond length of 2.52 Å which compares with the sum of metallic radii of 2.67 Å. A recent quantitative SATLEED analysis for 0.5 ML Pd/Cu{100} was performed in an earlier study (Barnes et al 2001 Surf. Sci.492 55). A similar structure to the Pt/Cu{100} has been retrieved with slight differences in the interlayer spacings. The ab initio density functional results are fully consistent with the experimentally determined structures. However, they reveal an interesting difference between the stability of the Pd and Pt systems and highlight the fact that the Pd/Cu structure is best thought of as a metastable structure occurring as an intermediate step in the diffusion of the transitional metal ion into the bulk of the Cu substrate.

The spatial scaling law and intermittency of the V₂O₅ surface roughness has been investigated by atomic force microscopy. The intermittency of the height fluctuations has been checked by two different methods, first, by measuring the scaling exponent of the q th moment of height-difference fluctuations i.e. C_q = ⟨|h(x₁) − h(x₂)|^q ⟩, and second, by defining the generating function Z(q, N) and generalized multi-fractal dimension D_q. These methods predict that there is no intermittency in the height fluctuations. The observed roughness and dynamical exponents can be explained by numerical simulation on the basis of the forced Kuramoto–Sivashinsky equation.

A study of the effects of electron bombardment on water adsorbed on Zr(0001) is reported. Zirconium surfaces are dosed with isotopic water mixtures at 160 K followed by electron bombardment (485 eV). The system is then probed by low energy electron diffraction, temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). No evidence is found that would indicate preferential mixing of hydrogen from the bulk with isotopic water dissociation products during TPD. However, electron bombardment results in the sharpening of a hydrogen/deuterium desorption peak near 320 K and the production of water near 730 K at low water exposures. In addition, although water does not oxidize Zr(0001) thermally, electron bombardment of adsorbed water induces a shift of about 2 eV in the Zr AES features indicating that the surface is partially oxidized by electron bombardment.

An ultrathin film of indium deposited on Cu(111) has been studied by an in situ combination of medium-electron energy diffraction, low-energy electron diffraction, scanning tunnelling microscopy and Auger electron spectroscopy. The surface alloys, Cu₂In and Cu₃In, which do not exist in these structures in the bulk, have been found in this system.

Transport and magnetic properties of La–Ba–Ca–Mn–O thin films fabricated by the pulsed laser deposition technique had been investigated systematically to see the effects of substitution of the small atom Ca by Ba which is much bigger. The induced insulator-to-metal (IM) transition was obtained not only in compositions near 0.5 and 0.18 which are boundaries between metallic and insulating phases but also in the heavily doped region. In the region of x > 0.5, the Ba doping causes an anomalous response of the system to the magnetic field and a positive magnetoresistance was observed. Besides, our results concerning the vicinity of 0.5 imply the existence of phase separation. As for x < 0.5, the doping enhances remarkably the paramagnetism–ferromagnetism transition and the IM transition temperatures.

The effects of different interlayer materials (Pd and Pt) deposited between Ni films and Si substrates on the NiSi thermal stability are discussed. Ni_0.943Pd_0.057Si and Ni_0.945Pt_0.055Si solid solutions were formed when the samples were annealed at high temperatures and the lattice parameters of Ni_0.943Pd_0.057Si were calculated according to Vegard's law. The NiSi thermal stability was enhanced by interposing a Pd or Pt interlayer, and the sample with the Pt interlayer had the highest NiSi thermal stability among all the samples studied. This is attributed to the reduction of the interface energy between NiSi and Si substrates and the decrease of the driving force for the nucleation of NiSi₂, induced by formation of the NiSi(200) preferred orientation and the solid solution respectively.

A visual transient of the growth mechanism from crystallization to fractal growth was observed clearly in a drop of boar bile. The growing crystals were replaced by treelike fractal structures during solidification of the sample. It is fascinating to compare the transient with the result observed in SnI₂ vapour. They were completely identical, and revealed that under certain conditions a linear growth could be transferred spontaneously into nonlinear growth. It may be possible to consider the transient as a 'bridge' between linear and nonlinear growth, and to develop a quantitative expression of transient dynamics.

The effects of Mn substitution on the structure and magnetic properties of the SeMO₃ (M = Cu_1−xMn_x) solid solution have been studied. Rietveld refinements of the x-ray diffraction patterns of these samples indicate that the manganese ions occupy copper sites. This replacement induces significant changes in the M–O bond lengths that give rise to abrupt decreases of the [M–O₆] octahedral distortion. In contrast, the M–O(1)–M and M–O(2)–M bond angles remain essentially constant. The magnetic behaviour of this solid solution was studied in the temperature range of 2 K < T < 300 K. The temperature dependence of the inverse magnetic susceptibility is well described by the Curie–Weiss law at high temperatures, in the composition range studied. We found that the substitution of Mn for Cu induces a sharp drop in the saturation moment of SeCuO₃. At about 10% of Mn there is a change from positive to negative Weiss constant θ_W that is mainly due to the [M–O₆] octahedral distortion introduced by the substitution of the larger Mn ions in the structure. The M– H isotherms show a weak ferromagnetism at about 52 K in SeMnO₃.

This paper presents the results of a simulation study of extrinsic defects in lead tungstate crystal. The results reveal that monovalent ions preferentially enter the Pb sites, whereas pentavalent ions preferentially occupy the W sites, and both of them will simultaneously produce oxygen vacancies to keep the charge neutrality. The solution energy of trivalent dopants is a strong function of the dopant's cation radius. They generally occupy the Pb sites, with the excessive charge mainly balanced by lead vacancies. In some cases, however, an oxygen interstitial ion might also coexist. Binding energy calculations demonstrate a strong tendency toward cluster formation of the trivalent dopant ions and the lead vacancies. The relationship between the aliovalent doping and the improvement of PbWO₄ (PWO) scintillation properties are discussed. This work enables us to comprehend the doping mechanism of PWO and has predicative value.

Photoluminescence (PL) properties of ZnO films grown on (001) InP substrates by thermal oxidization of metallic Zn films, in which oxygen vacancies and interstitial Zn ions are compensated by P ions diffusing from (001) InP substrates, are investigated. X-ray diffraction spectra indicate that P ions have diffused into the Zn films and chemically combined with Zn ions to form Zn₃P₂. Intense free exciton emission dominates the PL spectra of ZnO films with very weak deep-level emission. Low-temperature PL spectra at 79 K are dominated by neutral-donor bound exciton emission at 3.299 eV (I₄) with a linewidth of 17.3 meV and neutral-acceptor bound exciton emission at 3.264 eV. The free exciton emission increases with increasing temperature and eventually dominates the emission spectrum for temperature higher than 170 K. Furthermore, the visible emission around 2.3 eV correlated with oxygen deficiencies and interstitial Zn defects was quenched to a remarkable degree by P diffusing from InP substrates.

The pressure dependences of the lattice parameters, electronic structure, and bonding properties of the layered ternary compound Ti₃SiC₂ were investigated by performing ab initio plane-wave pseudopotential total energy calculations. The material exhibited elastic anisotropy. The lattice constants and axial ratio were studied for different pressures, and the same trend was obtained as is measured in experiment. It was found that although the structure was stable at high pressure, the electronic structure and atomic bonding were definitely affected. The electrical conductivity was predicted to reduce with pressure, which was interpreted by analysing the band dispersion curve and density of states at the Fermi level. The strengths of the atomic bonds in Ti₃SiC₂ were considered by analysing the Mulliken population and by examining the bond length contraction for various pressures. A redistribution of charge density that accompanied high pressures was also revealed.

The T²-coefficient of the electrical resistivity, A, is compared with the electronic specific heat coefficient, γ, for a number of Yb-based compounds. It is revealed that many systems, including YbCuAl, YbInCu₄, YbAl₃, and YbCu₅, show A/γ² values close to 0.4 × 10⁻⁶ μΩ cm mol² K² mJ⁻², which are remarkably small compared to those obtained from an expression known as the Kadowaki–Woods relation: A/γ² = 1.0 × 10⁻⁵ μΩcm mol² K² mJ⁻². Empirically, the compounds with the smaller A/γ² values appear to show weak intersite magnetic correlation and/or to have almost fully degenerate (J = 5/2 or 7/2) ground states.

There have been recent predictions of topologically unavoidable branch crossing in the energy band structure of solids. In this paper calculations of energy dispersion relations are carried out in a number of crystals with orthorhombic space group (SG) symmetry. Our calculations verify the topological crossings and are in good agreement with the predictions based on representations of SGs.

The temperature-dependent Verwey transition in a 500 Å (111) thin film of Fe₃O₄ (magnetite) has been studied using soft-x-ray emission spectroscopy at room temperature and below the transition temperature T_V. The Fe L_2,3 x-ray emission spectra show an increase in the intensity of the L₂ emission relative to the L₃ emission below T_V. This is independent of the excitation energy and is attributed to a metal–insulator transition across T_V. Comparison of the Fe L₃ emission and O Kα spectra with LDA band structure calculations supports the suggestion of charge ordering in Fe₃O₄ at low temperature.

The creation and relaxation of secondary excitons in solid Kr was investigated using energy-and time-resolved luminescence spectroscopy in the vacuum ultraviolet region. The spectrally selected emission of the free exciton (FE) was used as a probe for an investigation of the different exciton creation processes. Delayed FE creation via electron–hole recombination and 'prompt' (in terms of the time-resolution of the experiment) creation of excitons were separated.

The 'prompt' creation of a FE appears in the region above threshold energy E_th, which is equal to the sum of the band gap energy and the free exciton energy. 'Prompt' creation of excitons above E_th is ascribed to a superposition of two processes: (i) creation of the electronic polaron complex (one-step process) and (ii) inelastic scattering of photoelectrons described in the framework of the multiple-parabolic-branch band model (two-step process).

In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton (STE) emission was analysed. The behaviour of the ratio spectrum is a proof that electron–hole recombination leads to STE states through FE states as precursors.

The magnetism and magnetotransport properties of doped perovskite LaMn_1−xZn_xO₃ (x = 0.05, 0.10, 0.15, 0.20, 0.30, 0.40) have been studied. All samples exhibit insulating behaviour under zero field and a 6 T field (except for x = 0.05, the weak insulating–metallic transition occurs near T_C under zero field), but there is a paramagnetic–ferromagnetic (PM–FM) transition and a large MR effect near T_C. The ρ– T curves are fitted well by a semiconductor-like model. We suggest that the MnO₆ octahedron is contorted by zinc doping and that this contortion will influence the Jahn–Teller (JT) distortion of Mn³⁺. The e_g carrier has to overcome the energy gap caused by the difference between the JT distortions induced by Mn³⁺ in the non-contorted MnO₆ octahedrons and the contorted ones. Infrared spectra and Raman spectra confirmed our ideas. In addition, the magnetism was investigated by ESR and M–T measurements. The Zn doping suppresses the PM–FM transition and T_C decreases with the increase of dopant. For x = 0.30 and 0.40, there exists cluster–glass behaviour in the low-temperature range.

The Raman scattering by phonons and magnons was studied for the first time in the polycrystalline solid solutions Ni_cMg_1−cO. The experimental Raman spectrum for c = 0.9 is similar to that of NiO and consists of six well resolved bands, whose origins are the disorder-induced one-phonon scattering (bands at 400 and 500 cm⁻¹), two-phonon scattering (bands at 750, 900, and 1100 cm⁻¹), and two-magnon scattering (the broad band at ∼ 1400 cm⁻¹). We found that the dependence of the two-magnon band in solid solutions on the composition and temperature is consistent with their magnetic phase diagram. We also observed that the relative contribution of two-phonon scattering decreases strongly upon dilution with magnesium ions and disappears completely at c < 0.5. Such behaviour is explained in terms of a disorder-induced effect, which increases the probability of the one-phonon scattering processes.

The crystal structure and magnetic properties of a new series of ordered double-perovskite oxides Sr₂Fe_xMo_2−xO₆ (0.8 ≤ x ≤ 1.5) have been studied. The crystal structure changes from a tetragonal I4/mmm lattice to a cubic Fmm lattice around x = 1.2. The degree of ordering in Sr₂Fe_xMo_2−xO₆ exhibits a maximal at x = 0.95 and decreases as x deviates from 0.95. The saturated magnetization increases from x = 0.8 to 0.95 and then decreases from x = 0.95 to 1.5. The Curie temperature exhibits an abrupt drop around x = 1.2, where the structural transition takes place. These complex behaviours are strongly correlated to antisite defect concentration in the compounds.

The recombination processes in the x-ray storage phosphor BaBr₂:Eu²⁺ were investigated by optical and magneto-optical methods. A structure-sensitive investigation of the defects involved in the recombination processes was performed by detecting the microwave-induced changes in the recombination luminescence in a high magnetic field. F centres as well as V_K hole centres are created after x-irradiation at low temperatures. The low-energy recombination band peaking at about 460 nm is due to F–V_K centre recombinations, whereas the two high energy bands at 282 and 315 nm are probably due to recombinations of self-trapped excitons.

Porous anodic alumina (PAA) films with ordered nanopore arrays have been prepared by electrochemically anodizing aluminium in oxalic acid solutions, and the role of the oxalic impurities in the optical properties of PAA films has been discussed. Photoluminescence (PL) measurements show that the PAA films obtained have a blue PL band with a peak position at around 470 nm; the oxalic impurities, incorporated in the PAA films during the anodization processes and already existing in them, could be being transformed into PL centres and hence responsible for this PL emission.

The quantitative band mapping for MgB₂, graphite and diamond are realized using resonant inelastic x-ray scattering (RIXS) measurements. RIXS shows distinct dispersive features when the excitation energy is tuned near B 1s and C 1s thresholds, which are assigned to the calculated energy bands using -momentum conservation. The agreement between experiment and theory suggests that electron–electron interactions are not important for MgB₂, which behaves like a conventional metal and is well described by band theory.

Cerium doped lutetium pyrosilicate Lu₂Si₂O₇:Ce³⁺ (LPS), a new inorganic scintillator, displays particularly promising performance. This material can be readily pulled from the melt. A high light output (average value: 26 300 ph MeV⁻¹), a relatively good energy resolution (9%) and a fast decay time (38 ns) without afterglow make this new scintillator very attractive, in particular for medical imaging. Optical characterizations and scintillation properties of LPS:Ce large single crystals are presented, including timing properties and study of the scintillation yields as a function of incident energy.

YBa₂Cu₃O_6+x nanopowder, prepared by the citrate sol–gel method, is segregated by sedimentation in ethanol into three size groups with average particle heights of 0.7, 1.6 and 2.3 nm. The structural properties and composition of the powders, investigated by x-ray diffraction, atomic force microscopy, Auger electron spectroscopy and EPR-spectroscopy, show no clear differences, except the size. According to investigations by magnetometry and by non-resonant microwave absorption the as-prepared powder contains weak links which, however, disappear during the segregation. The magnetic susceptibility of the samples decreases with the decreasing particle size, in agreement with the susceptibility values calculated from the London equations for cylindrical particles smaller than the London penetration depth. In all three size groups the critical temperature of superconductivity is 92 K.

Silicate oxyapatite La_9.33(SiO₆)₄O₂:A (A = Eu³⁺, Tb³⁺ and/or Ce³⁺) phosphor films and their patterning were fabricated by a sol–gel process combined with soft lithography. X-ray diffraction(XRD), Fourier transform infrared spectroscopy, atomic force microscopy, optical microscopy and photoluminescence spectra, as well as lifetimes, were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 ^°C and the crystallinity increased with the increase in annealing temperatures. Transparent nonpatterned phosphor films were uniform and crack-free, which mainly consisted of rodlike grains with a size between 150 and 210 nm. Patterned thin films with different bandwidths(20, 50µm) were obtained by the micromoulding in capillaries technique. The doped rare earth ions(Eu³⁺, Tb³⁺ and Ce³⁺) showed their characteristic emission in crystalline La_9.33(SiO₆)₄O₂ phosphor films, i.e. Eu³⁺⁵D ₀ −−⁷F_J(J = 0, 1, 2, 3, 4), Tb³⁺⁵D _3,4 −−⁷F_J(J = 3, 4, 5, 6) and Ce³⁺ 5d(²D)−− 4f(²F _2/5,²F _2/7) emissions, respectively. Both the lifetimes and PL intensity of the Eu³⁺, Tb³⁺ ions increased with increasing annealing temperature from 800 to 1100 ^°C, and the optimum concentrations for Eu³⁺, Tb³⁺ were determined to be 9 and 7 mol% of La³⁺ in La_9.33(SiO₆)₄O₂ films, respectively. An energy transfer from Ce³⁺ to Tb³⁺ was observed in the La_9.33(SiO₆)₄O₂:Ce, Tb phosphor films, and the energy transfer efficiency was estimated as a function of Tb³⁺ concentration.

A theoretical model is suggested which describes a new type of topological defect in condensed matter, namely the hyperdislocations in networks of misfit dislocations at film/substrate interfaces. Formulae for elastic characteristics of misfit dislocation (MD) networks with hyperdislocations, such as elastic moduli and the hyperdislocation energy, are found. With these formulae, the anisotropy of the MD networks and the dependence of the hyperdislocation energy on the film thickness are analysed.