Table of contents

Using synchrotron radiation and an imaging plate the authors have performed a high-angular-resolution powder X-ray diffraction study of the 'distorted FCC' high-pressure phase of praseodymium. The crystal structure of this phase is identified as the trigonal space group R3m (D_3d⁵) with eight atoms in the rhombohedral unit cell. The observed pressure dependence of atomic positional parameters provides quantitative evidence of an occurrence of the continuous distortion of the FCC lattice. The observed atomic displacement pattern implies that the softening and condensation of a zone-boundary transverse phonon mode at the L point of the Brillouin zone drives this phase transition.

The authors have investigated visible photoluminescence from Si⁺-implanted SiO₂. It is found that a luminescence band observed around 2.0 eV in as-implanted specimens disappears on annealing to 500 degrees C and then a band around 1.7 eV appears on annealing to 1100 degrees C. They discuss the origin of the luminescence hands in terms of the defects in SiO₂ and the Si nanocrystals grown in SiO₂.

The X-ray absorption near-edge structures from the Si 2p edge have been measured at high resolution, and compared to a theoretical calculation using a cluster multiple-scattering model. The absorption features near threshold are shown to be sensitive to the degree of short-range crystalline order, over a range of up to 40 AA. The real-space scattering cluster calculation establishes the connection between the short-range order, as reflected in cluster size or final-state electron damping, and the line-widths of sharp absorption features near the 2p edge. Results are shown for amorphous and crystalline Si, and silicon oxides in the form of quartz, obsidian, and thin films on single-crystal Si substrates.

A long-wavelength model of polar optical modes coupling the vibration amplitude u (the relative displacement vector) and the electrostatic potential phi leads to a system of coupled differential equations. This system is here solved for a quantum well without approximations and with simultaneous satisfaction of mechanical and electrostatic matching boundary conditions. Explicit solutions for u and phi are given, and the resulting eigenmodes for GaAs-based quantum wells are studied in detail. The model gives modes with a mixed character describing the coupling between u and phi . Thus one single model yields confined quasi-longitudinal (strictly longitudinal for in-plane wavelength vector kappa =0), confined quasi-transverse (strictly transverse for kappa =0) and interface modes. The dynamical structure, spectral strength and spatial dependence of the relevant amplitudes are studied in detail. The results are in good agreement with available Raman experimental data and have all the basic features present in microscopic calculations. Some comments are made on the limitations of purely dielectric or purely mechanical models.

The authors have treated the relaxation and the dynamics of the (111) surfaces of the fluorides CaF₂ and SrF₂ in the framework of shell models, in which the short-range interactions are represented by Born-Mayer potentials. The relaxation in the outer layer of the crystal, consisting of three closely spaced F-Ca/SrF layers, is significant. The calculated normal vibrations of a 19-layer slab are presented in 3-dimensional diagrams of the normalized surface squared amplitude, which provide a very direct and pictorial display of the surface localization of vibrational states. For CaF₂ the calculations are compared with the HREELS data of Longueville et at. (1990).

The growth behaviour of barium on silver(111) at room temperature is investigated by AES, XPS, MEIS, TDS, work function measurements and by monitoring the secondary-electron-emission crystal current. It is shown that, for low coverage, initially a 2D monolayer is formed. For higher coverages the overlayer formed resembles a structure formed by Poisson or Stranski-Krastanov growth with small islands. As evaporation continues the islands become very large or the sticking coefficient drops to zero. Annealing the Ag with more than one monolayer of Ba on top shows a large work function decrease of 0.6 eV to an unusually low value of 1.9 eV. The lowering of the work function can partly be explained by the formation of large clusters. Coadsorption experiments with H2 indicate that the H₂ sticking coefficient on Ba/Ag(111) is constant and that the H₂ adsorbs dissociatively into one binding state. From the experiments an inelastic mean free path of 11+or-1 AA for 351 eV electrons and 17+or-1 AA for 1113 eV and 1119 eV electrons is deduced.

The authors study the surface states at the Gamma point of (001) GaAs/GaP semi-infinite superlattices basing an empirical tight-binding (ETB) Hamiltonian together with the surface Green function matching (SGFM) method. The strain due to lattice mismatch is explicitly included in the calculations. The authors study the effects of the period length and the layer forming the surface on the energy value and attenuation of the surface state.

The influence on superlattice surface states of a magnetic field B parallel to the interfaces and of a crossed electric field F and magnetic field B is considered theoretically. The degeneracy of the surface states is shown to be lifted in the magnetic field, and a picture of interaction with bulk levels in this case is given. In crossed fields the energy levels have a characteristic anticrossing behaviour as functions of B as well as F. Comparison of electric and magnetic quantization is performed.

Magnetic properties of the reentrant Ni₇₆Mn₂₄ alloy have been studied as a function of temperature by using both ferromagnetic resonance (FMR) and DC magnetization techniques. The experiments were performed either by applying the DC field after cooling the sample to 4 K (ZFC case) or by cooling the sample in the presence of an external field (FC case). In the FC case, the field-induced unidirectional anisotropy field acting on the surface spins is much stronger than that acting on the inner spins, probably due to the lower symmetry of the structure at the surface. This effect causes surface-induced modes to appear in addition to usual FMR bulk mode, when the measuring field is applied opposite to the field in which the sample is cooled. Introducing a unidirectional surface anisotropy energy into the (Rado and Weertman) general exchange boundary conditions, the authors analysed the experimental data and obtained magnetic parameters such as the surface-induced exchange anisotropy, bulk anisotropy and exchange stiffness constant as a function of temperature. A strong correlation has been observed between the field-induced exchange anisotropy deduced from FMR and the hysteresis effect observed in DC magnetization data.

For pt.I see Phys. Rev. B, vol.43, p.3171 (1991). In earlier studies, it was found that for terbium overlays on Ni(111) and Fe(100), the substrates induced some ferromagnetic ordering in the rare-earth film, at temperatures above the Tb Curie temperature. A study is given of the magnetization of ferromagnetic films on ferromagnetic substrates, at temperatures below both Curie temperatures. Expansions are developed for the magnetization within the film as a power series in the distance z to the interface, using Ginzburg-Landau theory. Empirical data for Tb and gadolinium films are found to be consistent with the general form of the expressions found from the theory. Relations are derived for the observed expansion coefficients in terms of the Ginzburg-Landau parameters of the film material. The form a/z for the magnetization is predicted to represent the magnetization well, for z located in an experimentally accessible range of values; data for the magnetization are consistent with this form.

The adsorption of NO₂/ads on a clean Au(111) surface has been studied using electron spin resonance (ESR) and thermal programmed desorption (TPD). NO₂/Au(111) appears to be an appropriate system in which to detect an ESR signal, because it is known from the literature that NO₂ adsorbs as a molecule and is bonded through its oxygen atoms to the surface, while the spin is more localized at the nitrogen atom. In agreement with former results, no resonance of a paramagnetic monolayer on a clean single-crystal metal surface is observed. However, a well resolved spectrum from a multilayer has been detected. Orientation of the molecules in the multilayer has been studied via angle-dependent measurements in combination with computer simulation. Temperature-dependent measurements are used to investigate reactivity and molecular motion.

The authors report on the Rietveld analysis of neutron powder diffraction data recorded on Th₂Fe₁₇D₅, and Th₂Fe₁₇C_0.6H_4.4 and Th₂Fe₁₇N₃, at 4 K and 300 K. They have determined the crystal structures and magnetic structures of these ternary interstitial compounds. The results are compared to those obtained for Th₂Fe₁₇, Th₂Fe₁₇C_x and other related ternary rare-earth compounds. The authors show that there exist several analogies in the magnetic properties between this series and the R₂Fe₁₄B(C) series, despite the differences in their respective crystal structures. The size of the iron moments is discussed in terms of interatomic metal-metal distances as well as in terms of the Fe-X (X=N, C or H) bonds.

The possibility of varying the compositional long-range order parameter s and the influence of s on the ferroelectric properties of pure and Li-doped Pb(Yb_0.5Nb_0.5)O₃ ceramics have been studied. X-ray diffraction and dielectric measurements were used. A compositional order-disorder phase transition was discovered at 1120 degrees C (in doped ceramics). It was shown that in the course of material synthesis the compositionally ordered perovskite structure is formed at comparatively low temperatures (800 degrees C) and the s value may be reduced during sintering at higher temperatures. The pure ceramic remains ordered after sintering because the rate of s change in it is much lower than in the doped material. Low-temperature sintered doped ceramics are characterized by spatial non-homogeneities of s, causing the appearance of two (or more) anomalies in the temperature dependence of the permittivity. The T-s phase diagram has been investigated. Materials with a low s (s<s_c) show a diffuse transition from high-temperature paraelectric phase to ferroelectric phase (at T_C=115 degrees C when s = 0). For materials with a high s (s>s_c) a paraelectric to antiferroelectric phase transition is observed. Thus there is a triple point at s=s_c. The Curie temperature T_C is quadratic in s and a minimum in the T_C(s) curve is observed at the triple point, which is in agreement with the earlier theoretical treatment of Bokov et al.

Two differently structured WSi₂ phases were formed by direct W ion implantation, for the first time, into silicon wafers using a metal vapour vacuum arc ion source. Implantation of W ions was conducted with an extract voltage of 40 kV, various beam densities from 50 to 115 mu A cm^-2 and a fixed dose of 5*10¹⁷ cm^-2. It was found that the formation of WSi₂ with either a hexagonal or a tetragonal structure depended on the ion current density. The temperature rise caused by beam heating and the beam-striking time related to the dose were calculated, and they were responsible for the formation and evolution related to the differently structured WSi₂ phases.

The maximum number of independent third-order elastic and second-order piezomagnetic constants required by the seven pentagonal point groups 5, 5, 10, 52, 5m, 52m, and 10m2 and the two icosahedral point groups 235 and 2/m35 that represent the symmetries of quasi-crystals in two and three dimensions are obtained in this paper. The non-vanishing and independent tensor components needed by each of these nine point groups with fivefold rotations are identified and tabulated. The results of this group-theoretical study are briefly discussed and summarized.

The connection between phase transitions (PTS) and calorimetric glass transitions (CGTS) is considered, first with reference to second order PTS as described by Ehrenfest, then by comparison with a rheological approach. This results in connecting the partition functions of the 'ideal' liquid with that of the 'ideal' glass, by means of a 'real' partition function, explicitly depending on the cooling (heating) rate. By requiring the entropy and heat capacity at any cooling rate in the rheological model to be positive, it turns out that the CGT sharpens with decreasing difference between the two ideal free energies, and not simply with decreasing cooling rate. In addition, it is impossible to make the two ideal free energies join at arbitrary low cooling rates without singularities, if their difference is non-vanishing at zero temperature. If this is not the case, a lower limiting critical cooling rate is predicted, at which a secondary (or beta ) relaxation toward the glass becomes a true PT. Under special conditions, the primary (or alpha ) relaxation can simulate a true PT, in which case one may speak of a 'false' PT. A classification of glass-forming materials is possible, based on the difference between false PTS simulating first-order PTS (class (A) materials), or second-order PTS (class (B) materials). The rheological model is also discussed in connection with the mode-coupling theory of the glass transition.

Transition state theory is frequently used to describe interstitial diffusion in solids. A basic assumption in this theory is that equilibrium statistical mechanics can be used to characterize the different configurations in the transition state, the region in configuration space which acts as a bottleneck for the motion of the interstitial. The authors have performed a detailed test of this assumption for hydrogen diffusion in palladium by combining the molecular dynamics and the Monte Carlo techniques. The study clearly confirms that equilibrium statistical mechanics can be used to characterize the different transition state configurations even though the presence of the hydrogen atom in the transition state strongly influences the fluctuations in the system and despite the fact that the time-scales for the motion of the H atom and the Pd atoms differ considerably.

A method of calculating the total energy for disordered alloys is presented. A formula for the potential in the self-consistent Korringa-Kohn-Rostoker coherent-potential approximation is also given by using the muffin-tin approximation after the variation of the total energy is made. This method was applied to Cu-Pd alloys. The numerical results indicate that the disordered state is stable over the whole concentration range of Pd. This agrees with experiments.

The authors have studied the lateral transport of electron-hole (e-h) pairs in GaAs/Al_xGa_1-xAs quantum wells (QWS) (0.3<or=x<or=1.0) as a function of well width, temperature and Al content in the barriers by a method based on transparent circular microstructured areas in otherwise opaque masks. In the experiment they observe an increase of the e-h-pair mobilities with both growing Al content and increasing well width. The experimental results are compared with theoretical model calculations for different scattering mechanisms such as acoustic-deformation-potential (AC), polar-optical (PO), barrier-alloy-disorder (BAL), impurity and interface-roughness (IR) scattering. For low temperatures (about 40 K<T<90 K) the increase observed in the experiment can most probably be related to efficient IR scattering. Model calculations of IR scattering were used to determine approximately the geometrical roughness parameters of the interfaces. Based on this analysis, the authors' samples investigated in this paper show average terrace heights of one monolayer, independent of well width. BAL scattering is found to affect the mobility of e-h pairs only for very narrow QWS at temperatures close to 100 K. At higher temperatures T>100 K AC scattering and PO scattering are found to dominate the scattering processes. In addition, a crossing of the theoretical electron and hole mobilities is observed for isolated BAL scattering for very narrow well widths or very low Al contents.

The electromagnetic (EM) waves associated with the polariton modes of bulk dielectrics are quantized using a procedure that accounts for both the EM and matter fields. The interaction of electrons with these polariton fields is described using the minimal coupling (e/m*)A.p interaction Hamiltonian where A is the transverse vector potential operator of the polariton field, p the electronic momentum and m* its effective mass. The electron-polariton interaction in the bulk is demonstrated to be closely linked to the behaviour of the polariton group and phase velocities. The same quantization procedure is then employed to describe the EM fields associated with the interface polaritons of a GaAs/AlAs quantum well system. The coupling leads to a dependence of the scattering rate on the group and phase velocities of the surface modes, just as it does for the bulk excitations. In contrast to the case in the bulk, it is shown that these modes are important for relaxing the electron energy in narrow wells.

Thermodynamic and ground-state properties of Cu clusters have been studied with the effective-medium theory including a tight-binding description of the one-electron spectrum. Simulated-annealing Monte Carlo calculations have been performed for cluster sizes between 3 and 29 to determine ground-state energies and structures. Finite-temperature ensembles have been generated for a range of temperatures. The magic numbers 8, 18 and 20 are reproduced and remain stable at temperatures close to 1000 K, where the clusters may be regarded as liquid. The coupling between the atomic and electronic degrees of freedom through a Jahn-Teller-like effect is shown to play a key role in understanding the stability of the magic clusters at high temperatures. For the non-magic clusters this Jahn-Teller effect gives rise to large Fermi gaps even at high temperatures and a pronounced stability of the even-sized clusters relative to those with odd sizes. Finite-temperature electronic spectra are calculated. The fluctuations in the atomic positions give rise to a large broadening of the electronic levels, in agreement with experimental observations. Cu₁₃ exhibits a rather sharp melting transition, whereas clusters of other sizes show more complex behaviour.

Absorption and magnetic circular dichroism (MCD) spectra of band C for the Sn²⁺ centre in alkali halide crystals have been calculated by means of the Monte Carlo integration method. The calculated spectra are compared with the asymmetric absorption and inversion-asymmetric MCD spectra observed in KCl:Sn²⁺ crystals. A good fit to the observation is obtained taking into account both the quadratic electron-lattice interaction with the T_2g vibrational mode and the charge-compensating vacancy in addition to the linear electron-lattice interaction. It is shown that, although the quadratic interaction gives rise to an asymmetric MCD lineshape, the vacancy is important to derive the spectra which agree with the observations for both the absorption and the MCD.

The dynamics of two-dimensional electrons in a circular ring in an external perpendicular magnetic field is investigated. The boundaries of the ring are approximated by infinite potential barriers. Within the quasiclassical approximation, a complete classification of the electron states generated by confinement and magnetic field is provided.

The authors undertake a theoretical study of an ideal lattice of circular hard wall antidots in a low magnetic field where classical skipping orbits can exist. They study the transmission of electrons through constrictions between antidots, and thence their paths through the lattice, which can take the form of a correlated random walk. Analytic expressions for the conductivity tensor are derived, and these give numerical values comparable with recent experiments.

The magnetocrystalline anisotropy and the magnetic phase diagram of the pseudoternary (Er_xHo_1-x)₂Fe₁₄B were studied by using the singular point detection technique, thermomagnetic analysis, low-field AC and DC susceptibility and polar dependence of M/sub /// and M_{perpendicular to} measurements. The competition among the different anisotropic contributions gives rise to a complex magnetic phase diagram in which a complete axis-cone-plane spin-reorientation transition is observed with decreasing temperature for x>0.6. A linear composition dependence of the anisotropy was found in the temperature range 200-293 K, which constitutes an indication of the single-ion origin for the anisotropy in these compounds. A microscopic single-ion crystal electric mean-field model has been used in order to explain the origin of the spontaneous spin-reorientation transitions as well as the thermal dependence of the measured anisotropy fields for all concentrations.

The authors have calculated the electronic self-energy of the two-dimensional Hubbard model as a function of frequency and momentum for various band fillings. They present this up to second order in the interaction parameter and also up to infinite order for ladder diagrams in the two-particle scattering channel. The authors have carried out direct k-space integrations using a triangle method. For a half-filled perfectly nested band, the imaginary part, lm Sigma (k_f, omega ), has a linear- omega dependence for nested Fermi surfaces which is consistent with the marginal Fermi liquid phenomenology for high-T_c superconductors. At, and near, half-filling there is a strong low-energy peak in lm Sigma ⁽²⁾(k, omega ) when k is off the Fermi surface. The physical origin of this peak is discussed. The peak is enhanced in the ladder approximation. The corresponding structure in Re Sigma ⁽²⁾(k, omega ) leads to multiple solutions of the Dyson equation for a wide range of values of the parameters. This leads to an appealing explanation of observed structure in some experimental angle-resolved photoemission spectra of high-T_c materials. The authors also find anti-bound states split off from the band in the two-particle scattering ladders consistent with the suggestion of Anderson.

The formation of localized moments of impurities in simple-metal hosts is investigated in the case of an Mn solute atom in jellia with continuously varying density. The mean-field critical behaviour of the transition from a spin-polarized to a non-spin-polarized state of the impurity, which is deduced from self-consistent local-spin-density (LSD) functional calculations, is analysed assuming a Landau-type expansion of the energy. This analysis is illustrated and supported by constrained LSD functional calculations.

The authors have investigated the linear and, in detail, the non-linear AC susceptibilities of two insulating spin glasses obtained from the dilution of the antiferromagnetic EuSe with diamagnetic SrSe. The systems studied are the mixed compound Eu_xSr_1-xSe with x=0.5 and 0.7. The linear susceptibilities of the samples show a single peak at their spin-glass freezing temperatures of 2.00+or-0.03 K and 2.88+or-0.03 K, respectively. However, the non-linear susceptibilities (third harmonics) of the systems exhibit double peaks: one at the spin-glass temperature, and the other at a higher temperature somewhat below the Neel temperature of pure EuSe which is 4.6 K. The authors believe that the origin of the second peak is the remaining antiferromagnetic phase which seems to persist in the systems down to a critical concentration below x=0.5.

The effects of finite size on the thermodynamic properties of ferromagnetic clusters have been calculated on the basis of a self-consistently solved spin-wave spectrum for clusters of various sizes. The finite size leads to an effective power law for the temperature dependence of the magnetization (M approximately T^alpha ; 3/2< alpha <3), and to a neutron-scattering cross section with a wavevector broadened discrete spin-wave spectrum very different from that of the bulk. Predictions of the implications of finite size are extended to the size range of nanoparticles. The effects should be readily observed in experiments.