Table of contents

Recent experimental data for the centrifugal sedimentation–diffusion profile of charged colloidal silica ( nm) in ethanol is analysed based on local charge neutrality and by either assuming a fixed particle charge or incorporating charge regulation. Especially at a high Debye length (>30 nm) the two models deviate at higher particle volume fractions ϕ (>0.1 vol%) with the data much better reproduced when charge regulation is included.

By a simple dynamical model it is shown that the radial breathing mode of single wall carbon nanotubes is not exactly radial: its longitudinal and circumferential components are non-vanishing and both diameter and chirality dependent. However, the radial component is only diameter sensitive; likewise the frequency.

The resistivity of a ferromagnetic wire with an extension in contact with a superconductor has been measured at various temperatures and magnetic fields. The distance from the ferromagnet to the superconducting contact was fabricated to be 250–400 nm, much larger than the coherence length in the ferromagnet, which was a few nanometres; nevertheless, we found that the resistivity increases at the superconducting transition. The result was obtained for different ferromagnets and superconductors. We establish that the resistivity increase is not due to a redistribution of magnetic domains as a result of the screening of magnetic flux by the superconductor, as suggested recently.

We present a model for the leakage current in ferroelectric thin-film capacitors which explains two of the observed phenomena that have escaped satisfactory explanation, i.e. the occurrence of either a plateau or negative differential resistivity at low voltages, and the observation of a positive temperature coefficient of resistivity (PTCR) effect in certain samples in the high-voltage regime. The leakage current is modelled by considering a diffusion-limited current process, which in the high-voltage regime recovers the diffusion-limited Schottky relationship of Simmons already shown to be applicable in these systems.

Diluted magnetic semiconductors currently attract immense experimental and theoretical attention as one of the most promising classes of materials for spintronic applications. We report a detailed density-functional-theory study of the influence of the clustering of the substitutional Mn impurities on the electronic structure, exchange interactions and Curie temperature of (GaMn)As. We demonstrate that the strong intra-cluster interaction of the Mn 3d states results in the change of the structure of the impurity band. The influence of this change on the exchange interactions and Curie temperature is revealed.

The dielectric and conductometric properties of aqueous polyelectrolyte solutions present a very complex phenomenology, not yet completely understood, differing from the properties of both neutral macromolecular solutions and of simple electrolytes. Three relaxations are evident in dielectric spectroscopy of aqueous polyelectrolyte solutions. Near 17 GHz, water molecules relax and hence this highest frequency relaxation gives information on the state of water in the solution. At lower frequencies in the MHz range, free counterions respond to the applied field and polarize on the scale of the correlation length. This intermediate frequency relaxation thus provides information about the effective charge on the polyelectrolyte chains, and the fraction of condensed counterions. However, the presence of polar side chains adds a further polarization mechanism that also contributes in this intermediate frequency range. At still lower frequencies, the condensed counterions polarize in a non-uniform way along the polyelectrolyte chain backbone and dielectric spectroscopy in the kHz range may determine the effective friction coefficient of condensed counterions. In this review, we analyse in detail the dielectric and conductometric behaviour of aqueous polyelectrolyte solutions in the light of recent scaling theories for polyelectrolyte conformation and summarize the state-of-the-art in this field.

Intensive experimental investigations have been performed on a typical liquid semiconductor As₂Se₃ over a wide temperature range under pressure, such as electrical conductivity, optical absorption coefficient, density, x-ray absorption fine structure, and x-ray diffraction measurements. With increasing temperature, it was found that liquid As₂Se₃ undergoes a semiconductor–metal transition at about 1000 °C, which is clearly seen in the electrical conductivity and optical absorption measurements. This transition is accompanied by significant structural and thermodynamical changes. The volume of the system apparently contracts near the transition temperature. The structural changes are observed especially in the intermediate-range order or the prepeak in the total structure factor characteristic to this glass-forming liquid. X-ray absorption fine structure (XAFS) and x-ray scattering results strongly suggest a formation of new As–As bonds in addition to the original As–Se bonds in the high-temperature metallic region. These experimental results are carefully discussed by comparing to theoretical results from an ab initio molecular dynamics (MD) simulation. Some new insights can be realized from this computational work. For example, a small portion of homopolar bonds already exist in the semiconducting region, and twofold coordinated As atoms existing in a chain structure with twofold coordinated Se atoms play a very important role for the metallization of this liquid. Some limitations of the MD simulation are also pointed out.

Decades of experimental and theoretical studies have brought some useful insights about what defines resistance to amorphization by radiation damage; however, the problem is still viewed as generally unsolved. I review ideas and concepts that have been put forward to help with understanding this problem. I then discuss how the type of interatomic force is relevant for resistance to amorphization, with covalency of bonding stabilizing the damage and making material amorphizable. On a more detailed level, I suggest that resistance to amorphization of a complex non-metallic material is defined by the competition between the short-range covalent and long-range ionic forces. I follow this with a review of experimental data on 116 materials, to illustrate that the type of interatomic force can generally explain the resistance to amorphization. I conclude by discussing how the proposed picture is related to models proposed previously, and by suggesting some possible future research.

When intense femtosecond laser pulses are focused on solid targets short-lived microplasmas are formed which emit bursts of x-rays with kilovolt photon energies. Under the proper conditions x-ray pulses as short as a few hundred femtoseconds can be produced. These x-ray pulses enable ultrafast x-ray spectroscopy using pump–probe schemes where the x-ray pulses serve as probe pulses. This article describes time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds. In particular, we have studied solid-to-liquid phase transitions in semiconductors induced by femtosecond photoexcitation and the accompanying thermoacoustic phenomena. We were able to monitor the changes in the atomic position underlying a coherent optical phonon mode. These and a number of other lattice dynamics experiments discussed here demonstrate the feasibility and usefulness of ultrafast time-resolved x-ray diffraction. Future applications in many other fields of science can be foreseen.

A two dimensional lattice gas model with a 'core-softened' potential is investigated. Two liquid phases and density anomaly are found. The demixing phase transition between the two liquid phases ends at a tricritical point that is also the terminus of a critical line. The density anomaly is shown to be related to this continuous line.

Ceramics in the (BiMnO₃)_x–(PbTiO₃)_1−x series were fabricated with compositions in the range . A macroscopically pseudocubic structure was observed for compositions x>0.4 while at x<0.4 the structure is tetragonal. The presence of a pseudocubic structure is attributed to frustration between different preferred environments of cations, leading to an absence of long-range ordered polar ionic displacements. Microstructures similar to those of relaxors were observed at x = 0.4 and evidence of incommensurate ordering of antiparallel cation displacements was found at compositions . Permittivity measurements for x = 0.3 and 0.4 revealed diffuse frequency-dependent maxima between 100 and 300 °C. In addition, compositions with exhibited a sharp peak at higher temperature. The broad peaks are considered to arise from different time constants attributed to the grain interiors and boundaries for x = 0.3 and 0.4 whereas the sharp peak is interpreted as the Curie temperature of a tetragonal to cubic phase transition.

We theoretically investigate under what conditions the director field in a spindle-shaped nematic droplet or tactoid obtains a twisted, parity-broken structure. By minimizing the sum of the bulk elastic and surface energies, we show that a twisted director field is stable if the twist and bend elastic constants are small enough compared to the splay elastic constant, but only if the droplet volume is larger than some minimum value. We furthermore show that the transition from an untwisted to a twisted director-field structure is a sharp function of the various control parameters. We predict that suspensions of rigid, rod-like particles cannot support droplets with a parity broken structure, whereas they could possibly occur in those of semi-flexible, worm-like particles.

We present an ab initio molecular dynamics study of pressure induced melting of an ice thin film confined between two parallel metal surfaces. The ice-to-water phase transition was observed at a pressure of roughly 0.5 GPa, when the film was compressed by 6.6%. The latter is in agreement with the volume change in the melting of bulk ice. The effects of nonadiabatic compression on the layer-dependent momentum distribution and the electronic redistribution at the interfaces are presented and discussed.

A microscopic density functional theory is used to investigate the liquid–vapour interface of fluids composed of short linear chains. We analyse the structure of the interface and evaluate the dependence of the surface tension and of the interfacial width on the temperature. The difference in chain length leads to differences in the thermodynamic properties of the fluids. The liquid-phase parts of the interfacial profiles of shorter chains exhibit oscillations at low temperatures. These oscillations vanish for longer chains. The surface tension and the interfacial width at a given temperature are found to increase with the chain length. Both the surface tension and the interfacial width scale as power laws upon approaching the critical point with critical exponents characteristic of mean-field-type theories and with prefactors depending on the chain length only.

An embedding scheme is developed for the Dirac Hamiltonian H. Dividing space into regions I and II separated by surface S, an expression is derived for the expectation value of H which makes explicit reference to a trial function defined in I alone, with all details of region II replaced by an effective potential acting on S and which is related to the Green function of region II. Stationary solutions provide approximations to the eigenstates of H within I. The Green function for the embedded Hamiltonian is equal to the Green function for the entire system in region I. Application of the method is illustrated for the problem of a hydrogen atom in a spherical cavity and an Au(001)/Ag/Au(001) sandwich structure using basis sets that satisfy kinetic balance.

Experimental data for proton nuclear spin relaxation and diffusion of H in HfTi₂H_x are analysed by simultaneously fitting the temperature-dependent relaxation and diffusion data with a common set of parameters. HfTi₂H_x has the C15 structure with the H occupying the inequivalent interstitial e and g sites. The fitting of the relaxation data uses a rigorous theory of nuclear spin relaxation between inequivalent sites and makes no assumptions about which types of H jumps are significant for the relaxation. The diffusion data is fitted by developing the theory of diffusion between the inequivalent e and g interstitial sites, which enables the diffusivity to be calculated rigorously as a function of temperature from the H jump rates in the low concentration limit. Monte Carlo simulations are used to estimate the effect of diffusion correlation effects at higher H concentrations. Models for diffusion between inequivalent sites involve a large number of parameters and density functional theory (DFT) calculations are used to provide constraints on them. Good fits to both the relaxation and diffusion data are obtained for energy parameters that are close to those from the DFT calculations. A complete set of jump parameters for H between the interstitial sites is deduced which provides a detailed microscopic description of the diffusion as a function of temperature.

The pressure–temperature phase diagrams of the heavy fermion antiferromagnet CeRhIn₅ and the heavy fermion superconductor CeCoIn₅ have been studied under hydrostatic pressure by ac calorimetry and ac susceptibility measurements using diamond anvil cells with argon as the pressure medium. In CeRhIn₅, the use of a highly hydrostatic pressure transmitting medium allows for a clean simultaneous determination by a bulk probe of the antiferromagnetic and superconducting transitions. We compare our new phase diagram with the previous ones, discuss the nature (first or second order) of the various lines, and the coexistence of antiferromagnetic order and superconductivity. The link between the collapse of the superconducting heat anomaly and the broadening of the antiferromagnetic transition points to an inhomogeneous appearance of superconductivity below P_c ≈ 1.95 GPa. Homogeneous bulk superconductivity is only observed above this critical pressure. We present a detailed analysis of the influence of pressure inhomogeneities on the specific heat anomalies which emphasizes that the observed broadening of the transitions near P_c is connected with the first-order transition. For CeCoIn₅ we show that the large specific heat anomaly observed at T_c at ambient pressure is suppressed linearly at least up to 3 GPa.

The crystal structure and magnetic properties of 6H-perovskites Ba₃MSb₂O₉ (M = Mn, Co, and Ni) were investigated. The M = Co and Ni compounds have a hexagonal structure with space group P6₃/mmc, while the Mn compound has a monoclinically distorted structure with space group C2/c. From the results of magnetic susceptibility and specific heat measurements, it was found that they show an antiferromagnetic transition at 10.0 K (for M = Mn), 3.8 K (Co) and 13.5 K (Ni), and the onset of the magnetic transition was observed much above T_N. The powder neutron diffraction measurements for Mn and Co compounds were carried out, and their magnetic structures were determined. Both compounds adopt the non-collinear 120° structure in the ab-plane. These magnetic properties show the existence of a geometric magnetic frustration derived from the triangular array of magnetic M ions in the ab-plane.

We have proposed the phenomenological description of dielectric hysteresis loops in ferroelectric semiconductors with charged defects and prevailing extrinsic conductivity. We have modified the Landau–Ginsburg approach and shown that the macroscopic state of the aforementioned inhomogeneous system can be described by three coupled equations for three order parameters. Both the experimentally observed coercive field values well below the thermodynamic values and the various hysteresis-loop deformations (constricted and double loops) have been obtained in the framework of our model. The obtained results quantitatively explain the ferroelectric switching in such ferroelectric materials as thick PZT films.

The crystal structures of M_1+xV₃O₈, where M_1+x = Ag_1.229 and Na_1.164, isomorphous to the Li_1+xV₃O₈ insertion electrode have been determined with x-ray four-circle diffraction. The networks of V ions are similar to each other, but significant differences for the M–O and M–M distances exist. The present results indicate that the Li insertion for the Ag system forces the Ag–O distance to be short, which results in an irreversible charge–discharge process accompanied with a deposition reduction of Ag ions. Both the Ag and the Na system exhibit that the donated electrons reside at specific V sites, which leads to quasi-one-dimensional electronic properties along the monoclinic b-axis notwithstanding the complicated structure. The semiconducting transport mechanism is of variable-range hopping in one dimension and the magnetic properties are explained with a one-dimensional Heisenberg-like model.

It is of high fundamental and practical importance to be able to control the formation and stability of the different crystalline phases of alumina (Al₂O₃). In this study, we have used density functional theory methods to investigate the changes induced in the thermodynamically stable α phase and the metastable θ phase as one eighth of the Al atoms are substituted for different additives (Sc, W, Mo, Cr, Cu, Si, and B). The calculations predict that the additives strongly affect the relative stability between the two phases. Most tested additives are shown to shift the relative stability towards, and in some cases completely stabilize, the θ phase, while Cu doping is predicted to increase the relative stability of the α phase. The reasons for these effects are discussed, as are possible implications on the growth and use of doped aluminas in practical applications. In addition, the effects of the additives on bulk moduli and densities of states have been investigated.

The specific heat has been measured for the relaxor ferroelectrics PbMg_1/3Nb_2/3O₃ and PbMg_1/3Ta_2/3O₃ and the low-dielectric-constant material BaMg_1/3Ta_2/3O₃ over a temperature range of 2–420 K. The behaviours of the specific heat for PbMg_1/3Ta_2/3O₃ and BaMg_1/3Ta_2/3O₃ crystals have been qualitatively compared. A large excess of specific heat of PbMg_1/3Ta_2/3O₃ over that of BaMg_1/3Ta_2/3O₃ has been revealed. The behaviour of the low-temperature specific heat of PbMg_1/3Nb_2/3O₃ has been analysed in the frameworks of the shell model, Debye model, Einstein oscillator, two-level systems approach, and fractal approach. The excess of the specific heat of PbMg_1/3Nb_2/3O₃ over the harmonic phonon contribution has been estimated, and its possible origin is discussed.

Recent studies have shown evidence of self-interstitial aggregation in ion-implanted Si, resulting in nanoscopic damage structures. Similarly, self-interstitial atoms are expected to play an important role for defect clustering in ion-implanted GaAs. We report results on stable neutral di-interstitial complex configurations in GaAs composed of both As and Ga atoms addressed by first-principles total-energy calculations based on density-functional theory.

The structural transformation of the quasi-one-dimensional charge-density-wave (CDW) conductor K_0.3MoO₃ blue bronze upon electron irradiation is reported. Evidence of the disappearance of Bragg spots corresponding to the C-centre symmetry is clearly demonstrated by the in situ recorded selected area electron diffraction (SAED) patterns in the transmission electron microscope, which acts as the electron irradiation facility as well as the structural analysis and recording instrument. Based on the experimental results and the crystal structure of K_0.3MoO₃ (Graham and Wadsley 1966 Acta Crystallogr.20 93), a hypothetical atomic model is proposed to interpret this kind of disordering. The simulated SAED patterns on the basis of the proposed atomic model agree with the experimental ones very well. This strong disorder may break both the coherence of the CDW along the chain direction and the phase order of the CDW in the transverse directions, and furthermore it may suppress the Peierls transition.

We calculate the change in the superconducting transition temperature T_c of MgB₂ caused by interband nonmagnetic impurity scattering using the Eliashberg theory for the two-band model of this compound. A much slower rate of T_c suppression is obtained compared to the prediction based on the BCS treatment of the two-band model which ignores renormalization and damping associated with the electron–phonon interaction. Hence, the interband impurity scattering rates deduced from experiments on MgB₂ using the formula which results from the BCS approach to the two-band model are underestimated. We generalize the BCS treatment of the two-band model to include renormalization effects of the electron–phonon interaction and find an excellent agreement with the full strong coupling calculation.

Recent proton spin–lattice relaxation-time (T₁) measurements on the ferrimagnetic chain compound NiCu(C₇H₆N₂O₆)(H₂O)₃·2H₂O are explained by an elaborately modified spin-wave theory. We give strong evidence of the major contribution to 1/T₁ being made by the three-magnon scattering rather than the Raman scattering.

An effect of the static long range cooperative Jahn–Teller (JT) distortion on the magnetic properties of (A =  La, Pr, Nd; , Sr) manganites with the orthorhombic structure of Pnma symmetry is studied on the basis of structural, magnetic and transport properties data. A direct relationship between the static JT distortion parameter and the Curie temperature was found. The relationship between the static long range cooperative JT distortion, strength of electron–phonon coupling effects and magnetic properties of manganites is discussed.

The multipole moments and multipole–multipole interactions of uniformly polarized particles have been calculated based on the fundamental theory of electrostatics. As the polarization of the particles is uniform, only surface charges are considered. The polarization may have its origin in magnetization or ferroelectricity or be an intrinsic property of molecules. It is demonstrated that, depending on the geometry of the particles, the higher order interactions can be comparable to or even stronger than the dipole–dipole interaction. The higher order moments give rise to an additional energy contribution in arrays of close packed polarized nanoparticles. The influence of particle aspect ratios as well as array periodicity is discussed.

Electron paramagnetic resonance (EPR) experiments on the dominant Fe³⁺ centre in near-stoichiometric LiTaO₃ crystals grown by the double crucible Czochralski method are reported. A near complete roadmap of EPR positions was obtained, and transitions from two magnetically non-equivalent sites clearly resolved in the zx plane, perpendicular to the glide plane. This allowed accurate determination of C₃ symmetry spin Hamiltonian parameters. Newman superposition model analyses of second and fourth order zero field splitting term parameters were performed to give further insight into the site of incorporation. The second order calculations provide evidence for Fe³⁺ substitution within the Li octahedron.

The changes in the dielectric constant of perovskite BiMnO₃ at about T_C, which are induced by the magnetic ordering as well as by the application of magnetic fields, indicate that coupling exists between the magnetism and the dielectric properties. The soft-mode theory based on the single-well potential model and the molecular-field approximation to the Heisenberg model are successfully applied to the electric and magnetic subsystems coexisting in the perovskite-type ferroelectromagnet BiMnO₃, respectively. By adding an appropriate coupling term, which is related to a combination of electrical polarization and spin correlation, we investigate the inherent coupling between the ferroelectric and ferromagnetic orders, and find that the application of magnetic fields induces a fairly large suppression of χ_p around T_C, and the field-induced change in the magnetocapacitance effect becomes maximal at T_C, which are in good agreement with experimental data in ferroelectromagnetic BiMnO₃.

Resonant Raman scattering spectra of glass-embedded CdS_1−xSe_x nanocrystals are measured and complemented with TEM and optical absorption as well as photoluminescence data. The selectivity of the resonant Raman process not only for the size, but also for the composition of nanocrystals within the ensemble, is directly observed in the dependence of phonon band frequency, linewidth and shape on the excitation wavelength.

We have used density functional pseudopotential calculations and molecular dynamics to predict new carbon structures of high stability. The new phases are strongly bound and involve the smallest radius nanotubes. It was found that it is possible to covalently link smallest/larger, smallest/smallest radius nanotubes together as well as larger nanotube/C₂₀ 1D-chains, resulting in extremely large interlinkage and consequent increase in the resistance to slippage. This procedure may enable the construction of extremely stiff nanotube bundles capable of making full use of the tensile properties of individual nanotubes, while enhancing the crystallinity of the material. Some of the carbon allotropes studied are the lowest energy non-diamond sp³ hybridized structures ever found.

We have studied the influence of a terahertz (THz) electric field and a magnetic field on the optical absorption in quantum wells. The studies show that the absorption spectrum becomes much richer in the presence of a THz field and a magnetic field. In a growth-direction polarized THz field, the main features of the quantum-confined Stark effect, such as the red-shift of the absorption edge, the reduction in the magnitude of the main excitonic peaks, and the appearance of additional absorption peaks due to transitions which are forbidden in the absence of a DC field, are retained. In addition, new asymmetric replicas emerge around these peaks. The perpendicular magnetic field leads to quantization of the in-plane motion, and thus enhances the main excitonic peaks and shifts the peaks to the blue, and discretizes the continuum of the absorption spectrum. The magnetic field may leads to the merging or broadening of peaks and replicas.

We have grown and characterized a SrWO₄:Nd³⁺ crystal. Photoluminescence and Raman spectra are exhibited. We have obtained end-pumped self-stimulated Raman scattering in the two and laser channels leading to 1.171 and 1.517 µm coherent radiations. The latter is close to the eye-safe range of wavelengths.