Table of contents

The thermal conductivity of a diacetylene monomer has been measured over the temperature range 2.8-25 K. The data are found to have a temperature dependence and magnitude very close to those previously obtained for the polymer form of the diacetylene. It is shown that the data support the model of acoustic phonon scattering by low energy optical modes associated with the side groups. The optical mode energies have been determined by far-infrared spectroscopy and it is shown that by using these values to model the data a good fit is obtained with each mode having an acoustic-optical phonon coupling of 6.3 meV.

A model of coherently diffracting domains with sinusoidal composition modulation waves that have a different wavelength in each domain is presented. Formulae describing the summed X-ray intensities diffracted independently by each domain are derived in the kinematical case. The model is experimentally verified for the alloy Alnico, in which a gaussian distribution of the modulation wavelength is assumed. The broadening of the satellite reflections in relation to the nodal reflection and the nonlinear dependence of the half-width of the satellite reflection on its order are shown for this case.

Damping and modulus measurements were made on high-purity, polycrystalline silver during 1.5 MeV electron irradiations in the temperature range 310 K to 590 K. A suggested interpretation of the pinning rate data gave a value of 0.24+or-0.01 eV for the difference in interstitial migration energies in the lattice and the dislocation line and an approximate value of 0.25 eV for the binding energy of a vacancy to the dislocation line. An initial increase in the decrement (peaking effect) at the onset of radiation was observed at different amplitudes and at temperatures as high as 590 K.

The authors devise a simple theory for the temperature dependence of the valence and conduction band offsets in semiconductor heterojunctions using the thermodynamic point of view. The temperature dependencies of the offsets originate from the separate contributions of holes in the valence bands and of electrons in the conduction bands of the two semiconductors to the temperature dependence of their respective band gaps. They use the earlier determination of these contributions by Heine and Henry (1975), from isotope shifts of luminescent lines due to impurities. By considering this temperature dependence of the band offset they suggest an explanation for the discrepancy between the determinations of AlAs-GaAs valence band offset by Wolford et al. (1986), and by Batey and Wright (1986). Whereas for most pairs of semiconductors the bands move in the same direction with varying temperature, for the particular case of HgTe-CdTe they move in opposite directions. From this they predict a much greater than usual temperature dependence for the band offsets for HgTe-CdTe junctions and reconcile the major discrepancy between valence band offsets determined by Kowalczyk et al. (1986), and by Chow et al. (1988).

A comparison between two of the so-called universal equations of state for solids is made. It is shown that although the Vinet equation and the Prieto-Renero equation are obtained on very different bases, their general form is very similar. Furthermore, the coefficients of the temperature dependent terms in the two equations are identical. This leads to a new relationship between the isothermal bulk modulus and other measurable parameters of the material. Numerical results obtained using both equations are shown to be in good agreement with some reported experimental data for gold, and for sodium chloride. The meaning of universality is different from one equation to the other; however, both of these meanings have been already accepted and used in relation to the scope of validity of equations of state. It is concluded that the use of one or the other equation depends more on the availability of the physical parameters needed than on the reliability of the equation itself.

Room-temperature isotherms of titanium in HCP and omega -phases are calculated by the first-principles linear muffin-tin orbital energy band method. Comparison with experimental data shows excellent agreement. Structural phase stability analysis by the Andersen force theorem shows that the omega -phase is the lowest-energy phase at 0 K and normal volume. The possibility that the s to d electronic transition is the cause of shock discontinuity at 17.5 GPa.

The authors use a local scaling approach to calculate the following properties near the Anderson transition: (i) the time-dependent pulse shape of the transmitted wave through a slab; (ii) the wavelength dependence of the intensity-intensity autocorrelation function C( Delta lambda ); (iii) the time dependence of the intensity-intensity autocorrelation function C( Delta t) for dynamic disorder; (iv) the correlation function for the memory effect. Their local scaling approach is shown to be consistent with Anderson's global scaling theory and yields the same scaling behaviour for the transmission coefficient. All the correlation functions are shown to depend explicitly on the averaged intensity pulse shape for small values of Delta lambda or Delta t.

Discommensurations have been studied by transmission electron microscopy in the ferroelectric commensurate phase of K₂ZnCl₄. Characteristic patterns show regularly arranged pairs of discommensurations and vortices where three pairs or six isolated discommensurations terminate. The results point to an attractive interaction between isolated discommensurations and to a repulsive interaction between pairs of discommensurations.

Non-self-consistent calculations of total energies using the Hohenberg-Kohn-Sham functional and the approximate functional of Harris have been made for a three-layer (111) slab of aluminium and compared to fully self-consistent calculations. The norm-conserving pseudopotential of Bachelet, Hamann and Schluter was used. The Harris total energy for the slab, which is calculated from an input charge density constructed by superimposing free atomic charge densities, is closer to the self-consistent energy for reasons which are discussed in detail. Nevertheless, the corresponding Harris surface energies are not sufficiently accurate to be useful. By renormalising the atomic charge densities which are superimposed to form the input charge density, in particular by pushing charge from the tails towards the cores, the surface energy from a self-consistent calculation can be accurately reproduced by the Harris functional. Furthermore the Harris functional with the same renormalised atoms accurately reproduces the surface-layer contraction and also the formation energy of a vacancy (to within 0.01 eV) obtained from self-consistent calculations.

The electronic structures of NiMnSb and CrO₂ were calculated using a spin-polarised scalar relativistic linear muffin-tin orbital (LMTO) method. The self-consistent calculations were performed for the observed experimental lattice constants. Cubic NiMnSb (MgAgAs-type structure) and rutile-structure CrO₂ are found to be 'half-metallic ferromagnets', i.e. they have a semiconducting gap in the minority-spin bands but metallic majority-spin bands. The calculated magnetic moments in both compounds were found to be in good agreement with the experimental values. The possibility of metastable magnetic states is investigated via extended Stoner factor calculations. It is shown that the position of the Fermi level with respect to the bandgap is extremely sensitive to the exchange-correlation potential approximation.

The authors discuss the decohesion and bond mobility models of impurity-promoted intergranular embrittlement. Both of these models are tested quantitatively using pseudopotential total energy techniques within the local density approximation. Energy calculations are for a highly simplified 'grain boundary' consisting of substitutional germanium or arsenic impurities in a crystalline aluminium lattice. They find that the germanium and arsenic impurities increase the ideal work of fracture for creating a (111) surface by up to approximately 8%. This result is inconsistent with the decohesion model of embrittlement. They also find that both impurities substantially increase the critical shear stress (by 25% for germanium and 58% for arsenic) and that arsenic impurities decrease the cleavage stress by 14%. All results are consistent with the bond mobility of intergranular embrittlement.

The authors report the results of first-principles pseudopotential calculations of the total energies of the (1), ( infinity ), (2211), (2) and (3) polytypes of ZnS. These results are compared with previous work on SiC polytypes. The lowest energy ZnS polytype is calculated to be ( infinity ), in agreement with the experimental observation that ( infinity ) is the stable low temperature modification. They find that, in contrast to SiC, all ZnS polytypes are very close in energy. They discuss the implications of their results for theories of polytypism in ZnS.

The authors have studied the temperature and magnetic field dependence of the conductivity of Au_xSi_1-x amorphous alloys (0.17<or=x<or=0.73) and Au/Si multilayers with individual layer thickness t_Si from 14 AA to 16 AA and t_Au from 5 AA to 16 AA. The three-dimensional electron localisation and interaction models correctly account for the data of both types of samples. Analysis of magnetoresistance and conductivity data yields consistent results which allow them to derive the temperature dependence of the inelastic scattering: close results are obtained both for amorphous alloys and multilayers.

The theory of relaxation of highly nonequilibrium persistent photoconductivity in inhomogeneous semiconductor material has been developed. For a wide range of current carrier concentrations, the influence of the statistical properties of stochastic electrostatic potential reliefs on the kinetics of persistent conductivity has been investigated analytically and numerically. The predicted temporal dependence of the instantaneous relaxation time is consistent with the results of known experiments performed far from equilibrium and may be used for their interpretation.

The phase diagram of the extended Hubbard model with on-site attraction and random inter-site Coulomb energies is studied for the half-filled band case. In the strong coupling limit the problem is mapped onto the system of hard-core bosons (bipolarons) on a lattice, described by the anisotropic pseudo-spin model with infinite-range random exchange interactions-in close analogy to the Sherrington-Kirkpatrick spin-glass approach. It is found that the disorder and frustration strongly affect properties of the system leading to the appearance of the bipolaronic superconducting phase, the bipolaronic superconducting glass state as well as the bipolaronic charge glass phase, depending on the temperature range and the degree of the disorder.

The author shows that SU(n) antiferromagnetic chains (or equivalently SU(2) spin-s-chains with Hamiltonians which project out singlet states) are exactly equivalent to the n²-state quantum Potts chain (obtained from the transfer matrix for the 2D classical Potts model), for arbitrary n (or s with n=2s+1). This implies that the models are spontaneously dimerised with a finite gap for n<2 but have a unique ground state and vanishing gap for n<or=2. The n to 0 limit may have some bearing on the quantum-Hall-effect localisation transition.

A magnetic phase diagram of Fe-rich amorphous Fe_100-xLa_x alloys has been determined from high-field magnetisation and AC susceptibility measurements. The alloys with x>10 are ferromagnetic, but show a reentrant spin glass behaviour at low temperature. The ferromagnetic state becomes unstable when x approaches 10. For x<or=10, the ferromagnetic state disappears and a direct transition from a paramagnetic to a spin glass state is observed. Extrapolation of the phase diagram to x=0 suggests that pure amorphous Fe is no longer a ferromagnet but a spin glass with a freezing temperature of about 110K. The irreversible process of magnetisation characterising the theoretically proposed spin glass picture has been observed in the low-temperature phases of the alloys.

The authors report magnetic and resistivity measurements between 2 and 300 K on well characterised CeSi_x single crystals (x=1.71 and 1.86). Both the magnetic and the transport properties of this ferromagnetic dense Kondo system are strongly anisotropic. They compare the resistivity results with those of NdSi_1.7 which crystallises with the same structure and exhibits a simpler magnetic behaviour. An anomaly is observed at high temperatures (230 K and 260 K respectively for x=1.71 and x=1.86) in the resistivity of CeSi_x which could result from some structural mechanism.

The hyperfine fields in the substitutionally disordered BCC alloy systems Fe_xCr_1-x and Fe_xCo_1-x have been studied by means of charge self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) electronic structure calculations. For the various components, the contribution to the Fermi contact hyperfine field due to core polarisation was found to be proportional to the corresponding local spin moment. The contributions of the conduction band, however, did not show such a simple behaviour and turned out to be dominated by the occupation of the s bands for spin up and down. To study the influence of relativistic effects on the hyperfine fields the corresponding matrix elements for the Fermi contact interaction have been calculated nonrelativistically as well as fully relativistically. The importance of contributions to the hyperfine fields coming from non-s electrons have been investigated by performing spin-polarised relativistic linear muffin tin orbital method (SPRLMTO) band structure calculations for hypothetically ordered alloys.

(LaS)_1.14NbS₂ is a misfit layer compound built of alternate double layers of LaS with La in square pyramidal coordination with sulphur and sandwiched with NbS₂ as occurs in 2H-NbS₂. The electrical transport properties and magnetic properties were determined using single crystals and powder compacts. The compound is a p-type metal (Hall coefficient R_H and Seebeck coefficient positive) with a strong anisotropy of the electrical resistivity: rho _{perpendicular to c}/ rho /sub //c/=5*10^-5. The electrical transport properties are interpreted as being due to holes in 4d(z²) band of the NbS₂ part of the structure. The 4d(z²) band which is half-filled in the case of 2H-NbS₂ contains 0.12 hole/Nb atom. This corresponds to a donation of 0.88 electron/Nb atom from the LaS part, the charge of La being +2.8. The magnetic properties are interpreted as being due to the LaS part. In the temperature region 40-300 K the curve of reciprocal magnetic susceptibility (not corrected) against T is linear with a Curie constant C of 1.4*10^-6 m³ K (mol La)^-1 corresponding to 0.14 4f electron at La, 0.86 electron being donated to the 4d(z²) band of the NbS₂ part. Down to 4 K there are no signs of superconductivity.

In order to get a good overall description of the electronic structure of CoGa, it has been calculated self-consistently by the LMTO method, including the 3d semi-core states of Ga as lower band states participating in the charge transfer. Using self-consistent potentials derived previously for the series TAl(T=Fe, Co, Ni), the dielectric functions of all these compounds are calculated by a tetrahedron method taking properly into account the cut-off of the Fermi surface.

Rare earths (REs) diluted into MBE grown and synthesised n-type InP samples create an acceptor-like level (at 30 meV for Yb and 60 meV for Er) below the conduction band; the activation energies are deduced from temperature dependent Hall effect measurements. From EPR experiments where the authors observe resonance due to the Yb³⁺ ground state in n-type samples, they deduce that this acceptor-like level is not the Yb²⁺/Yb³⁺ acceptor level. They propose that the RE creates an attractive potential for electrons in III-V semiconductors. They discuss the consequences of this property for the luminescence excitation mechanism of the luminescence.

The melting temperature of a 2D electron solid, density 8*10¹² m^-2, on liquid helium has been found to be almost independent of magnetic field from 0.5 to 7 T.

The electronic density of states of paramagnetic and ferromagnetic ordered Pt₃Mn alloys is calculated using the linear muffin tin orbital method (LMTO) and the atomic sphere approximation (ASA). The Fermi level in the ferromagnetic system is located at the dip valley between two peaks. The magnetic moments on Mn and Pt atoms are 3.64 mu _B and 0.22 mu _B respectively. These values are close to the experimental ones. The calculated value of the gamma -coefficient is similar to the observed value.

It is shown that the macroscopic equations describing the quantum Hall effect are Lorentz invariant. The influence of finite frequency on the symmetries underlying the QHE is studied and the role of the polarisation currents in the plateau regime is discussed.

The authors have developed a quantum theory of tunnelling plasmons in a quasi-(0+1)-dimensional superlattice (also called a lateral multiwire quantum dot superlattice). The analytical expressions of dispersion for both intrasubband and intersubband tunnelling plasmons are derived using a tight-binding approximation. The connection between a quasi-(0+1)-dimensional superlattice and an anisotropic array of quantum dots is discussed.