Table of contents

By combining quantum ab initio calculations and coarse grained polymer models we employ a multiscale approach to study site specific adsorption of polymers (polycarbonate) to a step on an otherwise perfectly flat nickel (111) surface. The presence of the defect leads to a well defined chain localization and ordering at the surface. This can be taken as a model situation for both surface site as well as chemical group (within the chain monomer) specific interaction of macromolecules with metal surfaces. The results shed some light on important possible mechanisms occurring at step defects and corners used to control mineralization processes and thus material properties.

We observed a field-induced staggered magnetization in the 2D frustrated dimer singlet spin system SrCu₂(BO₃)₂ by ¹¹B NMR, from which the magnitudes of the intradimer Dzyaloshinsky–Moriya interaction and the staggered g-tensor were determined. These anisotropic interactions cause singlet–triplet mixing and eliminate a quantum phase transition at the expected critical field H_c for gap closing. They provide a quantitative account for some puzzling phenomena such as the onset of a uniform magnetization below H_c and the persistence of the excitation gap above H_c. The gap was accurately determined from the activation energy of the nuclear relaxation rate.

Recent studies of electromagnetic response at microwave- and millimetre-wave frequencies of the high-temperature cuprate superconductors and related materials are reviewed, with special interest in the experimental papers. These include the superfluid response in the superconducting state, quasi-particle responses below T_c, and characteristic charge excitations in related materials. We also discuss the electronic structure in the vortex core and superconducting fluctuation.

We review the sequences of structural states that can be induced in colloidal suspensions by the application of flow. Structure formation during flow is strongly affected by the delicate balance among interparticle forces, Brownian motion and hydrodynamic interactions. The resulting non-equilibrium microstructure is in turn a principal determinant of the suspension rheology. Colloidal suspensions with near hard-sphere interactions develop an anisotropic, amorphous structure at low dimensionless shear rates. At high rates, clustering due to strong hydrodynamic forces leads to shear thickening rheology. Application of steady-shear flow to suspensions with repulsive interactions induces a rich sequence of transitions to one-, two-and three-dimensional order. Oscillatory-shear flow generates metastable ordering in suspensions with equilibrium liquid structure. On the other hand, short-range attractive interactions can lead to a fluid-to-gel transition under quiescent suspensions. Application of flow leads to orientation, breakup, densification and spatial reorganization of aggregates. Using a non-Newtonian suspending medium leads to additional possibilities for organization. We examine the extent to which theory and simulation have yielded mechanistic understanding of the microstructural transitions that have been observed.

There has been substantial renewed interest in negative thermal expansion following the discovery that cubic ZrW₂O₈ contracts over a temperature range in excess of 1000 K. Substances of many different kinds show negative thermal expansion, especially at low temperatures. In this article we review the underlying thermodynamics, emphasizing the roles of thermal stress and elasticity. We also discuss vibrational and non-vibrational mechanisms operating on the atomic scale that are responsible for negative expansion, both isotropic and anisotropic, in a wide range of materials.

A Bi 2 × n surface net was grown on the Si(001) surface and studied with inverse photoemission, scanning tunnelling microscopy and ab initio and empirical pseudopotential calculations. The experiments demonstrated that Bi adsorption eliminates the dimer related π₁^* and π₂^* surface states, produced by correlated dimer buckling, leaving the bulk bandgap clear of unoccupied surface states. Ab initio calculations support this observation and demonstrate that the surface states derived from the formation of symmetric Bi dimers do not penetrate the fundamental bandgap of bulk Si. Since symmetric Bi dimers are an important structural component of the recently discovered Bi nanolines, that self-organize on Si(001) above the Bi desorption temperature, a connection will be made between our findings and the electronic structure of the nanolines.

The surface stress was evaluated employing a composite elastic model using the first-principles full-potential linearized augmented plane-wave (FP-LAPW) method for 5, 7 and 9 monolayer (ML) Mo(001) thin films. The surface stress calculated from previously derived slope and curvature methods yields a very similar result. The surface stress is found to vary with the number of the Mo thin film layers. This variation is found to be consistent with the existence of quantum well states in these thin films.

We present here results of Brillouin scattering from bulk and surface phonons propagating in a well known ferroelectric–ferroelastic crystal β-Gd₂(MoO₄)₃, in the temperature range covering the phase transition. Temperature dependences of the velocity of Rayleigh surface acoustic waves, propagating in a few planes of this crystal, have been calculated. The surface phonon velocities determined experimentally have been found to show a different character of temperature dependences, especially in the phase transition range.

A method of resistivity measurement for insulators, previously described by Liesegang and Senn (1995 J. Appl. Phys. 77 5782; 1996 J. Appl. Phys. 80 6336), has been demonstrated to produce measurements not only in good agreement with established results in the literature but also with improved accuracy. The method is based on monitoring the discharge of a charged sample of an insulator through a grounded boundary. The measurements, however, were performed under atmospheric conditions and so the results were influenced by atmospheric effects such as temperature and moisture. By confining a sample to a vacuum and charging it via a custom built electron gun, we here extend the established method of Liesegang and Senn to a more controlled environment. In this paper, we numerically solve the potential and transport equations for the axial–cylindrical geometry of the system and present comparisons of theory and experiment through derived values of the resistivity, carrier depth and diffusion constant for a range of insulator samples. We also use asymptotic solutions via a theory of Burgers equation shock waves, as discussed in Holcombe et al (2004 J. Phys. Condens. Matter16 5999), to derive resistivity values and again draw comparisons. The results presented here are also compared with previously obtained or literature measurements of resistivity.

A theoretical model of two-phase crystal–glass composite materials is suggested which is based on the disclination description of glassy structures. In the framework of the model, the crystal–glass composite is described as a solid that contains nanoscale configurations of disclination–dislocation loops in the glassy phase. Specific defects at the crystal–glass interface are defined and theoretically described which are misfit disclinations generated as extensions of parent wedge disclinations present in the glassy phase. In doing so, the defect ensemble in the crystal–glass composite is characterized by nanoscale inhomogeneities in both the defect density and stress in the vicinity of the crystal–glass interface. The formulae for the stress fields created by ensembles of the disclination–dislocation loops in crystal–glass composites are derived. With these formulae, the energy of the crystal–glass interface is estimated.

The interaction of oxygen with evaporated Ni films on an Fe-doped SrTiO₃(100) substrate was investigated by means of LEED, AES and work function measurements (WF) at room temperature. The adsorption of oxygen takes place on the nickel overlayer firstly by chemisorption on nickel step sites, accompanied by a reduction of the WF, and secondly on terrace sites, followed by a WF increase. After the chemisorption phase, the oxidation of the nickel overlayer starts with NiO island formation followed by bulk NiO development, which is marked by a second WF reduction. The adsorption phases of oxygen correspond closely to those of oxygen on single crystals of nickel. This indicates that the character of the Ni predeposited layers on strontium titanate seems to be metallic.

We discuss and test a combined method to efficiently perform ground-state and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principles treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio. The errors incurred from the relaxed structure are found to be non-negligible but controllable.

The dielectric response of strontium barium niobate with 61% Sr has been studied from kHz to THz frequencies by means of several techniques (IR and time domain THz spectroscopy, the coaxial technique and dielectric spectroscopy) over a wide temperature interval: 20–600 K. A strong dielectric anisotropy is present in all the results. Relaxor ferroelectric properties were detected in ε_c^*(T,ν). At very high temperatures a strong relaxation appears in the microwave and THz range, which shifts and broadens to lower frequencies on cooling and then splits into two components. The high frequency one is seen in the THz range at high temperatures and the strong low frequency one weakens below the temperature T_m of the smeared permittivity maximum and broadens extremely in the spectra leaving a constant-loss background at very low temperatures. This relaxation is responsible for the dielectric anomaly near the ferroelectric transition. No anomalies in phonon frequencies were observed, which gives evidence relating to the order–disorder mechanism of the phase transition. The response perpendicular to the polar axis shows anomalous features at low temperatures, which could be connected with the shift of the low frequency limit of a broad dispersion (much weaker than along the polar axis) from the GHz range at 200 K to the kHz range at 40 K. There is no clear evidence of a new phase transition below 100 K suggested by some authors.

We present a calculation of the ground-state binding energy of an impurity magnetopolaron confined in a three-dimensional (3D) parabolic quantum dot potential, in the framework of a variational approach based on two successive canonical transformations. First, we apply a displaced-oscillator type unitary transformation to diagonalize the relevant Fröhlich Hamiltonian. Second, a single-mode squeezed-state transformation is introduced to deal with bilinear terms arising from the first transformation. Finally, the parameters of these transformations together with the parameters included in the electronic trial wavefunction are determined variationally to obtain the ground-state binding energy of an impurity magnetopolaron confined in a 3D parabolic quantum dot potential. Our approach has two advantages: first, the displaced-oscillator transformation allows one to obtain results valid for whole range of electron–phonon coupling strength since it is a special combination of Lee–Low–Pines and Huybrechts (LLP–H) canonical transformations, and second, the later transformation improves all-coupling results. It has been shown that the effects of quadratic terms arising from the all-coupling approach are very important and should be taken into account in studying the size-dependent physical properties of nanostructured materials.

We report on the first measurements of the anisotropy of the thermal conductivity of a single crystal of the ferromagnetic superconductor UGe₂, with the heat current applied parallel to the three orthorhombic main axes of the unit cell. The thermal conductivity was measured over the temperature range 4.2–300 K. The results obtained are discussed in the framework of various contributions to the total thermal conductivity of magnetically ordered material. As a reference compound, polycrystalline ThGe₂ has been used.

The Pr suppression of the superfluid density in Bi₂Sr₂Ca_1−xPr_xCu₂O_8+δ single crystals is discussed using the zero-temperature in-plane penetration depth extracted from the second peak field in magnetization curves. It is found that the results fit better to the modified dirty d-wave model than the usual model. The relationship between the superfluid density and the critical temperature is also discussed in the underdoped region and one finds that the phase fluctuation model cannot account for the critical temperature in the system, which may be due to the interaction between the thermally excited quasiparticles and the impurity scattering induced quasiparticles.

We study the point-contact tunnelling between a normal metal and a ferromagnetic superconductor. In the case of magnon-induced pairing the tunnelling conductance is a continuous and smooth function of the applied voltage. For small values of the applied voltage the Ohm law holds. We show that one can obtain the magnetization and the superconducting order parameter from the tunnelling conductance. In the case of paramagnon-induced superconductivity the tunnelling does not depend on the magnetization. We argue that the tunnelling experiment can unambiguously determine the correct pairing mechanism in the ferromagnetic superconductors.

Magnetic susceptibility, transport and heat capacity measurements for single-crystal Na_xCoO₂ (x = 0.71) are reported. A transition to a spin density wave (SDW) state at T_mag = 22 K is observable in all measurements, except χ_ac data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible along the c-axis. The system also exhibits a substantial positive magnetoresistance below this temperature. Calculations of the electronic heat capacity γ above and below T_mag and the size of the jump in C indicate that the onset of the SDW brings about the opening of a gap and the removal of part of the Fermi surface. Reduced in-plane electron–electron scattering counteracts the loss of carriers below the transition and as a result we see a net reduction in resistivity below T_mag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity.

The temperature variation of the fine and hyperfine parameters of Mn²⁺ in single crystals of PbWO₄ in the low temperature range reveals the presence of a resonant mode of frequency ω = 8.8 × 10¹² rad s⁻¹. Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T⁻² variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn²⁺ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.

In a certain growth mode, V pits have been observed in Al_yIn_xGa_1−x−yN grown by metal–organic chemical vapour deposition. Room temperature photoluminescence (PL) spectra from these quaternary alloys show strong band-to-band emission with equally intense yellow luminescence (YL). Simultaneous measurements of the PL intensity and topography using ultraviolet–visible near field scanning optical microscopy show that the enhanced YL intensity originates from the V pits whereas the intensity of the band edge PL decreases in the vicinity of pits. The strong YL emission can be related to the defect clusters associated with impurity decoration at dislocation lines and sidewalls of the pits. A higher degree of Al incorporation at partially filled V pits leads to the multiple near band edge PL peaks.

In the applications of thermoluminescence (TL) in dosimetry and archaeological and geological dating, a desirable dose dependence of TL intensity is a monotonically increasing function, preferably linear. It is well known that in many dosimetric materials, nonlinear dependence is observed. This may include a superlinear dependence at low doses and/or sublinear dose dependence at higher doses, where the TL intensity approaches saturation. In quite a number of materials, non-monotonic dose dependence has been observed, namely, the TL intensity reached a maximum value at a certain dose and decreased at higher doses. This effect is sometimes ascribed to 'radiation damage' in the literature. In the present work we show, both quasi-analytically and by using numerical simulation, that such dose dependence may result from a simple energy level scheme of at least one kind of trapping state and two kinds of recombination centres. One does not necessarily have to assume a destruction of trapping states or recombination centres at high doses. Instead, the main concept involved is that of competition which takes place both at the excitation stage and the read-out stage during the heating of the sample. This may explain the fact that the phenomenon in question, although very often ignored, is rather common. Cases are identified in which competition during excitation dominates, and others in which competition during read-out dominates.

Thin films of Ti_1−xFe_xO₂ (x = 0 and 0.05) have been prepared on sapphire substrates by the spin-on technique starting from metal–organic precursors. When heat treated in air at 550 and 700 °C respectively, these films present pure anatase and rutile structures as shown both by x-ray diffraction and Raman spectroscopy. Optical absorption indicates a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1 × 10⁻⁵ Torr at 500 and 600 °C respectively, the magnetic moment, at 300 K, is strongly enhanced, reaching 0.46 μ_B/Fe for the anatase sample and 0.48 μ_B/Fe for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K. When the ferromagnetic rutile sample is reheated in air, the magnetic moment reduces strongly. The data seem to indicate that oxygen defects created as a result of vacuum annealing may be responsible for the observed ferromagnetism in our samples.

LiLuF₄ scheelite (I4₁/a, Z = 4) has been investigated at high pressures using synchrotron angle-dispersive x-ray powder diffraction in a diamond anvil cell at room temperature. At 10.7 GPa, it reversibly undergoes a tricritical phase transition to the fergusonite structure (C 12/c 1, Z = 4), a distorted modification of the scheelite type. No other phase transition occurs in this material up to 19.5 GPa, the highest pressure in this study. Such a high-pressure behaviour is compared with the pressure-induced transformations in LiYF₄ and LiGdF₄, adding on to our knowledge of the structural systematics in LiMF₄ compounds.