Table of contents

The application of the specular reflection of neutrons to the study of surface and interfaces is described. The theoretical and experimental background to the technique is presented. A range of recent experimental results in surface chemistry, solid films and surface magnetism is discussed. In surface chemistry the results include adsorption of surfactants at the air-solution interface, insoluble monolayers and polymers at the air-liquid interface, soap films and adsorption at the liquid-solid and liquid-liquid interfaces. In solid films results on Langmuir-Blodgett films, hard carbon films, polymer films and some semiconductor layers are discussed. In surface magnetism experimental data which illustrate the nature of magnetism in magnetic multilayers and ferromagnetic films, and which describe flux penetration in superconductors, are presented.

An exact formula for the expansion of an outgoing spherical electron wave around another centre is presented from which new approximation for spherical-wave scattering are derived. These expressions allow a considerably higher accuracy in high-speed numerical multiple-scattering calculations than methods existing hitherto. This is demonstrated for tungsten in the energy range from 50 to 5000 eV. The improvements of the given formulae over earlier approaches are greater the higher the energy. In particular the errors in the forward scattering direction are very small.

Kinetic analysis of experimental results shows that an independent two-level-systems (TLS) model can account for the effects of irreversible structural relaxation on the electrical resistance of Pd₈₂Si₁₈, provided that a broad spectrum of activation energies of the TLSS is invoked. Using Eyring's theory of the rates of chemical reaction, the authors show that the real TLS transition attempt frequency nu ₀ is up to two orders of magnitude less than the apparent attempt frequency v and that v₀ is much lower than a typical Debye frequency.

The perturbed angular correlation measurements were performed with ⁹⁹Rh in dilute PdX (X identical to Cd, In, Sn, Sb) alloys and with ¹¹¹In in dilute Pd-Rh alloy. The rhodium atoms attract Cd, In, Sn and Sb impurities and impurity pairs of Rh-Cd, Rh-In, Rh-Sn and Rh-Sb are formed in a Pd matrix. Their binding energy was derived from the impurity pair concentration measured as the function of temperature. The negative binding energy contradicts the repulsive interaction predicted for those impurities by the semi-empirical model developed in the literature.

The author discusses the nature of phonon spectra near interfaces and show by general thermodynamic arguments and by detailed analysis of linear chain models how it is that localised measurement of atomic motion on one side of an interface yields no information about the phonon structure of the material on the other.

The authors have determined the phonon dispersion curves in GaAs at 12 K for wavevectors along the six directions Gamma -( Delta )-X-( Sigma )- Gamma -( Lambda )-L-X-(Z)-W-(Q)-L by high-precision inelastic neutron scattering. Various existing lattice-dynamical models are used to reproduce the experimental data. The relative merits and drawbacks of these models are discussed.

Phonon frequencies on two dispersion sheets for TA modes have been determined by inelastic neutron scattering on the (001) and the (011) planes at 12 K. Phonon group velocities for these phonons have been derived. The results are used to calculate heat transport properties. Comparison with results from phonon-focusing experiments shows good agreement and yields an estimate of the lifetime of ballistic phonons.

A variety of physical properties (low-temperature X-ray diffraction, electrical resistivity, thermal conductivity, thermopower, specific heat, thermal expansion and susceptibility) of orthorhombic SmCu₂ have been studied. A significant contribution of the J=⁵/₂ Hund ground state has been observed. An estimation of the overall crystal-field splitting obtained from the termal expansion experiment yields a value of 110 K. The magnetic entropy indicates a doublet ground state, since the observed value (S_m=5.50+or-0.2 J mol^-1 K^-1) is very near to the theoretical R ln 2. An additional influence of the J=⁷/₂ multiplet is visible in the resistivity and susceptibility results. The pronounced increase of the thermal conductivity in the magnetically ordered region (Neel temperature T_N=23 K) is discussed within the framework of magnon-heat conductivity.

A numerical method is presented for the calculation of the longitudinal relaxation time of proton diffusion in parallel hexagonal lattices of calcium hydroxide. A comparison between theoretical results and experimental data shows that the probability of a single proton jump is very high while the probability of two or more jumps can be almost neglected.

The authors present an extension of classical molecular dynamics (MD) to include the forces calculated from electronic degrees of freedom using the tight-binding (TB) approximation. The combined MD-TB problem is solved using simulated annealing techniques. As an example they study the structures and energetics of small silicon clusters, containing up to 10 Si atoms.

The electronic structure of group IV impurities in InP has been analysed using a discrete variational X_alpha self-consistent field method in the embedded scheme within the local-density framework. The pure semiconductor gap is calculated to be 1.44 eV in very good agreement with the experimental value of 1.41 eV. The defected clusters are studied by substituting C, Si, Ge and Sn at the central In site. All the impurities but C show a donor-like behaviour with states introduced deep in the gap region of the host semiconductor. The charge distributions and the density of states are calculated. Comparison with experimental results are made wherever available.

The authors report on spin polarised energy band calculations for the cubic Laves phase compound NpAl₂. The calculations were performed within the local spin density formalism, with a correction that takes into account the different intra-atomic Coulomb interactions for states with different azimuthal quantum numbers. The spin-orbit interaction was included in the band Hamiltonian and thus the parameter-free calculations took into account Hund's first, second and third rules. A description of the formalism is presented and it is found that only when treating all of Hund's rules simultaneously do they obtain good agreement between the theoretical and experimental magnetic moments. A prediction of the magnetic form factor is also presented. It is found that the cohesive properties are strongly affected when introducing the orbital polarisation, and only when doing this do the theoretical and experimental volumes compare well.

The authors examine some properties of the electronic self-energy potential for metallic systems within the GW-plasmon-pole approximation. The self-energy is found to be highly state dependent. States with bonding character are depressed relative to states with less bonding character when compared to density functional eigenenergies, irrespective of whether they are occupied or not.

The Hamiltonian of a molecular chain, describing the interaction of excitons (Pauli quasi-particles) with acoustic phonons, after a suitable unitary transformation is diagonalised using the Bethe ansatz. The spectrum of the collective excitations at T=0, including strings of excitons, is calculated and compared with the energies of classical solitons. Relevant experiments are discussed.

The author derives the Fokker-Planck equation for the distribution of the phase of the reflection amplitude for a disordered conductor of length L, using the invariant embedding method. The limiting (L= infinity ) stationary distribution in the strong-reflection regime is uniform provided that the localisation length is large compared with a Fermi wavelength. Next the author studies the joint distribution of the phase shift and of the time delay experienced by an incident electron in the conductor before being back-scattered. He obtains the explicit form of the marginal distribution for time delays for large L and strong reflections, using the uniformity of the phase distribution. The author applies it to study the spectral density of the surface charge fluctuation noise. An approximate analytical calculation yields an f^-12/(^{3- alpha} 2)/ spectrum where alpha >or=1.

The influence of inelastic processes on the coherence in the semiballistic regime of mesoscopic transport is investigated by calculating the AC conductance of electrons in a single-channel quantum wire, coupled to a bath of longitudinal acoustic phonons. Numerical results illustrate the situation in GaAs structures.

Resonance scattering of electrons on impurities is shown to play a crucial role in nonlinear conductivity under quantum Hall effect conditions at zero temperature. The dissipative current as a function of electric field and Landau level filling factor is calculated.

Fluctuation effects in nematic-type phases in quantum systems of spin 1 and ³/₂ are examined. The analogue of the Mermin-Wagner theorem is formulated and the perturbation theory is shown to be logarithmically divergent under certain conditions. The low-energy excitations depend on the parity of 2S.

The antiferromagnetic structure of triclinic Cu₃(PO₄)₂ (T_N=22.2(5)K) has been determined at 2 K by unpolarised neutron diffraction from powdered and single-crystal samples prepared by hydrothermal synthesis. The triacute reduced chemical unit cell has a=4.855(1), b=5.288(1), c=6.184(1) AA, alpha =72.34(1), beta =86.99(1) and gamma =68.54(1) degrees at ambient temperature and a=4.848(1), b=5.280(1), c=6.183(1) AA, alpha =72.30(1), beta =86.90(1) and gamma =68.59(1) degrees at 6 K: it contains one formula unit and its space group is P1. The magnetic propagation vector is (00¹/₂) referred to this cell and, within the accuracy of the determination, the magnetic structure is collinear with equal moments of 0.68(1) mu _B on each of the two crystallographically inequivalent Cu²⁺ ions. The Cu1 ion is at an inversion centre which is preserved in the magnetic space group P_2c1. The Cu1 moment points in a direction theta =72(2) degrees , phi =135(2) degrees with the Cu2 moments antiferromagnetically coupled to the Cu1 moment; here theta and phi are the polar angles referred to the orthogonal crystal axes with (001) as the polar axis (z), and x along a^*. The phi -angles are measured from the x, z plane. The magnetic form factor is found to be expanded by 14(3)% relative to that calculated for free Cu²⁺ ion wave functions. The magnetic exchange paths through the structure are discussed.

Mechanisms of thermoluminescence (TL) in CaSO₄:Dy are studied using electron spin resonance (ESR) in single-crystal samples. The ESR analysis indicates the presence of several variations of a distorted SO₄^- centre locally stabilised by a nearest neighbour Ca vacancy. This centre's structure, which has not been reported previously, relates to several centres observed in this system and could also explain results in related compounds. The presence of the Dy impurity, which increases the concentration of Ca vacancies due to charge neutrality requirements, enhances both the concentration of these centres and the intensity of the major TL peaks near 220 and 350 degrees C. The specific role of these and other previously reported centres in the TL of this material is followed using heat treatments. This enables one to identify centres that change in the range of the observed TL peaks as well as centres that seem to be related to one another by charge transfer or rearrangement. The behaviour of the observed centres and their relationship to the TL behaviour of the system reemphasises the complexity of this system.

The authors discuss positron behaviour and in particular the ground state positron probability in high-T_c materials. Special attention is paid to La₂CuO₄, for which they report detailed high-resolution two-dimensional angular correlation of positron annihilation radiation spectroscopy measurements in three sample orientations and as a function of temperature. Besides a large isotropic core-like electron contribution ( approximately=85%), the remaining anisotropic contribution is attributed to valence electrons which are modelled by a linear combination of atomic orbitals-molecular orbital method in conjunction with a localised ion scheme, within the independent particle model approximation. The covalency structure so derived is used to discuss the case of the parent doped material and YBa₂Cu₃O₇. The authors also comment on the application of the Lock-Crisp-West theorem to La₂CuO₄, before adding some concluding remarks.

Current controversy concerning the nature of the motion of charge carriers in polydiacetylene single crystals is examined. New results using picosecond time-resolved photocurrents are able to distinguish whether the observed saturated drift velocity is due to the motion of a solitary-wave acoustic polaron or a carrier moving in a distribution of traps and barriers, and indicate the former case. The relevance of the Onsager geminate recombination theory in these materials is also demonstrated unequivocally.

The authors report on some pressure-related properties of the organic superconductor kappa -(BEDT TTF)₂Cu(SCN)₂ in the range 1 bar-3 kbar. The pronounced resistance maximum at around 100 K is very sensitive to the pressure and so is T_c, which decreases rapidly at a rate dT_c/dP=-3.6 K kbar^-1. The resistivity anisotropy parallel to the transverse ratio does not show any anomalous behaviour across the resistance maximum. Instead, it shows a zig-zag shape when the resistance decrease is steepest.

Magnetic properties of disordered Ising antiferromagnets with frustrated intra-sublattice exchange interactions are studied. The theory explains satisfactorily some experimental results.