Table of contents

It is shown that very near the metal-insulator transition in a three-dimensional system at very low temperatures (T), the inelastic diffusion length (L_i) is reduced and may become the smallest length scale. In this region the conductivity, sigma , follows the law sigma = sigma ₀+mT^1/2 where the T^1/2 dependence arises primarily from a diffusion length L_i approximately sigma ₀³2//T^1/2 which is smaller than the interaction length L_T approximately sigma ¹2//T^1/2 when sigma ₀ is small. The authors present experimental evidence for these results. In particular, for InSb they find m varies as sigma ₀^-3/2 and for compensated GaAs they find negative magnetoresistance near the transition, which yields L_i approximately T^-1/2.

The conductivity of a system of 2D electrons interacting in the presence of impurities in a magnetic field is studied theoretically. It is shown that the current obeys a sum rule similar to the one found in the case of the integer quantum Hall effect. The theoretical results are illustrated with the aid of finite-size computations and their relevance to the fractional quantum Hall effect is discussed.

The formation of the Al/GaAs(110) interface has been studied at room temperature using synchrotron-radiation-excited soft-X-ray photo-emission spectroscopy. It is demonstrated that the Al 2p core-level emission, at low Al coverages, provides evidence for at least three distinct Al phases. These are isolated Al adatoms, Al atoms in surface Ga sites and Al metallic clusters. The last two phases are observed at higher Al coverages and have been extensively studied by other groups. However, the nature of the first phase and the transition from the first to the second phase have been the subject of only a few studies. In this paper it is shown that the change from the first to the second phase, as determined from the Al2p emission, is similar to that observed when Al is deposited on InP(110). Additionally it is argued that the low-coverage Al2p core-level photo-emission spectra, which have been used previously to support the view that the Al adatoms are weakly interacting with the GaAs substrate, can also be interpreted in terms of sub-monolayer Al chemisorption.

Photo-induced electronic and vibrational absorption of polydiacetylene (10H) in the infrared (400-4000 cm^-1) is found to be consistent with photo-generation of bipolarons. Overlap of electronic and vibrational transitions occurs due to strong electron correlation effects which reduce the electronic transition energies. Electron-phonon interaction produces anti-resonances in the bipolaron electronic absorption.

Using the incoherent quasi-elastic neutron scattering (IQNS) technique, the motions of 1-cyanoadamantane C₁₀H₁₅CN are investigated below 170K in the glassy phase obtained by rapid quenching of the room-temperature orientationally disordered phase. The broadening of the experimental spectra is evidence for the existence of rotations of the molecules about their symmetry axes on the timescale 10^-11-10^-9 s. However, from a direct Fourier transform analysis, it is demonstrated that the experimental data cannot be described in terms of any simple jump model involving a single correlation time. In accordance with the conclusions of the studies by NMR and X-ray techniques, an interpretation based upon a distribution of the correlation times is proposed. Finally, the authors discuss whether this time distribution is a characteristic feature of this glassy state.

An electron microscopy study of a new polytype of ZrTe₅ is reported. The space group, determined by convergent beam diffraction, is monoclinic P2₁/m, with lattice parameters a=4.12 AA, b=13.7 AA, c=7.5 AA and beta =111 degrees . This cell is compared with that for the known orthorhombic form (Cmcm, D_2h¹⁷), and a plausible structure is suggested; the polytypic nature of the material (including the third form reported by Okada and co-workers) is addressed. The thermal behaviour of the new monoclinic form has been investigated. A 2c₀ superlattice appears at 70K. This is about the temperature at which Brill and Sambongi have reported an anomaly in the Young's modulus, but well below the temperature of the strong peak in electrical resistivity ( approximately=140K). The effect of the polytypism on the detailed form of the band structure (and hence on the interpretation of these events) remains to be resolved.

The author develops a thermodynamic fluctuation theory for mixtures of finite size ions embedded in a responsive neutralising background of electrons. He characterises the electron gas response through a generalised dielectric function and an associated screening length for which a long-wavelength sum rule is obtained. He shows that, in the case of partially stripped ions, the presence of bound electrons induces an electrostrictive behaviour in the response of the system. The screening length and the correlation length for the ions is also calculated. Furthermore the author obtains long-wavelength expressions for the partial structure factors in the case of rigid as well as responsive backgrounds.

Conditions for a transition from ergodic to non-ergodic dynamics in the scalar lattice phi ⁴-model are investigated. Within the mode-coupling approximation it is found that for sufficiently high strength and anisotropy of the couplings the system exhibits non-ergodic behaviour due to localisation of order parameter fluctuations in a relatively broad region around the ferrodistortive phase transition point. The appearance of precursor clusters and the anomalous narrow central-peak phenomena are explained naturally in terms of dynamic non-ergodicity of local order parameters.

The temperature dependence of the D modes of partially deuterated NH⁺ ions in ammonium sulphate has been investigated with a view to examining the nature of the changes in these ions. It is inferred that the changes in these ions as well as those in the H bonds at T_c are minor and gradual. In contrast, a similar study of the nu ₁(SO₄^2-) mode reveals that the SO₄^2- ion undergoes a significant and sudden change. These observations have been used to estimate the heat of transition, giving the result 4.2+or-0.4 kJ mol^-1; this agrees well with the experimentally observed values ((3.89-4.27)+or-0.02 kJ mol^-1). These inferences support the assertion that a molecular-distortion-type (MD-type) microscopic mechanism is responsible for the transition.

Electronic band structures of intercalation compounds of 3d transition metals with TiS₂ are calculated for the non-magnetic state by a self-consistent augmented-plane-wave method. The energy dispersion and density of states calculated for Fe₁3/TiS₂, FeTiS₂ and CrTiS₂ are discussed in connection with chemical bonding and magnetic ordering.

The authors discuss the calculation of Fermi liquid parameters for the Anderson impurity model. An earlier calculation of de Chatel (1982) is extended to a finite f-f-Coulomb interaction U and corrections of order 1/N_f are studied, where N_f is the degeneracy of the localised level in the Anderson model. The lack of momentum conservation in the impurity model is found to be a complication, since the low-lying excited states are not automatically orthogonal in approximate calculations. In low-order 1/N_f calculations the impurity induced quasi-particle density of states is found to have a singularity at the Fermi energy. The origin of this singularity is traced. The complications in obtaining thermodynamic properties via the calculation of Fermi liquid parameters is contrasted to the accuracy obtained in a 'direct approach', in which the calculations are performed to orders (1/N_f)⁰, (1/N_f)¹ and (1/N_f)².

The authors have used the complete neglect of differential orbital (CNDO) method to investigate the two models proposed by Shi et al. (1984, 85) to account for the two intense Si-H infrared stretching bands in the floating zone melted silicon crystal grown in hydrogen atmosphere (FZ-Si:H). Their results are such that the two models proposed give minimum total energies and the Si-H bond length is found to be about 1.6 AA with the H atom located at the anti-bonding site, consistent with the experimental value determined by Picraux and Vook (1975).

Hydrostatic pressure (up to 16 kbar) has been used to investigate the transport properties of GaInAs-AlInAs heterojunctions for the ranges B<or=18 T and 4.2<T<300K. The authors demonstrate that a parallel-conduction process is the cause of the absence of plateaus in the Hall effect at low pressure. The hydrostatic pressure acts on the Si donor level in AlInAs, reducing the free-electron concentration in this layer and the charge transfer to the quantum well. The conductivity of the AlInAs layer becomes negligible at approximately=7 kbar, and this leads to the first observation of quantum Hall plateaus in the system. A simple model based on the triangular-well approximation is used together with low- and high-magnetic-field measurements to extract the concentration of the bulk layers to conduction over the whole of the ranges of pressure and temperature.

Measurement of the room temperature conductivity and Seebeck coefficient for the magnetoplumbite system BaFe_12-xMn_xO₁₉, 0<or=x<9, shows that Mn substitution increases the p-type conductivity caused by an enhancement of hole concentration, presumably Fe⁴⁺ or Mn⁴⁺ ions. This observation supports the deduction from neutron diffraction data that the Mn in the tetrahedral sites 4f_iv of the spinel blocks is Mn²⁺. Such a formal-valence distribution is shown to be possible because of a lack of local charge neutrality in the structure. The intergrowth of (BaFe₆O₁₁)^2- R blocks and (Fe₆O₈)²⁺ S blocks creates an internal electric field parallel to the c axis that shifts the redox potentials at 4e(1/2) sites relative to those at 4f_iv sites. Measurement of the temperature dependence of the conductivity gives Arrhenius plots with two activation energies; E₁ for T<T_c and E₂ for T>T_c, where T_c is the Curie temperature. E₁ approximately=0.37 eV appears to be a motional enthalpy which is essentially independent of x, but E₂>E₁ at x=0 changes to E₂<E₁ at larger x.

The transferred hyperfine interaction parameters have been measured at 4.2K by ENDOR for five shells of ¹⁹F neighbours around Gd³⁺ in CsCdF₃:Gd and CsCaF₃:Gd. The parameters for the octahedrally coordinated ligands are rather different from those previously determined from the transferred hyperfine structure of EPR spectra at 300K by Buzare and co-workers (1977). The authors' own EPR measurements show that the separations of the transferred hyperfine-structure lines are rather smaller than those predicted using the ENDOR data. They are only just consistent for CsCdF₃, and they are slightly outside the estimated uncertainty of the EPR data for CsCaF₃. There is no evidence of temperature dependence of the separation of the transferred hyperfine-structure lines in the EPR spectrum. Local distortions are deduced from ¹⁹F second-shell parameters and these distortions and the ligand-shell parameters are compared with those for octahedrally coordinated ¹⁹F ligands in CaF₂ and CdF₂.

Structural and magnetic changes with composition in the (Fe_1-xMn_x)₂P system with 0.01<or=x<or=0.85 have been investigated by means of X-ray diffraction and ⁵⁷Fe Mossbauer spectroscopic techniques. (Fe_1-xMn_x)₂P crystallises with the hexagonal Fe₂P-type structure for x<0.24 and x>0.70 and with the orthorhombic Co₂P-type structure for 0.26<x<0.68. Manganese substitutes preferentially for iron at the Fe(2) position in both the Fe₂P and Co₂P structures. Magnetic hyperfine fields, centroid shifts and quadrupole splittings are reported, as well as the results of magnetisation measurements for (Fe_0.97Mn_0.03)₂P at 4.2K in fields up to 12 T.

Since haematite ( alpha -Fe₂O₃) microcrystals are available with well defined shape (sphere, spindle and disc) and size homogeneity, the authors have been able to check the validity of the classical theory of absorption of radiation by small particles in the infrared. The agreement between observed and calculated infrared spectra is very good once particle aggregation is taken into consideration. Comparison between calculated and experimental IR spectra also affords a useful method for determination of the relative orientation of the crystallographic axes in the morphological coordinate frame.

Precipitation of divalent strontium ions in monocrystalline KCl and KBr has been analysed using the Eu²⁺ ion as an optical and a paramagnetic probe. It has been ascertained that different kinds of Sr second-phase precipitates can be formed in the alkali halide matrix depending on the annealing temperature to which the crystals are subjected. From the optical (emission and excitation) and electron paramagnetic resonance data obtained, some characteristics of these precipitates could be inferred. In particular, the annealing of KCl:Sr:Eu at 473K produces the formation of SrCl₂ precipitates, the europium ions being incorporated into these precipitates. The annealing at lower temperatures ( approximately 343K), however, induces the growth of another type of precipitate in which the Sr(Eu) ion occupies a cubic symmetry site with six-fold coordination. The structure of these precipitates could be that of K₄SrCl₆. A similar situation was found in KBr. The data obtained in this host crystal indicates that the Sr precipitates which are formed by annealing at 343K are different to those which nucleate at 473K. However, it appears that, in both cases, the precipitates have a cubic structure; the cubic site occupied by the Sr²⁺ ion in the 343K precipitated phase being of a six-fold coordination, while that in the 473K second-phase precipitate being of an eight-fold coordination. Some tentative proposals for the structures of these Sr precipitates are given.