Table of contents

From dielectric permittivity measurements, we show that deuteration yields a large increase of the transition temperature for the charge ordered state of (TMTTF)₂X (X = AsF₆, SbF₆, ReO₄) salts. We propose an explanation of this phenomenon, suggesting that deuteration induces a modification of the (TMTTF)₂X crystal unit cell.

A half-metal has been defined as a material with propagating electron states at the Fermi energy only for one of the spin directions. But is it fully half-metallic, that is without electrons with opposite spin at that energy? We have studied the spin-conserving process of tunnelling between La_0.7Sr_0.3MnO₃ half-metallic electrodes across an ultrathin SrTiO₃ insulator. This experiment demonstrates that the class of half-metallic materials indeed exists at non-zero temperatures, even at interfaces. It also shows that a fully spin-polarized tunnelling current may persist at large bias.

Dielectric dispersion studies in the frequency range between 220 Hz and 1 MHz have been performed on pentakis(imidazolium) undecachlorodibismuthate(III), (C₃N₂H₅)₅Bi₂Cl₁₁, single crystals. The measurements have been made over the temperature range between 300 and 140 K. It is found that the dielectric dispersion, observed along the b-axis of the monoclinic symmetry (P 2₁/c), is monodispersive between 166.5 and 168 K, and polydispersive in the close vicinity of T_c = 166 K, for T<T_c + 0.2 K. The observed dielectric dispersion is analysed using a single relaxation mode of the Debye type. Over the paraelectric phase the process of the dielectric critical slowing down is observed. This is characteristic of the order–disorder ferroelectric phase transition.

We analyse the magnetic susceptibility χ and the specific heat C of the heavy-fermion material URu₂Si₂, assuming a variety of the crystal field level schemes. The heavy-fermion behaviours of χ and C above T_N = 17.5 K are shown to be reproduced fairly well in the singlet–singlet–singlet (Γ₃–Γ₁–Γ₂) level scheme proposed by Santini and Amoretti, and also in a novel level scheme of Γ₁-singlet ground state with Γ₅-doublet excited state. We discuss that the novel singlet–doublet level scheme is favourable for the close proximity of weak antiferromagnetism to the hidden order in URu₂Si₂, and the most probable candidate of the hidden order is a J_x(J_y²−J_z²)-type octupolar ordering.

Liquid foams can behave like solids or liquids, depending on the applied stress and on the experimental timescale. Understanding the origin of this complex rheology which gives rise to many applications and which resembles that of many other forms of soft condensed matter made of closely packed soft units requires challenging theoretical questions to be solved. We briefly recall the basic physics and physicochemistry of foams and review the experiments, numerical simulations and theoretical models concerning foam rheology published in recent years.

We present new data for the electrical conductivity of foams in which the liquid fraction ranges from two to seventy per cent. We compare with a comprehensive collection of prior data, and we model all results with simple empirical formulae. We achieve a unified description that applies equally to dry foams and emulsions, where the droplets are highly compressed, as well as to dilute suspensions of spherical particles, where the particle separation is large. In the former limit, Lemlich's result is recovered; in the latter limit, Maxwell's result is recovered.

We report on the use of optical probes to determine the thermal transition temperatures of spin-coated films of the prototypical fluorene based conjugated polymer poly(9,9-dioctylfluorene) (PFO). In particular we focus here on the use of temperature dependent photoluminescence (PL) measurements that are well suited to the study of PFO and other intrinsically fluorescent polymers and blends. The integrated PL intensity reveals clear signatures of the glass transition temperature (T_g), the onset of crystallization (T_c) and subsequent melting into the nematic liquid crystalline mesophase (T_m). The PL intensity determined transitions are shown to be consistent with an independent optical probe, namely spectroscopic ellipsometry, and with differential scanning calorimetry measurements on bulk samples. The especially strong contrast in PL intensity at T_c and T_m is shown to be a consequence of changes in light out-coupling in the direction of PL detection, a conclusion that is confirmed by measurements of edge emitted waveguided PL and amplified spontaneous emission, and by analysis of ellipsometry data.

A new analytical concept is applied to the kinetics of the shrinking process of poly(N-isopropylacrylamide) (PNIPA) gels. When PNIPA gels are put into hot water above the critical temperature, two-step shrinking is observed and the secondary shrinking of gels is fitted well by a stretched exponential function. The exponent β characterizing the stretched exponential is always higher than one, although there are few analytical concepts for the stretched exponential function with β>1. As a new interpretation for this function, we propose a superposition of step (Heaviside) function and a new distribution function of characteristic time is deduced.

Well-defined hydrophobic polyelectrolytes adsorbed onto oppositely charged solid surfaces at the solid/liquid interface have been investigated. We have used in situ high-energy x-ray reflectivity to study how the thickness h of the adsorbed layer depends on the polymer effective charge fraction f_eff and on the chain length N. We have found , suggesting a pearl-necklace conformation for the chains in the adsorbed layer.

We describe the sedimentation–diffusion equilibrium of binary mixtures of charged colloids in the presence of small ions and for non-dilute conditions, by extending the work of Biben and Hansen (1994 J. Phys.: Condens. Matter 6 A345). For a monocomponent system, they included a Carnahan–Starling hard-sphere correction and a pressure term due to the small ions. We extend this approach to mixtures of spheres of unequal size, and implement the fact that the effective buoyant mass of a particle is based on the difference in mass density between the particle itself and the local average mass density, and not on the difference with the mass density of the pure liquid.

Without the three volume effects (hard-sphere repulsion, ion pressure, buoyant particle mass based on local, average, mass density), the lighter particle (buoyant mass m_L, charge z_L) only levitates from the bottom (with a maximum in concentration displaced upwards) when z_L/m_L> z_H/m_H (with H indicating the heavier particle). With these volume effects included the fractionation is much sharper and occurs even for .

For certain parameter settings we find a bimodal distribution of the heavier particles with most of them in the bottom region, but with a small fraction forming a thin layer higher up in the column. This second layer is not found when the buoyant particle mass is based on the mass density difference with the pure liquid and/or when the ion pressure is neglected, suggesting that it is due to a subtle interplay between these two contributions.

Drainage is one of the driving forces for the temporal instability of molten metal foams. For usual aqueous foams this phenomenon is well examined and understood on both the experimental and the theoretical side. The situation is different for metallic foams. Due to their opaque nature, the observation of drainage is only possible by either measuring the density distribution of solidified samples ex situ or by x-ray or neutron radioscopy. Up to now there exists just one theoretical study describing the drainage behaviour of metallic foams incorporating the drainage equation, the temperature dependence of the viscosity and thermal transport. This paper will present results on the drainage behaviour of aluminium foams grown by a powder-metallurgical production route. For this purpose an experiment which allows the observation of drainage in cylindrical metal foam columns has been developed. Experimental density profiles after different drainage times are measured ex situ and compared to numerical results of the standard drainage equation for aqueous foams. This first comparison between the density redistribution of metallic aluminium foams and numerical solutions shows that the standard drainage equation can be used to explain the drainage behaviour of metallic foams.

Rapidly quenched ZrSiO₄ melts produced by CO₂ and pulsed YAG lasers have been analysed using infrared (IR) and Raman spectroscopy. The quenched melts exhibit decomposition into binary oxides (ZrO₂ and SiO₂), while other phases or complex SiO₄ tetrahedron networks are also observed. The local structures and the phases of the quenched melts depend strongly on the quenching rate and melting conditions. Monoclinic ZrO₂ are found to be the main ZrO₂ phase in the samples treated by CO₂ laser, although tetragonal ZrO₂ was found near boundaries between the untreated and melted regions. High concentrations of tetragonal ZrO₂ were detected in the samples treated by pulsed YAG lasers. Our observations indicate that the formation of tetragonal ZrO₂ is related to relatively high quench rates. Micro-IR and Raman data from areas near the boundaries between the quenched melts and untreated zircon show systematic variations of local structures and compositions. We observe a small region with relatively low density between the untreated and melted boundaries, which consists of tetragonal ZrO₂ or glassy ZrO₂, and SiO₂. Broad vibrational bands occur in the wavenumber region where the characteristic frequencies of zircon are located. This observation could indicate the possible existence of small amounts of glassy ZrSiO₄ in the melt state of zircon, although zircon tends to decompose above the melting point. These observations are in sharp contrast with the behaviour of radiation-damaged zircon, the so-called metamict zircon, where no phase separation occurs in samples which have not been subsequently annealed.

Transparent conducting oxides (TCOs) are evaluated as selective filters in thermophotovoltaic (TPV) devices using theoretical considerations. The TCOs plasma frequency and the optical properties in the near infrared are calculated using the Drude theory for free carriers. To improve the radiation transmission for energies higher than the band gap of the cells and the reflection back to the emitter for sub-band gap radiation the TCO's optical properties are studied as a function of electron concentration. Taking into account the photocell and emitter spectral response radiation, the optimal free carrier concentration and the thus thickness of the TCO to be used as selective filter are calculated.

We report a combined pyroelectric, dielectic and structural characterization of PZN single crystals in field cooling (FC) run along [001]. The pyroelectric current and the dielectric permittivity show a new characteristic temperature, at T≈370 K, related to a macroscopic competition between two induced ferroelectric tetragonal (T) and monoclinic (M) phases. The phase transition, which starts around T_C−R = 385 K, spreads over a large temperature range and some amount of T-phase is still observed at room temperature (RT). The M–T phase mixture, induced below T_C−R, is stable after field removal. According to a previous Raman scattering work, a fourfold symmetry of M-domains is proposed; the residual T-phase is stabilized by the coexistence of four symmetry-allowed M-domains arranged around [001]. Our results confirm that in a restricted temperature range (T_{C −R}− 20 K, T_C−R+20 K) the free energy of the relaxor-ferroelectric PZN is almost independent of the direction of polarization and accordingly the material can switch from one polar order to the other.

Room-temperature Mn K-edge extended x-ray absorption fine structure (EXAFS) studies were carried out on La_1−xSr_xMnO_3+δ (x = 0.1–0.9) compounds. It is found from the detailed EXAFS analysis that the local structure around Mn sites is different from the global structure inferred from x-ray diffraction, especially for x≤0.4, indicating the presence of local distortions in MnO₆ octahedra. For the rhombohedral compounds, x = 0.1 to 0.3, the distortion is maximum for x = 0.1 and two bond lengths are seen: a short one in the basal plane and a long one in the apical plane. For compounds with x = 0.4–0.8 two short bonds in the basal plane and four long bonds (two in the basal plane and the remaining two in the apical plane) are seen. For the compounds with compositions up to x = 0.3, the long bond length decreases and the short bond length increases with increase in x, whereas for the compounds with 0.4≤x≤0.8 both types of bond length decrease. Such behaviour of bond lengths is an indication of the changed nature of distortion from Jahn–Teller type to breathing type at x = 0.4 composition.

Polycrystalline CrN/Si₃N₄ multilayer films were deposited by using two reactive magnetron sputtering targets, Cr and Si, respectively, and the as-deposited multilayer coatings were annealed at 900 °C for 2–4 h in vacuum. We investigated the effects of mixed discharge gas pressure, bias voltage, substrate temperature and annealing temperature on the microstructural and interfacial properties of polycrystalline CrN/Si₃N₄ multilayer films. X-ray diffraction and x-ray reflectivity measurements showed that the layer structure of the coatings is maintained even after annealing at 900 °C for 4 h when the Si₃N₄ layer thickness is 0.3 nm, which meant that with proper control of the Si₃N₄ thickness CrN/Si₃N₄ multilayer coatings had an excellent thermal stability and were potential candidates for high-temperature tribological applications.

We report on electronic band structure calculations, using the ab initio full potential linearized augmented plane wave (FLAPW) method, for Sr₂FeMoO₆ (SFMO) double perovskites presenting antisite defects or/and oxygen vacancies. We exhibit that the half-metallic character is preserved for SFMO containing only nearly isolated oxygen vacancies, indicating that the control of oxygen content in such compounds is less critical for the transport properties than the control of the antisite defects. We show that the total magnetic moment is much more reduced for structures with oxygen vacancies (approximately 2 µ_B per vacancy) than for structures without. We also investigate the stability of an antiparallel (AP) magnetic moment on Fe antisites and we show that these solutions are highly unstable relative to the parallel (P) solution, even if the presence of O vacancies reduces the difference between P and AP solutions.

We report high-resolution Compton profiles (CPs) of Ni₇₅Cu₂₅ and Ni₇₅Co₂₅ disordered, ferromagnetic alloys measured along [100], [110] and [111] crystallographic directions. The directional CP anisotropies, profile derivatives and reciprocal form factors B(z) are analysed in detail and compared with first-principles KKR-CPA (Korringa–Kohn–Rostoker coherent potential approximation) calculations. Good overall agreement is found between the experimental and theoretical anisotropies. It follows from experiment that the majority-spin Fermi surface of the Ni₇₅Cu₂₅ and Ni₇₅Co₂₅ alloys occurs at higher momentum than the theory predicts. Also, a splitting in the secondary maxima in experimental B(z) functions of the alloys is observed, which is not predicted by the KKR-CPA calculations.

We present here a calculation of the configuration averaged optical conductivity of random binary alloys CuAu and AgAu. Our formulation is based on the augmented space formalism proposed by Mookerjee (1973 J. Phys. C: Solid State Phys. 6 1340) and the optical conductivity is obtained directly through a recursive procedure suggested by Viswanath and Müller (1993 The User Friendly Recursion Method (Troisième Cycle de la Physique, en Suisse Romande)).

Highly oriented thin films of LaNi_1−xM_xO₃ (M = Mn, Co) are grown on LaAlO₃(100) substrate by pulsed laser deposition. They undergo a metal to insulator transition when the Mn or Co concentration is increased. The observed conduction pattern is highly sensitive to the doping concentration in these thin films. The conduction pattern also varies as the doping element is varied from Mn to Co. There is a large dominance of electron–lattice interactions in the conduction mechanism of the charge carriers. While the metallic thin films of LaNi_1−xCo_xO₃ show a linear variation of resistivity with temperature, LaNi_1−xMn_xO₃ thin films exhibit a prominent square-root dependence of resistivity on temperature. At high concentrations of Mn or Co, the conduction takes place via a polaron hopping mechanism, which suggests that lattice polarization may be present in these films. The change observed in the transport properties is attributed to the charge disproportionation between the Ni³⁺–Ni²⁺ pairs, which are favoured more in Mn doped thin films. The photoelectron spectroscopic studies give evidence of charge disproportionation present in these films.

We present the first molecular dynamics simulation of the vacuum deposition of amorphous selenium films. We compare the classical, tight-binding and Hubbard-term corrected tight-binding molecular dynamics simulation methods. Densities, coordination defects, radial distribution functions, bond angles, dihedral angles, intrachain and interchain atomic correlations were investigated in the obtained amorphous films. Local atomic arrangements were compared to results of diffraction measurements.

Muon spin relaxation (μSR) experiments on the geometrically frustrated spinel oxide, Li₂Mn₂O₄, show the development of spin correlations over a range of lengthscales with decreasing temperature. Increased relaxation below ∼150 K is consistent with the onset of spin correlations. Below 50 K, spin order, on a lengthscale which is long range for the μSR probe, appears abruptly in temperature, consistent with prior neutron diffraction results. The oscillations in the zero field asymmetry are analysed using a three frequency model. By locating the muon site, this is shown to be consistent with the unexpected 2D structure on the Kagomé planes proposed originally from neutron data. Longitudinal field data demonstrate that some spin dynamics persist even at 2 K. Thus, a very complex magnetic ground state, featuring the coexistence of long lengthscale 2D ordering and significant spin dynamics, is proposed. This is unusual considering the 3D topology of the Mn³⁺ spins in this material.

A combined experimental and theoretical Brillouin light scattering study of thermally excited spin waves in Fe/Co multilayers with three and five magnetic layers in direct contact is presented. A large number of discrete, well-resolved peaks, classified as either surface or bulk standing modes of the stack, appear in the measured spectra. The investigation of their frequency dependence on the magnetic field intensity and incidence angle of light allowed us to determine the complete set of magnetic parameters of the multilayer. The interlayer exchange coupling and perpendicular interface anisotropy are discussed in detail. The profiles of the dynamic magnetization associated with different spin wave modes are calculated and utilized for the calculation of the Brillouin scattering cross section. The result, compared with experimental spectra, allowed the determination of the ratio between the magneto-optic constants of Fe and Co.

The effect of treatment conditions on the magnetic and magnetotransport properties of A-site ordered PrBaMn₂O_6−δ manganites is examined. The parent oxygen-stoichiometric A-site ordered PrBaMn₂O₆ samples were prepared from oxygen-stoichiometric A-site disordered Pr_0.50Ba_0.50MnO₃ ones by using a 'two-step' synthesis method. The most significant structural feature of the A-site ordered manganites is that the MnO₂ sublattice is sandwiched by two kinds of rock-salt layers, PrO_x and BaO. The oxygen-stoichiometric A-site ordered PrBaMn₂O₆ demonstrates a ferromagnetic metal to paramagnetic insulator transition with the Curie point at about 320 K. These compounds are stable in air up to 1300 °C. The A-site ordered PrBaMn₂O₆ samples were next reduced in flowing argon as well as being treated under high pressure conditions. The reduction of the A-site ordered PrBaMn₂O₆ samples leads to the appearance of an antiferromagnetic state with a Néel point of about 140 K, while the A-site order remains. Upon high pressure treatment the degree of A-site order decreases, which reduces T_C to 180 K. The magnetotransport properties of A-site ordered manganites treated under different conditions are discussed in terms of the manganese valence, oxygen content and degree of A-site order.

The Jahn–Teller energy, bandwidth and Curie temperature T_C of LaMn_1−xM_xO₃ (M = Ni, Co, 0≤x≤0.208) are calculated using the atomic simulation technique. The simulated Jahn–Teller energy is in agreement with the data deduced from experimental results. It is found that there is a strong relation between the doping-introduced strains and T_C. The calculated T_C agrees well with experimental results.

The magnetic properties of maghemite particles hosted in silica aerogel pores are studied in depth, with most of the focus on the magnetic relaxation mechanisms. It is shown that at room temperature, the system behaves as an ensemble of non-interacting superparamagnetic particles, indicating that particle aggregation can be avoided by using a sol–gel preparation method with supercritical drying. In the temperature range from 15 to 300 K the magnetic ac susceptibility χ(T) displays a broad peak that shifts to higher temperatures on increasing the ac applied field frequency. Transmission electron microscopy reveals acicular shaped maghemite particles exhibiting unimodal size distributions, from which energy barrier distributions can be inferred. The distribution of activation energies has been independently obtained from scaling plots of the frequency dependent out-of-phase ac susceptibility component using τ₀ = 10⁻¹¹ s. The value obtained is in agreement with the one derived from the particle size distribution. Combining the two distributions and assuming magnetic volume anisotropy, an effective anisotropy constant was determined for 5 × 20 nm and 4 × 20 nm average dimensions respectively. It is also shown that the temperature dependences of the relaxation time τ as obtained from Néel analysis of the magnetic absorption χ''(T,ω) and Cole–Cole frequency dependence analysis are in good agreement with the value derived from Mössbauer effect spectroscopy at room temperature.

Transparent Ni-doped SnO₂ thin films grown by the pulsed laser deposition technique on LaAlO₃, SrTiO₃ as well as R-cut Al₂O₃ substrates all show room-temperature ferromagnetism (FM). While the Ni-doped SnO₂ films on LaAlO₃ substrates have a large magnetic moment of about 2 µ_B/Ni, films grown under the same conditions on SrTiO₃ and Al₂O₃ substrates have a magnetic moment of one order smaller. Magnetic force microscopy measurements confirmed that the Ni:SnO₂ films on LaAlO₃ are magnetically homogeneous at nanometre-scales, and the FM in the films comes from the doped matrix.

A systematic analysis of the electrostatic effects on the effective masses of holes in In_yGa_1−yAs_1−xN_x/GaAs quantum-well structures was performed. A 10-band k·p Hamiltonian matrix was used in the calculations and solved self-consistently with the Poisson equation. Numerical results have been presented for a large range of material and structural parameters. Our results show that significant variation in the effective masses is possible by adjusting the relevant parameters and that the effects due to self-consistency are small for most subbands.

In this paper, the spectroscopic properties of Er³⁺-doped oxyfluoride glass ceramics containing CaF₂ nano-crystals are systematically investigated. X-ray diffraction (XRD) and transmission electron microscope (TEM) experiments confirmed the formation of CaF₂ nano-crystals in the glassy matrix. Based on the Judd–Ofelt theory, the intensity parameters Ω_2,4,6, spontaneous emission probability, radiative life, radiative quantum efficiency, width of the emission line, stimulated emission cross-section and gain spectrum were evaluated, and the effect of Er³⁺ doping concentration on the spectroscopic properties of the transition is discussed. For glass ceramics, intense red and weak green up-conversion luminescence were observed and ascribed to two-photon absorption processes.

Real-time inelastic neutron scattering has been used to study the kinetics of the demixing process in AgBr–NaBr mixed single crystals. The variation of transverse acoustic phonon spectra provides most direct information about the microscopic non-equilibrium behaviour. The phase separation is monitored by a well defined splitting of phonon peaks that reflects the different dynamical behaviours of the constituents. As a function of temperature, time evolution of the phonon spectra changes drastically, thus proving unambiguously that the demixing process is dominated by nucleation in the high-temperature regime and by concentration fluctuations at low temperatures.

Mixed-cation superionic fluorites are exceptionally good F⁻ ion conductors due to the mixed nature of the cation sublattice. In this study, we have used extended x-ray absorption fine structure (EXAFS) spectroscopy to gain insights into the local environments of the Cd and Pb cations (as a function of composition and temperature) in CdF₂(xPbF₂) mixed-cation superionic fluorites. A high degree of disorder is shown around both cations. However, the extent of disorder is even larger around the Pb²⁺ in the entire series of solid solutions, with the highest disorder noted for the x = 60 mol% PbF₂ sample. The large values of the Debye–Waller factors of the Pb–F interactions are consistent with the low amplitudes of their Fourier transforms. This can be correlated with relative ionic conductivities of each composition in these materials. Molecular dynamics (MD) has been used as a complementary technique on structural properties; systems were modelled at temperatures ranging from liquid nitrogen temperature, 77 K, to 300 K. Our MD results confirm that disorder is greatest at concentrations x≈60 mol% PbF₂.

We report high-pressure Raman measurements up to 20.2 GPa on the perovskite-type oxide (Na_1/2Bi_1/2)_0.89Ba_0.11TiO₃ (NBT–BT_0.11), which is of interest as a potential lead-free piezoelectric material. Distinct changes of the Raman spectra with increasing pressure illustrate that NBT–BT_0.11 shows at least two phase transitions. First, the application of low pressure progressively reduces the tetragonal signature in NBT–BT_0.11 and leads at around 1.8 GPa to a Raman spectrum reminiscent of rhombohedral NBT. Second, with further increase of pressure, the pressure-dependent behaviour of NBT–BT_0.11 resembles that of NBT and transforms to a non-cubic high-pressure structure at around 9 GPa. These observations suggest that a progressive substitution of Na⁺/Bi³⁺ by Ba²⁺ pushes the pressure-induced structural instabilities of NBT towards higher pressure, corresponding to a Ba²⁺-induced negative (tensile) pressure within the structure.