Table of contents

The superconducting behaviour of highly disordered bulk electronic systems close to the metal-insulator transition (MIT) is considered. It is argued that these systems are frustrated even in the absence of a magnetic field. If disorder fluctuations are larger than some critical value a superconductor glass phase is predicted. Technically the results are based on Thouless-Anderson-Palmer-like mean-field equations that are derived to describe the proposed glass transition.

Critical exponents of magnetization beta below T_N in the weakly ferromagnetic layer compounds MnC_nH_2n+1PO₃.H₂O have been measured by SQUID magnetometry for n=2-4. In all three compounds crossovers are observed in beta as follows ( beta ₁, beta ₂): 0.21(2), 0.73(2) (n=2); 0.18(1), 0.42(6) (n=3), 0.18(1), approximately 0.6 (n=4). The crossover occurs at values of the reduced temperature epsilon =(T_N-T)/T_N that become smaller as the separation between the magnetic layers increases.

The recently discovered giant magneto-impedance (GMI) effect has been measured as a function of circular driving-field frequency and applied tensile stress on two near-zero-magnetostriction amorphous wires. The effect of different orientations of the induced magnetoelastic anisotropy has been verified, for the first time, by using wires with opposite magnetostriction constant, lambda s, signs (Fe_4.9Co_71.8Nb_0.8Si_7.5B₁₅, lambda _S=1.5*10^-7, and Co_68.1Fe_4.4Si_12.5B₁₅, lambda _S=-4*10^-8). GMI ratios up to 300% were found in the magnetically softer (lower-magnetostriction) wire. The frequency dependence of GMI has been found to be strongly influenced by the magnetoelastic anisotropy induced in the amorphous wires. Results are interpreted in terms of changes in the magnetic penetration depth by modifications in the circumferential permeability originated by the action of external agents as field and mechanical stresses. AMI is therefore found to be largely determined by the magnetic domain configuration and relative contributions of both domain wall motions and magnetic moment rotations to the overall magnetization process.

The structure of a common batch of amorphous hydrogenated carbon samples (a-C:H) has been studied in detail using time of flight neutron diffraction, inelastic neutron scattering, NMR and molecular dynamics (MD) simulation. Supplementary work has included differential scanning calorimetry (DSC), infrared (IR) spectroscopy and combustion analysis. A summary of the results is presented as evidence for a new structural model for a-C:H.

A molecular dynamics simulation of the alpha - and beta -phases of cristobalite has been performed. The simulated beta -phase was found to be much more disordered than the alpha -phase, in agreement with evidence from many experimental techniques. The nature of the disorder was found to be dynamic, in agreement with results from many experimental techniques, but no distinct domains were detected. The results are in agreement with the rigid unit mode (RUM) model of framework structures, and recent inelastic measurements.

By applying a phenomenological theory for long-wavelength polar optical oscillations to mesoscopic layered semiconductor structures, we calculate the normal modes of a quantum wire and of a free-standing wire, the cylindrical geometry is adopted with circular cross-section of radius r₀. The displacement field u and the electric potential phi are calculated for the different modes, as well as the dispersion relation curves. The case of the GaAs/AlAs structure is analysed. We limit ourselves to the study of oscillations perpendicular to the wire axis. The electron-phonon interaction Hamiltonian is derived for the present problem using the second-quantization formalism.

We point out the possibility that the coupling between d hole and Jahn-Teller distortion plays an important role for the large T-linear specific heat ( gamma ) of DCNQI-Cu; the Coulomb repulsion between d holes (U) is not important for the enhancement of gamma . On the contrary, U is the origin of the large T² term of the electrical resistivity (A). Yet, gamma and A have a correlation (A alpha gamma ²). We present a picture in which all these facts can be explained without any contradiction. On the basis of the picture we can understand naturally the essential properties of DCNQI-Cu in the metallic phase, including the anomalous behaviour near the metal-insulator transition, although there remain many details to be confirmed with the developing experiments.

The linear muffin-tin orbital (LMTO) method has been used to calculate energy and momentum distributions of valence electrons in diamond, silicon, germanium and grey tin along (100), (110) and (111) directions and as the spherical average over the irreducible wedge of the Brillouin zone. These data can be used for interpreting results of (e,2e) experiments on single-crystal, polycrystalline and amorphous targets.

Band-structure calculations based on the augmented-plane-wave method and the rigid-band model are used to investigate the ferromagnetic 4d transition metals in the constrained crystal structure. The densities of states of 4d transition metals at the Fermi energy are estimated from those calculated for BCC and FCC Rh in paramagnetism. The results show that BCC Ru seems to be ferromagnetic using the Stoner criterion because of the high density of states. The band structure of Ru is studied in detail using the first-principles total-energy-band calculation by the Korringa-Kohn-Rostoker method based on the local-density approximation, whether ferromagnetic BCC Ru exists or not. It is concluded that BCC Ru has a magnetic moment of about 1_{mu B} at 5% expanded lattice constant.

We have carried out an elaborate study of electrical conduction in the generalized Thue-Morse (GTM) lattice. We have studied (i) the Landauer resistance, trace map and localization length of GTM structures, and (ii) the effects of deviations of inter-barrier distances from ideal GTM structures on electrical conduction. Among other things, our results indicate clearly the conditions under which a GTM lattice is likely to be most akin to a periodic system.

The relaxational dynamics of weak itinerant magnets below the Curie temperature is investigated. The calculation is based on the non-linear equations of motion for the magnetic order parameter. It is shown that besides the conventional linear relaxation mechanism due to the Landau damping of spin fluctuations (SFS) which gives rise to overdamped paramagnons, there may exist a different mechanism which is due to non-linear three-mode interactions of spin fluctuations. The interplay between these two mechanisms is analysed and it is argued that three-mode scattering with emission, or absorption, of a longitudinal SF by a magnon may dominate. This results in a new type of overdamped longitudinal SF propagating close to the magnon dispersion, which has probably been observed in recent neutron scattering experiments.

We present a theoretical framework for describing atomic short-range order and its effect upon such quantities as magnetization and hyperfine fields in magnetic alloys. All electronic effects are accurately described from a 'first-principles', density functional formalism, within the restriction of a rigid, uniform lattice. These effects include the filling of the spin polarized electronic states, Fermi surface contributions, and the rearrangement of charge and changes to the magnetization as the chemical composition of the alloy fluctuates. We have calculated the magnetochemical response for bulk Fe₈₇V₁₃ and Cr₇₀Fe₃₀ magnetic alloys to compare to those obtained from spin polarized neutron scattering experiments. We also show the utility of these response functions for investigating the changes in, for example, the moments and hyperfine fields for multilayers with varying textures in the case of FeV.

A detailed study of the magnetic properties of the tetragonal rare earth compound DyRu₂Si₂ is presented. Magnetization measurements on a single crystal as well as specific heat and resistivity measurements on a polycrystal show features which are typical of a long-period commensurate system in the presence of a huge uniaxial anisotropy. Multistep metamagnetism occurs below the Neel temperature T_N=29 K, in particular at low temperatures. In the low-field low-temperature region, a new magnetic phase has been discovered with an anomalous behaviour.

The magnetization of NdCu₂ has been measured from 0.3 to 7.0 K, in fields up to 12 T. The correspondence of the magnetization with a zero-field neutron diffraction study is considered. A model is proposed in order to explain the various magnetic phases observed in the magnetization data.

In this work, an ENDOR analysis of the O₂^- defect in RbI is presented. A complete angular variation of one set of ⁸⁵Rb and ⁸⁷Rb ENDOR transitions is shown. The corresponding hyperfine and nuclear quadrupole coupling tensors are determined. The orientation of the principal axes of these tensors is in good agreement with the overall D_2h symmetry of the O₂^- ion in the crystal. The ENDOR results can only be explained assuming a monovacancy model, wherein the O₂^- molecule is replacing a single I^- ion. A comparison with a previous ENDOR study of the S₂^- centre in RbCl is presented. The linewidth of the EPR signals can be explained using the ENDOR results.

The metastable 304 stainless steel film with BCC structure, fabricated by using a vapour quenching method, is strongly ferromagnetic. The effect of the local environment on the hyperfine parameters, and the influence of the heat treatment and pressure on stability in the metastable phase, were studied by means of conventional and high-pressure ⁵⁷Fe Mossbauer spectroscopy. Each non-iron nearest-neighbour atom (Cr, Ni and Mn) of Fe atoms decreases the hyperfine field by an average of about 2.3 T and the isomer shift by about 0.01 mm s^-1. Moreover, each non-iron next-nearest-neighbour atom is estimated to reduce the hyperfine field by an average of about 1.2 T. It is found that the transformation from the metastable BCC state to the FCC state begins near 500 degrees C and is completed near 800 degrees C. No FCC phase appears under a pressure smaller than 36 kbar. In both cases, however, the magnetization direction of the sample is observed to reorientate, and shows a stronger tendency to the out-of-plane after annealing and under pressures up to 8 kbar, while it is closer to the in-plane under pressures larger than 8 kbar.

Mossbauer spectra of NH₄Co_0.994Fe_0.006Cl₃ have been recorded in the temperature range 1.3 to 250 K. The spectra are similar to those of isostructural CsCo_0.99Fe_0.01Cl₃ which is a quasi-one-dimensional Ising-like antiferromagnet. For NH₄Co_0.994Fe_0.006Cl₃ the transition to a partially disordered magnetic state occurs at T_N1=(27+or-2) K and to the fully magnetically ordered state at T_N1=(11+or-2) K. As for CsCo_0.99Fe_0.01Cl₃ the relaxation processes evident in the spectra were analysed using the stochastic model of Blume and Tjon with electronic relaxation occurring for Fe²⁺ in the three-dimensionally ordered chains below T_N1 and magnetic field reversal, attributed to solitons, in the one-dimensionally ordered chains in the partially disordered phase. Models developed for CsCo_0.99Fe_0.01Cl₃ were extended to include relaxation involving all the Fe²⁺ occupied electronic levels below T_N1 (i.e. the three lowest levels) and to allow both electronic and soliton relaxation to occur in the one-dimensionally ordered chains. The data for NH₄Co_0.994Fe_0.006Cl₃ and previous data for CsCo_0.99Fe_0.01Cl₃ were both analysed with these extended models. Relaxation rates were lower than those predicted theoretically for pure CsCoCl₃. This may be due to the presence of iron in the cobalt chains changing the soliton dynamics.

We have re-examined the second-order Raman spectra in a thick AlAs layer grown on a GaAs substrate by polarized Raman spectroscopy. So as to reproduce the spectra, the force constants have been refined and then the phonon dispersion has been calculated within a modified version of the adiabatic bond-charge model. We present phonon frequencies at critical points, e.g. Gamma , X, L and W, and the force constants for AlAs. The phonon dispersion is compared with a previous ab initio result and it is shown that the procedure presented herein is useful. A brief discussion is also given of the analysis of the second-order Raman spectra of AlSb.