Table of contents

The covalent interaction between localized transition metal d states and extended metalloid sp states, which plays an important role in the formation of alloys of transition metals with simple metals or metalloids, is considered from the point of view of the Anderson impurity model. The authors demonstrate how by means of concepts derived from this impurity model a basic understanding of the electronic structure of transition metal-metalloid alloys can be obtained. In addition, the experimental investigation of the electronic structure of such compounds by means of high-energy spectroscopies is discussed. In particular, they consider the interpretation of experimental results from photoelectron spectroscopy and X-ray absorption spectroscopy.

An unusual feature of the mixed-valance compounds alpha -Fe₂(PO₄)O is the presence of zig-zag chains of face-sharing octahedra running parallel to (010) and containing, alternately, Fe²⁺ and Fe³⁺ ions. These chains form sheets with the composition (FeO₂)_n and, because of the presence of face-sharing octahedra, have a very short Fe²⁺-Fe³⁺ distance of 2.92 AA. The magnetic properties of alpha -Fe₂(PO₄)O have been studied by powder neutron diffraction between 4.2 and 280 K and by the Mossbauer effect between 4.2 and 295 K. Below the Neel temperature of 218(3) K, the neutron diffraction data indicate a magnetostriction predominantly along the c-axis, magnetic moments at 4.2 K of 3.82 (4) mu _B for Fe²⁺ and 4.26(5) mu _B for Fe³⁺ oriented parallel to (010), and the presence of ferromagnetic sheets, containing the zig-zag chains, which are coupled into an overall antiferromagnetic structure. The net antiferromagnetic coupling is the result of a dominant 134 degrees Fe³⁺-0(1)-Fe³⁺ intersheet superexchange pathway. Ferromagnetic coupling dominates within the sheets and, as a result, the entire magnetic structure avoids any spin frustration. Above 220 K, the Mossbauer effects spectra are paramagnetic and show two quadrupole doublets with hyperfine parameters corresponding to the discrete Fe²⁺ and Fe³⁺ valence states. Below 220 K, the Mossbauer effect spectra show two magnetic components with hyperfine parameters characteristic of discrete Fe²⁺ and Fe³⁺ ions and in agreement with the corresponding paramagnetic values and the magnetic structure.

The total energy profile or surface for Al-H complexes in a large silicon cluster has been calculated using a self-consistent semi-empirical method. The diffusion path of hydrogen appears to be confined to a low-energy channel in the vicinity of the Si atom which is bonded directly to the Al atom at a substitutional site. The most stable configuration of this Al-H system results when the H atom is located at the bond-centred (BC) site. The system probably contains a Si-H-Al three-centre bond structure of bridging nature. This finding is compared with the Al-H pair assignment based on infrared experiments given by Stavola and co-workers which is based on the Al-H frequency calculated by DeLeo and Fowler.

The authors present measurements of the elastic constants on a single crystal of the shape-memory alloy NiTi in a wide temperature range. Step-like anomalies and hysteresis are detected at the austenitic-martensitic phase transition which provide evidence for the strain-order parameter coupling in the pre-martensitic phases. The results are discussed in the framework of existing Landau theory models.

First-principles total energy calculations are used to derive the anharmonic coupling parameters for the phonon-phonon interactions in molybdenum. The calculated frequency shifts from 10 to 295 K are in reasonably good agreement with experiment. The derived anharmonic forces are analysed in terms of a central potential and bond-bending terms which are found to be significant.

The soft X-ray L₂₃ emission spectra for various hydrogenated amorphous silicon (ASIL) and crystalline silicon samples have been analysed. The significant differences in the valence band structure for p-type compared with n- and i-type ASIL films are ascribed to greater structural disorder in the former. The presence of apparent high densities of states in the ASIL energy gap region is ascribed to cross transitions from nearest-neighbour hydrogen states to shifted core states of silicon. Both B and P dopant atoms have been directly detected from their emission spectra. Comparison of results for ASIL films made with 1% and 20% phosphine-in-silane show a gas atom incorporation ratio that is seven times higher for the 20% film.

Cluster generalization of the CPA method, based on the variational calculation of the alloy thermodynamic potential, is considered. Various possible choices of the variational parameters are discussed. One of them leads to an approximation that is analogous to the Bethe approximation for the Ising model. The technique developed is applied to the calculation of the density of states in the Ni_1-cCu_c alloy by the LMTO-ASA method.

A detailed experimental investigation of the temperature dependence of the resonance anomaly on the dispersion curves of optical conductivity has been carried out on single crystals of Gd, Tb and Dy. This dependence is related to electron transitions between valence states split by the exchange field of the 4f shell. Over the entire temperature range of existence of the optical conductivity peak, its position on the energy scale is stable. Magnetic phase transitions of the ferromagnet-spiral type have not brought about any noticeable changes of peak parameters. The theoretical interpretation of the properties of the observed anomaly is based on direct calculations of the electronic structure and interband density of states for ferromagnetic and spiral configurations of magnetic moments. It is shown that, in the electronic spectrum of rare-earth metals, there is a region that remains relatively stable in the course of changes of the magnetic configuration and is responsible for the stability of the energy localization of the investigated peak.

The possibility of studying impurity passivation complexes in semiconductors by quadrupole resonance spectroscopy is examined. The problem is illustrated for the case of boron in silicon passivated with hydrogen or, equivalently, with muonium, since the radioactive light isotope in principle offers a greater sensitivity for detection of the spectra. Ab initio calculations on suitable cluster models of the passivation complexes provide estimates of the electric field gradients at the quadrupolar nuclei, and thereby predictions of the quadrupole resonance frequencies. Detection via cross-relaxation techniques is proposed, notably muon level crossing resonance ( mu LCR), and illustrated by calculation of the time dependence of the muon polarization function. Possible reasons for the absence of quadrupolar resonances in mu LCR spectra recorded in exploratory experiments are discussed: these include the existence of a local tunnelling mode for the lighter isotope.

The numerical solution of the Bethe ansatz equations of the n-channel Kondo problem is presented. The thermodynamics of the impurity is obtained as a function of temperature, external field, impurity spin S and the number of channels. Three situations have to be distinguished: (i) If n=2S the conduction electrons exactly compensate the impurity spin into a singlet at low temperatures, (ii) if n<2S the impurity spin is only partially compensated (undercompensated), and (iii) if n>2S the impurity spin is said to be overcompensated giving rise to critical behaviour. The results are briefly discussed in the context of magnetic impurities, the quadrupolar Kondo effect, an impurity spin embedded in the Takhtajan-Babujian model and a two-level system interacting with conduction electrons.

Results are presented for phonons in a 1D diatomic model superlattice. Each frequency omega corresponds to two Bloch wavenumbers Q₁ and Q₂. For alpha <0.25, where alpha is the ratio of next-nearest to nearest force constants, the results are essentially a perturbation of those found with nearest-neighbour forces only. For alpha >0.25, however, qualitatively new results are found, and in particular both Q₁ and Q₂ can be real over some frequency range. Illustrative dispersion curves are presented.

Large time-independent conduction fluctuations were observed as a function of transverse electric field in thin (25 nm) and narrow (60 nm) bismuth wires. The conduction of leads far away from a gate capacitor was influenced by changes in the gate voltage. The effects are interpreted as being due to a variation in the Fermi wavelength caused by gate-induced changes in the charge concentration of the leads rather than an electrostatic Aharonov-Bohm-type interference. The screening of charge is strongly reduced in narrow wires.

The vibrational motion of Fe atoms in Bi₂Sr₂Ca₁Cu_1.96Fe_0.04O_y studied by means of Mossbauer spectroscopy was found to be very anharmonic. This anharmonic behaviour can be explained by local structural excitations between two or more states supposedly existing in the material rather than by the usual phonon-phonon interactions. The increase in T_c for the annealed Bi₂Sr₂Ca₁Cu_1.96Fe_0.04O_y is attributed to the increase in local structural instabilities which enhances the effective electron-electron pairing.

The extended Hubbard model with nearest neighbour inter-site correlations has been studied by mapping it onto a spin-Hamiltonian. In the case of a half-filled band the problem is equivalent to that of an anisotropic Heisenberg model in the absence of a field. The phases in the U-Q plane obtained here are in good agreement with those obtained by Monte Carlo simulation. The antiferromagnetic XY-ordered phase of the model is shown to be very similar to an RVB phase. Extending the analogy away from half-filling, a possible dependence of the critical temperature T_c with the doping concentration delta in a high-T_c system is suggested.

Magnetic phase diagrams of disordered Heisenberg antiferromagnets with mixed uniaxial anisotropy are calculated within a local molecular field theory. The phase diagrams numerically obtained for sets of parameters relevant for the system Fe_1-xNi_xCl₂ are in very good agreement with recent experimental results. It is demonstrated that by changing the concentration x, or the ratio of the anisotropies, a rich variety in the physical behaviour of these systems can be expected.

Detailed X-band EPR studies of the Gd³⁺ ion have been carried out in a europium ammonium sulphate single crystal between liquid-helium and room temperature. The behaviours of the overall splitting of EPR lines and the EPR linewidth indicate the occurrences of two first-order phase transitions at 273+or-0.5 and 166+or-0.5 K, respectively. The spin-Hamiltonian parameters for Gd³⁺ at room temperature have been evaluated. The intrinsic parameter b₂ for Gd³⁺ in NH₄Eu(SO₄)₂.4H₂O crystal has been calculated by the use of the superposition model of Newman (1975), and found to be -5.802 GHz, with the exponent t₂=9.0.

The influence of applied static electric fields on the SrTiO₃:V⁴⁺ Jahn-Teller system has been studied by means of EPR at liquid helium temperatures. The experimental results can be accounted for by considering a ²T_2g electronic ground state showing a strong Jahn-Teller interaction with a localized e_g mode. In the presence of electric fields, the degeneracy of the vibronic ground state is lifted appreciably by strain. This strain results from strong electrostrictive couplings in SrTiO₃.

The authors have investigated the mixed crystal system Rb₂Mn_xCr_1-xCl₄ with competing exchange interactions and competing anisotropies over the whole concentration range x by means of magnetic resonance in the millimetre wave range. The measurements have been performed for several configurations of the external magnetic field. The analysis of their experimental data is based on calculations with a modified average or virtual crystal model. The results of their investigation confirm for high Mn^2- (0.86<or=x<or=1.0) and for high Cr²⁺ concentrations (0<or=x<or=0.13) the antiferromagnetic and ferromagnetic order, respectively, as observed by neutron diffraction. The essentially new result of their study is that their experimental results and the model data based upon them also provide information about the intermediate-concentration range where the mixed crystals exhibit pronounced disorder phenomena and spin-glass behaviour.

The response function of some simple fractal circuit systems are discussed. In addition to the ubiquitous constant phase angle response (CPA), a second behaviour class is identified and its dynamic scaling equation determined. The conditions under which each of the invariant behaviour classes occur have been derived. Computed response functions are used to illustrate the formal theoretical results. An outline is also given of the relationship of the circuit systems to experimental dielectric responses for which both behaviour classes can be observed.

The authors propose a new choice of the reference bridge functions to be employed in the modified hypernetted-chain (MHNC) approximation for determining the structure of a liquid alloy. The reference system they choose is the one interacting with the repulsive part of the interatomic potentials. The new scheme is applied to a liquid Na_0.5K_0.5 alloy and it is shown that the calculated radial distribution functions are in excellent agreement with those of the molecular dynamics simulation. The results are also compared with those of the HNC and PY approximations as well as those of the MHNC scheme in which the bridge functions of the additive hard-sphere mixture are used.

Adsorption phase transitions between distinct partial wetting states (or 'thin-thick' transitions at bulk two-phase coexistence) are studied systematically in the phenomenological Landau theory for Ising models, as well as in the Sullivan model, a more microscopic Van der Waals theory for fluids. In the Landau theory the previously proposed mechanism of competing surface fields is examined in detail and confirmed. In the generalized Sullivan model with combined exponential and square-well wall-fluid potentials thin-thick transitions are found already if the potential is purely square-well. A mapping of the Van der Waals to the Landau theory suggests that the mechanism of competing surface fields can largely explain the new phase transitions.

The low-temperature RF penetration profile is obtained for the particular case of impurity NMR probes diffused into the surface of a metallic host specimen. Conditions under which an exponential RF profile description remains appropriate are presented and the significance of the results for a previous analysis of single-passage NMRON data for ¹²⁵SbFe is assessed.

The anisotropic ionic diffusion coefficients in model electrochemical cells in the molten-salt regime for the electrolyte are evaluated from the ionic density profiles reported in the simulation work of Grout and co-workers. A local description of the diffusion processes for counter-ions and co-ions in the electrical double layer is obtained from the data.

The authors calculate the effects of introducing muonium (a positive muon plus one electron) into trans-polyacetylene and cis-polyacetylene. They find that the energetically favoured site for the muon produces an sp³ hybridized carbon atom and an associated spin distribution. The spin is bound to the muon in cis-polyacetylene but is free to propagate in trans-polyacetylene. The presence of the muon acts as a barrier to the motion of mobile electronic defects.

Brillouin spectroscopy in amorphous media for different angles offers a direct way to determine the difference between changes in dielectric constant along and perpendicular to induced strain. Here the method is applied to quartz glass and supercooled liquid o-terphenyl. It is found for liquid o-terphenyl that the change in dielectric constant along the induced displacement is smaller than the change in dielectric constant perpendicular to the induced displacement, as it is also in quartz.