Table of contents

X-ray diffraction measurements were carried out for amorphous alloys Fe_1-xTi_x with x from 0.2 to 0.75. The radial distribution function, g(r), shows a clear first peak and a shoulder beyond the second peak. The g(r) is insensitive to the alloy concentration, which is explained by a simple model based on the spiral tetrahedral stacking. No chemical short-range order has been detected.

Experimental results on neutron quasi-elastic diffuse scattering in the plastic phase of 99% deuterated succinonitrile are reported. Very strong correlations of steric origin occur between orientationally disordered molecules. Their influence on the diffuse scattering is analysed by a mean-field treatment of the hard-core repulsions.

Based on an improved semi-empirical effective Hamiltonian method, computed electronic structures of clusters and linear chain crystals are reported. The ionization energies of clusters as a function of cluster size are examined to find correlation with bulk values as the cluster size further increases. The improved method is shown to remedy some failures of conventional semi-empirical methods. Some of the salient features from the present study are the improvement of the first ionization energies, the satisfactory prediction of the work function, a newly found linear relationship between the ionization energies and the inverse of the cluster size, 1/n, for the molecular clusters of (H₂O)_n. In addition, the predicted band structures for the one-dimensional organic crystals and ice are discussed.

Infinite polyacenacene has been studied by the same method as applied by Salem and Longuet-Higgins (1960) to infinite polyacetylene and polyacene. Polyacenacene can be considered as three polyacetylene chains joined by cross links. It is predicted that the most stable configuration is the equilibrium configuration exhibiting glide plane symmetry, in which the C-C bonds of three chains all have the same length, being slightly shorter than the cross bonds. This stable configuration of equilibrium has zero gap and zero slope at the Fermi level.

Resistivity measurements carried out in the temperature range 80 mK to 1 K on silicon bolometers-phosphorus doped by ion implantation-at concentrations near the metal-insulator transition, exhibit variable range hopping (VRH) conduction in the whole observed temperature range, with a T^-1/2 dependence, in accordance with the Coulomb interaction model for low-temperature conductivity in disordered systems. Samples which apparently show a different behaviour, intermediate between VRH and metallic conduction, can be modeled by a metallic resistance in parallel with an active layer which follows the classic exp(T₀/T)^1/2 law. The observed behaviour can be explained in terms of residual radiation damage induced by the ion implantation process.

Adiabatic potential energy surfaces (APESs) for self-trapped excitons (STEs) (incipient Frenkel pairs) in alpha -quartz and alpha -alumina are deduced from experimental temperature dependences of the decay constants for transient volume change, optical absorption and luminescence, associated with the lowest STE state, as induced by an electron pulse irradiation. APES profiles along some effective 'reactive coordinate' are obtained by processing the temperature data by means of the reaction rate method. The resulting profiles suggest that STEs and defect pairs (DPs) in alpha -quartz, each incorporating an interstitial oxygen (O_i^-) and a singly charged oxygen vacancy (E'₁ centre), are likely to form as two energetically non-equivalent pseudo-Jahn-Teller distortions of the quasi-symmetry of the saddle-point configuration between STEs and DPs. The APES for alpha -alumina appears to be inherently different from that for alpha -quartz. Now the pseudo-Jahn-Teller conjecture is not likely to apply immediately, owing to a very small crossover splitting. Implications for the defect formation process in quartz will be discussed.

Measurements have been made of the electrical resistivity rho , thermal expansivity Delta L/L and elastic modulus E of gamma -Fe-Mn alloys containing 0-8.73 at.%. Ge. Anomalies in rho , Delta L/L and E are observed in the vicinity of the Neel temperature T_N for all these alloys. The effect of the Ge content on the anomalous behaviour of Fe-Mn alloy is essentially similar to those of the Al or Si content. The experimental results are qualitatively discussed on the basis of two postulates, i.e. firstly that the valence electron band of Ge overlaps the d band of the matrix so as to induce or enhance the localized moment on the Fe atom sites and secondly that the increase in the Ge content weakens the itinerant electron character of antiferromagnetic gamma -Fe-Mn alloys.

A possible explanation for the electroconductivity activation energy temperature dependence observed in transition metal oxides is proposed. The activation energy according to the authors' approach is determined by the mobility edge position in the conduction (valence) band. The dependence of the mobility edge on the magnetic order is obtained. Hence the magnetic order temperature dependence determines the activation energy temperature dependence.

By use of angle-resolved inverse photoemission spectroscopy (ARIPES) in the isochromate mode using 9.88 eV photon energy the authors have studied the unoccupied electronic structure of 1T-VSe₂ along the principal symmetry directions Gamma M, Gamma M' and Gamma K. The data are found to be in good agreement with the recent ARIPES study of Law et al (1990) employing 20 eV photon energy. The experimental bands are reasonably well described by band-structure calculations but viewed together with the 20 eV data appear to be less dispersive perpendicular to the surface than theoretically predicted. A structure at 5 eV above E_F for normal incidence can be identified as an image potential state.

The Ising ferromagnet with fluctuating exchange integrals has been investigated using the two-spin cluster approximation. The phase diagrams were obtained for both two- and three-dimensional systems. They show a re-entrant behaviour only for a square lattice. It is argued that this behaviour is expected in disordered systems where one has competition between ferromagnetic and antiferromagnetic interactions in a certain range of the concentration of the bond mixture.

An inhomogeneous Heisenberg spin Hamiltonian with single-ion anisotropy is used to investigate the non-linear excitations in a ferromagnetic chain. By means of the Holstein-Primakoff transformation and Glauber's coherent-state representation, the equation of motion for the annihilation operator a(j) is reduced to a non-linear Schrodinger-like equation with variable coefficients. Its non-linear modified terms are strongly restricted by the relation between the continuum approximation ( eta =a/ lambda ₀, where eta is the 'degree' of the long wavelength, a is the lattice constant and lambda ₀ the characteristic wavelength of the excitation) and the semiclassical approximation ( in =1/ square root S, where in is the degree of truncation of the operator expansion and S is the spin length). When assuming that eta =O( in ) and after retaining the terms to O( in ⁴), the motion of the coherent amplitude for the homogeneous case satisfies the non-linear Schrodinger equation. The single-soliton and two-soliton bound-state solutions are given. The results show that the magnon localization and two-magnon bound state are possible in the chain. Other relations between eta and in ( eta =O( in ^1/2), eta =O( in ^3/2 and eta =O( in ²)) are also discussed.

Representative first-order metamagnetic transitions with a large hysteresis have been observed for x<or=0.06. The transition field H_c for x=0.06 is 220 kOe at 4.2 K. Simple but strong ferromagnetism is found for x>0.12. A mixing state of ferromagnetic and paramagnetic states of Co is assumed for 0.06<or=x<or=0.12. The paramagnetic component in the state also exhibits the metamagnetic transition. By surveying the temperature dependence of metamagnetism, a boundary temperature of the first- and second-order metamagnetic transitions, and a first-order magnetic transition in thermomagnetic curves are found for the first time. In relation to the magnetic phase transitions the critical parameters T₁ and H₁ are introduced and discussed. Finally a magnetic phase diagram is presented.

The author attempts to determine how much of the Ising ground state there is in the ground state of an Heisenberg antiferromagnetic. Square planar clusters of 5, 13 and 25 spins of ¹/₂ are considered, using approximate ground states chosen as a result of symmetry arguments. Various physical quantities, including bond energies which are more negative than those found with the corresponding Ising states, then become relatively easy to evaluate. The author concludes that the chosen states are better approximations than the Ising states. It seems that as the number of spins increases the probability of finding the Ising state tends to zero, while remaining the most probable state, in a site representation.

The temperature dependence of the Mossbauer spectra of the iron(III) trisdithiocarbamate complexes, Fe(R₂dtc)₃, where R represents N,N'-disubstitution with methyl, ethyl, isopropyl, benzyl, n-hexyl and cyclohexyl groups, has been measured from 1.3 or 4.2 K to room temperature. A consideration of these spectra in terms of spin-state relaxation indicates that the hyperfine parameters for the high-spin and low-spin states at equilibrium in some of these complexes must be quite similar. The Mossbauer absorption area shows a strong temperature dependence, which has been analysed with the Debye model. At lower temperatures the spectra show broad asymmetric linewidths, which result from the onset of slow paramagnetic relaxation on the Mossbauer timescale. An analysis of the observed Mossbauer spectral lineshape with either a Blume or Bradford and Marshall relaxation profile indicates that the paramagnetic relaxation is almost isotropic in Fe((methyl)₂dtc)₃, has a larger transverse than longitudinal component in Fe((benzyl)₂dtc)₃ and is predominantly transverse in Fe((ethyl)₂dtc)₃ and Fe((cyclohexyl)₂dtc)₃. Below 90, 40, 25 and 15 K for the methyl, ethyl, benzyl and cyclohexyl derivatives, respectively, the paramagnetic relaxation rate is independent of temperature and dominated by a spin-spin relaxation mechanism. Above these temperatures the paramagnetic relaxation is dominated by a two-phonon spin-lattice relaxation mechanism.

The room-temperature pressure dependence of the Mossbauer spectra of the iron(III) trisdithiocarbamate complexes, Fe(R₂dtc)₃, where R represents the N,N'-disubstitution with methyl, ethyl, benzyl, n-hexyl and cyclohexyl groups, has been measured between zero and ca. 80 kbar. As expected, the Mossbauer-effect isomer shift decreases slightly with increasing pressure, whereas the quadrupole splitting, the linewidth and the recoil-free fraction increase with increasing pressure. The pressure dependence of the Mossbauer absorption area is modelled by using the Debye theory of lattice vibrations, this analysis being consistent with that of the temperature dependence of the Mossbauer absorption area and also with the mean square displacements of the iron(III) ions, as determined from single-crystal X-ray thermal parameters. The Mossbauer spectra do not show any changes with pressure that can be associated exclusively with changes in the electronic spin state with increasing pressure. The authors conclude that these compounds are completely low-spin even at small applied pressures of a few kilobars. Experimental techniques required to obtain reproducible Mossbauer spectra at high pressure, through the use of an opposed diamond-anvil cell, are critically evaluated.

Electron paramagnetic resonance spectroscopy of Mn-doped Bi₄Ge₃O₁₂ single crystals has been studied at room temperature in the X band. The spectra obtained have been attributed to Mn²⁺ ions in the Bi³⁺ site without local charge compensation having trigonal symmetry. Moreover, the spin-Hamiltonian parameters have been interpreted by means of the superposition model in order to obtain information about the relaxation of the lattice in the neighbourhood of the impurity.

A theoretical approach providing the shape of the spectrum of dipolar fluctuations (Fourier transform of the longitudinal component of magnetization) and shape of the wings of NMR absorption in condensed matter has been elaborated. It is shown that the spectral contours are exponentials with the same decay constant. The results obtained explain for the first time the known experimental data and establish a connection between two groups of different experiments. The theoretical prediction is in good agreement with experimental results.

A Mossbauer effect measurement of ⁵⁷Fe has been performed for the Fe-N austenite in order to study the interaction between nitrogen atoms and the distribution among the octahedral sites of the FCC lattice. The spectrum for Fe-N austenite is decomposed into one singlet gamma ₀ and two sets of doublets gamma ₁ and gamma ₂ which are identified as being caused by iron atoms with different configurations of nitrogen atoms at the nearest-neighbour sites. The fractional intensities for the three components are precisely determined by taking into account the effect of the absorber thickness. It is concluded from the analysis that the interaction between the first-nearest-neighbour nitrogen atoms is strongly repulsive and that between the second nearest nitrogen is weakly attractive.

Dielectric relaxation measurements were carried out on three isomers 2,3'-dicyanodiphenyl, 3,3'-dicyanodiphenyl and 4-cyanodiphenyl in p-xylene in the temperature range 293-323 K for static frequencies up to 10 GHz. The analysis of the dielectric dispersion of these compounds was carried out and the mechanism of the internal rotation for these molecules has been proposed.

The temperature dependence of positron trapping at voids in molybdenum has been carefully studied from 7 to 360 K. The trapping was studied before and after decorating the voids with hydrogen or nitrogen. The influence of positron-diffusion-limited trapping was extracted and found to be independent of void decoration, as expected. The voids were also studied by transmission electron microscopy and the results confirm that 4 pi rnD₊ is the correct term for describing the diffusion-limited trapping rate at spherical defects.

The EMF of the liquid Na-Bi system has been measured as a function of temperature and pressure in detail by the method of an electrochemical cell using beta -alumina as the electrolyte. The authors have obtained the associated thermodynamic quantities of mixing such as the free Gibbs energy Delta G, entropy Delta S, enthalpy Delta H, excess specific heat Delta C_p and excess volume Delta V. The isothermal compressibility chi _T and the concentration-concentration fluctuation S_cc(0) in the long-wavelength limit are obtained. From these, a strong short-range order was found at the composition of Na₃Bi and a rather weak short-range order was found around the composition of NaBi.

A series of experiments on liquid gallium is briefly reviewed. A new approach to the behaviour of liquid gallium during the crystallization is explained and a model of kinetics of heterogeneous nucleation is presented. The experimental results obtained by use of the simple thermal analysis have fully confirmed this model.

The Knight shifts of As and Ge in liquid Ge have been determined by TDPAD (time-differential perturbed angular distribution) measurements over a temperature range of 1210-1370 K, using the isomeric nuclei ^73mAs and ^69mGe as probe atoms. The Knight shifts have also been measured in liquid Ga (295-1175 K), Ga_0.33Ge_0.66 (1010-1210 K), Ge_0.66Pb_0.33 (1170-1215 K) and GaSb (975-1135 K). For both As and Ge the shifts are essentially metallic in these matrices. However, even in definitely metallic conditions for As, the shift is only about ²/₃ of that of Ge. Non-metallic references for the Knight shift of ^73mAs have been established in liquid GaI₃ (515 K) and solid GaAs (1440-1510 K); for Ge reliable reference data were available.

The authors propose a very simple method of calculating short-range order parameters alpha _ij in disordered alloys using a series expansion in powers of a parameter gamma =exp(-1/ xi ) where xi is the dimensionless correlation length of the pair correlation function. In authors' approximation the sum rule alpha _ii=1 is satisfied exactly, unlike in previous theories. In the zeroth order their approach leads to the spherical model results. The high accuracy of the theory developed is illustrated by comparing its results with those of Monte Carlo simulation and estimating the pair interactions from diffuse scattering data. In the latter problem pair-interaction potentials are explicitly expressed in terms of experimentally determined quantities.

It is shown that in the presence of weak quantum mechanical tunnelling, an orthogonality catastrophe type of effect is found in Coulomb glass systems in 3D and 2D, under the application of a long-ranged 1/r type potential. Some physical implications of the effect are discussed.

The previously accepted spatially modulated magnetic structure of CeAl₂ is shown to be incorrect. Neutron diffraction measurements in an applied magnetic field imply that the zero-field magnetic structure is spiral: however, the moments on the two Ce sublattices rotate in opposite senses. In the author's model, the magnitude of the Ce ordered moment varies little from site to site; a non-magnetic ground state for the Ce³⁺ ion is no longer required, and the extreme weakness of the harmonics is explained.

The authors investigate the electronic screening at semiconductor heterojunctions, and through calculations over selected systems they show that local-field effects are capable of affecting the band offset by some tenths of an eV. Their results indicate the limits of accuracy attainable with models that emphasize the line up of some reference level and implicitly use only macroscopic screening.

The intrinsic stacking fault in the (111) plane of SiC, which is closed by two Shockley partial dislocations, has been observed by high-resolution electron microscopy. The energy of the intrinsic stacking-fault of SiC is experimentally determined and compared with the theoretical value calculated by Denteneer et al. (1987).