Table of contents

Neutron scattering experiments have been carried out on the heavy fermion antiferromagnetic (AFM) superconductor CePt₃Si with T_N = 2.2 K and T_SC = 0.75 K. We observed clear AFM Bragg reflections with Q₀ = (001/2) below and above T_SC, indicating microscopic coexistence of AFM order and heavy fermion superconductivity. The AFM structure, of two interleaved ferromagnetic sublattices of local Ce 4f moments, has inversion symmetry under simultaneous space–time reversal. However, hybridization with Pt and Si breaks this degeneracy and a combination of these two competing effects may be relevant to an understanding of the simultaneous occurrence of superconductivity and AFM order. The observed magnetic moment 0.16(1) μ_B/Ce is strongly reduced from the Curie–Weiss effective moment 2.54 µ_B/Ce. Clear crystal field excitations at 1 and 24 meV were observed. The magnetic susceptibility can be well explained in a level scheme assuming the Γ₇ ground state, Γ₆ and Γ₇ first and second excited states, respectively.

We investigated the exchange bias effect for Fe/Gd ferrimagnetic multilayers. Our results show that there is a −cosine type of oscillatory exchange bias with temperature for the [(Fe 4 nm/Gd 4 nm)₄/Gd 16 nm] multilayer. For the first time, exchange bias was observed to oscillate between positive and negative fields with a period of approximately 130 K. In addition, the coercive field was observed to oscillate within itself for the same temperature range studied. We speculate that oscillatory exchange bias is related to the underlying magnetic phase diagram of the ferrimagnetic Fe/Gd multilayer. When the [(Fe 4 nm/Gd 4 nm)₄/Gd 16 nm] system is cooled under a high magnetic field, negative exchange bias is forced to shift to positive values irregularly. Nevertheless the system shows oscillatory behaviour.

Dielectric permittivity measurements show that (1− x)NaNbO₃–(x)Gd_1/3NbO₃ crystals, at x = 0.09, exhibit a first-order phase transition near 150 °C that is diffuse over a wide temperature interval. The area of the thermal hysteresis loop was found to depend both on annealing and cooling temperatures. Optical studies in polarized light show a decrease of the domain sizes at this phase transition. The results obtained are fitted assuming a spatial distribution of Curie temperatures.

With a variety of experimental techniques we studied the formation of particle networks when a suspension of colloids in nematic liquid crystal is cooled through the isotropic–nematic transition. Rheological data suggest that, contrary to previous expectation, network formation is possible with particles over a broad range of sizes. Only little dependence on particle size is observed. NMR data indicate the presence of a significant fraction of isotropic material down to 10 K below the bulk nematic transition of the liquid crystal. On the basis of calorimetric findings we suggest that small amounts of alkane impurities, carried originally by the dispersed particles, are present. These molecules act as a second solvent. They play a crucial role in 'tuning' the kinetics of phase separation to allow network formation by (i) opening up a biphasic region and (ii) wetting the particles with a layer of isotropic material.

Insight into the forces governing a system is essential for understanding its behaviour and function. Calorimetric investigations provide a wealth of information that is not, or is hardly, available by other methods. This paper reviews calorimetric approaches and assays for the study of lipid vesicles (liposomes) and biological membranes. With respect to the instrumentation, differential scanning calorimetry (DSC), pressure perturbation calorimetry (PPC), isothermal titration calorimetry (ITC) and water sorption calorimetry are considered. Applications of these techniques to lipid systems include the measurement of thermodynamic parameters and a detailed characterization of the thermotropic, barotropic, and lyotropic phase behaviour. The membrane binding or partitioning of solutes (proteins, peptides, drugs, surfactants, ions, etc) can also be quantified. Many calorimetric assays are available for studying the effect of proteins and other additives on membranes, characterizing non-ideal mixing, domain formation, stability, curvature strain, permeability, solubilization, and fusion. Studies of membrane proteins in lipid environments elucidate lipid–protein interactions in membranes. The systems are described in terms of enthalpic and entropic forces, equilibrium constants, heat capacities, partial volume changes etc, shedding light also on the stability of structures and the molecular origin and mechanism of structural changes.

Biomolecules have the well-known ability to build reversible complexes. Indeed, antigens and antibodies or adhesion molecules are able to recognize one another with a strong affinity and a very high specificity. This paper first reviews the various techniques and related results about binding and unbinding, at the scale of a unique ligand/receptor couple. One important biotechnological application arising from these recognition phenomena concerns immuno-diagnosis, which is essentially based on the formation of these specific complexes. We show how the physics of colloids associated with the growing scientific background concerning molecular recognition helps in rationalizing and inventing new diagnostic strategies. Finally the concept of colloidal self-assembling systems as biosensors is presented as directly impacting the most important questions related to molecular recognition and their biotechnological implications.

This article presents a topical review of coarse grain simulation techniques. First, we motivate these techniques with illustrative examples from biology and materials science. Next, approaches in the literature for increasing the efficiency of atomistic simulations are mentioned. Considerations related to a specific coarse grain modelling approach are discussed at length, and the consequences arising from the loss of detail are given. Finally, a large number of results are presented to give the reader a feeling for the types of problem which can be addressed.

We have used spectroscopic methods to investigate the effect of cross relaxation on the 1470 and 1800 nm emissions in Tm³⁺:TeO₂–CdCl₂ glass for Tm³⁺ ion concentrations of 0.05, 0.1, 0.25, 0.5 and 1.0 mol%. The emission spectra obtained under 800 nm excitation reveal the existence of energy transfer via cross relaxation among the Tm³⁺ ions. As a result, as the thulium concentration is increased, the strength of the 1470 nm emission is found to decrease in relation to that of the 1800 nm emission. The analysis of the time evolution of the ³F₄ emission due to the transition further shows that the electronic mechanism responsible for the ion–ion interaction can be identified as an electric dipole–dipole energy transfer process and occurs in the diffusion-limited relaxation regime. The average critical distance parameter which provides a measure for the strength of cross relaxation was further determined to be 17.9 Å.

Suspensions of anisometric particles in the nematic phase of a liquid crystalline host solvent were prepared. We chose Claytone AF, a commercial quaternary ammonium surfactant treated montmorillonite, with an aspect ratio of up to 1:2000, and dimethyldioctadecylammonium bromide treated Laponite, with an aspect ratio of 1:8 as the dispersed particles. K15, a nematogenic compound (also known as 5CB), was the dispersing medium. The suspensions were characterized by small angle x-ray scattering (SAXS). The liquid crystal delaminates the clays well, but the scattering curves from Claytone suspensions have prominent first and second order pseudo Bragg peaks, indicating that stacking of clay plates has occurred. We report a model for fitting SAXS data based on Hosemann's theory for suspensions of plane parallel sheets.

Patterns formed by the flow of an inhomogeneous fluid (suspension) over a smooth inclined surface were studied. It was observed that fractal patterns form. There exists a threshold angle for the inclination above which global fractal patterns are formed. This angle depends on the particle size of the suspension. We observed that there are two fractal dimensions for these patterns, depending on the area from which the pattern is extracted. If the pattern is taken from the top which only consists of the beginning steps of the pattern forming, one finds two fractal dimensions, i.e. 1.35–1.45 and 1.6–1.7, in which the first one is dominant while, if the entire pattern is taken, then a fractal dimension 1.6–1.7 is observed. The first fractal dimension belongs to the class of the flow of water over an inhomogeneous surface and the second one corresponds to the river network. This may imply that both universality classes are present. However, here disorder is present in the fluid and is transferred to the surface.

We have carried out ab initio molecular dynamics simulations on Al–15% Si liquid alloy at different temperatures. The temperature dependences of energy, volume, diffusion coefficient and structure factor have been studied. The theoretical structure factors are in agreement with the experimental data. The obtained results suggest that the structure properties near 1100 K have different behaviour from that at higher temperature.

We investigate interfacial structural and fluctuation effects occurring at continuous filling transitions in 3D wedge geometries. We show that fluctuation-induced wedge covariance relations that have been reported recently for 2D filling and wetting have mean-field or classical analogues that apply to higher-dimensional systems. Classical wedge covariance emerges from analysis of filling in shallow wedges based on a simple interfacial Hamiltonian model and is supported by detailed numerical investigations of filling within a more microscopic Landau-like density functional theory. Evidence is presented that classical wedge covariance is also obeyed for filling in more acute wedges in the asymptotic critical regime. For sufficiently short-ranged forces mean-field predictions for the filling critical exponents and covariance are destroyed by pseudo-one-dimensional interfacial fluctuations. We argue that in this filling fluctuation regime the critical exponents describing the divergence of length scales are related to values of the interfacial wandering exponent ζ(d) defined for planar interfaces in (bulk) two-dimensional (d = 2) and three-dimensional (d = 3) systems. For the interfacial height , with θ the contact angle and α the wedge tilt angle, we find β_w = ζ(2)/2(1−ζ(3)). For pure systems (thermal disorder) we recover the known result β_w = 1/4 predicted by interfacial Hamiltonian studies whilst for random-bond disorder we predict the universal critical exponent even in the presence of dispersion forces. We revisit the transfer matrix theory of three-dimensional filling based on an effective interfacial Hamiltonian model and discuss the interplay between breather, tilt and torsional interfacial fluctuations. We show that the coupling of the modes allows the problem to be mapped onto a quantum mechanical problem as conjectured by previous authors. The form of the interfacial height probability distribution function predicted by the transfer matrix approach is shown to be consistent with scaling and thermodynamic requirements for distances close to and far from the wedge bottom respectively.

Sm³⁺-doped glass sol was prepared by a sol–gel method and coated on a bare Si substrate and an Al coated Si substrate (Al/Si). The coated glass films were heat treated in a hydrogen atmosphere or air to reduce the Sm³⁺ to Sm²⁺ and then the optical properties were examined through a photoluminescence (PL) experiment. While the glass films coated on a bare Si substrate or an Al/Si one were well reduced in hydrogen atmosphere, only the glass films coated on the Al/Si substrate were reduced in air. We thus suggest two possible reducing mechanisms related to hydrogen and the metal aluminium and we found that the metal aluminium coated on a Si substrate plays an important role in the reducing process.

The possibility of deriving the contact potentials between amino acids from their frequencies of occurrence in proteins is discussed in evolutionary terms. This approach allows the use of traditional thermodynamics to describe such frequencies and, consequently, to develop a strategy to include in the calculations correlations due to the spatial proximity of the amino acids and to their overall tendency of being conserved in proteins. Making use of a lattice model to describe protein chains and defining a 'true' potential, we test these strategies by selecting a database of folding model sequences, deriving the contact potentials from such sequences and comparing them with the 'true' potential. Taking into account correlations allows for a markedly better prediction of the interaction potentials.

Molecular dynamics simulations with two different embedded-atom-method (EAM) potentials are applied to calculate the density, specific heat and self-diffusion coefficient of liquid cobalt at temperatures above and below the melting temperature. Simulation shows that Pasianot's EAM model of cobalt constructed on the basis of a hcp structure is more successful than Stoop's EAM model in the framework of a fcc structure in predicting the thermophysical properties of liquid cobalt. Simulations with Pasianot's EAM model indicate that the density fits into ρ = 7.49–9.17 × 10⁻⁴(T− T_m) g cm⁻³, and the self-diffusion coefficient is given by D = 1.291 × 10⁻⁷exp(−48 795.71/RT) m² s⁻¹. Dissimilar to the linear dependence of the density and the Arrhenius dependence of the self-diffusion coefficient on temperature, the specific heat shows almost a constant value of 38.595 ± 0.084 J mol⁻¹ K⁻¹ within the temperature range of simulation. The simulated properties of liquid cobalt are compared with experimental data available. Comparisons show reasonable agreements between the simulated results from Pasianot's EAM model and experimental data.

Development of a quantum cluster embedding scheme based on non-orthogonal localized orbitals requires an efficient numerical method for calculating the electron density of the reference periodic crystal. We demonstrate that an existing method due to Löwdin based on the inverse overlap matrix expansion can only be used when the orbitals are highly localized (e.g. ionic systems). In other cases including covalent crystals or those with an intermediate type of chemical bonding this method may be either numerically inefficient or fail altogether. Instead, we suggest an exact and numerically efficient method which can be used for orbitals of practically arbitrary localization. Theory is illustrated by numerical calculations on a model system.

Aluminium oxide nanocrystals have been prepared via a straightforward reaction, initiated at low temperatures (<300 °C), between aluminium nitrate and hydrazine. The initial pressure parameter is found to be responsible for the variations of the particle size (ranging from nanocrystalline to sub-microcrystalline) and for the resulting crystalline phase (γ- or α-Al₂O₃) of these powders. The fibre-like morphology obtained for the as-synthesized γ-Al₂O₃ permits the synthesis of nanocrystalline α-Al₂O₃ ( nm) even after a high temperature treatment at 1200 °C. The findings suggest a promising approach for controlling the size and crystal phase of the particles.

Optical absorption, electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to characterize Ni⁺ ions substituting for Ag⁺ ions in AgGaSe₂ crystals. These Ni⁺ ions are responsible for a strongly polarized optical absorption band peaking near 2.2 µm (the maximum absorption occurs with ). Phonon structure is resolved at low temperature. This infrared band, which is present even in undoped crystals because of trace amounts of Ni, can significantly degrade the performance of optical parametric oscillators using AgGaSe₂. Sets of EPR and ENDOR angular dependence data were taken in the (010), () and (001) planes. This allowed the g matrix, the ⁶¹Ni hyperfine and nuclear quadrupole matrices, the ⁷⁷Se hyperfine matrix for the four nearest neighbours and the ^69,71Ga hyperfine and nuclear quadrupole matrices for four neighbours to be determined. Of the eight gallium near neighbours, only the four in the basal plane containing the Ni⁺ ion have a significant interaction. Large hyperfine interactions with ^107,109Ag nuclei were not observed.

Phonon dispersion curves of austenitic stainless steels Fe–18Cr–16Ni–10Mn and Fe–18Cr–12Ni–2Mo have been measured by triple-axis neutron spectroscopy. The data were analysed using Born–von Karman interactions as well as calculations including the contribution of conduction electrons on the lattice dynamics. An appropriate description of the experimental data was obtained by taking into account two-neighbour shells plus the contribution of the electron gas. The elastic constants and moduli obtained are close to reported results by ultrasonic studies on polycrystalline samples. The phonon densities of states in both systems calculated from the dispersion curves agree well with results obtained by time-of-flight neutron spectroscopy on polycrystalline samples. The Debye temperature Θ(T) shows a minimum around 40 K, similar to copper and nickel.

We report the ionic conductivity relaxation in fast ion conducting glasses in the system xAgI–(1− x)(0.40Ag₂O–0.60TeO₂) in the temperature range 128–303 K and in the frequency range 10 Hz–2 MHz. We have observed that the conductivity relaxation can be described by the highly non-exponential Kohlrausch–Williams–Watts function. The Ag⁺ ions have to overcome the same barrier while relaxing as well as while conducting. We have further observed that the migration of Ag⁺ ions in the glassy matrix is highly cooperative and the cooperation is extended as the AgI content is increased.

Experimental studies show that the rate of electron diffusion in mesoporous nanocrystalline TiO₂ may depend on the electrolyte concentration. In the available literature, this effect is attributed to ambipolar diffusion. We present arguments and a kinetic model indicating that it might instead be connected with different arrangements of cations near the traps occupied by an electron before and after tunnelling. With a reasonable choice of the ratio of the kinetic parameters, in agreement with experiment, the model predicts that the electron diffusion coefficient at high electrolyte concentrations may be about five times higher than at low concentrations.

We have studied the effect of Lu substitution on the spin dynamics of the Kondo insulator YbB₁₂ to clarify the origin of the spin-gap response previously observed at low temperature in this material. Inelastic neutron spectra have been measured in Yb_1−xLu_xB₁₂ compounds for four Lu concentrations, x = 0, 0.25, 0.90 and 1.0. The data indicate that the disruption of coherence on the Yb sublattice primarily affects the narrow peak structure occurring near 15–20 meV in pure YbB₁₂, whereas the spin gap and the broad magnetic signal around 38 meV remain almost unaffected. It is inferred that the latter features reflect mainly local, single-site processes, and may be reminiscent of the inelastic magnetic response reported for mixed-valence intermetallic compounds. On the other hand, the lower component at 15 meV is most likely due to dynamic short-range magnetic correlations. The crystal-field splitting in YbB₁₂ estimated from the Er³⁺ transitions measured in a Yb_0.9Er_0.1B₁₂ sample has the same order of magnitude as other relevant energy scales of the system, and is thus likely to play a role in the form of the magnetic spectral response.

Electrical resistivity ρ(T) measurements on polycrystalline samples of the Ce (Pt_1−xNi_x)₂Si₂ system exhibit dense Kondo behaviour for alloys with . The temperature at which a maximum appears in ρ(T) shifts from T_max = 65 K for x = 0 to T_max = 290 K for x = 0.6 as the system evolves towards the intermediate-valence regime for the CeNi₂Si₂ compound. At low temperatures the Ce (Pt_1−xNi_x)₂Si₂ alloys show a Fermi liquid T² contribution to ρ(T) below the coherence temperature T_coh. It is observed that , with a transition that takes place from a dense Kondo regime of T_K< T_CF () to a regime of T_K> T_CF (). T_K () and T_CF denote the Kondo and crystalline electric field splitting, E_F and E_R are the Fermi and the Abrikosov–Suhl resonance energies and j denotes the total angular momentum. An electronic Grüneisen parameter value of 19 has been obtained for the alloy system which is in reasonable agreement with Ω = 28 found from pressure studies on CePt₂Si₂. It is argued that volume effects are mainly responsible for the hybridization in the Ce (Pt_1−xNi_x)₂Si₂ system and application of the compressible Kondo lattice model to T_max data yields |JN(E_F)| = 0.309 ± 0.008, where J is the on-site exchange interaction between conduction electrons and the localized 4f spin and N(E_F) is the density of states at E_F.

Low temperature measurements of the Hall effect were carried out on photoexcited electrons in EL2-rich semi-insulating GaAs before and after photoquenching of the EL2 defects. A model of conductivity in the magnetic field is proposed that takes into account essential physical aspects of the electron transport in a fluctuating electrostatic potential and describes experimental data. Both experimental results and model calculations show a decrease of the amplitude of fluctuations upon photoquenching of the EL2. However, infrared shallow donor spectroscopy measurements showed an increase of the amplitude of fluctuations upon photoquenching of the EL2. This contradiction is explained by showing that observed properties of a fluctuating pattern depend on a spatial extent of a wavefunction that probes fluctuations.

It is usually believed that Néel and spin-Peierls orderings cannot co-exist. Very recently a transition to an ordered magnetic Néel state was observed in a pure organic spin-Peierls system p-CyDOV, and in one doped with p-CyDTV. While known theories can describe the onset of a Néel ordering in spin-Peierls compounds by the influence of impurities (e.g., in doped CuGeO₃), they exclude the possibility of onset of a Néel state in spin-Peierls pure systems. We propose a simple theoretical model, which manifests the onset of a Néel ordering in a spin-Peierls ordered state in the ground state and for nonzero temperatures, and calculate critical temperatures for phase transitions in those states.

We use a variational mean-field-like approach, the coupled cluster method (CCM), and exact diagonalization to investigate the ground-state order–disorder transition for the square lattice spin-half XXZ model with two different nearest-neighbour couplings J and . On increasing the model shows in the isotropic Heisenberg limit a second-order transition from semi-classical Néel order to a quantum paramagnetic phase with enhanced local dimer correlations on the bonds at about . This transition is driven by the quantum competition between and J. On increasing the anisotropy parameter Δ>1 we diminish the quantum fluctuations and thus the degree of competition. As a result the transition point is shifted to larger values. We find indications for a linear increase of with Δ, i.e. the transition disappears in the Ising limit .

Detailed magnetic and transport properties of electron-doped Bi_0.125Ca_0.875 MnO₃ materials are reported. Low field magnetization measurements provide evidence for ferromagnetic Curie–Weiss behaviour at higher temperatures and an antiferromagnetic state below T_N = 109 K, which supports a ferromagnetic component. High field () DC magnetization measurements (5–300 K) show a strongly nonlinear field response below and far above the ordering temperature. The residual magnetic moment and coercive field, in the ordered state, are, however, exceptionally small. These results are discussed in terms of the FM coupling of spins into clusters. The possible coupling of these clusters to the AF staggered magnetization (below T_N) and to AF fluctuations (above T_N) is discussed. The magnetic effects appear to be consistent with canted antiferromagnetism, however, FM segregation is not ruled out. The high field magnetoresistance (up to 30 T) appears to be governed by a field-induced reduction in the doped carrier localization. The glassy character of the cluster magnetic response was investigated by measuring the frequency dependent AC susceptibility and time/history evolution of the low field magnetization. Glassy behaviour in the cluster response is indeed observed and is discussed. The differentiation of cluster glass versus canted-AF origins of the time/history effects is still an open issue.

The Fe³⁺/Fe²⁺ ratioin natural ilmenite (FeTiO₃) samples shows rapid increase under pressure from ambient up to  GPa, as deduced from ⁵⁷Fe Mössbauer pressure studies. The ratio, which is initially 0.2 at ambient pressure, saturates at beyond 2–4 GPa to the highest pressure of 14 GPa in this study. Dramatic changes occur at low pressure where the compressibility of the unit cell is appreciably anisotropic, the c-axis being more compressible than the a-axis, and coincides with the rapid decrease observed in the electrical resistance under pressure. Pressure induced intervalence charge transfer away from the ferrous sites, conceivably across regions of octahedra, to the empty 3d manifold of the Ti⁴⁺ cation in an adjacent face-sharing layer along the c-axis may account for the change in the Fe³⁺/Fe²⁺ ratio.

Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na_0.5Bi_0.5TiO₃ complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

From a phenomenological point of view, the dielectric dispersion function of a medium can be defined by the location of its zeros and poles in the complex-frequency plane. This general approach led to the use of a factorized form of the dielectric function to fit the experimental infrared reststrahlen data. Recently, such an approach has been generalized to the description of relaxation.

At low frequency ranges, the factorized form for relaxation may be expressed as a product of Debye relaxors. This product form corresponds to the factorization of the conventional sum model and provides a way to describe the contribution of the different Debye relaxing units in a poly-dispersive system. It also allows us to estimate the importance of the interaction between the different relaxing units.

The present work summarizes the fundamentals of the product model for relaxation and describes its application to the analysis of the dispersion observed in the vicinity of the paraelectric to antiferroelectric phase transition in the mixed crystal (betaine phosphate)_0.75 (betaine arsenate)_0.25 and at the structural phase transition of betaine potassium iodide dihydrate.

Using CaCO₃, metal oxides (all dissolved by nitric acid) and tetraethoxysilane Si(OC₂H₅)₄ (TEOS) as the main starting materials, Ca₂R₈(SiO₄)₆O₂:A (R = Y,La,Gd;A = Eu³⁺,Tb³⁺) phosphor films have been dip-coated on quartz glass substrates through the sol–gel process. X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the 1000 °C annealed films are isomorphous and crystallize with the silicate oxyapatite structure. AFM and SEM studies revealed that the phosphor films consisted of homogeneous particles ranging from 30 to 90 nm, with an average thickness of 1.30 µm. The Eu³⁺ and Tb³⁺ show similar spectral properties independent of R³⁺ in the films due to their isomorphous crystal structures. However, both the emission intensity and lifetimes of Eu³⁺ and Tb³⁺ in Ca₂R₈(SiO₄)₆O₂ (R = Y, La, Gd) films decrease in the sequence of R = Gd>R = Y>R = La, which have been explained in accordance with the crystal structures.