Table of contents

Blends of the low-bandgap polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5- (4',7'-di-2-thienyl-2',1',3'-benzothiadiazole] (APFO-3) and the fullerene derivative [6,6]-phenyl–C₆₁–butyric acid methyl ester (PCBM) were spin-coated from chloroform solution into thin films, which were examined with dynamic secondary ion mass spectrometry. For blends with high PCBM content, the depth profiles show composition waves that were caused by surface-directed phase separation during spin-coating. The formation of such multilayer structures by spontaneous self-stratification is likely to have implications for optimization strategies for the performance of organic solar cells.

One of the most spectacular phenomena in physics in terms of dynamical range is the glass transition and the associated slowing down of flow and relaxation with decreasing temperature. That it occurs in many different liquids seems to call for a 'universal' theory. In this article, we review one such theoretical approach, which is based on the concept of 'frustration'. Frustration in this context describes an incompatibility between extension of the locally preferred order in a liquid and tiling of the whole space. We provide a critical assessment of what has been achieved within this approach and we discuss its relation with other theories of the glass transition.

Motivated by the melting transition of DNA, we study genuinely three-dimensional models for two interacting open, flexible and homogeneous macromolecular chains, bound or unbound to each other, at thermal equilibrium from about room temperature up to about the denaturation temperature (T_un). In each chain, angular constraints on bond angles (due to covalent bonding) determine monomers: each monomer contains n_e nucleotides and has an effective length d_e. These monomers could remain practically unaltered for temperatures in a range above and below T_un, down to 300 K. Estimates for n_e and d_e are provided and justified. Upon proceeding from Quantum Mechanics to the classical limit and using suitable large-distance approximations (partly, due to those monomer configurations), we get a generalization of Edwards' model, which includes effective potentials between monomers. The classical partition function for the two-chain system is reduced to an integral of a generalized and discretized two-chain Green's function. We analyse conditions for the denaturing transition. The fact that each single chain is an extended one-dimensional system modifies their mutual global interaction, in comparison with typical potentials between nucleotides: this is simply illustrated by computing a global effective potential between the two chains. Applications for Morse potentials are presented. Our models seem to be physically compatible with some previous one-dimensional ones and could allow us to efficiently extend the latter to three spatial dimensions.

We study the stability of mixtures of highly screened repulsive charged spheres and non-adsorbing ideal polymer chains in a common solvent using free volume theory. The effective interaction between charged colloids in an aqueous salt solution is described by a screened Coulomb pair potential, which supplements the pure hard-sphere interaction. The ideal polymer chains are treated as spheres that are excluded from the colloids by a hard-core interaction, whereas the interaction between two ideal chains is set to zero. In addition, we investigate the phase behaviour of charged colloid–polymer mixtures in computer simulations, using the two-body (Asakura–Oosawa pair potential) approximation to the effective one-component Hamiltonian of the charged colloids. Both our results obtained from simulations and from free volume theory show similar trends. We find that the screened Coulomb repulsion counteracts the effect of the effective polymer-mediated attraction. For mixtures of small polymers and relatively large charged colloidal spheres, the fluid–crystal transition shifts to significantly larger polymer concentrations with increasing range of the screened Coulomb repulsion. For relatively large polymers, the effect of the screened Coulomb repulsion is weaker. The resulting fluid–fluid binodal is only slightly shifted towards larger polymer concentrations upon increasing the range of the screened Coulomb repulsion. In conclusion, our results show that the miscibility of dispersions containing charged colloids and neutral non-adsorbing polymers increases upon increasing the range of the screened Coulomb repulsion, or upon lowering the salt concentration, especially when the polymers are small compared to the colloids.

On the basis of Brownian dynamics computer simulation studies, we investigate the structure and dynamics of two-dimensional mixtures of rods driven by an external torque and spheres. First, we show that the tracer long-time diffusion of spheres in a system of rotating rods can be efficiently tuned via the rod density. It behaves non-monotonically for increasing rod density. In particular, the sphere diffusion becomes strongly enhanced across the jamming transition of the rods. Second, we show that rotating rods in a dense suspension of spheres form aggregates of rod clusters which are rotating jointly. The cluster size is in reasonable agreement with the predictions of a simple theory.

Potentially, lipid membranes possess a high tangential conductivity and permittivity due to their surface charges and the in-plane orientation of the headgroup dipoles. Electrically, membranes exhibit a sandwich structure with a largely isotropic centre formed by the fatty acid chains and confined by two anisotropic headgroup layers. Accordingly, we described spherical vesicles by an aqueous core covered by three shells. For a theoretical comparison, models with an anisotropic single shell and anisotropic homogeneous spheres were also considered. Two effects can be clearly demonstrated. (1) High tangential conductivities or permittivities may lead to cyclic variations in the phase of the electric potential in the radial direction, resulting in a hemi-shell structure of the electric potential inside the objects with oppositely charged facets. The thickness of the anisotropic shell restricts the number of phase oscillations. (2) Despite the strong local field inhomogeneities, an isotropic homogeneous Maxwellian equivalent body with an identical external field distribution exists for any of the anisotropic models. Its properties can be found from a comparison of the numerically calculated surface potential and the classical expression of the Clausius–Mossotti factor at any given frequency. The permittivity conductivity pairs obtained exhibit a sigmoidal-like frequency dependence.

The possibility that vibrational excited states (VESs) are the drivers of protein folding and function (the VES hypothesis) is explored to explain the reason why Gln- and Asn-rich proteins are associated with degenerative diseases. The Davydov/Scott model is extended to describe energy transfer from the water solution to the protein and vice versa. Computer simulations show that, on average, Gln and Asn residues lead to an initial larger absorption of energy from the environment to the protein, something that can explain the greater structural instability of prions. The sporadic, inherited and infectious character of prion diseases is discussed in the light of the VES hypothesis. An alternative treatment for prion diseases is suggested.

Depletion of the liquid density near a solid surface with a weak long-range fluid–surface interaction was studied by computer simulations of the liquid–vapour coexistence of a Lennard-Jones (LJ) fluid confined in slitlike pores. In a wide temperature range the liquid density decreases towards the surface without the formation of a vapour layer between the liquid and the solid surface. This evidences the absence of a drying transition up to the liquid–vapour critical point. Two contributions to the excess desorption Γ_tot were found. The first one, Γ_ξ∼ρ_bulkξ, exists at any temperature and diverges as the bulk correlation length ξ when approaching the liquid–vapour critical temperature T_c. The second contribution, Γ_L∼ρ_bulkL₀, originates from a microscopic drying layer near the solid boundary. At high temperatures the thickness L₀ of the drying layer increases in accordance with the power law L₀∼−ln(1−T/T_c), indicating a drying transition at T_c. The drying layer can be suppressed by strengthening the fluid–surface interaction, by increasing the fluid–surface interaction range or by decreasing the pore size.

The surface tension and density of the liquid Sn₆₀Pb₄₀, Sn₉₀Pb₁₀, Sn_96.5Ag_3.5 and Sn₉₇Cu₃ solder alloys (wt%) have been determined experimentally over a wide temperature interval. It is established that the surface tension of liquid Sn₉₀Pb₁₀ is about 7% higher than that of a traditional Sn₆₀Pb₄₀ solder and that the surface tension of Sn_96.5Ag_3.5 and Sn₉₇Cu₃ alloys is about 12% higher than that of Sn₆₀Pb₄₀. The analytical expressions for the temperature dependences of the surface tension and density are given.

The frequency-dependent magnetic relaxation of Co_xFe_3−xO₄ nanoparticles stabilized by N-methyloleamidoacetic acid is investigated in three apolar suspending media (toluene, cyclohexane and decane). Hereby, the dependence of the crossover from a Brownian to a Néelian relaxation mechanism on the cobalt content is observed. The experimental results obtained from the complex magnetic susceptibility confirm the theoretically expected crossover between both relaxation mechanisms. The topological characterization is performed by means of x-ray diffraction, photon correlation spectroscopy and—in the Brownian regime—by magnetic relaxometry, allowing the comparison of crystallographic and hydrodynamic particle sizes.

In order to get better glass-forming abilities (GFAs), Ni atoms are partially replaced by Cu and Co atoms in Al₈₄Ni₁₂Zr₄ alloys. Thermal analysis shows that the reduced crystallization temperature T_rx has no direct correlation with the GFA of the alloys. However, it is notable that prepeaks have been found in the total structure factors of the amorphous Al₈₄Ni_(12−x)Zr₄Cu_x and Al₈₄Ni_(12−x)Zr₄Co_x alloys. In addition, the results prove that the intensity of the prepeaks influences the GFA powerfully. The amorphous alloys with larger intensity of the prepeak show better GFA. The influence of prepeaks on the GFA can be explained by the atomic configuration difference among the liquid, crystal and glass states.

The neutron double-differential cross-section of molecular hydrogen at low density has been measured at two rather low scattering angles and different final neutron energies by means of three-axis spectrometry. This first inelastic scattering determination of the single-particle roto-translational dynamics of room temperature H₂ allows for a detailed test of the theoretical modelling of the spectral line-shapes of such a fundamental molecule, performed by referring both to a careful quantum-mechanical treatment and to a simpler semi-classical approximation. A comprehensive report on the neutron measurements and data analysis is presented, along with an overview of the theories used for comparison with the experimental results. An encouraging picture of the present capabilities in the calculation of the true dynamic response of hydrogen gas to slow and thermal neutrons is obtained, opening new perspectives for accurate data calibration in inelastic neutron spectroscopy, with special relevance for small-angle experiments.

In the present work the effects produced by cation substitution on the dynamics of water in Linde type A (LTA) synthetic zeolites were investigated by means of inelastic neutron spectroscopy (INS). In particular, we performed measurements on fully hydrated Na-A and Mg-exchanged-A zeolites. The collected INS spectra showed two broad bands due to the vibrational (water–cation and hydrogen bond stretch) and librational (rock, twist and wag) modes of water. To quantitatively assign these various modes, the spectra were decomposed into Gaussian components and compared with the INS spectra of ice and of water in other zeolites. The observed shifts of the band positions for different temperatures were discussed. In particular, a slight shift of the three librational modes towards higher frequencies with increasing temperature was revealed for both samples. The hydrogen bond stretching and librational modes of water in Na-A and Mg50-A zeolites shift to lower frequencies with respect to ice. Furthermore, in the case of Mg50-A zeolite, we observed a shift upwards in frequency of librational bands with respect to Na-A zeolite. The obtained results were discussed in relation to our previous elastic and quasi-elastic neutron scattering (ENS and QENS respectively) measurements on the same samples.

We explore the possibility of using a simplified version of the multicomponent hypernetted-chain (HNC) integral equation for charged colloidal suspensions. The method presented here is an extension of a previous one (Anta et al 2003 J. Phys.: Condens. Matter15 S3491) for systems with added salt. Within this theoretical scheme, colloid–colloid, colloid–microion and microion–microion correlations are treated with different levels of approximation. Thus, we describe microion–microion correlations in the random phase approximation (RPA) whereas colloid–microion and colloid–colloid correlations are solved in the HNC approximation. This strategy reduces the numerical demands of the full HNC equation and makes it possible to get a solution at thermodynamic states not included in the original solution region. In addition, it is found that we can extend even further the range of applicability of the theory by solving the colloid–microion part of the theory for a fixed colloid–colloid radial distribution function. This is obtained in turn from the solution of the one-component HNC equation for the effective DLVO potential. Using these strategies we obtain a semi-quantitative description of the colloidal system as well as an indication of a salt-driven phase transition at low ionic strengths. This phase transition is associated to charge inversion, as inspection of the colloid–microion density profiles reveals. However, the theory fails to reproduce reliable effective-pair potentials in the vicinity of the non-solution region. The applicability and usefulness of integral equation theories when applied to charged colloids is discussed.

The crystal structure of the γ-brass phase Cu₅Zn₈ is confirmed with single-crystal x-ray diffraction and CCD detector to be cubic with 52 atoms in the unit cell, space group , and the refined atomic positions are in good agreement with previously reported data. The structural behaviour of α-(fcc), β-(bcc), and γ-brass (cI52) phases of the Cu–Zn alloy system has been studied under pressure using diamond anvil cells and powder x-ray diffraction with synchrotron radiation. The appearance of additional peaks in the diffraction patterns of α- and β-phases indicates the beginning of transitions to new phases at 17 and 37 GPa, respectively. The complex cubic γ-brass phase is observed to be stable up to at least 50 GPa. The bulk modulus K₀ was determined as 140(4) GPa for α-, 139(5) GPa for β-, and 121(2) GPa for γ-phase assuming K₀^' = 4. The structural stability of brass phases of the Cu–Zn system under pressure is discussed in terms of a Hume-Rothery mechanism.

The electronic and magnetic properties of BaTbO₃ are studied by using the local-spin density approximation method where on-site Coulomb interaction U is included. The ground magnetic phase is found to be G-type antiferromagnetic with lattice constant of 4.278 Å, which is consistent with experimental results. The states of Ba in the compound show obvious ionic characteristic, while very strong hybridization between Tb 4f and O 2p orbitals is found in the compound. The on-site Coulomb interaction U is found to have significant influences on the state distribution of Tb 4f. With the increase of U, most of the spin-up and spin-down states of Tb 4f move away from the Fermi level drastically, giving rise to the change of band gap of the compound and the variation of magnetic moment of Tb ions. The optimized value of U is determined to be 4.0 eV, and the corresponding band gap of BaTbO₃ is 0.61 eV, which seems to be a narrow gap semiconductor rather than a good insulator.

The specific heat oscillation in the mixed state of type II superconductors is studied theoretically when rotating a field within a plane containing a gap minimum and maximum. The calculations are performed microscopically by solving the quasi-classical Eilenberger equation for vortex lattices. The field dependence of the oscillation amplitude can discriminate between the nodal and anisotropic gap with a finite minimum and the oscillation phase gives the gap minimum position on the Fermi surface. These also provide a way to separate out the anisotropic behaviour due to the Fermi velocity.

The structural and compositional changes during mechanical milling of Fe–Cr powder (10 wt% Cr) were followed by scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), Mössbauer spectroscopy and magnetization measurement. All these techniques give complementary information about the alloying process during milling. Different particles are cold welded and elongated to form composite granules in the initial stage of milling (5–10 h). The crystalline order is also destroyed heavily and nanosize crystallites are formed during early milling. Chromium diffusion in the iron lattice starts somewhat late (after 20 h of milling) when composite granules flatten, reducing the distance between iron and chromium layers. Though compositional homogeneity is achieved up to nanometre scale in 65 h of milling, atomic diffusion of the species still continues up to 100 h of milling.

The electronic and magnetic properties of both LaCoO₃ and La_0.5Ca_0.5CoO₃ have been investigated by means of ab initio full-potential augmented plane wave plus local orbitals (APW+lo) calculations carried out with the Wien 2k code. The functional used is the local-density approximation LDA +U. Doping with Ca²⁺ introduces holes into the Co–O network. We analyse the densities of states and we confirm that the intermediate state (IS) is stabilized by the Ca²⁺ substitution. This intermediate state in our results turns out to be metallic, and has a large density of states at the Fermi energy. The calculated magnetic moment in La_0.5Ca_0.5CoO₃ is found to be in good agreement with experiment.

Optically active defects in diamond can be excited by photons with an energy less than that of the zero-phonon transition. We show that this process is thermally activated. For diamonds in which one type of defect is dominant, the activation energy is equal to the energy difference between the zero-phonon line and the photons used for excitation.

The dependence of Ga K-edge multiple-scattering extended x-ray absorption fine structure (MS-EXAFS) effects on the nearest neighbours in GaP, GaAs and GaSb semiconductor compounds with the zinc blende structure has been comprehensively investigated by considering the coordination environment within the first three shells around the Ga atoms. It is revealed that in the case of GaP with a light element as the first neighbour of the Ga absorber, the MS-EXAFS effects are negligibly weak with respect to the single-scattering (SS) contribution. For GaAs and GaSb compounds with heavier elements as the first neighbour of the Ga absorber, the MS effects become increasingly important and are dominated by a triangular double-scattering path DS2 (). The EXAFS contribution of the DS2 path destructively interferes with that of the second shell single-scattering path (SS2), with the amplitude ratio of DS2 to SS2 rising from 7% for GaP to 25 and 70% for GaAs and GaSb, respectively. This indicates that the second shell peak magnitude for these compounds is increasingly damped by the MS effects as the first nearest neighbour goes from P to Sb. Based on these results, we present a generalized and simplified high-shell MS-EXAFS analysis method for compounds with the open structure of zinc blende.

Magnetic properties of the hexagonal compounds of YMn_1+xO₃ (0≤x≤0.15) have been studied by measuring the dc magnetization, ac magnetization, magnetic hysteresis, and relaxation of the magnetization. All the results give evidence of a spin glass (SG) state at low temperatures in samples with x≥0.08. This is attributed to the competition between the antiferromagnetic (AFM) matrix and the ferromagnetic (FM) phase induced by the excess Mn. Moreover, careful investigations on the thermoremanent magnetization (TRM) of the sample with x = 0.10 indicate that this SG state is an ideal state with nearly infinite equilibration time and that it shows almost perfect full ageing.

We report on the magnetic, magnetotransport and structural characterization of (Ca_1−yNd_y)₂Fe_1+xMo_1−xO₆ (x<0.5) ferromagnetic double perovskites. It is found that the presence of an excess (x>0) of Fe ions in the metallic sublattice produces a remarkable increase, by more than 90 K, of the Curie temperature. Mössbauer spectroscopy data indicate a reinforcement of the magnetic interactions. We argue that this dramatic enhancement of the ferromagnetic order is due to the strong antiferromagnetic superexchange coupling between near-neighbour Fe–Fe occupying regular and antisite positions in the structure. Moreover, the results indicate that the excess of magnetic ions (Fe) is essential to overcome the dilution effects caused by antisite defects.

The electrical resistivity of a metal close to a weakly first-order paramagnetic-to-ferromagnetic transition at T = 0 is determined assuming that droplets of magnetic order remain in the paramagnetic state. Using an elementary model of spin-diffusion to describe the spin dynamics, it is found that, at low temperatures, the resistivity ρ varies as ρ = ρ₀+aT^3/2. The temperature-dependent term occurs because of the scattering of electrons by low-frequency spin fluctuations propagating normal to the surface of the ferromagnetic regions. The temperature dependence is in agreement with recent measurements made on MnSi at pressures above the critical pressure. The pressure dependence of the model prediction for a is also found to be in qualitative agreement with the variation observed experimentally above the critical pressure.

The value of m_h = 0.33 m₀ has been experimentally obtained for hole effective mass in a tunnel-thin (2–3 nm) SiO₂ film. The use of this value ensures the adequate modelling of a direct-tunnelling hole current in MOS devices. For the first time, in order to determine m_h, the characteristics of a MOS tunnel emitter transistor have been mathematically processed, that allows for the precise estimation of the effective oxide thickness, as the electron effective mass in SiO₂ is independently known from the literature. The formulae for simulation of currents in a tunnel MOS structure are listed along with the necessary parameter values.

The single-crystalline x-ray storage phosphor BaCl₂:Ce³⁺ has been investigated by photo- and x-ray luminescence, and also by observing the afterglow and the photostimulated luminescence (PSL). A single active Ce³⁺ site has been observed with a characteristic emission doublet at 349 and 373 nm at room temperature. The PSL stimulation spectrum shows a clear resemblance to F-type absorption spectra in undoped BaCl₂, indicating that the PSL-active electron centres are F-type centres. The results are compared with previous photoluminescence, PSL and new x-ray luminescence data on BaBr₂:Ce³⁺. The integral PSL efficiency in the chloride is found to be somewhat smaller than in the bromide, which is still large enough for storage phosphor applications, while the efficiency in the bromide may be even larger or comparable to that of the commercial x-ray storage phosphor BaFBr:Eu²⁺.

Following previous work on Hubbard and Anderson models, we introduce a simplified three-band Hubbard model for CuO₂ planes of high-T_c superconductors. This allows for exact evaluation of single-particle Green's functions within the dynamical mean field approach. Keeping information about the lattice structure through the appropriate bare density of states, we study the existence of magnetic solutions, and their stability with respect to variations of the doping fraction, interaction parameters, and temperature. Normal-state properties of known superconducting cuprates are reproduced, and we show that the model has room for substantial enhancement of Néel temperatures.

We study the role played by the magnetic frustration in the antiferromagnetic phase of the organic salt κ-(BEDT–TTF)₂Cu[N(CN)₂]Cl. Using the spatially anisotropic triangular Heisenberg model we analyse previous and newly performed NMR experiments. We compute the 1/T₁ relaxation time by means of the modified spin wave theory. The strong suppression of the nuclear relaxation time observed experimentally under varying pressure and magnetic field is qualitatively well reproduced by the model. Our results suggest the existence of a close relation between the effects of pressure and magnetic frustration.

The thermal quenching of Eu²⁺ 5d–4f emission on Ba, Sr, or Ca sites in compounds is often attributed to a large displacement between the ground state and excited state in the configuration coordinate diagram. This work will demonstrate that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism. A model is proposed to explain why in some types of compounds the quenching temperature decreases when going from the Ba variant via the Sr variant to the Ca variant and in other types of compounds the reverse behaviour occurs. The nature of the bottom of the conduction band plays an important role in this.

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