Table of contents

We report measurements of low-frequency dielectric permittivity in two-dimensional θ-(BEDT-TTF)₂RbZn(SCN)₄ organic compound. The real part of the dielectric permittivity ε' exhibits a sharp peak in the extreme vicinity of the charge ordered phase transition at T_CO. The very low magnitude of ε' below T_CO results from the lattice dimerization asssociated with the charge ordered phase. We ascribe the temperature-dependent permittivity above T_CO to pre-existing charge inhomogeneities—charge disproportionation—as revealed previously by NMR measurements. A qualitative comparison between the dielectric response due to charge order in quasi-one-dimensional conductors and in this two-dimensional system is presented.

Both neutron and x-ray powder diffraction studies have been carried out for two high temperature quantum paraelectrics, Na_1/2Nd_1/2TiO₃ and Na_1/2Pr_1/2TiO₃. Although visual inspection of the diffraction patterns suggests a 'nearly cubic' like structure, careful analysis of powder neutron diffraction patterns and comparison of Rietveld refinement results for competing models reveal that both compounds belong to the orthorhombically distorted GdFeO₃ type of structure (Pbnm space group, a⁻a⁻c⁺ tilt system in Glazer notation). The full set of refined structural parameters is reported for the first time. The relationship between octahedral tilt angles and dielectric softness of these compounds is briefly discussed.

Scott's (2006 J. Phys.: Condens. Matter18 7123) recent review of Kleemann et al' s (2002 Europhys. Lett. 57 14) critical exponents of strontium barium niobate is shown to be misled by erroneous input parameters. Although the observed set of exponents reflects, indeed, the absence of true three-dimensional (3D) random-field Ising model critical behaviour, it cannot be compatible with the proposed domain wall model in d = 2.5 dimensions or with Levanyuk and Sigov's (1988 Defects and Structural Phase Transitions (London: Gordon and Breach)) defect model. As was argued independently by Kleemann et al (2006 Phys. Rev. Lett. 97 065702), it is rather in agreement with the pure two-dimensional (2D) Ising model.

Nb₂AsC showed superior mechanical properties to those of other layered ternary carbides (Kumar et al 2005 Appl. Phys. Lett. 86 111904). In the present density functional calculation, the underlying mechanism is interpreted by astonishing bonding features of Nb₂AsC. The Nb d–As (p_x+p_y) and Nb d–As p_z bonding states locate in the same energy range as those of Nb d–C p bonding, which indicates that the Nb–As bond has similar bonding strength as the Nb–C bond does; and thereafter, Nb₂AsC has improved mechanical properties compared to the others. The present reported bonding features are interestingly different from those experienced by T₂AlC (T =  Ti, V, Cr, and Nb), wherein the weak T–Al coupling was separated from T–C bonding states in the higher energy level by a pseudogap. This work proposes an effective method to strengthen the relative weaker T–A (A is the A-group element) bonding in layered ternary carbides.

We summarize procedures for producing 'nanoemulsions' comprised of nanoscale droplets, methods for controlling the droplet size distribution and composition, and interesting physical properties of nanoemulsions. In contrast to more common microscale emulsions, nanoemulsions exhibit optical transparency at high droplet volume fractions, ϕ, surprisingly strong elasticity at low ϕ, and enhanced diffusive transport and shelf stability. For these reasons, nanoemulsions have great potential in a wide range of industries including pharmaceuticals, foods, and personal care products.

Molecular dynamic simulation has been done to determine the dynamic and local structure of liquid alumina at 3000 K. Fourteen different systems at densities ranging from 2.5 to 4.5 g cm⁻³ was prepared by compressing the low-density melt. Two kinds of pore aggregation, pore cluster and pore tube, were examined. Clear evidence was found of structural transformation from a tetrahedral to an octahedral network. For a low-density system there was a large pore tube, which involved 93% of oxygen-vacancy-like pores and spread over the whole simulation cell. Conversely, in a high-density system the largest pore tube contained less than 1% of all oxygen-vacancy-like pores. A similar trend also was observed for other pore kinds such as aluminium-vacancy-like pores and large pore clusters. The diffusion constants significantly decreased in the region of the structural transformation. The diffusion mechanism in low- and high-density systems was examined and is discussed here.

A realistic model to study the properties of an aqueous electrolyte surface has been developed. The complex liquid surface consisting of a large number of interacting particles, ions and dipoles, is modelled using a Monte Carlo technique considering grand canonical sampling. The possible interactions existing in the system are charge–charge, charge–dipole, charge–quadrupole and dipole–dipole. The concentration dependence of the diffusion coefficient suggests a first order phase transition (structural transition), while its temperature dependence indicates the existence of a second order phase transition. A critical analysis of the effect of decreasing temperature on the samples with added cations to limit motion of the particles in the surface reveals an interesting feature—a signature of glass transition.

We formulate a density functional approach for arbitrarily branched liquid-crystalline (LC) heteropolymers consisting of elongated rigid rods coupled through elastic joints. The theory exactly accounts for the energetic and entropic single-chain effects, whereas the interchain excluded volume effects are treated within the Onsager approximation. We apply the theory to finite-length main-chain LC polymers composed of rigid mesogens coupled by flexible spacers of finite length, modelled initially as chains of thin rods. The theory then allows an easy passage to the wormlike chain limit for the spacers. Employing a bifurcation analysis we analytically obtain the stability boundaries of the isotropic phase towards the nematic liquid crystalline phase, as a function of the relative size of the mesogens with respect to the spacers and the spacer flexibility. From the same analysis we also obtain the distribution of the incipient nematic ordering at the spinodal density as a function of position along the chain, including the end-effects.

We apply a density functional approach for arbitrary branched liquid-crystalline (LC) heteropolymers consisting of elongated rigid rods coupled through elastic joints developed in a companion paper (Wessels and Mulder 2006 J. Phys. Condens. Matter. 18 9335) to a model for side-chain liquid-crystalline polymers. In this model mesogenic units are coupled through finite-length spacers to a linear backbone polymer. The stereochemical constraints imposed at the connection between spacer and backbone are explicitly modelled. Using a bifurcation analysis, analytical results are obtained for the spinodal density of the I–N transition and the variation of the degree of ordering over the various molecular parts at the instability as a function of the model parameters. We also determine the location of the crossover between oblate and prolate backbone conformations in the nematic phase.

Heat capacities and enthalpy relaxation rates were measured for crystalline [Ni(cyclam)(H₂O)₂]₃(TMA)₂·24H₂O, where cyclam is 1,4,8,11-tetraazacyclotetradecane, TMA is 1,3,5-benzene tricarboxylic acid, and 24H₂O represents the water forming a nano-channel. A phase transition was found to occur at 196.9 K, and a glass transition at 87 K. A potential picture is given for the progress of the ordering of water molecules and hydrogen atoms in the channel. At room temperature, interfacial water molecules form hydrogen bonds with the oxygen atoms of the channel wall, and the aggregation of water molecules is recognized, as the average structure, to be in a crystalline state with a long-range order. The bond formation plays important role in the stabilization of the crystalline framework with a channel structure. The aggregate transforms to a more stable crystalline state at 196.9 K. All the water molecules should be fixed completely there. The positions of the hydrogen atoms on the network are, however, essentially in the disordered state while keeping an ice rule, and freeze at 87 K only with a short-range order in the arrangement. The progress of such ordering of channel water reveals a striking contrast to the behaviour of the water within meso-porous silicas, in which the molecules in the interface are always in the non-crystalline state and those in the pore centre tend to crystallize only when the pore diameter is greater than 2 or 3 nm.

Melting of a finite size binary system consisting of two types of particles having different charges and/or masses, confined in a two-dimensional (2D) parabolic trap, is studied. The melting temperature is obtained for different values of the ratio between the charges and/or masses of the two types of particles. The two types of particles melt at different temperatures; e.g., particles with smaller charge melt first. The importance of the commensurate/incommensurate configurations and the different normal modes to the melting phenomenon is studied. When the ground state consists of a nonsymmetric arrangement of particles new thermally induced structural phase transitions are found. In addition, a remarkable temperature induced spatial separation of the two types of particles is found.

We report on the wrenching of a metal-capped periodically corrugated polymer bilayer when the bilayer is annealed at a temperature far above the glass transition temperature of the polymer layer. Certain corrugation geometries such as several micrometre line width of groove and ridge and step heights, thicknesses of the metal film, and extents of reduction of the elastic modulus of the polymer layer by increasing the annealing temperature give rise to a wrenching pattern in the metal surface. This wrenching pattern was characterized by the critical wrenching angle. The critical wrenching angle could be theoretically determined by calculating the mechanical energy required for the wrenching of the metal film and polymer layer in order to relax the thermal compressive in-plane stress. An increase in the annealing temperature incorporated with a decrease in the corrugation period yields a smaller critical wrenching angle. For the critical wrenching angle larger than a certain value, the wrenching wave pattern was directed by the wave interaction relationship between the corrugation and the intrinsic buckling wave of the metal-capped polymer bilayer.

Dielectric investigations of the photoinduced effects in the vicinity of the smectic A (Sm-A)–antiferroelectric smectic C (Sm-C_A^*) transition of a guest–host system consisting of photoactive azobenzene-based guest molecules and non-photoactive host molecules have been carried out. The frequency-dependent dielectric data is analysed in terms of a molecular mode and a high-frequency mode. We observe that the dielectric parameters associated with the two modes show primary as well as secondary photoferroelectric effects, in agreement with the effects observed in our previous study (Nair et al 2006 Phys. Rev. E 73 011712, referred to as [1] in the present article) for polarization and response time. Here the primary photoelectric effect is associated with changes in the polar ordering and/or transverse molecular dipole moment at a constant reduced temperature and the secondary effect is due to the radiation-induced reduction in the transition temperature. The temporal variation of the relaxation parameters shows that the photoinduced isothermal transition as well the thermal back relaxation occurs on fast timescales.

The electronic states of finite carbon nanotubes in the presence of electric and magnetic fields are calculated by the tight-binding model. Electronic properties such as state energy, energy gap, and density of states are mainly determined by the transverse electric field, the magnetic field, the Zeeman splitting, and the nanotube length, as well as the transverse geometric structure. The electric field could induce the destruction of state degeneracy, produce more low-energy states, and lead to significant changes in energy spacing. Complete energy-gap modulations exist during the variation of the electric field. Such effects are enhanced by the magnetic field.

This paper deals with the effects of some material properties on the J–V characteristic of hydrogenated amorphous silicon (a-Si:H)-based p–i–n solar cell. The factors considered are the free carrier's mobilities, the capture cross sections of the gap states and the bulk density of states (i-layer DOS). Accurate investigation of the cell photo-parameters' sensitivity to these factors is carried out using a simulation program, previously developed by our group. The model is based on a complete set of Poisson and carrier continuity equations taking into account the defect pool model for the a-Si:H gap density of states. Ours results reveal the important role of the hole mobility when it takes low values, as given frequently in the literature, on the recombination rate at interface regions as well as in the bulk. This considerably affects the short-circuit current density (J_sc), the open-circuit voltage (V_oc), the fill factor (FF) and the conversion efficiency (η). Moreover, by changing separately the capture cross sections of the band's tails and those of dangling bonds we found more precisely that J_sc, V_oc, and η are more sensitive to the interface recombination while the FF seems to depend more on the bulk recombination. Ultimately, when the i-layer DOS is higher than 10¹⁶ cm⁻³, the recombination rate, which was first limited by the valence band tail's states, becomes limited by the dangling bond's states. Then, any further increase significantly deteriorates the solar cell performance.

We examine the transverse magnetoresistance of three-dimensional bi-component composites Au_p–(PrBa₂Cu₃O₇)_1−p. This property, seldom measured on percolation systems, has been measured below 5 K and up to 1.5 T. Contrarily to well known transport properties, the magnetoresistance of these compounds exhibits a metal volume fraction's threshold p_c linked to the onset of disorder rather than to percolation.

Magnetic anisotropy and magnetization reversal in Fe₃Si and Fe films grown on GaAs(113)A substrates are studied using the planar Hall effect (PHE). The PHE in this orientation exhibits an antisymmetric component in addition to the usual symmetric component. The relative magnitude of symmetric and antisymmetric components in the PHE is affected by the composition of the Fe₃Si films and the thickness of the Fe films, which lead to a complex behaviour of the planar Hall resistivity in the low magnetic field region below saturation. However, irrespective of the composition/thickness of the films, magnetization reversal can be described qualitatively within a single domain by the simple Stoner–Wolfarth model of magnetization reversal. This allows us to determine the magnetic anisotropy properties of these films in good agreement with the experimental results of anisotropic magnetoresistance and superconducting quantum interference device magnetometry.

The magnetic moment and spin fluctuation temperature T_SF of isolated Fe atoms in a number of Pd-based binary alloys, namely Pd_0.95M_0.05 (M = Ni, Rh, Mo, Ag, Cd, In, Sn, Th and U), have been determined from the local susceptibility χ_loc(T) of ⁵⁴Fe probe nuclei measured by means of the time differential perturbed angular distribution (TDPAD) technique. Depending on the element M added to the Pd matrix, the results derived from Curie–Weiss analysis, χ_loc(T) = C/(T+T_SF), reveal strong enhancement or suppression of the ferromagnetic host spin polarization S_h associated with the giant moment of Fe. Concurrent with the variation in host spin polarization, we have observed a large change in the spin fluctuation temperature T_SF, showing an exponential rise with diminishing value of S_h. The results, analysed on the basis of the Kondo model, indicate that spin fluctuations caused by the antiferromagnetic interaction associated with the negative conduction electron spin polarization are suppressed progressively by an incremental addition of a positive contribution to the effective exchange interaction.

We present results of magnetization measurements in very close temperature intervals in the superconducting mixed state of CeRu₂. While the temperature (T) dependence of magnetization in low applied magnetic fields (H) is typical of that of a type-II superconductor with moderate pinning, the character changes markedly in high applied H, showing clear evidence of a first-order phase transition. Isothermal field-dependent magnetization studies have suggested earlier the possibility of such a first-order phase transition in the low-T high-H part of the superconducting mixed state of CeRu₂. The present results for the first time provide the evidence of this first-order phase transition in a temperature-dependent study of magnetization. The possible role of paramagnetic impurities in the observed phase transition is discussed.

The pressure-induced phase transition in zinc sulfide is studied using a constant-pressure ab initio technique. The reversible phase transition from the zinc-blende structure to a rock-salt structure is successfully reproduced through the simulations. The transformation mechanism at the atomistic level is characterized and found to be due to a monoclinic modification of the simulation cell, similar to that obtained in SiC. This observation supports the universal transition state of high-pressure zinc-blende to rock-salt transition in semiconductor compounds. We also study the role of stress deviations on the transformation mechanism and find that the system follows the same transition pathway under nonhydrostatic compressions as well.

We propose a simple method for calculating the ground state wavefunction of an electron–hole pair confined in a heterostructure formed by a flat quantum dot deposited on a thin wetting layer and imbedded in a matrix made of other material. The calculations of the exciton ground state energy, the electron–hole space pair correlation function and the density of the charge distribution have been performed for In_0.55Al_0.45As–Al_0.35Ga_0.65As and Ga_0.7Al_0.3As/GaAs quantum discs and rings. It is shown that the increase of the wetting layer thickness and the strength of the external magnetic field applied parallel to the axis lead to a lowering of the inner barrier height reinforcing the tunnelling of the particles into the central hole region. We also analyse the charge distribution related to the spatial separation of the particles due to the difference between masses of the electron and the hole. The variation of the quadrupole momentum sign with the increase of the ring inner radius and the appearance of the dipole momentum oriented in the crystal growth direction with the increase of the wetting layer thickness in the presence of the electron–hole pair are predicted.

The adsorption of hydrogen molecules on the titanium metallocarbohedryne (met-car) cluster has been investigated by using the first-principles plane wave method. We have found that, while a single Ti atom at the corner can bind up to three hydrogen molecules, a single Ti atom on the surface of the cluster can bind only one hydrogen molecule. Accordingly, a Ti₈C₁₂ met-car can bind up to 16 H₂ molecules and hence can be considered as a high-capacity hydrogen storage medium. Strong interaction between two met-car clusters leading to the dimer formation can affect H₂ storage capacity slightly. Increasing the storage capacity by directly inserting H₂ into the met-car or by functionalizing it with an Na atom have been explored. It is found that the insertion of neither an H₂ molecule nor an Na atom could further promote the H₂ storage capacity of a Ti₈C₁₂ cluster. We have also tested the stability of the H₂-adsorbed Ti₈C₁₂ met-car with ab initio molecular dynamics calculations which have been carried out at room temperature.

The thermal expansion has been investigated for a highly crystalline hexagonal BC₂N compound synthesized by the compression of a turbostratic B–C–N precursor with iron catalyst at the high temperature of 1500 °C and the high pressure of 5.5 GPa. The thermal expansion in the c direction is large and linear with an expansion coefficient of 35.86 × 10⁻⁶ K⁻¹ up to 1000 °C, while in the basal plane, the a dimension displays a slight linear contraction up to 750 °C with a contraction coefficient of −8.76 × 10⁻⁷ K⁻¹, but above 750 °C a linear expansion is observed with a larger expansion coefficient of 1.52 × 10⁻⁶ K⁻¹.

We discuss the application of the variational cluster perturbation theory (VCPT) to the Mott-insulator-to-superfluid transition in the Bose–Hubbard model. We show how the VCPT can be formulated in such a way that it gives a translation invariant excitation spectrum—free of spurious gaps—despite the fact that it formally breaks translation invariance. The phase diagram and the single-particle Green function in the insulating phase are obtained for one-dimensional systems. When the chemical potential of the cluster is taken as a variational parameter, the VCPT reproduces the dimensional dependence of the phase diagram even for one-site clusters. We find a good quantitative agreement with the results of the density-matrix renormalization group when the number of sites in the cluster becomes of order 10. The extension of the method to the superfluid phase is discussed.