Table of contents

Amphiphilic molecules with hydrophobic tails and hydrophilic heads form micelles of various shapes and sizes above a minimum threshold concentration known as the critical micelle concentration (CMC). The CMC as well as the size and the shape of the aggregates formed depend on various factors, e.g., the length of the amphiphiles, their internal rigidity, and temperature. In this letter we report the results of a detailed investigation of the dependence of the CMC on temperature for different lattice models of the amphiphilic self-assembly. Ensuring that the CMC can be unambiguously associated with a peak in the heat capacity as a function of the amphiphilic concentration, we show that for the amphiphiles of different lengths and head-to-tail ratios, the CMC decreases rapidly as a function of the chain length, consistently with the experimental results. However, for a given chain length, different lattice models predict that the CMC is always an increasing function of temperature. We point out that these lattice models, although widely used, are inadequate to explain the decrease of the CMC with temperature, seen experimentally for non-ionic surfactants.

One of the most difficult problems in condensed matter physics is describing the microscopic nature of the liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss a particular microscopic structure; only ensemble averages can be specified. Such averages can be performed via well crafted molecular dynamics simulations: the length of the simulation, the size of the ensemble and the nature of the interatomic forces must all be carefully analysed. Historically, a problematic issue in doing such simulations is that of how to describe the interatomic forces in the liquid state. This matter is especially challenging for the melt of semiconductors, such as silicon or gallium arsenide, where the chemical bond contains a strong covalent component. It is difficult to use pairwise interatomic potentials in such cases. Although many-body potentials can be utilized for simulations of these materials, one must map quantum phenomena such as hybridization onto classical interatomic potentials. This mapping is complex and difficult. In this review, we illustrate how one can avoid this problem by utilizing quantum forces to simulate liquids. Our focus is on the pseudopotential-density functional method. Within the pseudopotential method, only the valence electrons are explicitly treated and within the density functional theory, exchange and correlation terms are mapped onto an effective one-electron potential. These two approximations allow one to extract quantum forces at every time step of the simulation. The pseudopotential-density functional method is highly accurate and well tested for semiconductors in the solid state, but has only recently been applied to liquids. In this review, we illustrate this approach for a number of semiconducting liquids such as liquid Si, Ge, GaAs, CdTe and GeTe. For these liquids, we will present results for the microstructure, the dynamical properties such as the diffusion constants and the electronic properties such as the conductivity.

Multiarm star polymers, consisting of a high number of linear homopolymer arms joined covalently to a central core, represent model soft 'hybrid' spheres encompassing both polymeric (arm) and colloidal (core) character. Due to this topology, the single star has a nonuniform monomer density distribution. In nondilute solutions, a liquid-like ordering occurs as a consequence of the enhanced osmotic pressure that outbalances the entropic stretching of the arms; this type of order persists in the melt as well, due to 'macromolecular excluded volume' effects. The resulting rich dynamic response, which is presented in this review, exhibits signatures of both polymeric and colloidal behaviour. In solution, concentration and number density fluctuations relax via cooperative diffusion, self-diffusion and structural relaxation. In the melt, the viscoelastic terminal relaxation involves arm relaxation (independent of arm number) and structural rearrangements of the stars (strongly dependent on arm number and size). The identification of the relaxation mechanisms in such complex soft spheres provides the necessary ingredients for the molecular design and control of novel composite materials combining properties of polymers and colloids.

An international conference on `Scattering Studies of Mesoscopic Scale Structure and Dynamics in Soft Matter' was held in Messina, Italy on 22-25 November 2000, organized by Francesco Mallamce and Sow-Hsin Chen. The theme of the conference was selected to correlate with the past and present research activities of one of the organizers (SHC), who has worked extensively on the structure and dynamics aspects of complex fluids and soft matter using light, x-ray and neutron scattering techniques. The conference turned out to be a success, owing largely to the high quality of the invited lectures. It has therefore been a pleasure to ask a number of the invited speakers to contribute feature articles describing their latest research results. We are grateful for these wonderful friends who took time out of their busy schedules to prepare for this special volume of Journal of Physics: Condensed Matter articles which summarize some of their scientific views and exciting discoveries.

Under a general heading: `Topics on Scattering Studies of Structure and Dynamics in Complex Fluids', we assemble in this volume the following eight feature articles:

(1) Antonio Coniglio, Percolation and critical points (2) G Gompper, D Richter and R Strey, Amphiphilic bloch copolymers in oil-water-surfactant mixtures: efficiency boosting, structure, phase behaviour and mechanism (3) C Varea and A Robledo, Theory of interfacial bending constants (4) M Nonomura and T Ohta, Kinetics of morphological transitions between mesophases (5) K A Dawson, G Foffi, F Sciortino, P Tartaglia and E Zaccarelli, Mode-coupling theory of colloids with short-range attractions (6) F Sciortino and P Tartaglia, Aging in simple liquids: a numerical study (7) G Ruocco and F Sette, High frequency vibrational dynamics in glasses (8) M C Bellissent-Funel, Structure of confined water

The subject matter of these articles include: (1) a theoretical explanation of the co-existing critical and percolation phenomena in colloidal systems with short-range (compared with their typical dimensions) attractive interactions, written by an expert who pioneered the theoretical explanation of the phenomena; (2) the elucidation of the physics behind the recent discovery of a tremendous emulsification efficiency boosting in non-ionic microemulsions by addition of small amounts of di-block copolymers, written by a group of experts which made the discovery; (3) a concise summary of the density functional theory for the interfacial bending constants, the fundamental phenomenological constants which enter into the effective Hamiltonian description of complex liquids, by two veterans of the subject; (4) a description of the theory and simulations of a fascinating class of morphological transitions between micro-phase separated meso-structures in complex fluids, by an author (T Ohta) who pioneered the theory for such transitions; (5) a summary of the recent achievements, in terms of the mode-coupling theory, by the same group of physicists, on elucidation of the fascinating phenomenon of the formation of an `attractive glass' and the possibility of a `glass-to-glass' transition, exhibited by colloidal systems with short-range attractions; (6) a concise summary of their recent extensive MD works on `aging' phenomenon in quenched non-equilibrium systems; (7) an authoritative review of the recent results of investigations of molecular-scale wave length collective dynamics in glass forming systems made possible by the development of a high-resolution inelastic x-ray scattering method. This powerful spectroscopic method was developed originally by the authors of the paper; (8) a concise summary of the known molecular-scale hydrogen-bonded structures of confined water near hydrophilic and hydrophobic surfaces by an author who is the foremost expert in experimental investigations of these types of structures using neutron and x-ray diffraction techniques.

This special issue is edited jointly by Sow-Hsin Chen and Marie-Claire Bellissent-Funel and each article was refereed. We would like to thank the Publisher, Dr Richard Palmer for his cooperation and patience throughout the lengthy editing and production period of this special issue.

The interplay between critical percolating clusters and critical points in physical systems and models is reviewed. The main emphasis is given to systems such as colloids and gels. Much information on the main mechanism leading to percolation in correlated systems can be obtained from the study of the Ising model. It is emphasized that different cluster definitions are necessary for studying different phenomena and that the possibility of experimentally detecting the percolation line is related to the cluster lifetime. Some results on the interplay between critical points and percolating clusters in fluids are also discussed.

The effect of amphiphilic block copolymers on the phase behaviour and structure of ternary microemulsions in water, oil and non-ionic surfactant mixtures is reviewed. Recent experiments have revealed that the addition of small amounts of polyethylenepropylene-polyethyleneoxide block copolymer to the ternary systems leads to a dramatic increase in the volumes of oil and water solubilized into a bicontinuous microemulsion for a given surfactant volume fraction. While phase diagrams directly show the power of the amphiphilic block copolymers as efficiency boosters, the theoretical analysis in terms of bending energy discloses the mechanism for the efficiency boosting as due to the variation of the surfactant film curvature elasticity by tethered polymers in the form of mushrooms at the interface. Neutron scattering experiments employing a high-precision two-dimensional contrast variation technique confirm this picture and demonstrate that the polymer molecules uniformly decorate the surfactant film.

We critically work over the density functional theoretical foundation of the interfacial free energy with curvature terms. For a spherical interface described by a free-energy functional with square-gradient and square-Laplacian terms we find that the grand potentials of the stationary states are given exactly (and only) by pressure-volume and interfacial tension contributions. On the other hand, when the density functional is partially optimized in the subspace of densities with fixed interface position, the resulting effective interface potential acquires in the limit of large radius the customary form of Helfrich. We illustrate our findings with a description for the nucleation of micelles.

We investigate the dynamics of morphological transitions between mesoscopic structures on the basis of the model equation for microphase separation of block copolymers. The phase diagram for the modulated structures is obtained by means of the single-wavenumber approximation. In order to study the time evolution of domains, we derive a coupled set of equations for the amplitudes of the modulated structures in the weak-segregation regime. Simulations of the transitions between lamellar and hexagonal structures are carried out in two dimensions and the results are compared with the theory. Simulations in three dimensions are also presented.

Within the framework of the mode-coupling theory of super-cooled liquids, we investigate new phenomena in colloidal systems on approach to their glass transitions. When the inter-particle potential contains an attractive part, besides the usual repulsive hard core, two intersecting liquid-glass transition lines appear, one of which extends to low densities, while the other one, at high densities, shows a re-entrant behaviour. In the glassy region a new type of transition appears between two different types of glasses. The complex phenomenology can be described in terms of higher order glass transition singularities. The various glass phases are characterized by means of their viscoelastic properties. The glass driven by attractions has been associated with particle gels, and the other glass is the well known repulsive colloidal glass. These correspondences, in association with the new predictions of glassy behaviour, mean that such phenomena may be expected in colloidal systems with, for example, strong depletion or other short-ranged attractive potentials.

We discuss the phenomenon of physical aging in a well studied atomic model liquid, a binary mixture of particles interacting with Lennard-Jones potentials. We put emphasis on the different dynamical behaviours of the model as well as on the relations between the experimental and numerical timescales. The physics of aging is discussed within a well characterized thermodynamic framework, based on properties of the model potential energy surface.

In this paper we present a review of the recent inelastic x-ray scattering (IXS) studies on the high-frequency collective dynamics in topologically disordered systems. At mesoscopic exchanged momenta (Q≈1-10 nm^-1), these IXS studies allowed experimental demonstration of the existence in glasses of propagating acoustic phonon-like excitations whose characteristics are qualitatively independent from the specific system investigated. These universal features are as follows: (i) on the small-Q side of the momentum region examined, the existence of a linear dispersion for the excitation frequency of the longitudinal excitations; this dispersion starts to bend downward with increasing Q-value; (ii) a quadratic Q-dependence of the excitation width, Γ(Q), in the Q-region where Ω(Q) is linear; (iii) temperature independence of this excitation; and (iv) the appearance - with increasing Q-value - of the signature of the collective transverse dynamics. In this paper, using the case of vitreous silica as a representative example, we present the IXS evidence for these universal features and we compare them with the outcome of computer simulations. Finally, we will show that all the observed characteristics of the high-frequency vibrations can be retrieved within the `harmonic' approximation of the glass dynamics.

Water is essential for the stability and function of biological macromolecules. In living systems, essential water-related phenomena occur in restricted geometry in cells, and at active sites of proteins and membranes or at their surface. In this paper, we present the more recent results on the structure of confined water as compared with that of bulk supercooled water. In particular, the combined effects of the hydration level and the temperature on the change of the structure of water are discussed in the light of powerful techniques.

Electrical conductivity σ(T) and thermo-emf α(T) measurements, performed in a wide temperature range (from 700 to 1700 K) for binary S_0.35Te_0.65 and ternary CuAsSe₂ and TlAsSe₂ liquid alloys, suggest that a transition from semiconducting to metal conductivity occurs. The metallization temperature determined by thermo-emf measurements is lower than that determined by electrical conductivity. A temperature-dependent transformation of the pseudogap width is interpreted in the frame of a screening mechanism of the bound states by the free electron gas in the pseudogap region.

The alternating-current (ac) conductivity σ(ω) and the dielectric permittivity of polyaniline doped with β-naphthalene sulphonic acid (NSA) have been extensively studied in the temperature range of 300 K to 80 K and in the frequency range of 500 Hz to 10 MHz. The ac conductivity follows a power law in frequency, σ(ω)∝ω^s, at high frequency, where s (≃0.85-0.88) is nearly independent of temperature. At low temperature the ac conductivity spectra show saturations for frequencies of the order of 10⁶ Hz. The ac conductivity exhibits a scaling behaviour in the form σ(ω)/σ₀ = 1 + (ω/ω₀)ⁿ with 0.8<n<0.9. The dielectric loss spectra reveal two peaks corresponding to two plateaus of σ(ω) at low and high frequencies. The broad and asymmetrical loss peaks of the dielectric spectra have been analysed on the basis of stretched-exponential decay of the electric field, ϕ(t) = exp -(t/τ₀)^β, where 0<β⩽1. The loss spectra at low temperatures collapse onto a single scaling curve determined by the height and position of the peak.

Using non-equilibrium computer simulations, it is shown that various phenomenological criteria for melting and freezing hold not only in equilibrium but in steady-state non-equilibrium as well. In particular, we study the steady state of charge-polydisperse Brownian particles shaken by a time-dependent oscillatory electric field. Among these criteria are the Lindemann melting rule, the Hansen-Verlet freezing rule and the dynamical freezing criterion proposed for colloidal fluids by Löwen, Palberg and Simon.

We determine the structural data of seven of the polymorphs of ice (ice Ih, ice Ic, ice IX, ice II, ice VI, ice VII and ice VIII) from ab initio calculations. The dynamical properties have been analysed within the harmonic approximation via a finite-difference evaluation of dynamical matrices from atomic forces. Supercells are used to model the various ordered and disordered phases considered. Calculations are done at zero pressure in order to compare directly with neutron scattering studies performed on recovered phases. The normal modes are resolved into projections chosen to display their intra- and inter-molecular character. Further projections are performed for ice VI, ice VII and ice VIII to probe the interactions between sub-lattices. Trends in the dynamical results are discussed in terms of changes in the structural complexity of the various phases considered.

The electronic structures and magnetic properties of layered perovskite LaSrMnO₄ and Ca₂RuO₄ have been determined using the full-potential linearized augmented-plane-wave method within the local spin-density approximation (LSDA) and the LDA + U approach (LDA standing for local density approximation). The results of LSDA and LDA + U total-energy calculations show that the antiferromagnetic state of these materials is stable compared with the ferromagnetic and paramagnetic states. The LDA + U calculation results show that Jahn–Teller distortion of Mn–O and Ru–O octahedra produces an energy gap, and 3z² − r² and xy orbital ordering for LaSrMnO₄ and Ca₂RuO₄, respectively. However, the LSDA calculation is not sufficient for describing the orbital ordering and fails to produce the band gap. The Jahn–Teller distortion stabilizes a two-dimensional planar antiferromagnetic state.

In this paper, we generalize the finite-U slave-boson mean-field theory, provided by Kotliar and Ruckenstein (Kotliar G and Ruckenstein A E 1986 Phys. Rev. Lett. 57 1362), to investigate the Kondo correlation effects on linear and non-linear transport in a quantum dot connected to reservoirs at zero temperature. A comparison between the present formulation and other slave-boson formulations shows that this approach provides a more precise description of Kondo-type transport through quantum dots. In addition, this approach naturally fulfils the Friedel-Langreth sum rule exactly. The numerical results for the linear conductance at zero temperature agree well with experimental data and the numerical renormalization group calculations. The zero-temperature non-linear differential conductance is also discussed for Kondo and non-Kondo systems. A pronounced zero-bias maximum in the Kondo regime and flat zero-bias minimum in the non-Kondo regime are predicted for the zero-temperature differential conductance.

We derive the Ginzburg-Landau theory of unconventional singlet superconductors in the presence of a Zeeman field and impurities, in order to examine the resulting Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases. We show that the behaviour of the FFLO phases in unconventional superconductors in the presence of impurities is qualitatively different from that found for s-wave superconductors.

The short-wavelength effects on the fluctuation-induced diamagnetism (FD) in bulk isotropic three-dimensional (3D) superconductors are taken into account by introducing in the Gaussian-Ginzburg-Landau approach different cut-off conditions. These calculations, which extend to the 3D case our previous results on layered superconductors, are then used to briefly analyse the FD data measured for the low-temperature superconducting alloy Pb-8 at.% In. These analyses confirm the adequacy of a total-energy cut-off for explaining, for low-temperature 3D superconductors also, the thermal fluctuation effects in the high-reduced-temperature region. These results thus provide further support to the recent proposal that, due to the localization energy, the size of the effective fluctuations cannot be appreciably smaller than the superconducting coherence length amplitude extrapolated to T = 0 K.

The itinerant-electron metamagnetic transition (MT) and the effects of hydrostatic pressure on the critical transition field B_C of the MT, on the spontaneous magnetization M_S and on the Curie temperature T_C have been investigated for well homogenized Lu(Co_1-xAl_x)₂ Laves phase compounds.

The critical field B_C decreases with increasing x, maintaining a linear relationship with the inverse susceptibility at the temperature where the susceptibility exhibits a maximum value, χ^-1(T_max). On applying pressure, the magnetization M of the ferromagnetic compound with x = 0.100 is drastically decreased at a critical pressure, resulting in a paramagnetic state. In addition, the metamagnetic transition from the paramagnetic to the ferromagnetic state is induced by applying an external magnetic field.

The effect of pressure on the Curie temperature T_C is extremely large and negative in the vicinity of the critical concentration for the onset of ferromagnetism. The pressure coefficient of the Curie temperature, ∂ln T_C/∂P, is much larger than that of the spontaneous magnetization, ∂ln M_S/∂P, below x = 0.150. These results can be explained by the theory for itinerant ferromagnets having a negative coefficient b of the fourth-order term in the Landau expansion. The Landau expansion coefficients estimated from the experimental results are in accord with the theories. From these estimated values, it is concluded that the magneto-volume effect decreases the critical transition field B_C. It has been confirmed that the results for Lu(Co_1-xAl_x)₂ are very much analogous to those for Lu(Co_1-xGa_x)₂.

In the pyrochlore-structure compounds R₂Ti₂O₇, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb₂Ti₂O₇, we combine ¹⁷⁰Yb Mössbauer spectroscopy, ¹⁷²Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb³⁺ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb³⁺-Yb³⁺ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature (<1 K) properties of Yb₂Ti₂O₇ in a separate publication.

X-ray powder diffraction experiments have been made for mixed-valence vanadium compounds of AV₆O₁₁ (A = Na, Sr, and Pb) between 12 and 700 K and their magnetic susceptibility has been measured between 5 and 700 K. It was revealed that each compound of AV₆O₁₁ indicates a structural phase transition at T_t = 245 K for A = Na, 320 K for A = Sr and 560 K for A = Pb. This transition is accompanied by atomic displacement, leading to the formation of triangular trimers of the V(1) atoms, which construct a two-dimensional kagome lattice in AV₆O₁₁. The Curie constant C, the Weiss temperature Θ_p and the temperature-independent susceptibility χ₀ have been obtained in two temperature regions, T < T_t and T > T_t. The abrupt reduction of C below T_t may indicate that the d-electrons of V(1) show a transition to a spin-singlet ground state at T_t.

Using helium as a pressure-transmitting medium in a diamond anvil cell enabled a high-quality sample of the antiferromagnet CePd₂Si₂ to be studied in a highly hydrostatic environment. With the aid of a low-temperature force-modulation device, ac calorimetric measurements were made up to 25 kbar and resistivity measurements up to 33 kbar, on both sides of P_C, found between 28 and 29.5 kbar. The narrow superconducting domain found around P_C indicates the connection between the magnetic instability and the formation of Cooper pairs. We assert that there is an influence of the residual resistivity both on the emergence of superconductivity, and on the corresponding transition width. Close to P_C, the temperature dependence of the resistivity in the normal state qualitatively agrees with the spin-fluctuation model. While standard Fermi liquid behaviour was found to break down in the immediate vicinity of P_C, it reappeared below 700 mK at 33 kbar, less than 5 kbar above.

A summary of detailed measurements of the field- and temperature-dependent ac susceptibility, the field-cooled and zero-field cooled magnetization, coercive field and the resistivity in zero field and applied field of 1.5 T is presented for hole-doped La_1−xMg_xMnO₃ (0.05 ≤ x ≤ 0.4). Measurements of the ac susceptibility enable estimates of the (effective) critical exponents γ, β and δ to be made, which, when combined with the magnetization data, enable a phase diagram to be proposed. The transport data highlight the suppression of a metal–insulator transition in these systems with small average A-site radius. Furthermore, the transport behaviour is shown to be consistent with the predictions for charge transport by conventional small polaron hopping in both the paramagnetic and ferromagnetic phases, but inconsistent with recent quantitative predictions for magnetic small polaron-mediated conduction in a phase-separated picture.

Using the Langevin dynamics, we have calculated numerically the temperature and concentration dependence of the complex alternating-current susceptibility χ(ω,T) for disordered systems of fine magnetic particles taking into account the dipolar interparticle interaction. We demonstrate that the behaviour of the χ(ω,T) dependencies with increasing particle concentration (which means increasing interaction strength) and with increasing frequency depends qualitatively on the single-particle anisotropy and on the damping parameter used in the corresponding Langevin equation.

Micro-Raman spectroscopy is applied to study structural changes in the vicinity of the domain walls in ferroelastic lead phosphate Pb₃(PO₄)₂. The Raman spectra measured in untwinned and in heavily twinned regions of monoclinic Pb₃(PO₄)₂ are studied and the observed differences are analysed on the basis of calculated vibrational modes of monoclinic structural units with different atomic arrangements. The wall-related Raman signals observed near 537, 65 and 51 cm⁻¹ result from static displacements of the Pb₂ atoms along the monoclinic binary axis. PO₄ tetrahedra of rhombohedral geometry can also contribute to the extra Raman scattering near 537 cm⁻¹. The deviations of the Pb₂ atoms from their monoclinic positions couple preferably within the Pb₂–O sheets parallel to the cleavage plane. Rearrangements of the Pb₂ atoms occurring across the domain walls affect the intensity ratios between out-of-plane and in-plane components of ν₄(PO₄) bending, PO₄ rotational and PO₄ translational modes.

ZrRu_1-xRh_xP alloys (x = 0, 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.3, 0.4, 0.5 and 1) have been prepared at around 1300 and 1600 °C under 4 GPa. ZrRuP (x = 0) prepared at these temperatures has a hexagonal Fe₂P-type structure at room temperature. When the Ru atoms in ZrRuP are slightly displaced by the Rh atoms, the structure of the alloys changes drastically from the hexagonal form to the orthorhombic Co₂P-type structure. The x-ray diffraction patterns of the alloys (x = 0.05, 0.1 and 0.15) mainly indicate the orthorhombic Co₂P-type structure, though there are several weak lines of the hexagonal phase. Above x = 0.2 the alloys have a single phase of the orthorhombic form. Electrical and magnetic properties of the alloys ZrRu_1-xRh_xP have been studied at low temperatures. The superconducting transitions of hexagonal and orthorhombic ZrRuP occur at around 12 and 4 K, respectively. The transition temperature, T_c, decreases rapidly with increasing x for both phases of ZrRu_1-xRh_xP. The Rh atoms suppress the onset temperature of the superconductivity in the alloys.