Table of contents

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

The molecular translation and rotational motions of liquid CCl₄ are studied by means of the concurrent use of computer molecular dynamics simulations as well as neutron quasi-elastic scattering. The relevant correlation functions have been analysed in terms of simple analytical models and the total spectra are compared with the measured ones. The data confirms the substantial departure from simple rotational or diffusion behaviour encountered in a previous work.

The magnetic behaviour of liquid gold-germanium alloys has been characterized by susceptibility measurements performed from room temperature up to a maximum temperature of 1600 K with special emphasis on the liquid state. Throughout the range of composition the diamagnetic susceptibilities were found to decrease with the temperature except for the very gold-rich side where a tendency of opposite sign was indicated. The variation of the susceptibilities with alloy composition was non-ideal with a flat minimum located at about 18 at.% Ge. The interpretation was given in terms of 'compound formation' around the stoichiometric compositions AuGe and Au₄Ge. Alternatively, an interpretation based on a critical electron concentration was presented.

The effect of annealing on the segregation in (Fe₆₄Mn₃₆)₃₅Y₆₅ metallic glass has been studied by anomalous small-angle X-ray scattering in conjunction with conventional X-ray scattering and differential scanning calorimetry measurements. From the Ornstein-Zernike theory, it has been found that the structural relaxation takes place through an increase in the concentration fluctuations (already present in the as-quenched ribbons) and a small decrease of their correlation length. The crystallization of HCP Y is never preceded by an amorphous phase separation but occurs by nucleation-and-growth processes. It is then a classical primary crystallization. From the anomalous dispersion technique the ratio of the atomic volumes in the amorphous alloy does not change significantly during the crystallization of Y.

The author calculates the transverse phonon spectrum of a three-dimensional Wigner crystal in a large magnetic field where the electrons occupy the lowest spin-polarized Landau level. The calculation differs from that for a conventional phonon spectrum because the lattice is stable only in the presence of the magnetic field. They give specific results for wavevectors parallel and perpendicular to the magnetic field for the phonon spectrum of a Wigner crystal state in the narrow gap semiconductor HgCdTe. The temperature dependence of the electron lattice specific heat is also calculated and varies as T³2/.

In this work, the aging damping associated with the discontinuous phase transformation was studied in two Al-Zn alloys using a forced-vibration pendulum. Correlating these results with an X-ray diffraction analysis and TEM observations the authors suggested that the aging damping was directly proportional to the total interface area which was related to the nucleation and growth during the discontinuous transformation.

A general method is proposed for estimating activation energies of diffusion in (amorphous) covalent solids. The parameters involved are the shear modulus, the effective area associated with a bond and the size of diffusion saddle-point doorways associated with the critical percolation pathway for diffusion in the structure. Calculations of the effective force constant for dilating the diffusion doorways in a model of amorphous silica, made using the Keating potential, are in very good agreement with the behaviour predicted by the model, and calculated activation energies for diffusion are in good agreement with experimental values for diffusion of rare gases in vitreous silica.

Values of the optical energy gap E₀ as a function of temperature T in the range 10 to 300 K were obtained by optical absorption and modulated reflectance measurements on samples of CuInSe₂, CuGaSe₂, CuInS₂ and CuGaS₂. The variations of E₀ with T were fitted well by a Manoogian-Leclerc equation of the form E₀(0, 0)-E₀(T, 0)=UT+V phi (coth phi /2T-1). Values of (dE₀/dT)₁, the electron-phonon interaction contribution to the variation of the energy gap with temperature and (dE₀/dT)_s the static component, were obtained from the V phi and U terms respectively. Comparison with values from the pressure coefficient dE₀/dP indicated that in addition to the lattice-dilation term (dE₀/dT)₂, (dE₀/dT)_s contains a further contribution, labelled (dE₀/dT)₃, attributed to a change with temperature of the position coordinate u of the anions. From the values of (dE₀/dT)₁ and (dE₀/dT)₂, values were determined for the acoustic deformation potentials of the conduction band C_e and of the valence band C_h.

The polaron self-energy due to phonon confinement in a quantum box or wire has been calculated as a function of the size of the quantum boxes or wires by the perturbative method using the framework of the effective mass approximation. It is found that the effects of phonon confinement are dominant in small boxes and wires. The results also show that it is a function of both width and thickness of the wire rather than simply the cross-sectional area.

Analytical and numerical investigations are carried out to reveal the frequency and magnetization orientation dependences of the conductivity tensor components. Analysis of the perturbatively obtained expressions shows that the peculiarities of the tensor components that are even in magnetization are immediately related with the spin-flip transitions. The corresponding identification of the peculiarities of the orientation magneto-optical Kerr effect can allow one to extract the magnitude of an exchange splitting from experimental data. The results of application to Ni are presented.

The local electronic structure of intrinsic defects, including antisites, interstitials vacancies and complexes with themselves or with Si impurities, in a (GaP)₁/(InP)₁ (001) strained-layer superlattice determined by the Keating model, is calculated using a developed recursion method. Besides the localized states obtained, attention is paid to the electronic occupancy on various defects. A general consequence of describing the influence of isolated defects on the electron distribution of their neighbours is given. It is found that the charged state of an atom will be changed when it becomes an antisite. A closely bound (I_P-P_III) (III identical to Ga or In) pair, formed by Coulombic interaction, will induce some states in energy gap. No evidence is found anywhere of EL2-like behaviour in this system. Some novel diffusion mechanisms, based on I_P and V_P complex formation and dissociation, are presented.

The authors present calculations of the differential conductance, G, of a constriction, defined by a saddle-point potential in a two-dimensional electron gas, in the non-linear regime of transport as a function of Fermi energy, source-drain voltage and magnetic field. The manner in which the potential is dropped along the device is considered phenomenologically. The dependence of G on the parameters that define the potential drop is investigated, extending the model proposed by Glazman and Khaetskii. A method for measuring the sub-band energies and spin-splitting energies in a bottle-neck of the constriction is also proposed. Finally, a comparison between experimental data and theoretical calculations is presented.

The results of electrical resistivity rho measurements in the temperature interval 1.5-300 K on the intermetallic compounds YCu₂Si₂, YNi₂Si₂, YCu₂Ge₂, YNi₂Sn₂, YPd₂Si₂, YPt₂Si₂, YRu₂, YPt₂, YPd₃ and ScAl₂ containing small amounts of Pr as well as on Sc_0.95Pr_0.05 are reported in order to look for the Kondo behaviour of Pr ions. The logarithmic variation in rho , however, is not apparent in the temperature range of investigation in any of the alloys, although these alloys are potential candidates for exhibiting Kondo behaviour.

For pt.I see ibid., vol.3, p.4389, 1991. In this paper the causes of the 'mathematical breakdown' of the random-phase approximation RPA' are analysed. Starting from an exact matrix formula, a diagram analysis of a functional-integral approach is developed with the following characteristics: (i) All the singularities of the integrands are cancelled completely; there is no problem of the limitation from the convergence radius of the formula used. (ii) The reality of the partition functions at every stage is guaranteed and all the benefits of the complex representation are preserved simultaneously. (iii) All the functional-integral series are transformed into two-dimensional integral series. (iv) A functional-integral approach, which can calculate the mixed mode contributions, is given for the first time. The diagrammatic rules of mixed mode contributions and some concrete examples are given. Some exact symmetry relations and expressions are suggested and proved. These symmetry relations are also preserved at every state in the author's diagram analysis. They are useful in practical calculations. A new exact relation is also derived.

The origin and magnitude of a paramagnetic contribution to the CuO susceptibility at T<150 K are studied by analysing electron paramagnetic resonance and static magnetic susceptibility measurements carried out on a set of CuO single crystals. This has allowed the authors to confirm a previous supposition about the behaviour of the susceptibility for T<150 K. The asymptotic finite value of the susceptibility in the antiferromagnetic phase is also discussed by means of a crystal-field calculation which seems to rule out contributions to the local magnetic moments from incompletely quenched orbital angular moments of the Cu²⁺ ions. Preliminary magnetization measurements suggest the presence of weak ferromagnetism, which may explain the unusual behaviour of the CuO susceptibility.

Expressions are derived for determining the angular variation in the intensity of the forbidden hyperfine transitions ( Delta M=1, Delta m=+or-2) in the EPR spectra of transition ions. The intensities calculated using these expressions are compared with the predictions of Bleaney and Rubins and of Bir. It is found that third-order contributions arising from the nuclear spin operator as well as the quadrupole term are important in interpreting the intensity of forbidden hyperfine lines.

Ferromagnetic resonance has been used to study the isothermal annealing behaviour of metallic glass Fe₂₀Ni₆₀B₂₀ at temperatures between 300 K and 473 K. The temperature dependence of the resonance field (H_res) and the linewidth ( Delta H) for both parallel vertical (PV) and parallel horizontal (PH) configurations suggest the formation of less random clusters leading to a second amorphous (paramagnetic) phase. Landau-Lifshitz-Gilbert theory has been used to obtain estimates of the effective magnetization (M_eff) and the Gilbert damping or relaxation parameter ( lambda ). Well below and near the Curie temperature (T_c), lambda varies linearly with saturation magnetization (M_s) whereas at intermediate temperatures the behaviour is non-linear.

In this work the authors make Mossbauer and magnetization studies of Sn⁴⁺-substituted Ni_1-yZn_yFe₂O₄ (y=0.3, 0.4) ferrite. In this range Mossbauer spectra are not relaxed and may be used to gain insight into the Sn⁴⁺ distribution process. It is found that H_eff(B) remains almost unchanged and H_eff(A) decreases as the concentration of Sn⁴⁺ ions increases in the lattice. It is evident that Sn⁴⁺ ions occupy only the octahedral B sites. The magnetization shows a non-linear behaviour with increase in the proportion of Sn⁴⁺ ions.

A 2*2 matrix method is developed and used to calculate the reflection and transmission amplitudes for normal-incidence reflection and transmission by uniaxial crystals and crystal slabs. The reflection is characterized by two reflection amplitudes (r and r') which give the amplitudes of the reflected field components along and perpendicular to the incident polarization direction. The reflection results take the form r=r₀ cos² phi +r_csin² phi , r'=(r₀-r_e)cos phi sin phi , where phi is the angle between the incident polarization and the ordinary electric field vector in the crystal, and r₀ and r_e are the reflection amplitudes of isotropic crystals or crystal plates, with appropriate refractive indices. Similar results are obtained for the transmission amplitudes.

Studies of the positron annihilation and the Seebeck effect were performed on silver-doped tungsten trioxide of composition Ag_0.01WO₃, on the substoichiometric tungsten trioxide phase WO_2.975 and on WO_2.90(W₂₀O₅₈). Both methods point out that there are some clusters of Ag⁺ ions in the first case and oxygen vacancies in the second case. The clusters have some internal substructure and the formation enthalpy of the vacancy (or a positron trap) in this substructure was obtained as equal to 0.39+or-0.01 eV. The measurements of the Seebeck effect showed that Ag_0.01WO₃ and WO_2.975 are normal n-type semiconductors whereas WO_2.90 exhibits metal-like properties.