Table of contents

We report measurements of the magnetic penetration depth λ in single crystals of Pr(Os_1−xRu_x)₄Sb₁₂ down to 0.1 K. Both λ and superfluid density ρ_s exhibit an exponential behaviour for the x≥0.4 samples, going from weak (x = 0.4,0.6) to moderate coupling (x = 0.8). For the x≤0.2 samples, both λ and ρ_s vary as T² at low temperatures, but ρ_s is s-wave-like at intermediate to high temperatures. Our data are consistent with the presence of an additional nodal low-temperature phase at T_{c 3}<0.6 K, for small values of x.

Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects—rather than a unique one—are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.

At large tilt angles the coupling between the spontaneous polarization and the tilt of ferroelectric liquid crystals exhibits nonlinear behaviour. This is theoretically described by the generalized Landau model, which introduces a bilinear and a biquadratic coupling term, the latter describing transversal quadrupolar order. We have investigated a ferroelectric liquid crystal with two chiral centres, systematically varying the chiral centres, to elucidate both the influence of molecular configuration and constitution on the coupling between polarization and tilt. It is shown experimentally that the bilinear coupling term depends strongly on the molecular configuration (chirality) of the mesogen, while the molecular constitution determines the achiral biquadratic coupling.

We present a model equation of state for C₆₀ based on a variational series mean spherical approximation for a double Yukawa fluid. This allows us to investigate the liquid–vapour coexistence curve and calculate the thermodynamic properties of liquid C₆₀. The comparisons with computer simulation results, based on the Girifalco potential, suggest the importance of treating the attractive tail of the potential accurately. The estimated critical parameters, T_c = 1943 K, ρ_c = 0.477 nm⁻³ and P_c = 34.2 bar, are in good agreement with Gibbs ensemble Monte Carlo simulation predictions. The results are discussed, making reference to previous studies.

A phenomenological generalization of the well known Vinen equation for the evolution of vortex line density in superfluid counterflow turbulence is proposed. This generalization includes nonlinear production terms in the counterflow velocity and corrections depending on the diameter of the tube. The equation provides a unified framework for the various phenomena (stationary states and transitions) present in counterflow superfluid turbulence: in fact, it is able to describe the laminar regime, the first-order transition from laminar to turbulent TI state, the two turbulent states, the transition from TI to TII turbulent states, and it yields a slower decay of the counterflow turbulence than the classical local description. Finally, a comparison with the experimental results shows that the contribution of the new terms is prevalent in the laminar and in the turbulent TI regime, while in the fully developed turbulent TII regime the equation reduces to the original Vinen equation.

The diffusive thermal conductivity tensor of the A₁-phase of superfluid ³He at low temperatures and melting pressure are calculated beyond the s–p approximation, by using the Boltzmann equation approach. The interaction between normal–normal, normal–Bogoliubov and Bogoliubov–Bogoliubov quasiparticles in the collision integrals are considered for important scattering processes such as binary process. At low temperatures, we show that the scattering between Bogoliubov and normal quasiparticles in binary processes plays an important role in the A₁-phase, and Bogoliubov–Bogoliubov interaction is ignorable.

We show that the two normal and superfluid components take part in elements of the diffusive thermal conductivity tensor differently. We obtain the result that the elements of the diffusive thermal conductivities, K_xx, K_yy and K_zz, are proportional to T⁻¹, and also that the superfluid components of the diffusive thermal conductivity tensor, and , are proportional to T³ and T, respectively.

Optical properties of asymmetrical multiple quantum wells for the construction of quantum cascade lasers are calculated, and expressions for the electronic states of asymmetrical multiple quantum wells are presented. The gain and differential cross-section for an electron Raman scattering process are obtained. Also, the emission spectra for several scattering configurations are discussed, and the corresponding selection rules for the processes involved are studied; an interpretation of the singularities found in the spectra is given. The electron Raman scattering studied here can be used to provide direct information about the efficiency of the lasers.

Al_1−xGa_xPO₄ solid solutions (x = 0.3, 0.7) with the α-quartz-type structure were investigated up to 1208 K by x-ray powder diffraction. The composition Al_0.7Ga_0.3PO₄ exhibits almost simultaneous transitions to the β-quartz and β-cristobalite forms at close to 1050 K. The tendency towards the β-quartz type structure is found to be much less marked for the Ga-rich composition (x = 0.7) based on the temperature dependence of the cell parameters, molar volume, fractional atomic coordinates and tetrahedral tilt angle. Direct transformation to the β-cristobalite form begins close to 1123 K. The β-quartz form exists as a stable phase only for values below x = 0.3.

Theoretical results of a first-principles plane-wave pseudopotential study on the phonon and dielectric properties for the nitrides, phosphides, and arsenides of Al, Ga, and In under hydrostatic pressure are presented. The pressure dependences of the dielectric constant, phonon frequencies at the Γ point, polarity, and localized and non-localized effective charges are calculated. We found that the dielectric constant, dynamic effective charge, and polarity decrease, while the localized effective charge and optical phonon frequencies increase with pressure for all these III–V phases. The distinctive behaviours as regards lattice dynamics of these compounds are explained on the basis of the non-localized to localized charge transference inside the crystal under pressure.

A problem of screening of electron–electron interaction by LO phonons is investigated for bound two-electron systems in bulk semiconductors and semiconductor quantum dots. We consider a D⁻ centre and a two-electron quantum dot and obtain the effective LO-phonon-induced interaction between the electrons, i.e., V_eff(r₁₂)∼e²/[ε_eff(r₁₂)r₁₂], where r₁₂ is the interelectron distance and ε_eff(r₁₂) is the effective phonon dielectric function. The calculated phonon dielectric function rapidly increases for small r₁₂ starting from the high-frequency dielectric constant, , and reaches some constant value, , at relatively small interelectron distances. We have found that—in most cases— is less than the static dielectric constant, ε_s. Only in the weakly ionic compounds, like GaAs, . We argue that—in the bound few-electron systems— better approximates the average LO-phonon-induced screening than the commonly used ε_s. We have also shown that the coupling with LO phonons leads to the increase of the binding energy of the two-electron system confined in the quantum dot.

An approach to creating the highest-order multiple-quantum coherences is developed. The method is based on using the multiple-quantum NMR technique and various initial conditions and can be considered as supplemental to earlier developed methods. Using the dipolar ordered state as the initial one, it is demonstrated that in a cluster of four and eight dipolar-coupled nuclear spins the highest-order multiple-quantum coherences can be created.

Dimethylammonium gallium sulfate hexahydrate (DMAGaS) and dimethylammonium aluminium sulfate hexahydrate (DMAAS) are isomorphous and ferroelastic at room temperature. But at lower temperatures their ordering behaviours are quite different. Whereas DMAAS shows only a single order–disorder-type transition into a ferroelectric phase, DMAGaS exhibits an exceptional sequence of commensurate and incommensurate phases with an antiferroelectric lock-in phase at low temperatures. The basic experimental findings from recent pressure and composition dependent dielectric investigations and from electron paramagnetic resonance studies are briefly reviewed. Experimental results are then discussed within the framework of the semimicroscopical extended discrete frustrated φ⁴ (DIFFOUR) model. The frustrating nearest and next nearest neighbour interactions between the dimethylammonium (DMA) dipoles in the –DMA_(i)–Ga/Al(H₂O)_6(i)–DMA_(i+1)–Ga/Al(H₂O)_6(i+1)– chains give rise to the observed complex phase sequence. It will be shown that the phase sequence of DMAGaS, its variation under pressure and with DMAAS admixture, as well as the new pressure induced phases of DMAAS can be well interpreted by theory.

The continuum percolation process of particles with a size distribution is investigated by Monte Carlo simulation. The critical area (volume) fraction of particles at the percolation threshold in two (three) dimensions is determined for various distribution functions. It is shown that the critical area fraction is a monotonically increasing function and the critical volume fraction is a monotonically decreasing function of the width of the distribution or the poly-dispersity and that the dependence on the poly-dispersity is well fitted by a universal function as long as the width is less than 15% of the average radius of the particles. It is also shown that the critical area (volume) fraction is not in proportion to the packing fraction and that it can be a double-valued function of the packing fraction.

We measured the electrical resistivity of an antiferromagnet Ce₂Ni₃Ge₅ with the orthorhombic crystal structure under pressure. The Néel temperature T_N = 5.2 K decreases with increasing pressure P and becomes zero at a critical  GPa. The A and ρ₀ values of the low-temperature electrical resistivity ρ = ρ₀+AT² in the Fermi liquid relation increase steeply above 3 GPa. A value of A = 10.7 µΩ cm K⁻² at 3.9 GPa iscomparable to A = 10 µΩ cm K⁻² in a heavyfermion superconductor CeCu₂Si₂. The heavy fermion state was found to be formed around P_c, in which pressure region superconductivity was found below 0.26 K.

We study the Kondo lattice model with the Heisenberg-type RKKY exchange coupling among localized f spins in the presence of a magnetic field. By means of an extended dynamical mean field theory combined with the non-crossing approximation, we investigate the one-particle spectral function and the dynamical spin correlation function in the Kondo insulating phase. It is shown that the magnetic field and the RKKY exchange interaction both cause instability to the antiferromagnetic order with enhanced transverse spin fluctuations, which give rise to a strong renormalization of quasi-particles as the system approaches the quantum critical point. This leads to a tendency to retain the Kondo insulating gap up to rather large fields.

In an earlier paper we presented a formulation for the calculation of the configuration-averaged optical conductivity in random alloys. Our formulation is based on the augmented-space theorem introduced by one of us (Mookerjee 1973 J. Phys. C: Solid State Phys. 6 1340). In this communication we shall combine the augmented space methodology with the tight-binding linear muffin-tin orbital technique (TB-LMTO) to study the optical conductivities of two alloys, CuAu and NiPt.

Much of our knowledge of the charge states, lattice site and behaviour of hydrogen in bulk semiconductors comes from observation of its muonium analogue. Here we present studies of muonium behaviour across the composition range in bulk, Czochralski-grown Si_1−xGe_x alloy material, focusing in particular on the muonium hyperfine parameters. For the bond-centred muonium species, a broad distribution of parameters is observed, consistent with a variety of bonding environments. The average value of the isotropic component of the bond-centred hyperfine parameter shows a linear variation with alloy composition, which might be expected based on the linear variation with composition of alloy bond lengths. In contrast, the hyperfine parameter of the tetrahedral-site muonium species (Mu_T) appears to vary non-linearly with alloy composition, and an explanation of this in terms of Mu_T mobility is provided. The temperature dependence of the Mu_T hyperfine parameter observed in several alloy compositions is compared with that seen in pure Si. Previous descriptions of the low-temperature behaviour of the Mu_T parameter in Si are discussed in the light of results from Si_1−xGe_x material.