Table of contents

The correction term in the Callaway model for phonon conductivity is separated into its longitudinal and transverse parts in the presence of phonon dispersion. This will provide, for the first time, a realistic opportunity to speak about the total individual contribution of the phonons, of different polarizations, to the total phonon conductivity.

Lattice softening is found to occur at about 30 K in Bi₄Sr₃Ca₃Cu_3.92Fe_0.08O_16.38. The average quadrupole splitting of Fe nuclei decreases with increasing temperature between 40 K and 297 K and its reduction follows a T^3/2 law. Below 30 K the rapid drop in the average quadrupole splitting is closely related to lattice softening. The lattice vibrational behaviours of Fe-doped Bi-4334 and Fe-doped Bi-2212 are quite different.

Results from a theoretical analysis of the Heisenberg model of a ferromagnet, including dipolar interactions, are successfully used to interpret recent measurements performed on EuS, just below the critical temperature, that probe the dynamics of long-wavelength spin fluctuations parallel (longitudinal) and perpendicular (transverse) to the spontaneous magnetization. Attention is given in the paper to longitudinal fluctuations following this and other related experimental investigations on the topic. With a view to future experiments, properties of the spin dynamics in the critical region are discussed within the framework of the coupled-mode approximation.

Results of scanning tunnelling microscopy (STM) show that two different kinds of surface structures coexist on the (110) surface of Pt₈₀Fe₂₀. The predominant structure corresponds well to the missing row type (1*2) reconstruction previously observed on, for example Pt(110), whereas the minority domains show a faintly buckled structure with approximately (1*1) geometry. Based on the STM images, the authors propose that the two domains have different surface chemical compositions and correspond to two different metallurgical phases known for the PtFe system at this composition.

The STM's ability to image adsorbates depends on the probability that the electronic states localized on the adsorbed molecule contribute to the tunnelling current. When the STM images are dominated by the substrate, any interpretation in terms of the actual positions of the atoms within the molecule is controversial. A criterion is presented for deciding whether an STM observation is of the adsorbed molecule directly or of the indirect effect of the molecule on the current from the substrate. It is based on analysis of the wave function associated with the tunnel current obtained by a self-consistent molecular orbital calculation, within the LCAO and cluster framework at CNDO level. The results for a range of molecules and polymers are in this case fully consistent with the criterion. The data suggest that any credible high-resolution STM observation of adsorbed species requires a contribution to the wave function not less than 50%. The notorious observed differences in contrast of co-adsorbed species are mainly attributed to consideration differences among individual contributions to the tunnel current.

The authors present a theoretical study of the effects of the STM tip on the geometry of Si(100) reconstructed surfaces. The energy barrier to switching between different reconstructions is also discussed. They use a molecular dynamics method and self-consistent forces to simulate the time-dependent behaviour of the surface atoms. The molecular orbital calculations are performed at the CNDO level using a cluster model. Their results indicate significant differences for positively and negatively biased tips. The thermally induced rocking of surface dimers is inhibited by the application of a positive bias to the tip and it is promoted by a negative bias. These bias-dependent effects may offer a plausible explanation for the bias dependence of STM images of this surface.

Considering the influence of interface roughness on phonon vibrational modes in the dielectric continuum model, electron-interface phonon scattering rates are calculated in a model GaAs/Ga_1-xAl_xAs quantum-well structure. The intrasubband and intersubband scattering rates are given as a function of quantum-well width. It is shown that interface phonon scattering is the dominant scattering mechanism in narrow quantum-well structures and electron relaxation is strongly dependent on interface roughness. For intersubband scattering, the infinite barrier height approximation can introduce a large error, in particular, in narrow quantum-well structures. The authors' results are in good agreement with recent experimental data.

Experimental scanning tunnelling microscopy data for the Si(111)5*5-2*1 phase boundary are analysed with reference to the structure of the 2*1 surface. The height change data are subject to significant correction factors, making interpretation difficult due to the small theoretical differences involved. The theoretical lateral differences are much larger, being 3.3 AA, and these are analysed.

The authors have used dynamical low-energy electron diffraction (LEED) to determine the structure of both the clean Ni(111) surface and the p(2*2) structure of potassium adsorbed on Ni(111). The result of the clean surface study indicates that the Ni(111) is essentially a truncation of the bulk crystal. The p(2*2) structure of potassium on this surface consists of the potassium atoms adsorbed on top of the Ni atoms with a slight reconstruction of the top-layer Ni atoms combined with vertical relaxations of the first and second layers of Ni. The potassium-nickel bond length is 2.82+or-0.04 AA corresponding to an effective potassium radius of about 1.57 AA. This result fits well in the bond length versus coordination number trend observed for other alkali metal overlayers.

The interaction of ethylene with the Si(100) 2*1 surface has been studied by a semi-empirical molecular orbital theory using a four-layer silicon cluster model Si₂₉H₂₄-C₂H₄. Ethylene is shown to create a di- sigma bond with the surface dimer sites, and to rehybridize to a near sp³ state. Its initial C-C double bond is reduced to nearly a single bond, while its C-H bonds are almost not weakened. Both the ethylene pi donation to the surface and the substrate back-donation to the ethylene pi * orbital result in the equilibrium di- sigma bonding geometry. The C₂H₄ decomposition behaviour is somewhat different from that for C₂H₂(ads) on Si(100), and C₂H₄(ads) on transition metals. Ethylene favours C-C bond scission first, and then dehydrogenation. The calculated vibrational properties of the C₂H₄(ads) and C₂H₄-induced species confirm the results of a recent HREELS study.

The bonding characterization of Br on the Si(100) surface has been studied by the atom superposition and electron delocalization molecular orbital theory, using 2*1 symmetric- and buckled-dimer models for this surface. From the total energy minima, the authors have found two stable adsorption sites for Br bonding in the symmetric-dimer model. One is along the surface dangling bond with a Br-Si bond length of 2.16 AA and a bond angle of 19 degrees to the surface normal; the other is the bridge site with a Br-Si bond length of 2.13 AA. In the buckled-dimer model, Br is predicted to adsorb onto the dangling bonds and further to enhance the buckling. The calculated bond lengths and angles are in good agreement with the recent X-ray standing-wave (XSW) experiment. Moreover, the authors give a new explanation for the low coherent fraction f_c in the XSW study.

High-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS) are employed to study the interaction between oxygen and potassium on graphite in the coverage range up to 2.5 monolayers and in the temperature range 160-800 K. Annealing gives rise to a succession of different KO_x complexes, partially induced by surface segregation of intercalated potassium. A vibrational excitation at 39 meV is attributed to K₂O formation, in agreement with a recent interpretation of XPS and HREELS spectra of oxidized single-crystal potassium. Two vibrational excitations at 30-31 meV and 180 meV are tentatively attributed to either KO₂ or K₂O₂. A high-temperature complex, which is an unidentified precursor to CO₂ formation/desorption, has a single electron energy loss at 27 meV.

It is well known that silver grows epitaxially on Si(111)-7*7. The authors deposited several monolayers of silver on silicon under UHV conditions (p=1*10^-10 mbar). Due to the perfect substrate they were able to observe electric conduction of one monolayer of silver. They made in situ measurements of the conductance during growth in the temperature range 50-130 K. The authors show that conduction starts at a critical coverage less than a monolayer, and the critical coverage decreases for increasing deposition temperature. They discuss a percolation model and several possible growth modes.

The macroscopic dielectric function of a superlattice is used to describe the angular dependence of the superlattice's confined phonons and their associated polaritons at long wavelengths. Results are shown for short-period GaAs/AlAs superlattices making use of a dielectric tensor derived from a linear chain model that can take account of the effects of interface roughness. The method gives a unified description of interface and confined optic phonon modes in the long-wavelength limit, and provides precise information about the angular dependence of mode frequencies that are accessible to micro-Raman and far infrared measurements.

Data on the magnetic susceptibility, low-temperature specific heat and X-ray absorption spectroscopy (Ce L_III and M_IV.V edges) for CePd₇ and its reference compound YPd₇ are presented. The respective Pauli susceptibilities chi ₀ of 0.11*10^-3 emu mol^-1 and 0.50*10^-3 emu mol^-1 and the respective Sommerfeld coefficients gamma of 9.8 mJ K^-2 mol^-1 and 35.8 mJ K^-2 mol^-1 are much smaller (per atom) than those of the corresponding components (Ce, Y and Pd). The valence extracted from the Ce L_III edge is 3.5. The chi ₀/ gamma -ratios are found to be 0.012 emu K^-2 J^-1 and 0.014 emu K² J^-1 for CePd₇ and YPd₇, respectively. Such a value for CePd₇ corresponds to that of a free-electron system instead of an intermediate-valence system (0.035 emu K² J^-1).

The strain-field expression of a small dislocation loop (SDL) in an elastically isotropic medium is extended to the case of icosahedral quasicrystals. The diffraction contrast images of SDL are simulated systematically for different experimental and SDL parameters. The main results of the simulation are explained semiquantitatively by the contrast expressions for the SDL deduced by a perturbation treatment.

The authors report results of atomistic simulation studies of the formation of intrinsic defect structures in YIG. Using calculated defect formation energies they obtain energies for defect reactions, from which they determine the most favourable defect processes in yttrium iron garnet. Finally, they comment on some electronic properties of YIG crystals.

The kinetic and equilibrium behaviour of the order parameter, Q_od, of Al,Si ordering in anorthite has been compared with the results of current rate theories (Ising-type Glauber, Ising-type Kawasaki and Landau-type rate equations). Although all rate equations can be adjusted to fit the time evolution for an accessible small interval of Q_od(t) between 0.76 and 0.88, significant systematic differences were found for the equilibrium behaviour Q_od(T). Q_od(T) is well described by Landau theory but not by Bragg-Williams equations. The rate laws are rather model-independent for Q_od>0.76, but significant differences are expected for the ordering behaviour of crystals with smaller values of Q_od (e.g. <0.5).

The structural properties and the phase transition between the B1 (rock salt) and B2 (CsCl) phases of NaCl have been investigated at an ab initio level. Total energy curves, obtained with periodic Hartree-Fock (HF) calculations, have been corrected a posteriori by integrating the HF charge density according to the correlation-only density-functional-like formulae proposed by Colle and Salvetti in 1975 and by Perdew in 1986 and 1991. The three functionals are 'non-local', containing terms that depend on the gradient of the electronic density. The correlation correction brings the HF data for the lattice energy (LE), lattice parameter (a_o) and bulk modulus (B) into good agreement with experiment, the best performance being obtained with the most recent functional, (the percentage error is +0.3%, -0.5% and +5.6% respectively for LE, a_o and B).

The competition between the elastic energy and the electrostatic energy induces particular shapes of the phase front at the KD₂PO₄ first-order transition. The coexistence of the two phases is reported for a paraelectric-ferroelectric transition; the relevant domain textures and dielectric properties are described. It is shown that the tilt angle between the phase front and the (001) plane never exceeds 22 degrees in the experiments.

The authors report on the first comprehensive theoretical study of the electronic, magnetic and structural properties of the quasi-two-dimensional magnetic insulators K₂NiF₄ and K₂CuF₄. The investigations were carried out by means of electronic structure calculations based on density functional theory within the local density approximation, and led to full agreement with experiment with regard to many important ground-state properties. In particular they result in a correct description of the two-dimensional electronic dispersion, the insulating behaviour, the magnetic order and the structural distortions inherent in these compounds. They were also able to study the delicate interplay of all these properties. Furthermore they arrive at the conclusion that band theory is well suited to describe magnetic systems, which possess localized moments and serve as almost ideal candidates for the Heisenberg model.

The spin-glass behaviour of amorphous Fe_xY_100-x alloys (70<or=x<or=90) has been investigated by means of differential magnetic susceptibility dM/dH. In low magnetic fields, the dM/dH versus T curves show the peaks, indicating that the spin-glass state occurs through two steps. Two freezing temperatures are defined from the two peaks in the curve, i.e. the higher temperature is T_g and the lower temperature is T_f. From the results of the magnetization measurement as a function of temperature, i.e. the M versus T curve, it is indicated that the magnetization starts to decrease at T_g and a thermal irreversibility appears below T_f with decreasing temperature. With increase in the applied magnetic field, a third peak appears at around 150 K in the dM/dH versus T curve for amorphous Fe₈₄Y₁₆ alloy. The position of this peak corresponds to the upper inflection point in the M versus T curve, being defined as the Curie temperature T_C. Therefore, re-entrant behaviour appears in this system on application of a magnetic field. The concentration dependences of T_C and T_f for a field of 500 Oe are very similar to those obtained in zero field for other amorphous Fe-RE alloys (RE: rare-earth metal) such as Fe-Lu and Fe-Ce systems.

The response of triads of strongly coupled spin-1/2 nuclei to the generalized Goldman-Shen pulse sequence ( alpha )_phi - tau ₁-( alpha )_{phi +180 degrees} - tau ₂-( beta )_{0 degrees} is reconsidered by taking into account the spin diffusion between the equidistant energy-level pairs of the spin-rotor system during the time tau ₂. The refined theory gives quantitatively correct model responses and sheds new light on the discrepancy between the previous theory for an ensemble of isolated triads and the analogous theory for pairs of coupled triads, in the limit of no coupling. Theoretical three-pulse spectra are compared with experimental responses of the tunnelling methyl protons in a CH₃COONa.3D₂O single crystal at 30 K.

The P2 centre was the first defect observed in diamond with electron spin resonance. Because of a very complicated ESR spectrum many years elapsed before the correct model was determined. Accurate spin Hamiltonian parameters have been determined from extensive ENDOR measurements at room temperature. These parameters are consistent with a model in which the defect consists of three nitrogens plus a vacancy. The observed ENDOR transitions do not follow the normal selection rules, and it will be explained how it can be predicted what frequencies will present in the ENDOR spectra.

The carbides RTiFe₁₁C_x (R=Y and Gd) have been prepared by heating powders of RTiFe₁₁ in CH₄ at 500 degrees C. For both the C concentration is about x=0.7-0.8 on the basis of the weighing method and Mossbauer spectra. X-ray diffraction indicates that these carbides retain the crystal structure of their parents, but the cell volume expands by about 3%. The Curie temperatures increase by 175 and 100 degrees C, respectively, as compared to those of their parents. The ratio of the relative change of the Curie temperature to the relative change of the cell volume, rho =( Delta T_c/T_c)/( Delta V/V)=12.4, is a constant for YTiFe₁₁C_x annealed at various temperatures. This result indicates that the increase of the Curie temperature is a volume effect. Mossbauer spectra at room temperature are fitted using seven subspectra for YTiFe₁₁C_0.7 and GdTiFe₁₁C_0.7, because the C atoms affect the hyperfine parameters. Their average hyperfine fields are 30.2 and 29.1 T, respectively, which is an increase of 30 and 15% as compared to those for the parents.

Far-infrared reflection spectra were determined, for the first time, for single crystals of guanidinium aluminium sulphate hexahydrate (GASH) and its solid solution for 5 mol.% chromium isomorph (GCSH). The infrared active transversal and associated longitudinal phonon frequency and the dielectric permittivity functions for E//C and E perpendicular to C have been determined using both Kramers-Kronig analysis and a fitting procedure based on a four-parameter model. The indices of refraction were also measured using the immersion method with Cargille liquid and the results were compared with data obtained by numerical analysis of reflectivity measurements. The optical transmission of the samples were measured in the visible range and three wide energy bands were observed at about 2.6 eV, 3 eV and 4.9 eV for GASH doped with Cr, below the conduction energy zone.