Table of contents

The authors investigate the sensitivity of the indirect interaction behaviour with the range of the adsorbate-substrate hopping integrals. They show that this sensitivity is quite large. It means that the common approximation which consists of restricting the adsorbate-substrate interactions to nearest neighbours is often incorrect. They show also that there is a general tendency to obtain a repulsive indirect interaction when electrons can go from an adatom to the other 'via' only one surface atom through direct electronic resonances. All these results are illustrated by a computation carried out in the case of the (100) surface of a single s band simple cubic crystal. As a consequence of their results, it can be emphasised that a careful computation of the adsorbate-substrate hopping integrals is necessary to obtain reliable results in the computation of the indirect interaction energy between adatoms.

The general problem involving an infinite potential barrier is treated by first constructing a pseudopotential H' that is shown to reproduce the effect of the barrier. A new procedure is then developed to handle the perturbative effect of H' since the standard formulae become invalid. The case of a finite barrier can then be solved by reducing it to that of an equivalent infinite barrier. The authors' result should also furnish a check for any specific problem involving a barrier solved by other approximate means such as the variational method.

For pt.I see ibid., vol.14, p.L525 (1981). It has been demonstrated that it is possible to predict methyl hopping rates in solids over a wide temperature range, from a single measurement of the methyl tunnelling frequency at low temperatures. Previous measurements, though, have been largely limited to tunnelling frequencies in excess of 100 MHz which are accessible to high-resolution inelastic neutron scattering. By exploiting the detection of weak tunnel sidebands of nuclear magnetic resonance spectra in methyl malonic acid, it is demonstrated that the theory works equally well for a system having a tunnel frequency of 68+or-2 kHz. The temperature dependences of both the proton spin-lattice relaxation time and the tunnel frequency agree well with the theory.

X-ray powder diffraction measurements have been made on both ordered and disordered phases of bicyclo-octane. It is concluded that the structure of the ordered phase is hexagonal, in agreement with earlier work. The high-temperature disordered phase is known to be face-centred cubic. Incoherent neutron quasi-elastic scattering measurements have been made throughout the disordered phase. These have been successfully analysed to show that the dominant motions are uniaxial rotations about the unique molecular axes (adequately described as C₆ rotations) plus C₄ rotations by which the unique molecular axes can reorient along the eight different diagonals (111) of the cubic cell. The correlation times are described by: tau ₆^-1/s^-1=(1.1+or-0.3)*10¹² exp((-5.3-or+0.5)kJ mol^-1/RT) tau ₄^-1/s^-1=(3.7+or-1.5)*10¹¹ exp((7.6-or+1.5)kJ mol^-1/RT).

Classification enables the maximum possible information to be obtained from experimentally determined force constant tensors. In finite systems, mathematical arguments show that there are only three distinct classes; the author argues that the 'best' physical classification corresponds to this, i.e. isotropic, traceless pairwise, and antisymmetric forces. The latter violate Newton's third law and imply nonrigid behaviour of the electrons. The way the classification distinguishes between the microscopic contributions to the force, i.e. the contribution of the different parts of the generalised pseudoatom, is shown. In particular the deformation of the pseudoatom is shown to give traceless pairwise and antisymmetric contributions and so cannot be described in terms of dipolar, quadrupolar, etc., contributions. In ionic crystals it is shown that it is not possible to move the ions independently, but that in all crystals it is still possible to define interionic force constants. These can be classified in the same way as in finite systems; the presence of real or imaginary parts of the crystalline force constants depends on the crystal structure. The classification also shows the need for more experimental data in order to determine the validity of various theoretical models.

The inner elasticity of corundum is treated by a pure point group method. It is also shown that the local strain and compliance tensors previously defined are compatible with the inner elasticity theory and can be calculated from its results.

The work is concerned with an ab initio calculation of the energy levels of Mn²⁺ ions in MnF₂. The essential idea is the application of an analytic approximation of Watson's SCF procedure to obtain the spin-orbit interaction and the electrostatic parameters for the isolated ions. Crystal field energy levels are then calculated using the nearest-neighbour point-ion approximation. Despite these approximations the results are quite good and indicate that the technique may be of general use as the starting point of more sophisticated calculations on the dⁿ ion in crystals. In addition, the bands at 39010 cm^-1, 41340 cm^-1 and 43200 cm^-1 are interpreted in terms of double exciton transitions, which are in good agreement with experimental findings.

For pt.I see ibid., vol.13, p.6167 (1980). The authors analyse the numerical results presented in part I of this paper for the electronic contribution to the photoionisation cross sections of deep levels of a₁ (s-like) and t₂ (p-like) symmetry in direct and indirect gap semiconductors. Whereas in part I they were mainly concerned with the technical aspects of their calculation, here they attempt to establish, in simple physical terms, a link with a wide range of experimental data. They consider several specific systems in Si, GaP and GaAs as well as some general properties of optical spectra reported in the literature. They find that the experimental evidence strongly supports the existence of the predicted band structure and defect-related effects influencing the spectral forms. However, a detailed comparison with experiment on a convincing scale has not been achieved. It would appear that the uncertainties in their theoretical predictions are less significant than those concerning the experimental data.

The authors introduce the finite-element method for the effective conductivity of two-dimensional composite materials. For the problem of two-dimensional percolation and conduction they use three kinds of composite material with regular cell structure and compute an upper and a lower bound on the effective conductivity of the materials by the finite-element method. The three models can be related to site percolation problems, and the relation between the computed bounds and two kinds of percolation problems (current percolation and field percolation) is made clear.

A series expansion is given to calculate the effective conductivity of disordered square resistor networks in which sites are broken at random (the site model). The multi-bond expansion proposed in the bond model is applied to the site model, taking into account the correlations between the values of bonds with a common site. Configurational averaging is studied over sums of two-bond, three-bond and multi-bond terms. Special configurations of sets of bonds are found to contribute to the first and second orders of the fraction c of broken sites for the effective conductivity. The expression for the effective conductivity is presented up to the second-order term of the fraction c for site-disordered resistor networks. It is shown that the dilute site percolation conductivity agrees with the dilute limit of the effective-medium approximation of Watson and Leath (1974).

Within the generalised mean-field approximation an integral equation for the static structure function S(k) has been obtained using sum rule arguments. The integral equation is solved self-consistently in the domain of the weak- and intermediate-coupling regimes. It is found that the static properties of a two-dimensional classical one-component plasma are in overall good agreement with the computer results of Totsuji (1978).

The two-sublattice Hubbard model has been solved in the Roth scheme of decoupling. It is found that antiferromagnetic solutions are in present in the strong-correlation limit. The stability of antiferromagnetic solutions is investigated.

The conductivity of tunnel junctions containing paramagnetic impurities of arbitrary spin in the dielectric layer is calculated. The causes of the anomalous broadening of the conductivity peaks are considered. It is shown that measuring the tunnel characteristics in the presence of a RF field makes it possible to investigate complicated EPR spectra of the impurities.

The ESR spectra of a cluster of three nitrogens observed in natural Cape yellow diamonds are interpreted. The ESR results show that the nitrogens lie in (110) planes. The spin Hamiltonian parameters are given and a model is proposed in which the nitrogens occupy substitutional sites with the paramagnetic electron occupying a pi -orbital in the plane of the nitrogens.

NMR shift at the Pb and Te sites and the linewidth of the Te resonance have been measured between T=300K and T=4.2K in a series of Mn-doped p-type semiconductors (PbTe)_100-x(MnTe)_x, with x=0, 0.2, 0.5, 1 and 2. An analysis of Pb Knight shift based on a molecular field model yields a value of J_sd=0.010+or-0.005 eV for the s-d exchange between Mn localised moments and conduction holes. The same model applied to the Te linewidth partially explains the measurements. The Te Knight shift is quite insensitive to the introduction of Mn impurities, even at highest concentration and lowest temperature. Hall effect measurements made on the samples at T=77K and t=300K prove that the presence of Mn up to 2 at.% does not alter the charge carrier density of the host PbTe.

The paper reports the investigations of the hyperfine interactions in DyMe₂Si₂ (Me=Mn, Fe, Co, Ni, Cu) tetragonal compounds. The large and monotonic decrease observed of the hyperfine field (overall difference of 60 T) as well as of the field gradient at ¹⁶¹Dy nuclei in the above series may be semiqualitatively explained by the influence of the crystalline field on Dy³⁺ ions. Similarly, the authors claim that the crystal field interactions are responsible for a pronounced increase of values of the hyperfine parameters of ¹⁶¹Dy in the intermediate phases Dy(Me'_1-xMe"_x)₂Si₂, where Me', Me" are two of the 3d metals adjacent in the periodic table. The Mossbauer effect for ⁵⁷Fe in DyFe₂Si₂ supports the assumed absence of the localised magnetic moment at the iron sites, and indicates the low-spin divalent 3d⁶ iron configuration.

It is shown that the strong short-range interactions which exist in hydrogen bond ferroelectrics, produce strong anisotropy in the wavevector (q) dependence in the polarisation fluctuations in these systems. It is suggested that this anisotropy is the origin of the observed strong suppression neutron intensity with wavevectors pointing along the ferroelectric axis of KD₂PO₄. Calculations of the q-dependent susceptibility has been made using an Ising model which includes four spins per unit cell representing the motion of the four deuterons surrounding each PO₄ group. The short-range spin-spin interaction reproduces the known energies of the different hydrogen bonds configurations in KD₂PO₄. The neutron scattering is then calculated using the mean-field approximation, and the anisotropy of the scattering is found to be in good agreement with experiments. Finally in the ice-rule limit of the model, the scattering has a dipolar singularity as q to 0.

The luminescence associated with exciton decay at an isoelectronic centre in beryllium-doped silicon has been studied by Zeeman spectroscopy. The centre is shown to have a (001) axis of symmetry and has been identified with a pair of beryllium atoms occupying a substitutional site (an interstitialcy). Unlike previous isoelectronic centres described in the literature, there is strong evidence for significant mixing of the Gamma ₇ and Gamma ₈ valence-band-derived hole states which is considered to be due to the small spin-orbit splitting of the beryllium atom. This, together with the phonon-assisted structure, suggests that the centre can be identified with an isoelectronic donor. However, it also renders the group theoretical formalism used for the analysis of the Zeeman data incompatible with the usual spin Hamiltonian approach. Nevertheless, it has been possible to derive approximate electron and hole g-values of 2.1 and 1.2, respectively.

The authors reinvestigate the interpretation of fine structures associated with beam emergence threshold in LEED. This work is based upon a new expansion of the total amplitude matrix of the crystal in a power series of the off-diagonal part of the bulk amplitude matrix. They clearly show that the pure resonance picture is incorrect, even in the favourable case where the fine structures fall inside a forbidden gap. Their alternative picture is that the dominant mechanism is an interference between the waves directly reflected at the barrier or at the bulk, and the waves associated with processes including on metal-metal reflection at the barrier. This interpretation, which proved to be adequate in their previous study of Al(001) and Ni(001), holds equally well for the fine structure measured by Adnot and Carette (1977) on W(001) in the presence of a band-gap. As a consequence the authors discuss why McRae's method (1967) to extract structural information from a 2D surface band structure cannot be used.

Ge thin films are grown using molecular beam epitaxy on clean cleaved GaAs (110) substrate at a temperature of 160 degrees C which ensures the formation of a smooth and crystalline layer with an abrupt interface. The film and the interface are characterised using low-energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The latter, measured as a function of film thickness from 10^-3 monolayer (1 ML=8.8*10¹⁴ atoms cm^-2) to 50 ML, brings a direct measurement of the energy separation between the valence band edges of the two semiconductors at the interface, Delta E_v=0.55 eV, and shows the existence of a filled band of interface states, the tail of which penetrates the lower part of the GaAs band gap. The escape depth of electrons in Ge at several energy values is also determined.