Table of contents

The authors calculate the density of states of a one-dimensional tight-binding hamiltonian with off-diagonal disorder only. The method consists in solving an integral equation for the self energy, from which the density of states is obtained by simple integration. The main feature obtained is a peak at the centre of the band, which does not appear for the anderson (diagonal) disorder. The results differ considerably from a previous study of the same problem.

The authors propose a method which enables one to apply the self-consistent boundary site approximation to a system of any dimensionality. The electronic density of states for a diamond lattice is calculated and compared with results from other theories.

For pt.i, see helsinki univ. Res. Inst. For theoretical phys., preprint 7-74. Positron annihilation is studied by using the boson description of the electron gas. The electron-electron interactions are taken in rpa, whereas the positron-electron interaction is more accurately treated. The scheme corresponds to calculating certain very large sets of perturbation diagrams. The authors give results for the positron annihilation probability, the pair-correlation function and the positron kinetic energy and chemical potential for various electron densities. New insight into the nature of the low-density divergencies occurring in many calculations of positron annihilation.

A numerical treatment of the eliashberg equations reveals that mcmillan's equation fails for lamda>1.5. For very large lamda, t_c scales as (lamda(omega²))¹2/, while for lamda<1.0, mcmillan's equation is reasonably accurate. The authors give a new approximate t_c equation which satisfies these limits, and show that the maximum t_c occurs at infinite lamda. A new tunnelling measurement for amorphous pb_0.45bi_0.55 gives lamda=2.59 and a t_c in excellent agreement with numerical solutions.

The possibility of superconductivity at metal-semiconductor interfaces brought about by exchange of surface plasmons is considered. It is shown using an approximation to the dielectric response in the interface region that the total interaction is less favourable than in the bulk metal. The importance of taking the full interaction rather than isolating the surface plasmons only is stressed.

It is shown that a special nonplanar two-dimensional ising model with short range two-spin interaction transforms to the spin equivalent of the baxter model. This is an example where the critical exponents depend on the value of the coupling constants in contrast to the universality so far assumed to be valid for this model.

A modification of domb's droplet model for low-temperature randomly dilute ising ferromagnets (percolation problem) suggests that asymptotically the ratio of 'surface area' to 'volume' of spin clusters is the same as for cayley trees on a bethe lattice, i.e. (1-p_c)/p_c where p_c is the percolation threshold. The similarity to clusters of overturned spins in usual (non-dilute) ising magnets near t_c is discussed quantitatively.

The magnetic exciton renormalization and soft-mode behaviour as the temperature approaches zero of the singlet-doublet magnet (dhcp)pr are accounted for by a selfconsistent rpa theory with no adjustable parameters. The crystal-field splitting between the ground state and the doublet is d=3.74 mev and the ratio between the exchange interaction and d is very close to unity. However, zero-point motion prevents the system from ordering.

New electro-absorption data are reported on the urbach edge in bulk and evaporated films of se and as-se alloys. A comparison with current microfield models suggests the presence of an additional absorption mechanism. A model is proposed, based on local optical anisotropy.

The angular average of the energy loss of h₂⁺ ions channelled through thin crystals is obtained, for incident energies in the mev range. The results show an important contribution to the energy loss coming from the correlated interaction of the protons with the valence electrons.

For electron micrographs taken with the offset bright-field configuration (interference microscopy), a transfer function is derived which includes the effects of angular and energy spread in the illumination on spherical aberration, defocussing and chromatic aberration of the objective lens. It is shown that this transfer function varies markedly across the plane of the objective aperture, acting like a 'fourier filter' which suppresses certain periodicities while leaving others relatively unaffected. The results support the view that 'lattice fringes' which have been observed in micrographs of amorphous ge may in certain circumstances be artefacts of the imaging process.

A formulation is given for computing the dispersion part of the binding energy of a molecular crystal taking the size of the molecules into account. This is done by folding the green function with a form factor representing the polarization density distribution induced on a molecule by the electric field. A model calculation for hcp and fcc crystals using a gaussian form factor reveals a structure dependence of the binding energy, giving a lower value for the fcc crystal. It is suggested that this structure dependence, unobtainable in a point molecule model, arises out of the many-body interactions which are automatically included through the form factor that takes into consideration the finite size of the molecules.

In the breathing shell model (bsm) the many-body interactions generated by the 'breathing' motion of the ions do not vanish for an alkali halide crystal under homogeneous deformation and hence contribute to many static properties of the lattice. Though this model has been quite successful in the region of lattice dynamics, no attempt has been made to apply them to lattice statics, except in so far as the elastic properties and indirectly the dielectric properties are concerned. It is now well known that no reasonable two-body central interaction is able to predict the correct structure for the caesium chloride structure alkali halides. The authors see whether the many-body interactions bsm purports to describe can yield the correct structure for them and at the same time explain the stability of the other members of the group. The results of the calculation show that within certain limitations bsm correctly predicts the structure of all the alkali halides considered in the present investigation.

For pt.II see abstr. A60612 of 1974. From measurements of the anelastic relaxation and the solubility for hydrogen in palladium, a spinodal curve for the PdH system has been determined. The critical point exponent, beta _s, for this spinodal, is within experimental error, the same as that of the coexistence curve and both are in agreement with predictions of mean-field theory. Comparison of the solubility data with the isotherm equation due to Lacher (based on a mean-field approximation) shows that the equation holds reasonably well for 0< rho <or=H/Pd=0.3 and temperatures within +or-10% of the critical temperature ( rho _c=0.29, T_c=566K).

The author reports preliminary investigations of superflow onset and third-sound velocity made using superconducting tunnel junction transducers in the temperature range 0.85-1.05k, a technique of comparable, if not superior, sensitivity to any used before. Several novel effects have been observed, notably a discontinuous variation of superfluid density wth film thickness, interpreted as reflecting a change of state in the helium film, and also finite resistance to dc film flow above the conventional onset point. The technique promises to be of significant importance in future investigations of superflow in the <1k range.

A set of boson models, incorporating an excitation spectrum which produces a linear relation between the temperature and the superfluid density in narrow channels at low temperatures as seen by pobell et al. (see abstr. A25131 of 1972), are presented. These energy states are interpreted as arising from adatom-substrate interactions resulting from the heterogeneity of the surface.

The authors present and discuss theoretical techniques that considerably increase the power of dynamical leed calculations, particularly in the layer treatment of surfaces. Their wide applicability makes them especially suitable for surface structural determination. The layer doubling method is an efficient and accurate way of calculating reflectivities off a stack of identical atomic layers requiring only finite absorption for convergence. Layer summation over surface-induced sublattices avoids the repetition of lengthy summations when the surface has a different two-dimensional unit cell from the substrate. The exploitation of symmetry finally can give spectacular reductions in computing time.

A manageable form is provided for two-centre integrals between localized functions appearing in calculations of electronic states. This result can be achieved by deriving general expressions for two-centre integrals between spherical harmonics and symmetry-adapted harmonics of any order. Their explicit dependence on the relative orientation of the centres is given. Previous expressions obtained by slater and koster (1954) for l?2 are recovered as particular cases.

The ground-state energy of an electron-hole liquid in n-type doped si and ge at high dopant concentrations is computed. The impurities are taken to be randomly distributed, linearly screened point charges and their effect on the energy calculated to second order in the impurity potential. The band structure is treated in the effective-mass approximation with the anisotropic conduction-electron effective masses and coupled light and heavy hole bands. The correlation energy is computed in the random-phase approximation with a spherically averaged conduction-electron polarizability. The binding energy per charge carrier increases in magnitude with impurity concentration, but is weaker than in the intrinsic case except for densities much higher than the mott density. Good agreement is found with experimental results for the shift in the optical band gap with doping and the shift in the luminescence edge when droplets are formed.

The transformation law for field-dependent tensors and the onsager reciprocity relations have been used to establish the form of the magnetoresistivity tensor rho_ij(b) for all 32 crystallographic point groups. Each component of rho_ij(b) is separated into even and odd terms in the magnetic induction b. The generality and simplicity of this formulation leads us to urge that the even components be defined as the magnetoresistance and the odd components as the hall effect. The form of rho_ij(b) can also be obtained from restrictions imposed by band structure and transport theory, a procedure which can lead to results of higher symmetry than the phenomenological forms. The umkehr effect is shown to occur in any component of rho_ij(b) which contains both even and odd terms in b and to be a direct result of crystal anisotropy.

Hall effect measurements at 296k and at hydrostatic pressures up to 15 kbar have been made on se-doped single crystals of GaAs_1-xp_x, where 0.30?x?0.42. Results have been interpreted in terms of the energies and effective masses of the gamma_1c and x_1c conduction-band minima. It is shown that such measurements provide a useful method of determining the composition where both bands have equal energies. This is found to occur at x=0.49 +or- 0.02 at an energy of 2.03 +or- 0.02 ev relative to the valence-band maximum. Two impurity levels, for samples with x_1c carrier concentrations near 10¹⁸ cm^-3 have activation energies of 0.04 and 0.10 ev below the x_1c minima. Results on stoichiometrically controlled samples indicate that the deeper level may be associated with a gallium vacancy-donor complex. The bowing of x_1c minima across the system should be lower than for the gamma_1c minimum in agreement with theoretical predictions.

Experiments on the transmission of negative ions from liquid neon to the vapour have been performed at various temperatures and at various liquid levels. The results show that the barrier is due to the dielectric discontinuity. The smoluchowski equation gives a good description of the results, provided that for the electric field e, the values of the field e_l which exists at the liquid side of the interface are used.

A pulsed, tuneable dye laser has been used to pump the optical energy levels of single crystals of rare earth compounds. Because of the short relaxation times, this results in fast heating of the crystals. The subsequent changes in magnetization have been recorded and explained using the known magnetic properties. The slow cooling is exponential provided the heat pulses are small and is limited by the kapitza boundary resistance (varying as t^-3) when the helium is superfluid. In normal helium the thermal resistance also varies as t^-3. Large heat pulses give anomalous magnetic signals, with very long delay times, and these are interpreted in terms of the formation of a gas film around the crystal surface.

The hyperfine interactions in ferromagnetic cr₂te₃, crte and cr₂te were studied by means of the mossbauer effect in ¹²⁹i and ¹²⁵te. The ¹²⁹i measurements in crte and cr₂te (at temperatures below t_c) revealed the existence of two equally populated sets of te(i) sites with different internal fields acting at the iodine impurities nuclei. This was explained as being due to nonstoichiometry and the presence of interstitial chromium ions in the tetrahedral holes. The magnetic fields measured on the ¹²⁵te nuclei in crte and cr₂te were found to be considerably lower than those measured on the ¹²⁹i impurities. A discussion of a transferred hyperfine interaction mechanism compatible with this trend, based upon the unpairing of the 5s electrons due to overlap with the 3d_e(g) orbitals is presented.

A new mechanism of absorption of infrared radiation at low temperatures in n-type GaAs, InP and CdTe is investigated. Photon energies are about two thirds of donor ionization energy. Donor concentrations up to 2*10¹⁴ cm^-3 in GaAs, 5*10¹⁴ cm^-3 in InP and 2*10¹⁵ cm^-3 in CdTe, and compensations up to 0.2 are considered. A model which is based on photon-induced transitions of electrons from 2p sigma _u- to 3d sigma _g-like states of ionized donor pairs is used. The formula for the absorption coefficient is derived and numerical results are presented. The transitions can be also detected as changes in DC conductivity.

Investigations on white-line in the k-absorption spectra of copper and its compounds have been made, using a modified 40 cm curved crystal (mica, 2?01 planes) transmission spectrograph of the cauchois type, and a molecular orbital interpretation is suggested for the white-line occurrence. It appears that there exists a possible correlation between the covalent character and the existence of white-line in X-ray absorption spectra of compounds. The concept of excitons and electron-phonon interaction is also discussed.