Table of contents

A quantitative description is proposed for the depletion-induced slip thickness and velocity profile of a polymer solution near the wall–fluid interface in simple shear flow. The inhomogeneous polymer density profile at a flat wall is inserted into the equations of motion for the flow of a polymer solution near the flat wall. The theory is in quantitative agreement with simulation results.

We have studied M₂AlN phases, where M =  Ti, V, and Cr, by means of ab initio total energy calculations. The bulk modulus of M₂AlN increases as Ti is replaced with V and Cr by 19.0% and 26.5%, respectively, which can be understood on the basis of the increased number of valence electrons filling the p–d hybridized bonding states. The bulk modulus of M₂AlN is generally higher than that of the corresponding M₂AlC phase, which may be explained by an extra electron in the former phases contributing to stronger chemical bonding. This work is important for fundamental understanding of elastic properties of these ternary nitrides and may inspire future experimental research.

Ab initio density-functional-theory has been used to investigate transfer doping between C₆₀ monolayers and the hydrogenated (100) diamond surface. An electron transfer from diamond to C₆₀ is predicted for a C₆₀ coverage of around one monolayer, and the possibility of electron transfer is expected to increase for higher coverages, leading to an accumulation of holes at the diamond surface. It has been found that the greater electron affinity of fluorinated C₆₀ is likely to enhance the effect. Results are reported on the structural and electronic properties of isolated C₆₀ and C₆₀F₃₆ molecules, solid C₆₀, and a monolayer of each material adjacent to the (100)-(2 × 1):H diamond surface.

Spin chains are the most elementary entities in the study of magnetic ordering behaviour. In this letter, we describe a route toward the realization of artificial XY spin chains. Using thin-film deposition and e-beam lithography, 300 and 550 nm diameter pillars consisting of alternating discs of permalloy and Al₂O₃ have been made. Shape anisotropy forces the magnetization in plane, creating a chain of two-dimensional magnetic moments . The discs couple to each other by their dipolar stray fields, resulting in an interaction of the form , with J_ij the coupling between discs i and j. This form closely resembles a chain of xy spins, coupled by effective interactions J_ij. We present a magnetic characterization of the pillars, and compare the results with model calculations. Also, spin dynamics and ordering phenomena are addressed.

The recent experimental discovery of ferromagnetism in organic C₆₀-based polymers has re-ignited interest in the topic of magnetism in organic and non-traditional inorganic materials. While the nature of the s–p magnetism of C₆₀-based polymers has some unique features not shared with non-traditional inorganic materials, a defect-based picture of the magnetism is found to provide a common thread in these materials. By analysing the common features seen in the magnetic properties of various types of unconventional systems we show that the magnetism in these materials can be considered as a generalized form of the well known McConnell ferromagnetism and that it has common origins with the traditional ferromagnetism of the transition metal systems.

Magnetoelectric films are proposed as key components for spintronic applications. The net magnetic moment created by an electric field in a magnetoelectric thin film influences the magnetization state of a neighbouring ferromagnetic layer through exchange coupling. Pure electrical control of magnetic configurations of giant magnetoresistance spin valves and tunnelling magnetoresistance elements is therefore achievable. Estimates based on documented magnetoelectric tensor values show that exchange fields reaching 100 mT can be obtained. We propose a mechanism alternative to current-induced magnetization switching, providing access to a wide range of device impedance values and opening the possibility of simple logic functions.

High molecular weight polymer solutions have a powerful tendency to deposit adsorbed layers when exposed to even mildly attractive surfaces. The equilibrium properties of these dense interfacial layers have been extensively studied theoretically. A large body of experimental evidence, however, indicates that non-equilibrium effects are dominant whenever monomer–surface sticking energies are somewhat larger than kT, a common case. Polymer relaxation kinetics within the layer are then severely retarded, leading to non-equilibrium layers whose structure and dynamics depend on adsorption kinetics and layer ageing. Here we review experimental and theoretical work exploring these non-equilibrium effects, with emphasis on recent developments. The discussion addresses the structure and dynamics in non-equilibrium polymer layers adsorbed from dilute polymer solutions and from polymer melts and more concentrated solutions. Two distinct classes of behaviour arise, depending on whether physisorption or chemisorption is involved. A given adsorbed chain belonging to the layer has a certain fraction of its monomers bound to the surface, f, and the remainder belonging to loops making bulk excursions. A natural classification scheme for layers adsorbed from solution is the distribution of single-chain f values, P(f), which may hold the key to quantifying the degree of irreversibility in adsorbed polymer layers. Here we calculate P(f) for equilibrium layers; we find its form is very different to the theoretical P(f) for non-equilibrium layers which are predicted to have infinitely many statistical classes of chain. Experimental measurements of P(f) are compared to these theoretical predictions.

Comparing the transmission electron microscopy and x-ray diffraction data for a disordered graphite-like (sp²-bonded) carbon phase (DGCP) synthesized from fullerite C₆₀ at high temperatures in the pressure interval from 1.5 to 8 GPa, we show that the nanocluster structure, long-range correlations for orientation of clusters, lamellar and tweed morphology, and possible macroscopic anisotropy are intrinsic properties of the DGCP, and that they can be recognized as a signature of stresses in the parent phase at different scales from the atomic level (atomic density modulation) to the macroscopic one (non-hydrostatic pressure environment). These data indicate that the mechanism of the formation of the DGCP from large-molecule crystalline carbon material, such as C₆₀, is quite uncommon in the standard classification for the first-order phase transitions since it displays features typical for both diffusion and martensitic mechanisms, e.g., a strong topological rearrangement of covalent bonds and a crystallographic-like relation between crystalline parent and disordered product phases, respectively.

Density-functional electronic calculations have been used to investigate the high-pressure behaviour of americium. The phase transitions calculated agree with the recent sequence obtained experimentally under pressure: double hexagonal close packed  face-centred cubic  face-centred orthorhombic  primitive orthorhombic (Pnma). In the first three phases the 5f electrons are found localized; only in the fourth phase (Am IV) are the 5f electrons found delocalized. The localization of the 5f electrons is modelled by an anti-ferromagnetic configuration which has a lower energy than the ferromagnetic ones. In this study the complex crystal structures have been fully relaxed.

In situ observations of the phase transition in Fe₂O₃ were carried out in a multianvil and in a diamond anvil cell high-pressure apparatus using synchrotron radiation. The phase transition of haematite was observed at high pressures and high temperatures. The phase boundary between the haematite and high-pressure phase in the temperature range of 800–2500 K was determined to be P (GPa) = 29.4(± 0.4)+0.0046(± 0.0015) × (T (K) −1000). This result should resolve a dispute regarding the transition pressure of haematite among previous studies on Fe₂O₃.

Low temperature Raman spectroscopic measurements are carried out on Sc₂(MoO₄)₃ to probe the orthorhombic–monoclinic phase transition. Phonon frequencies and their temperature coefficients are found to change discontinuously across the transition. Several modes are found to disappear while some new modes appear across the transition. The evolution of intensities of the modes suggests a two-phase coexistence over a temperature interval of about 20 K both in the cooling and heating cycles. The transition is found to occur at 142 ± 10 K upon cooling while the reverse transition occurs at 174 ± 11 K during the heating cycle, i.e., the system exhibits a hysteresis of 32 K. In contrast to the usual behaviour, most of the phonon modes in the monoclinic phase exhibit softening upon further cooling. Separation of the temperature dependence of Raman mode frequencies into quasi-harmonic and purely anharmonic contributions using reported high pressure Raman data reveals that bending modes of MoO₄ ions are the most anharmonic among the modes studied.

We present here a generalization of the recursion method of Haydock et al (1972 J. Phys. C: Solid State Phys. 5 2845) for the calculation of Green matrices (in angular momentum space). Earlier approaches concentrated on the diagonal elements, since the focus was on spectral densities. However, calculations of configuration averaged response functions or neutron scattering cross-sections require the entire Green matrices and self-energy matrices obtained from them. This necessitated the generalization of the recursion method presented here, with examples.

Single crystals of the heavy-fermion compound YbAgGe have been studied by neutron scattering. The magnetic ordering occurring below  K is characterized by a commensurate propagation vector k = (1/3,0,1/3) and the moments in the basal plane of the hexagonal structure. The dynamic magnetic susceptibility is dominated by quasielastic spin fluctuations with a characteristic energy Γ_q(T) of the order of 1 meV. The spins fluctuate predominantly in the basal plane. No spin-wave excitations are observed in the magnetically ordered phase. Below the Kondo temperature,  K, Γ_q(T) shows a strong q-dependence for wavevectors along the direction, but is q-independent in the basal plane. Γ_q(T) initially shows a rapid increase with temperature T at the antiferromagnetic zone centre, but follows a standard law for other q values and for T>T_K in general. These observations classify YbAgGe as a well-behaved heavy-fermion compound with a particular q-dependence of the antiferromagnetic spin fluctuations, possibly related to the geometrical frustration of the Y b³⁺ ions.

The crystal structure and electric transport properties of the compounds CeRu_2−xCo_xGe₂ (x = 0.0–2.0) have been investigated. The magnetic behaviour of this system is correlated with the structural parameters. Based on these studies, a magnetic phase diagram has been proposed for this system. A transition from a magnetic ground state to a non-magnetic ground state has been observed with increasing Co concentration. It is found that the Ce–Ru/Co bond distance plays a critical role in driving the system from an RKKY dominated regime to a Kondo regime.

We study numerically the superconductivity in a system whose normal state is characterized by the presence of a phenomenological pseudogap, E_g/t, in the energy spectrum, for . T^* is called the crossover temperature and it is defined in the high-temperature superconductors (HTSC) where the static spin susceptibility, χ(T), is a maximum. At T^* one also observes the formation of a pseudogap in the density of states around the chemical potential (Maier et al 2002 Preprint cond-mat/0208419). In order to fix ideas, we have chosen the pseudogap and the superconducting gap to have the same symmetry. We have adopted the scenario where the pseudogap and the superconducting gap are independent of each other (Tallon and Loram 2001 Physica C 349 53), for which the pseudogap enters in the superconducting phase going down to zero at zero temperature. We have found that we require a critical value of the superconducting interaction, V/t, to produce a finite superconducting critical temperature, T_c/t, and the superconductor order parameter at T/t = 0, Δ₀/t. These results have been obtained for both μ/t = 0 and . We have obtained a phase diagram, namely, V/t versus E_g/t, at half-filling. We have compared our results with the analytical calculations of ţifrea et al (2002 Physica C 371 104), the recent work of ţifrea and Moca (2003 Preprint cond-mat/0307362 (2004 Europhys. J. B, at press)) and other relevant theoretical results.

We investigate the phase diagram of the Heisenberg antiferromagnet on the square lattice with two different nearest-neighbour bonds J and J' (J–J' model) at zero temperature. The model exhibits a quantum phase transition at a critical value between a semi-classically ordered Néel and a magnetically disordered quantum paramagnetic phase of valence-bond type, which is driven by local singlet formation on J' bonds. We study the influence of spin quantum number s on this phase transition by means of a variational mean-field approach, the coupled cluster method and the Lanczos exact-diagonalization technique. We present evidence that the critical value increases with growing s according to .

We have measured the reflectivity spectra of La_1−xSr_xMn_1−zB_zO₃ (B = Cu, Zn; ; ) manganites over wide frequency (100–4000 cm⁻¹) and temperature (80–300 K) ranges. Besides the previously observed infrared active modes or mode pairs at about 160 cm⁻¹ (external mode), 350 cm⁻¹ (bond bending mode) and 590 cm⁻¹ (bond stretching mode), we have clearly observed two additional phonon modes at about 645 and 720 cm⁻¹ below the temperature T₁ (T₁<T_C), which coincides with the phase transition temperature when the system transforms from ferromagnetic metallic into a ferromagnetic insulator state. This transition is related to the formation of short range charge/orbitally ordered domains. The temperature T₁ of the phase transition is dependent on the doping concentration and for optimally doped samples we have found that . Electrical resistivity and magnetization measurements versus temperature and magnetic field support the short range charge/orbital ordering scenario.

Dielectric dispersion and Raman studies are carried out on the solid solution series (Ba_xPb_1−x)(Yb_0.5Nb_0.5)O₃ from x = 0.0 to 0.3. The x-ray diffraction analysis indicates that with the substitution of Ba²⁺, the intensity of the peaks corresponding to antiparallel displacement of Pb²⁺ decreases and the structure becomes cubic. The low frequency dielectric dispersion studies indicate that the series undergoes a transition from an antiferroelectric to a relaxor ferroelectric with variation of x. Raman spectra of the compositions indicate the presence of a greater number of modes for the compounds with cubic structure (). A new mode at 420 cm⁻¹ is observed for the compounds with Ba²⁺ substitution. The intensity of this mode increases monotonically with increase in Ba²⁺ substitution and this has been attributed to the commencement of local distortion. This could be responsible for the appearance and increase in the diffuseness around T_max, the temperature corresponding to .

Neutron diffraction, ac susceptibility and low field dc magnetic measurements were performed on the ternary intermetallic phase Y₃Co₈Sn₄. This compound displays a ferromagnetic transition at 53 K and then undergoes a further transition below 10 K into an incommensurate antiferromagnetic modulated structure. The low temperature magnetic structure is a long period sine or helical modulation with a propagation vector of [0.095, 0, 0] corresponding to a modulation length of about 80 Å at 1.6 K. A possible mechanism for such a peculiar behaviour is a change from a high temperature itinerant character of the magnetic interactions to a low temperature localized magnetic state in a multi-sublattice system.

A novel ternary structure type has been determined from single crystals of Y b₂Zn₃Ge_3.1 grownfrom zinc flux solvent. Y b₂Zn₃Ge_3.1 crystallizes in a novel monoclinic structure type (a = 1.5804(2) nm, b = 0.429 70(1) nm, c = 1.1524(1) nm; β = 126.14(1)°) with space group C 2/m,Z = 4. The large ytterbium atoms are at the centres of pentagonal pyramids formed by Zn/Ge atoms. Zinc atoms are centred in distorted triangular prisms and polyhedra around germanium atoms are related to octahedra. The void at the centre of the Zn octahedra is only partially (20%) filled by Ge atoms. There are two positions for Yb atoms in the unit cell, which contain ions with valency slightly higher than 2+, as evidenced by x-ray absorption spectroscopy and bulk magnetic measurements. The compound exhibits metallic-like electrical conductivity, and its Seebeck coefficient shows a temperature variation characteristic of metals, being, however, fairly enhanced, as expected for intermediate valence systems.