Table of contents

For original paper see ibid., vol.17, p.L551 (1984). Delley has attempted to explain the observation of a lower size limit in mass spectra of multiply charged Pb clusters. Cluster ions are produced by electron impact, ions Pb_n^z+ are only observed above a critical size n_min(z)=30, 45, and 72 for z=2, 3, 4 respectively. Echt accepts that the vertical ionisation and subsequent charge redistribution result in excited cluster ions, but argues that Delley's quantitative analysis of n_min(z) is based on invalid assumptions.

The origin of diffuse signals observed in time-of-flight phonon-reflection experiments is investigated theoretically. In this connection, it is shown that the mode-converted surface phonons are responsible for the anomalous energy transfer across the solid-liquid He interface.

The magnetic and charge susceptibilities of the one-dimensional mixed two-band Anderson model are studied from the Bethe ansatz exact solution. The results show that the ground state of the symmetric model is the coherent Kondo state for the strong coupling and the insulating state for vanishing coupling.

Using magnetic translation symmetry, the Hall conductance of an isolated magnetic band in units of e²/h is shown to satisfy the Diophantine equation p sigma +qm=1, where p and q are relatively prime integers giving the number of flux quanta per unit cell area, phi =p/q, and m is an integer. This equation holds for a general periodic Schrodinger Hamiltonian with an arbitrary magnetic field and is a direct consequence of the q-fold degeneracy of magnetic bands. Extension to general real phi gives the equation phi sigma _H- rho =integer with sigma _H the Hall conductance and rho the number of electrons per unit cell, from which sigma _H is uniquely determined once rho , phi and the gap structure are given.

The structure of a concentrated aqueous solution of LiCl has been investigated by neutron diffraction. The methods of first- and second-order differences were used. It is concluded that structural features characteristic of both the molten salt and the dilute solution coexist in concentrated solution and that each Li⁺ ion is surrounded, on average, by three water molecules and three chloride ions.

The ground-state properties of the He IV-A crystal are studied. The analytic form of a two-body force between pairs of He* atoms is derived from existing data. This function is then used to compute the classical cohesive energy of the crystal for several types of cubic lattices. Quantum mechanical ground-state energy calculations involving cluster expansion techniques and the method of correlated wavefunctions are also presented. On the basis on these calculations the FCC lattice structure of He IV-A appears to be energetically preferred.

The relative stability of a set of possible structures for the group IV elements has been investigated by using a semiclassical approximation. Independently computed pseudopotentials are the only input of the method. Calculations have been done for all group IV elements, including Sn and Pb. The experimentally observed structural trends are correctly described by the numerical results. In particular the authors explain the facts that graphite is the stable form of C and FCC the stable structure of Pb.

Using a simple 'static' model of the F centre, an analytical expression for the Stark peak shift is obtained for all powers of F. The available experimental and empirical data are used to compare the higher order effects to the quadratic one. It is found that the ratio Delta E(F⁴)/ Delta E(F²) becomes appreciable for F_0-p>or approximately=30 kV cm^-1 and that one gets a purely quadratic effect only in the limit of strong coupling. A method is suggested for measuring these higher order effects.

The temperature dependence of the infrared reflectivity spectra of NaNO₃ is reported in the whole ordered phase, from 10 to 549K. Reflectivity spectra from 5 to 600 cm^-1, are fitted with a model of the factorised form of the dielectric function including the relaxation mode. The temperature dependence of the infrared active lattice mode frequencies, dampings and oscillator strengths is reported. The ionic effective charges are calculated from the TO-LO splitting of the polar modes. The temperature dependence of the relaxation parameters confirms the gradual phase transformation due to the growing of the disorder in the crystal structure above 423K.

The augmented space technique introduced by one of the authors is used here to study the vibrational properties of Ni_xPt_1-x alloys at various concentrations. The 1CPA has proved to give results in poor agreement with experiment. Strong disorder in both mass and force constant has been quoted as being responsible. The CCPA results confirm this and are found to be in much better agreement. The authors incorporate statistically correlated diagonal and off-diagonal disorders correctly, as well as the force constant sum rules.

The influence of bond and site dilution on the phase transitions of a semi-infinite three-dimensional Ising model is investigated. To determine, in particular, phase diagrams two different techniques have been used: an extension of Migdal's approach to disordered systems and a more tractable approximation introduced recently. Temperature-concentration phase diagrams for fixed values of the ratio of surface and bulk interactions can exhibit five different types of phase transitions.

A new method is developing for finding the quasiparticle density of states in an infinite system containing a region where electron-electron interactions occur. Density fluctuations in this region couple the Schrodinger equations for the components of a generalised quasiparticle amplitude. The method is applied to the Anderson model of an adsorbate, the discrete channels coupled by the density fluctuations being determined from a cluster calculation. The resulting densities of states are satisfactory for all strengths of coupling between the adsorbate and substrate, and electron-electron interaction.

The authors adapt the split-step Fourier transform (SSFFT) algorithm to the problem of calculating the energy band diagrams and associated wavefunctions of solid-state lattices. The analysis is accompanied with a study of the accuracy of the technique in several test cases. They conclude from these calculations that the SSFFT method can be applied to a wide variety of solid-state physical problems.

A nonlinear theory for steady state electronic transport is suggested for electron-phonon-impurity systems in the presence of both an electric field and a crossed magnetic field. An electron temperature T_e is introduced without preassumption of any kind of equilibrium-type distribution function. Thus the formulation based on it should be applicable for both high and low carrier density in both classical and quantum limits in a magnetic field. A force-balance equation and an energy-balance equation are derived in steady state for determining T_e and current density. It is shown that the well known formula for transverse resistivity and Hall coefficient apply equally well for the nonlinear case if the electrons move in a single, isotropic and parabolic band. For moderate magnetic fields at about room lattice temperature, the longitudinal and transverse conductivity are calculated as functions of drift velocity assuming non-polar optical phonon scattering. The cooling effect of a magnetic field is also studied.

In a recent publication the authors have proposed a linear response formula for the Hall coefficient which only depends on the states at the Fermi energy. In this paper they consider this derivation using the Wigner representation which clarifies the issue and brings out a number of subtle points concerning the Hall coefficient.

The author derives a formula for the plasma oscillations in a two-dimensional electron gas (2DEG) when a periodic potential is applied in one of the dimensions. The result shows an anisotropy of the dispersion which in turn should show up as a frequency shift of the maximum infrared absorption when the plane of polarisation is varied.

The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni₃Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (T_c), a nearly linear inverse susceptibility well above T_c, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni₃Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature T_c, for the ratio p_eff/p₀ of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T²) variation of the magnetisation with temperature well below T_c, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.

A strongly enhanced superhyperfine interaction and large g shifts are observed by EPR on Ru³⁺ (S'=¹/₂) diluted into the Van Vleck paramagnet Tm₃Al₅O₁₂. The enhanced superhyperfine interaction (I^(Tm)AS'^(Ru)) is very anisotropic with components of A reaching 0.017 cm^-1. This is bigger than the self-ion hyperfine interactions for both Tm³⁺ and Ru³⁺. The g shift is also anisotropic with Delta g_z=-0.80 and Delta g_{perpendicular to} =4.64. The experimental data are interpreted in terms of a phenomenological Ru³⁺-Tm³⁺ exchange interaction tensor and some information is obtained linking components of this tensor.

Zeeman excitation and emission experiments have been used to study ⁵D and ⁷F multiplets of Eu³⁺ ion in the G1 centre in CaF₂. The characteristics of the Eu³⁺ energy levels deviate strongly from those of the free ion, and this is attributed to the presence of the neighbouring O^2- ion which provides charge compensation.

The authors report measurements of the silicon X-ray K emission bands of the crystalline polymorphs of silica (SiO₂): alpha -quartz, beta -cristobalite, beta -tridymite, coesite (all having 4:2 coordination), stishovite (6:3 coordination) and of the vitreous forms Infrasil and Suprasil (a-SiO₂). For alpha -quartz and coesite the Si L_2,3-emission bands are also presented. The K spectra of the various polymorphs of 4:2-coordinated silica exhibit small but distinct differences; the L spectra are virtually identical. The spectra are discussed along with available calculations of the electronic structure of alpha -quartz, beta -cristobalite and a-SiO₂. While for alpha -quartz and a-SiO₂ reasonable overall agreement with the theoretical results is found, the calculated width of the gap between the bonding and non-bonding valence bands in beta -cristobalite is incompatible with experiment.