Table of contents

We report the formation of nanoparticles of silicon (thread-like nanostructures of diameter approximately=2 nm and length a few microns, as if nanocrystals are linked together along curly tubules), the occurrence of a tubule-like configuration, whiskers and a new phase with large periodicity, a=14.25 AA, f.c.c. silicon, when evaporated under helium ambience (100 Torr) and characterized through transmission electron microscopic techniques, revealed several curious characteristics as mentioned above. We have suggested the possibility of a new phase with a=14.25 AA for the f.c.c. lattice, being formed by an assembly of Si₂₈ clusters. The geometrically calculated lattice parameter of the f.c.c. lattice resulting from an assembly of Si₂₈ clusters is in reasonable agreement with the observed lattice parameter.

We present a density functional study of the expanded FCC to condensed-FCC transition of a system of hard spheres with an additional short-range attractive Yukawa potential. The approach rests on a separation of the interaction into a hard-sphere reference part, treated nonperturbatively by means of the modified weighted-density approximation, and a mean-field treatment of the attractive Yukawa tail, in analogy with earlier work on the short-range square-well potential. The method confirms the existence of an FCC-FCC isostructural transition terminating at a critical point, and yields results for the critical temperature that are in very good agreement with simulations.

An overview on the theoretic formalism and up to date applications in quantum condensed matter physics of the effective potential and effective Hamiltonian methods is given. The main steps of their unified derivation by the so-called pure quantum self-consistent harmonic approximation (PQSCHA) are reported and explained. What makes this framework attractive is its easy implementation as well as the great simplification in obtaining results for the statistical mechanics of complicated quantum systems. Indeed, for a given quantum system the PQSCHA yields an effective system, i.e. an effective classical Hamiltonian with dependence on h(cross) and beta and classical-like expressions for the averages of observables, that has to be studied by classical methods. Anharmonic single-particle systems are analysed in order to get insight into the physical meaning of the PQSCHA, and its extension to the investigation of realistic many-body systems is pursued afterwards. The power of this approach is demonstrated through a collection of applications in different fields, such as soliton theory, rare gas crystals and magnetism. Eventually, the PQSCHA allows us also to approach quantum dynamical properties.

Data are reported for numerical simulations of the triple differential ( gamma ,e gamma ) cross-section for inelastic X-ray photon scattering with coincident analysis of recoil electrons from metal foils and thin crystals. Preliminary measurements and calculations have shown that the cross-section is sensitive to the three-dimensional electron momentum density distribution and Fermi surface of the target. These simulations show that a useful signal carried by unscattered recoil electrons is produced from a region near the back of the target and sits on a homogeneous background of multiply scattered electrons. Even for thick samples this background remains sufficiently low due to saturation scattering of the emergent electrons for the umklapp Fermi surfaces in transition metals to be observable with only moderate requirements in statistical accuracy. The present work examines the experimental triple differential cross-section for pseudo-single-crystal targets of Cu and details the resolution and sensitivity of the technique to umklapp regions of the Fermi surface. Figures are presented for the feasibility of measurements at the high-energy beamlines of various synchrotron laboratories.

Amorphization was achieved in the Hf-Nb system with a positive heat of formation (+6 kJ mol^-1) by room-temperature 200 keV xenon ion mixing of multilayered films. On the basis of thermodynamic calculation, in which the interfacial free energy of multilayered films was specially taken into account, a Gibbs free-energy diagram was constructed and a relevant interpretation to the observed amorphization behaviour was proposed together with the kinetic considerations for the ion irradiation process. Also, two metastable FCC phases of Nb-rich and Hf-rich concentrations with different sizes were obtained by ion mixing.

It has become easy to calculate numerically self-energy corrections in second-order perturbation theory. In order to find out its capabilities and limitations, we analyse the weak-coupling approach for a system of one-dimensional correlated fermions. We compare the self-energy contributions due to different scattering processes of left- and right-moving particles which can be treated analytically. We find that for Tomonaga-Luttinger processes second-order perturbation theory gives reliable results and show how deviations from Fermi-liquid behaviour and the separation of spin- and charge-density excitations can be detected from the numerical data. Choosing the Hubbard model as a typical example we present numerical results for the self-energy and the spectral density. Special attention is paid to the case without particle-hole symmetry. The insulating behaviour at half filling is outside the scope of the weak-coupling method applied.

Inelastic neutron scattering experiments have been performed on some members of the tetragonal RET₄Al₈ series of compounds (RE=Tb, Ho or Er; T=Mn, Fe or Cu) in order to determine the crystal-field potential at the RE site. A consistent set of the crystal-field parameters has been obtained from the experimental data by a least-squares fit procedure, using information on the second-order term deduced from Mossbauer spectroscopy measurements in ¹⁵⁵GdT₄Al₈. The results obtained have been used to estimate the rare-earth contribution to the magnetic anisotropy constants and to discuss the characteristics of possible spin reorientation processes.

It has been shown that there exists an additional magnetic phase transition of the magnetization curve for the S=1 SU(3) antiferromagnetic spin chain at zero temperature. As a the magnetic field decreases from the saturation field, there is a phase change at a critical field h_c, where the magnetization curve versus the magnetic field has a cusp. The critical field is a boundary between two states, one of which contains the particles with the spin +1 and 0 and the other with the spin +1, 0 and -1. In this paper we estimate the critical field h_c of the phase transition by examining the stability of these states.

The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K₂NiF₄ have been investigated using the ab initio periodic Hartree-Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is more stable than the ferromagnetic phase by 0.0216 eV per Ni pair, which is almost exactly two thirds of that found in KNiF₃, in agreement with the hypothesis of additivity of the superexchange interaction with respect to the number of Ni-Ni neighbours. K₂NiF₄ turns out to be a two-dimensional antiferromagnet, the calculated interlayer superexchange interaction being at least three orders of magnitude smaller than that within the layers; the latter is shown to obey a d^-12 (d is the shortest Ni-Ni distance) power law as suggested in the literature, and as verified in a previous study on KNiF₃. The two apical F ions of the NiF₆ octahedra not involved in the Ni-F-Ni superexchange path play an important role in determining the ferro magnetic-antiferromagnetic energy difference, which on the other hand is insensitive to large geometrical modifications that leave the octahedra unaltered. Charge- and spin-density maps are used to illustrate the electronic structure of the system and the superexchange mechanism.

Spin excitations in the Kondo semiconductor CeNiSn have been studied in a wide Q-range and in the energy range of h(cross) omega =1.2-7 meV by means of single-crystal neutron scattering. The magnetic fluctuation at low temperatures in this energy range is dominated by the easy a-axis component Im chi ^ad. Below the coherence temperature of 20 K, two dynamic antiferromagnetic correlations develop as excitation peaks at h(cross) omega =2 and 4 meV. The 4 meV excitation appears at Q=(Qa. 1/2 +n, Qc) where Qc and Qc are arbitrary and n is an integer, which indicates that the correlation is quasi-one dimensional along the b-axis. The 2 meV excitation appears around Q=(0,0,I) and (0,1,0), which shows three-dimensional Q-dependence. These two excitations reflect the nature of the Kondo coherent state of CeNiSn.

The influence of multiple-scattering contributions in the X-ray absorption fine structure (XAFS) above the iridium L_III edge is discussed for the iridium oxide IrO₂. It is shown that multiple scattering in the IrO₆ cluster cannot explain the occurrence of some features, as previously proposed by comparing with rhenium and tungsten oxide studies. Both the accurate recording of spectra and theoretical calculations lead us to propose that these previously observed features occur from poor signal extraction, especially if the iridium backscattering amplitude is truncated.