Table of contents

The authors have shown through first-principles total-energy calculations on sodium that at low temperatures there is a nearly-zero barrier path of continuous orthorhombic deformations leading from the body-centred cubic (BCC) to the more stable hexagonal close-packed (HCP) structure. The observed diffraction patterns may now be explained without the aid of stacking faults that are needed to account for deviations from the calculated pattern if a 9R structure is assumed.

Within the slave-boson approach to the t-J model of high-T_c superconductivity, the vertex of the spinon-mediated holon-holon interaction is exploited in detail to find out the critical temperature of holon pairing.

Elastic and quasielastic neutron scattering measurements have been performed in the S=1/2 square Heisenberg antiferromagnet copper(II) deuteroformate tetradeuterate, Cu(DCO₂)₂.4D₂O. The sublattice magnetization was measured below T_N=16.54(5) K, and the critical exponent beta was found to cross over from 0.23(1) to 0.32(2) at a reduced temperature epsilon =(T_N-T)/T_N of 0.06 as T_N was approached from below. The temperature dependence of the inverse correlation length of the two-dimensional antiferromagnetic correlations and of the sublattice susceptibility above T_N was consistent with the predictions of the quantum non-linear sigma model of Chakravarty (1989) and co-workers for a spin-wave stiffness of 13.4(4) K.

Benard-Marangoni convection, in containers with large aspect ratio, exhibits space-filling cellular structures, highly deformable, but crystallized. They contain dislocations and grain boundaries generated and moved by elementary topological transformations, and are subjected to a weak shear stress due to the earth's rotation. The cellular structure and its fluctuations are analysed from a crystallographic viewpoint, by using two complementary approaches. One is a global analysis of cellular structures in cylindrical symmetry. Their structural stability and defect pattern are obtained as topological mode-locking of a continuous structural parameter. The other, a local, molecular dynamics of the cells, gives a realistic parametrization of the forces and the transformations by generalizing the Voronoi cell construction in one extra dimension.

Phase stability in the Ni-Al binary system is investigated using linear muffin-tin orbitals total energy (LMTO) calculations. They provide total energies for the different existing compounds and, using Connolly-Williams inversion, the many-body interactions occurring in the FCC and BCC lattices. These interactions are used in conjunction with the cluster variation method (CVM) to calculate the phase diagram. The computed phase diagram agrees very well with the experimental one.

The formation of X-ray standing wave field XRSW under Bragg diffraction conditions was demonstrated using such non-traditional secondary radiation as X-ray photoluminescence.

The neutron-weighted phonon density of states of the superconducting Ba_0.6K_0.4BiO₃, BaPb_0.75Bi_0.25O₃ and BaPb_0.75Sb_0.25O₃ perovskites and their non-superconducting analogues, BaPbO₃ and BaBiO₃, have been measured by the neutron time-of-flight technique between 5 and 300 K. The spectra show very pronounced changes in the energies and intensities of the oxygen bending and breathing phonons, in sharp contrast to the more subtle situation encountered with the cuprate superconductors. There are also changes in the low-energy transfer region, reflecting the structural and electronic variations in the series. The results are consistent with the predictions of theoretical models for a primarily phonon-mediated mechanism for superconductivity.

A multiphonon mechanism of neutron scattering in glasses is considered. It is due to the interaction of the neutrons with strongly fluctuating double-well potentials characteristic of glasses which may be of importance at not too low temperatures (above the liquid helium temperature). The cross section of this process is calculated and its dependence on the temperature and the neutron energy transfer is found. It is shown that, contrary to the relaxation mechanism, the multiphonon mechanism produces a cross section without a maximum in its temperature dependence. This, as well as some other features, corresponds better to the experimental observations.

The absorption of electromagnetic waves and Raman scattering in glasses are calculated. The principal scattering mechanism considered is the interaction of the electromagnetic waves with atoms tunnelling in strongly fluctuating double-well potentials which are characteristic of glasses. The temperature and frequency dependences of the absorption coefficient and the cross section of the small frequency shift Raman scattering are obtained. The results are fitted to the experimental data.

A recently published interpretation of diffuse LEED patterns as electron holograms as well as the subsequent reconstruction of real-space images is extended to use conventional LEED spot intensities. The procedure is based on the fact that ordered adsorbates produce superstructure spots which sample the diffuse intensity distribution appearing in the case of disordered adsorption. This circumvents experimental difficulties in measuring diffuse (i.e. low-level) intensities, particularly at higher energies. It also opens up the application of electron holography to ordered structures. Using experimental data the authors show that laterally resolved images can be reconstructed from the spot intensities. However, atomic positions can be displaced and vertical cut images are of insufficient quality.

Nd₂Fe₁₄B permanent magnets with different load lines were irradiated with 20 MeV protons at temperatures between 300 and 15 K, and the flux loss was measured as a function of the irradiation temperature. The results show that the magnetic flux loss depends drastically on temperature and on the shape of the sample. A new theoretical model which explains the observed temperature dependence of irradiation effects is introduced.

An attempt has been made to carry out an overall calculation of the various damping rates of phonons in order to highlight the dynamic picture of non-equilibrium phonons arising in a metal film excited by laser. The calculations are compared with the experimental results and found to be consistent.

The layered compound KFeF₄ that exhibits a structural phase transition at T_c approximately=380 K from Bmmb symmetry (phase I) to Pcmn symmetry (phase II) is studied as a function of temperature by polarized Raman scattering. All the Raman lines are attributed on the basis of (i) symmetry analysis of the normal modes of vibration in the aristotype phase (phase I) and the compatibilities relations with the symmetries in phase II; (ii) comparison with RbFeF₄ in which the phonon spectrum can be calculated. The phase transition is clearly associated with a soft mode but a residual Raman signal is still observed above T_c; this is attributed to the existence of a disordered phase. Two additional modes exhibit a soft behaviour connected with a (virtual) martensitic transformation.

Ionization probabilities of imaging gas molecules under field ion microscope conditions are calculated and compared with previous calculated and experimental results. It is shown that it is necessary to include the possibility of ionization before reaching the region close to the sample in the calculation of the curves rather than just considering the local ionization rate constant as was done previously. The presence of structure on these curves, not related to resonance states, is also shown and is interpreted as being the result of the local enhancement of the electric field close to the surface and of the cut-off of the potential barrier at points very close to the sample.

Electron correlation on metallic surfaces is studied for the first time with the use of the slave-boson functional integral method. The ground-state properties of the semi-infinite simple-cubic model in the non-magnetic state are investigated. The double occupancy, delta _n, and the band narrowing factor, q_n, on layer n are calculated as a function of the electron interaction. It is shown that delta ₁ and q₁ on the surface show a peculiar behaviour depending on the ratio U₁/U_b where U₁(U_b) denotes the interaction on the surface (in the bulk).

Electronic distributions of various symmetries have been investigated for both valence and conduction bands in a series of Al-Mn and Al-Mn-Si alloys of different structural states (amorphous, quasi-crystalline or crystalline) and Mn concentrations ranging from 14 to 22 at.%. Experiments have been carried out by means of soft-X-ray emission and photoabsorption spectroscopies. This allowed the authors to provide a complete description of the valence band states and to confirm the progressive opening of a small gap in the Al-sp DOS at the Fermi level with increasing Mn concentration in the alloys. At a given Mn concentration the gap is wider for the quasi-crystalline phase than for its crystalline counterpart.

A new version of the tight-binding representation of the linear muffin-tin orbitals method based on the direct-screening of the muffin-tin orbitals (LMTO-TB-DS) is presented. The equations for the screened basis wavefunctions, the screened structure matrix and the corresponding Hamiltonian are obtained. Test calculations of the chromium and YBa₂Cu₃O₇ band structures proved the validity and accuracy of the method.

The band structures of GeSe, GeS, SnSe and SnS layered semiconductor compounds have been calculated by the EO LCAO method in a single-layer approximation. The Madelung energy of electron in the field of all lattice ions was taken into account. The results of band-structure and density-of-states calculations are in good agreement with earlier calculations carried out by the pseudopotential method. It is shown that, unlike Ge chalcogenides, the minimal energy gap for Sn chalcogenides is situated on the symmetry line Lambda (k, 0, 0) and not on V(0, 0, k). The effective masses of electrons and holes near band extrema have been calculated.

The authors report numerical results for localization induced by disorder on a continuous random network. A simple tight-binding model and a Hamiltonian with off-diagonal disorder are used to calculate the localization properties of electronic states. For both models, localization occurs at the upper band edge. In addition, off-diagonal disorder leads to localization at the lower band edge. For both models the density of states, mobility edges and localization lengths have been calculated, leading to significant differences. They discuss the possibility of extending the model to systems in three dimensions, like amorphous semiconductors.

Neutron diffraction experiments have been performed on a TbMn₂ single crystal and on Tb(Mn_0.96Fe_0.04)₂ powder samples. The magnetic structure of TbMn₂ is metastable poised between two structures, S₁ with propagation vector (2/3 2/3 0) and S₂ with propagation vector (1/2 1/2 1/2). A transition from S₁ to S₂ can be induced either by an applied field of 4.5 T at 25 K or by chemical pressure induced by substitution of Mn by Fe. The S₂ structure has been studied in Tb(Mn_0.96Fe_0.04)₂. The transition to this structure is accompanied by a huge rhombohedral distortion and the structure itself is notable for the coexistence of magnetic and non-magnetic manganese atoms. This peculiar feature is attributed to instability of the Mn moment combined with frustration of the Mn itinerant antiferromagnetism.

Single-crystal neutron diffraction measurements on the Kondo lattice compound CeCu₂, using a zero-field neutron polarimeter have confirmed that CeCu₂ orders at T_N=3.5 K to an antiferromagnetic structure. The sublattice magnetization is not saturated at the lowest temperature, 2.5 K, attained during the experiment. The magnetic moments of Ce atoms are 0.33(2) mu _B at 2.5 K and are oriented parallel and antiparallel to the orthorhombic c axis.

EXAFS at the iodine L₃ edge in beta - and gamma -AgI has been studied. The EXAFS Debye-Waller factors of the first and second coordination shells of iodine have been extracted by a standard analysis based on the harmonic approximation. No difference has been found between the temperature dependence of mean square relative displacements (MSRD) of the two beta and gamma phases. The harmonic approximation has been checked against more complete anharmonic EXAFS and X-ray diffraction analyses; it has been found adequate to interpret the MSRDs of AgI for temperatures lower than 300 K. The experimental MSRDs for the two distances I-Ag and I-I have been compared with MSRDs calculated on the grounds of the available lattice dynamics models. The sensitivity of EXAFS to phonon polarization properties has been made evident.

Ultraviolet photoemission experiments using radiation from a synchrotron source have been used to study details of the valence bands of amorhous Nb_xSi_1-x on either side of the metal-insulator transition. The alloys were prepared in situ by RF magnetron sputtering with surface and bulk characterization obtained by Auger electron spectroscopy and EXAFS respectively. The variation of the intensities of several features in the UV spectra with photon energy are due to the presence of resonant photoemission and Cooper minima in photoionization, and these effects can be used to identify the atomic origins of the partial densities of states in different parts of the valence bands. In the metallic phase such measurements show 'pure' Nb 4d states close to the Fermi edge, whereas for the insulating material there is no Fermi edge and little evidence for the Nb resonance.