Table of contents

Proposes a new method for extracting the interatomic potential from liquid structure data. The method, which is based on the Weeks-Chandler-Andersen theory, appears to be more reliable than the Percus-Yevick and hypernetted chain methods.

Important discrepancies have been reported between two previous measurements of the phonon dispersion curves of strontium titanate. New measurements are reported here which are found to be in good agreement with the previous data of Stirling (1972).

In molecular solids, symmetrical groups like CH₃ and NH₄ undergoing torsional oscillation and tunnelling rotation in a hindering potential, also experience temperature-dependent inertial forces due to the lattice vibrations. The main effect can be described in terms of a fluctuating displacement of the hindering potential. The stationary states in this fluctuating potential are temperature-dependent mixtures of the torsional oscillator states of the static potential. As a result, the frequency of tunnelling rotation of the lowest stationary state is a thermal average of the tunnelling frequencies of the torsional oscillator states of the static potential. This tunnelling frequency falls with increasing temperature, while remaining well defined, in agreement with experimental observations.

The ENDOR technique has been used to investigate the electron distribution in CsCdF₃ crystals containing Mn²⁺, Ni²⁺ and Co²⁺. The bond structure is of the form M²⁺-F^--Cd²⁺, where M stands for Mn, Ni or Co, and the measured isotropic ¹¹¹Cd and ¹¹³Cd hyperfine interaction is taken to be proportional to the unpaired spin fraction, f_sigma , in 2p_sigma orbitals on F^-. The results suggest that for the Mn²⁺-F^- bond f_sigma =4.3%, which is much larger than estimates from neutron scattering data though in reasonable agreement with work by Rinneberg and Shirley (1973, 1976) who used a method involving perturbed angular correlation of gamma -rays from ^111mCd.

Electric-field-induced absorption of a type IIa diamond has been measured at various resolutions over a wider spectral range (2300 to 2700 cm^-1) than hitherto. A new feature has been observed at 2335 cm^-1 close to the Raman-active but otherwise infrared-inactive band at 2339 cm^-1. Higher-resolution (5 cm^-1) measurements in the neighbourhood of the k approximately=0 overtone resulted in a band being observed at exactly twice the Raman frequency, namely 2664 cm^-1. This frequency is compared with a previous field-induced infrared result at 2660 cm^-1 measured at a resolution of 28 cm^-1 and a Raman scattering result at 2667 cm^-1 measured at a resolution of 8 cm^-1. The anisotropy of the overtone band is also found to agree with theoretical predictions.

Reports uniaxial stress measurements on the 30307 cm^-1 absorption line of the A aggregates of nitrogen in diamond. The A aggregates are shown to be trigonal. This result together with published data suggests that an A aggregate is a pair of nitrogen atoms substituting for two carbon atoms.

A physical explanation is proposed for the influence of the initial-hole location in the band-like versus quasiatomic characteristics of Auger spectra involving quasilocalized levels. Interatomic processes not competing with intra-atomic ones are shown to always sample the band of the solid in view of their much slower transition rates. Interatomic transitions between substrates and adsorbed layers are thought to be connected with d-band metal electrons in different bonds.

Comparison of the high-resolution L_2,3M_4,5M_4,5 Auger spectra and the L₂,L₃ XPS spectra of Zn and ZnO clearly shows a reduction in the L₂L₃M_4,5 Coster-Kronig transition probability in the oxide. A simple calculation indeed shows this transition to be nearly at threshold for ZnO.

It is shown that by destroying valence bonds the absorption of light changes the frequencies of the lattice modes of vibration as a result of the change in the binding energy of the atoms. A differential Raman analysis is used for detecting the small changes in the frequencies of the lattice vibrational modes.

The dynamics of the phase transition in SrTiO₃ (T_c approximately 105K) is complicated by the presence of a central mode in the scattering at the R point of the cubic Brillouin zone in addition to the expected soft-mode scattering. Experiments using Mossbauer gamma rays are reported in which 'elastic' scattering with an energy change of less than 1.5*10^-8 eV is detected at the R-point for temperatures as high as 170K. Furthermore, by using X-rays of different incident energy, it is shown that above T_c the scattering power of the crystal at the R point is depth-dependent. The intensity of the scattering above T_c can be altered by annealing the crystal. The results are interpreted in terms of a depth-dependent transition temperature; the crystal begins to transform to the lower-temperature phase first at the surface, and as T_c is approached from above the transformed surface region extends further into the bulk.

The velocity of second sound in ³He-⁴He mixtures was measured near their lambda transition temperatures under their saturated vapour pressure with molar concentrations of ³He of 0.449, 0.513, 0.544 and 0.560. Together with the data for lower concentrations previously reported, this gives information on the concentration dependence of the critical exponents, the amplitudes of second sound and the superfluid fraction. In particular, for the critical exponent, zeta , the universality concept seems to be satisfied for the concentration range up to 0.560 along the path of constant chemical potential difference, phi .

A tight-binding Hamiltonian in a bonding-antibonding basis is used to evaluate the optical dielectric response, within the random phase approximation, for diamond and silicon. The authors include the effects of local microscopic fields, and compare the results with experiment. They also discuss theoretically how local fields should affect the optical response, and why the next term beyond the random phase approximation gives only a negligible contribution to the response.

A method is proposed for calculating the energy spectrum for a rather general type of well, of which the potentials encountered in molecules and thin films may be considered representative. The procedure is to introduce a simple reference system and corresponding perturbation series. The parameters defining the reference potential are chosen by optimizing the perturbed energy, to first order, which is of standard variational form. Then higher terms in the series provide systematic corrections. Tests on simple cases, where exact solutions are known, suggest that second-order energies should be nowhere in error by greater than 4%. The procedure is then applied to a linear chain of pseudoatoms and the band structure for sodium is considered. The result is a very free-electron-like picture for the occupied states but with (m*/m)-1 strongly influenced by virtual transverse excitations.

The effect of uniaxial compressive stress of up to 6*10⁹ dyn cm^-2 on free-carrier absorption in various doped Ge, Si and GaAs crystals has been investigated. Absorption measurements were made at 18.5K and 79K over the wavelength range 8 mu m to 24 mu m with radiation polarized parallel or perpendicular to stress. The latter was applied along (111), (110) or (100) directions in Ge, (110) or (100) directions in GaAs but (100) directions only in Si. Changes in absorption produced in all but one of the germanium crystals are explained by a simple electron-transfer model in which the total electron concentration in the conduction band remains constant and electrons merely transfer between the (111) multivalley states as they are split by stress. For the other germanium crystal, the behaviour under (111) stress was similar to that found in silicon under (100) stress.

The probability of free-exciton decay in the presence of an ionized donor in a crystal, which leads to the neutralization of the ionized donor and the creation of a free hole, is suggested to be of the order of 0.5*10^-5n_d⁺ cm^-3 s^-1 in Si and GaP, and 0.2*10^-4n_d⁺ cm^-3 s^-1 in Ge, where n_d⁺ is the concentration of ionized donors. These results are derived by means of a modification of a theory by Trlifaj, the limitations of which are pointed out.

An investigation is carried out on the effect of velocity dependent screening of the Fermi surface form factors for non-local potentials. Application is made to the resistivity of liquid aluminium.

Four functions are introduced to describe the response of classical paramagnets to fluctuations in basic 'mechanical' and 'thermal' wavelength-dependent variables, and the response functions reduce, in the long-wavelength limit, to the specific heats at constant field and magnetization and the isothermal and isentropic susceptibilities. A response function is proportional to the mean square fluctuation in its associated variable, which is analogous to the well known results obtained from the theory of thermodynamic fluctuations in a fluid. The wavelength, temperature and field dependences of the response functions are studied for the one-dimensional classical Heisenberg model, for which exact results can be obtained. Approximate analytic results are derived for the low-temperature inverse correlation lengths and isothermal susceptibilities of ferro- and antiferromagnetically coupled systems in finite applied fields. Particular attention is paid to the behaviour, as a function of field, of an antiferromagnetically coupled model, emphasizing the rather striking features which occur at field strengths typifying a change from antiferromagnetic to ferromagnetic character.

For pt.I see ibid., vol.9, p.3639 (1976). A description of the dynamic response of a classical paramagnet to a time and spatially varying perturbation is formulated, which takes into account coupling between the magnetization and energy densities induced by an applied magnetic field. The coupling is manifested implicitly, via thermodynamic quantities which enter the theory, and this aspect is included fully by a renormalization of the energy density. There is also an explicit modification of the dynamical equations, which is studied within the context of a generalized Langevin equation, using the formulation proposed by Mori. It is shown that a judicious choice of dynamical variables enables one to identify dynamical effects resulting from the coupling and, in particular, modifications to the frequency and width of a collective mode. In order to illustrate the consequences of this formalism, results are presented of explicit calculations for one-dimensional ferro- and antiferromagnetically coupled classical Heisenberg magnets, for which all static spin correlation functions of interest can be calculated exactly.

An explanation of the recently measured static and dynamical properties of KDP-DKDP mixed crystals is presented. The Kobayashi model for this type of ferroelectric is assumed and, under approximations justified here, the problem of disorder is reduced to a form where a theory developed by the authors in a previous paper can be applied. The virtual crystal approximation is used to study the concentration dependence of the static and dynamic properties of the mixed system both above and below the transition. Numerical results for the Curie temperature, Curie constant, saturation polarization and mixed-mode frequencies and their pressure derivatives are presented and are generally in good agreement with observation.

The first-order Raman spectrum of cubic zinc metaborate has been measured at room temperature, and the number of q=0 phonon frequencies found exceeds the group theoretical prediction by one. The additional band appears as an interference feature at 122.5 cm^-1 and is thought to be attributable to some impurity introduced during crystal growth. The results are in reasonable agreement with an earlier qualitative investigation of the infrared transmission spectrum of this material, and the reported fluorescent properties are verified. A comparison between the Raman results for zinc metaborate and those for Cr₃B₇O₁₃Cl shows some correlations. A simple force-constant calculation identified four bands in each compound that can be associated with vibrations of BO₄ tetrahedra contained within the boron-oxygen framework.