Table of contents

The reversible magnetization M(T,H) in the mixed state of the quasi-two-dimensional superconductor Bi₂Sr₂CaCu₂O₈ appears to factorize into a product f(T)g(H). It was recently argued that this implies a puzzling temperature-independent critical field H_c2, at least in the region where scaling is observed. We propose an alternative explanation in which this scaling property is approximate and results from the presence of vortex fluctuations, keeping a conventional temperature dependence for H_c2(T). New data in the BiSrCaCuO (2212) and TlBaCaCuO (2201, 2212) systems that obey to a good approximation the factorization M(T,H)=f(T)g(H) are presented.

Data that pertain to experimental X-ray and inelastic neutron scattering studies of liquid sodium are obtained from an ab initio molecular dynamics simulation. A long run was used, so the statistical precision of the calculated quantities is as good as obtained experimentally. The long run is made possible by the use of a recently developed modification of the Car-Parrinello method which uses an orbital-free density functional to prescribe the electron energetics. Agreement with experiment is excellent.

We present a novel tight-binding scheme designed for total-energy and molecular-dynamics simulations in III-V compounds. The role played by orbital overlap and charge transfer is critically addressed and suitably modelled. Comparisons to first-principles density-functional and Hartree-Fock data are presented and commented on in view of the interpolative character of the present method.

We apply the tight-binding scheme developed in the preceding paper (see ibid., vol.6, p.5243, 1994) to molecular-dynamics simulations of liquid GaAs and GaSb. We investigate structural, electronic and dynamical properties of both liquid samples and present a comparison to experimental and ab initio data. Finally, we discuss the reliability of the present tight-binding method for molecular dynamics in III-V compounds.

The optical absorption coefficient alpha of liquid sulphur has been measured in a wide absorption range from 5.5*10^-2 to 2*10⁵ cm^-1 at temperatures from 130 to 450 degrees C. For measurement in the high-absorption range beyond 10⁵ cm^-1, we have developed an optical cell of our own design with a sample thickness of about 3000 AA. It was found that the absorption spectra are substantially changed at the polymerization temperature, especially in the high-absorption region; a new absorption band appears at around 3.25 eV and increases with increasing temperature. We discuss the origin of this new absorption band in the high-absorption region together with the details of the spectral changes in the intermediate and low-absorption regions.

We have performed measurements of acoustic attenuation and sound velocity at ultrasonic frequencies in a molecular liquid (m-fluoroaniline) across its calorimetric glass transition T_g. This liquid offers the opportunity to observe its dynamical response continuously over an extended temperature domain where no crystallization is to be feared. The temperature dependence of the longitudinal moduli components M'(T) and M''(T) were derived and analysed with the Cole-Davidson and the Kohlrausch-Williams-Watts functions where a Vogel-Fulcher-Tamman expression and a power law were employed to link the main relaxation time to the temperature. The experimental behaviour of this molecular liquid is related to its 'fragile' character.

Measurements of the dielectric constant of poly(n-butyl acrylate) have been taken in the frequency range 100 Hz-3 GHz, for temperatures between 254 and 359 K. Analysis of dielectric data and their comparison with results recently obtained on the same system by Brillouin light scattering have evidenced a two-step relaxation process. Dielectric spectra account for the behaviour of the slower process, i.e. the structural relaxation, while Brillouin spectra are particularly sensitive to the faster process, i.e. the secondary relaxation. The temperature behaviour of these processes is not in accordance with the predictions of the mode-coupling theory, while it can be suitably interpreted by a model that relates the high-frequency dependence of the dielectric loss curve to the local chain dynamics and the low-frequency dependence to the intermolecular correlation.

We have applied Sullivan's model to study the interfacial properties of a spherical drop embedded in a one-component fluid using the mean field approximation. We have examined the effect of drop size (expressed by the equimolar radius R_e) on the density profile, pressure tensor, pressure difference Delta p across the interface, surface tension, Tolman's length, and density, pressure, and normal component of pressure tensor at the centre of the drop. Delta p is found to be an ill defined quantity in the sense that it can be defined in various ways, whose results coincide for large drops. The surface tension is a non-monotonic function of R_e; it increases slowly from its flat interface value as R_e decreases until a maximum value is attained, then it decreases rapidly. For small supersaturations, the drops are under tension and compression while at large supersaturations they are only under tension. The results of this theory are compared qualitatively with previous molecular dynamics simulations and theoretical calculations for drops.

The liquid-expanded (LE)-liquid-condensed (LC) phase transition in monolayers of amphiphilic molecules is investigated in the pair approximation of the cluster variation method. The model analysed is the model proposed by Firpo et al. (1984) which can be mapped into the Blume-Emery-Griffiths Hamiltonian. The signatures of the LE-LC transition are searched for as break points on the isotherms of the surface pressure-molecular area diagram. The results obtained in the pair approximation are improved with respect to those obtained in the Bragg-Williams approximation for a set of typical energy and entropy parameters in the case of pentadecanoic acid. In particular, the LE-LC phase transition is shown to be weakly first order. A comparison with experimental results gives satisfactory agreement.

Measurements of the electrical conductivity sigma , thermoelectric power S and magnetic susceptibility of liquid Ag_1-xSe_x alloys have been measured as a function of composition and temperature. The unusual maximum in sigma , along with the negative d sigma /dT observed around stoichiometry for Ag-S alloys is also observed in this system. The thermopower shows a p-n transition at x=0.36 with a magnitude indicating that this is a liquid semiconductor with an essentially zero energy gap. It is demonstrated that the results cannot be explained using the Kubo-Greenwood expressions for sigma and S if rigid band behaviour is assumed. The origins of this unusual behaviour are discussed in terms of an enhanced carrier mobility near the stoichiometric composition.

We present model calculations for the lattice dynamics of berlinite (AlPO₄). The short-range potentials used for the calculation are based on the ones determined for quartz (SiO₂). The polarizability of the oxygen ion is taken into account by an adiabatically moving shell. A good description both of the experimental frequencies and the high-frequency dielectric constants is obtained. Estimates of the ionic as well as effective charges of the constituent ions are given. The deduced charges in conjunction with the obtained polarizability indicate a similar charge state of the oxygen ion in quartz and berlinite. The ionic charges of phosphorus and aluminium differ strongly. The same observation is made for the short-range forces between the cations and oxygen. On the basis of these results the picture of an oxygen framework crystal is discarded. Special effort is devoted to the investigation of equilibrium conditions. The stability of the respective tetrahedra turns out to be of central importance for the dynamics of both quartz and berlinite. Consequences for the ( alpha - to beta -phase) transition are discussed.

The blue and infrared (IR) bands of the photoluminescence spectra in the temperature range 11-430 K and excitation bands at 77 and 300 K of zinc tungstate single crystals have been studied. The line shape, obtained from the theory of lattice relaxation and multiphonon transitions, is compared with the experimental results. The temperature dependences of the intensity, the peak position and the line width of the zero-phonon line associated with the IR emission band are investigated experimentally. The results obtained are discussed in terms of the theory of linear and quadratic electron-phonon interactions.

Phase relations and thermodynamic properties of sodium fulleride (Na_xC₆₀)have been measured up to x=6 by the electromotive force (EMF) method. Na beta -alumina was used as the solid electrolyte and Na was added to the cell containing a fixed amount of fullerene by coulometric titration. The phase diagram is characterized by alternate single-phase regions (EMF varying with composition) and two-phase regions (EMF plateaus). At 599 K, the Gibbs energies of formation of Na₃C₆₀ and Na₆C₆₀ are 113.2 and 94.7 kJ/mole of Na, respectively. A model is proposed for the Na activity (or EMF) as a function of composition based on nearest-neighbour interactions, orientational disorder and a random distribution of Na ions. The ionic size and the hindrance of rotation of C₆₀ accounts for the miscibility gap in alkali fullerides in this model.

Na inclusions with diameters in the range from 2 nm to 15 nm have been made by ion implantation of Na into 70 nm thick single-crystalline Pt foils followed by annealing. The structure of solid inclusions and the diffusion of molten inclusions have been studied by transmission electron microscopy. At room temperature the inclusions are faceted and crystalline with a BCC structure and they are aligned topotactically with the Pt (FCC) matrix. The diffusion of inclusions in the liquid state was investigated by annealing at temperatures of 1227 K, 1432 K and 1534 K. The diffusion coefficient was found to have a dependence on the inclusion radius and an activation energy of 2.8+or-0.2 eV, this being consistent with a mechanism by which volume self-diffusion of Pt controls the diffusion rate of the inclusions. Diffusion of Pt at the interfaces between the Pt matrix and the liquid Na inclusions was found to be strongly inhibited compared with the free surface self-diffusion of Pt. The results are used to propose a method to produce sources for positron annihilation spectroscopy at high temperatures.

Waveguiding epitaxial LiTaO₃ films have been prepared on sapphire substrates by the excimer laser ablation technique. The as-grown films were characterized by Raman scattering, XRD and SEM techniques, which revealed that epitaxial (001)-oriented and (012)-oriented LiTaO₃ films with small surface roughness were achieved on sapphire (001) and (012) substrates, respectively. Optical waveguiding properties were demonstrated by m-line measurement of TM and TE multimodes.

A new scheme for constructing approximate effective electron potentials within density-functional theory is proposed. The scheme consists of calculating the effective potential for a series of reference systems, and then using these potentials to construct the potential of a general system. To make contact with the reference system the neutral-sphere radius of each atom is used. The scheme can simplify calculations with partial wave methods in the atomic-sphere or muffin-tin approximation, since potential parameters can be precalculated and then for a general system obtained through simple interpolation formulas. We have applied the scheme to construct electron potentials of phonons, surfaces, and different crystal structures of silicon and aluminium atoms, and found excellent agreement with the self-consistent effective potential. By using an approximate total electron density obtained from a superposition of atom-based densities, the energy zero of the corresponding effective potential can be found and the energy shifts in the mean potential between inequivalent atoms can therefore be directly estimated. This approach is shown to work well for surfaces and phonons of silicon.

Using a model of random one-dimensional wires, we determine an exact recursive formula for the transmission probability of a Bethe lattice. We use this formula, in numerical simulations, to study the metal-insulator transition. In particular, we examine the form of the conductance distribution and find that it does not obey the predictions given in other studies.

We study the Hubbard model in dimension d= infinity , within an approximation that simultaneously reproduces the atomic limit and the weak-coupling limit up to second order in the interaction strength U. This method, which is a slightly modified version of an approximation suggested by Edwards and Hertz, is applied to the calculation of the self-energy and of the spectral function in the paramagnetic regime for different U, band fillings n, and temperatures T. We find Fermi-liquid behaviour for small U and (for half-filling n=0.5) a metal-insulator (Mott) transition at a critical value U_cr, but also a metallic non-Fermi-liquid phase for intermediate values U<U_cr. We compare our results with results obtained within the second-order perturbation theory in U and (at finite temperature) with the (essentially exact) quantum Monte Carlo (QMC) results. Qualitatively, the approximation reproduces the correct tendencies obtained by QMC, but in detail there are still discrepancies, in particular for intermediate values of U.

The ground-state characterization of the polaron problem is retrieved within the framework of a variational scheme proposed previously by Devreese et al for the bound polaron. The formulation is based on the standard canonical transformation of the strong coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Specializing our considerations to the bulk and strict two-dimensional polaron models we see that the theory yields significantly improved energy upper bounds in the strong coupling regime and, moreover, extrapolates itself successfully towards the well-established weak coupling limits for all polaron quantities of general interest.

The T(X)(e+2t₂) Jahn-Teller (JT) system is studied analytically in strong coupling on a tetrahedral cluster model by transformation methods. Using an energy minimization procedure, the system is shown to be localized in potential-energy minima of tetragonal, trigonal or orthorhombic symmetry depending upon the relative strengths of the e and the two t₂ coupling constants and on the quadratic coupling constants. The addition of an extra t₂ mode to the T(X)(e+t₂) system does not introduce any extra potential-energy minima although their respective depths are increased. Also derived are expressions for symmetry-adapted vibronic states and their energies for the T(X)2t₂ JT system. Projection operator techniques are used starting from the exact vibronic states in the infinite-coupling limit. These results give an insight into the effect of including additional modes from the remainder of the crystal into the JT effect, and hence indicate how a full multimode model can be formulated. The calculation of symmetry-adapted excited states means that it will be possible to undertake analytical calculations of reduction factors. This in turn will provide a new insight into the derivation of improved models for explaining experimental data obtained on specific systems.

In this paper, we study the persistent current in a normal conductor ring coupled to a reservoir with an ideal lead and investigate the effect of an electromagnetic potential on free electrons in a normal-conductor one-dimensional ring. The electrostatic potential on the ring modulates the amplitude, phase and frequency of the Aharonov-Bohm flux current. We discuss the effect of reflection on both sides of the gate electrode on the persistent current in the presence of an electromagnetic potential. The oscillation amplitude of the persistent current and the density of states decrease as the electrostatic potential increases at constant magnetic flux.

Longitudinal and transverse magnetoresistivity measurements have been performed on a-Fe_40-xNi₄₀Mn_xB₁₂Si₈ alloys (0<or=x<or=5.5) in fields up to 40 kG, at 4.2 K, 100 K, 200 K, and 300 K. Above technical saturation, all the alloys exhibit a negative slope, i.e. the resistivity decreases as a function of the magnetic field. The high-field magnetoresistivity slope becomes more and more negative with increasing Mn concentration. Assuming that the negative slope of magnetoresistance is caused by the suppression of spin waves, the temperature and composition dependence of magnetoresistivity of the present series could be qualitatively understood. The spin-wave contribution to the zero-field resistivity has been estimated using a simplified approach. Furthermore, the ferromagnetic anisotropy of resistivity (FAR) has been calculated and the results are analysed in the light of a two-current conduction model generalized to weak ferromagnets and amorphous alloys.

Ballistic electron transport in resonant quantum-wire structures (double point contacts in series) is investigated taking into account charge build-up effects. It is shown that when the resonant condition holds (i.e. when the Fermi energy of incoming electrons coincides with the energy of the quasibound state), the charge stored in the structure may lead to a bistable behaviour of the system. In the linear-response approximation, when the current flowing through the structure is described by the two-terminal Landauer formula, a closed non-linear equation is derived relating the transmitted current to the energy of the resonant level and the width of the resonance Gamma . It is shown that the condition that must be satisfied for bistability to occur is e³V/ pi C Gamma ²>or approximately=1, where V is the applied bias and C is the capacitance of the structure. The dependence of the width of the resonant level and its position on the device geometry is investigated using the match-mode technique for calculations of the electron transmission coefficient. The width of the resonance is shown to increase with increasing length of the contacts, but the position of the resonant level (in units of the propagation threshold energy in the contact regions) is insensitive to the length of the contacts as well as to the lengths of the side arms. On the basis of the results obtained, the bistability in the I-V_g characteristic (current-gate voltage) is studied for structures of realistic dimensions.

The coupled-cluster method of quantum many-body theory is applied to 1D and 2D spin- 1/2 systems with nearest-neighbour and next-nearest-neighbour isotropic exchange. Several simple approximation schemes based on a Neel model state are investigated and are found to give satisfactory results in the antiferromagnetic regime. Results are given for the ground-state energy, the order parameter and the sublattice magnetization. We obtain evidence of a phase transition.

A study of the magnetic properties of the hexagonal compound CeCu₅ reveals antiferromagnetic order below T_N=3.9 K. The magnetic structure comprises moments along the c-axis with a propagation vector k=(0,0, 1/2 ). The presence of the Kondo interaction reduces the magnitude of the moments from about 0.42 mu _B, expected for a mod +or- 1/2 ) doublet, to about 0.36 mu _B, observed from elastic neutron scattering experiments.

We report the structural, magnetic and transport properties of (Co₇₀Fe₃₀)_xAg_1-x heterogeneous alloys, with x varying between 27 and 47 at.%. The samples were prepared by DC triode sputtering at room temperature (RT) and liquid-nitrogen temperature. Annealing processes lead to phase segregation and grain growth, resulting in the formation of small and compact Co₇₀Fe₃₀ precipitates. The resulting evolution of the magnetic properties has been investigated by SQUID magnetometry, Mossbauer spectroscopy and ferromagnetic resonance. The largest magnetoresistance amplitudes at room temperature have been obtained for a magnetic concentration x of around 36 at.% in both as-deposited and annealed samples. Amplitudes Delta R/R(H=0) of 15% in the as-deposited samples and 21% in samples annealed at 600 K for 10 min have been observed at RT. At 4 K these amplitudes reach 43% (equivalent to Delta R/R(H=2 T)=75%).

On the basis of a model that was proposed to explain a large isotope effect on the temperature (T_c) of the transition from a ferroelectric phase to a paraelectric phase in KH₂PO₄ (KDP), the motion of electric dipole moments is examined in the framework of the harmonic approximation; it is concluded that, in the ferroelectric phase, there are electric dipole waves, by which the vibration motion of a proton is induced along the direction of an oxygen-hydrogen-oxygen (O-O) bond. Scattering of thermal neutrons induced by this motion of protons (deuterons) is also examined.

In this paper, we study the dynamic and infrared activity of pure trans- and cis-polyacetylene, doped systems, and systems with cis-trans defects. First we study the behaviour of the Goldstone mode frequency of a simple linear chain. We find that this frequency depends strongly on the value of the dimerization parameter u, in contrast with the experimental results, where this mode is particularly stable. Thus, taking u as a free parameter, we develop several realistic models for neutral perfect cis and trans chains and doped systems. Our results are in very good agreement with experimental results. We show that for the doped chain the transverse component of conductivity is about 1% of the longitudinal component. The presence of the cis-trans defect allows us to give the origin of several peaks found experimentally and not interpreted until now. We show that in the presence of cis-trans defects the photoinduced infrared activity may appear in systems with charged solitons and antisolitons on the same neutral chain. Computed results are also in very good agreement with the experimental data.

Fluctuations of order parameters Q near a tricritical point coupled with strain e( varies as eQ²) lead to fluctuation-induced first-order phase transitions. Anisotropic fluctuations with strong correlations along a 'hard' direction show a much reduced first-order behaviour. In the case of improper ferroelastics the fluctuation-related renormalizations become immeasurably small and a classic tricritical behaviour is expected. Experimental observations of such fluctuations are facilitated if the fluctuating order parameter couples bi-quadratically with a second, non-critical order parameter which is used as tracer. The fluctuation-induced renormalization of the tracer order parameter is calculated.