Table of contents

Recent measurements of the scattering law, S(Q, Omega ), for liquid nitrogen (Sinclair et al. 1975) which were interpreted as corresponding to the scattering as for free nitrogen nuclei at 77K are re-examined. It is found that the experimental results correspond more closely to scattering by effectively rigid nitrogen molecules. This interpretation is in better accord with our present knowledge of liquid nitrogen.

A simple recurrence relation connecting the lattice Green's function at (l, m, n) and the first derivatives of the lattice Green's function at (l+or-1, m, n), is presented for the simple cubic lattice. By making use of that recurrence relation, the lattice Green's functions at (2, 0, 0) and (3, 0, 0) are obtained in closed forms, which contain a sum of products of the complete elliptic integrals of the first and the second kind.

An extension of the MSX alpha cluster technique is proposed for the calculation of the energy levels in extended structures containing large numbers of atoms, using the example of an adsorbed layer on an adsorbent surface. The valence levels in these systems are delocalized throughout the whole of the structure and it is suggested that the electron content of any model cluster must be referenced to the real structure and determined on the basis of an accurate representation within any chosen cluster for the extended system.

A recent letter about the effect of Coulomb interaction on variable-range hopping is discussed.

An insulator-metal-insulator sequence of transitions has been observed as a function of pressure in a freshly prepared single crystal of CuCl. The metallic phase is evidenced by a sudden drop in resistance by a factor of approximately 10⁷ between approximately 40 Kbar and approximately 55 Kbar.

The question of local field corrections to the hopping conductivity of small polarons in a polarizable dielectric is investigated. It is shown that the Lorentz contribution to the dipolar potentials cancel in taking the inter-site energy difference, and that the electric field appearing in the hopping conductivity is just the macroscopic field.

The conductivity of n channel MOSFET devices has been measured in the temperature range 4.2K<or=T<or=77K in samples with varying densities of interface charge. Contrary to previous results in this temperature range, the conductivity obeys the law lg sigma varies as T^-1/3 indicative of a variable range hopping mechanism. The minimum metallic conductivity is found to vary linearly with the average separation of the oxide charge, contrary to prediction.

ESR studies on Cu²⁺ doped in Mg(H₂O)₆H₂EDTA crystals are reported, in which the hyperfine structure for the complex Cu²⁺(H₂O)₆ at room temperature is observed for the first time. This has been attributed to the large scale hydrogen bonding involving the water protons in this crystal. The spin Hamiltonian parameters are found to be considerably different from the corresponding values of Cu²⁺(H₂O)₆ in other matrices, due to the large covalent bonding present in this crystal. g_z=2.286+or-0.002 g_y=2.095+or-0.002 g_x=2.193+or-0.002 A_z=87+or-2G A_y=77+or-2G A_x=28+or-2G.

The authors propose a new model of the photothermal ionization mechanism of shallow donor states in CdTe. Assuming that the electron in an excited donor state is released to the conduction band by the absorption of one LO phonon, various mechanisms of the 2p state decay were investigated. Results show that for temperatures greater than 10K the thermal ionization probability of the 2p excited state in CdTe is about unity.

Previous work by Loh (see abstr. A34348 of 1973) is discussed, in which the vibrational spectra of sheet silicates is interpreted in terms of weakly coupled molecular units. An alternative view is proposed.

A simple analytic expression is derived for realistic optical cross sections, associated with deep impurity states in semiconductors, and applied to GaP:O.

The neutron scattering from deuterated thiourea, SC(ND₂)₂, was examined in the antiferroelectric and paraelectric phases using single crystals mounted on triple-axis spectrometers. The long-period order in the antiferroelectric phase gives rise to satellite Bragg peaks, the intensities of many of which were measured in the (hkO) and (Okl) zones. A model of the structure in which the lattice contains a sinusoidal displacement of the molecules as rigid units was developed and fitted to the data with reasonable success. Quasi-elastic diffuse scattering was detected in phase V which was strongly temperature dependent. A mechanism for the transition based on an overdamped soft rotational lattice mode is presented and supported by calculations of the Coulomb lattice energy of a thiourea crystal distorted by such a mode as a function of the wavevector. The temperature dependence of the relaxation time of the ferroelectric mode is given.

The symmetry of the normal modes of vibration of CS₂ and Cl₂ are analysed at points and lines of high symmetry in and on the surface of the Brillouin zone. The results of Waeber (1969) for Ga which has the same structure as Cl₂ differ from the authors in certain small but important respects.

Extending the phenomenological law due to Machlup and Onsager, the authors present a model for systems undergoing distortive structural phase transitions. It is then shown that the critical dynamics reduce to that of the kinetic Ising model. In a certain limit, the model can be solved exactly.

Measurements are presented of the temperature variation between 2K and 100K of the thermal conductivity, kappa of several samples of extruded polyethylene. At high temperatures ( approximately 100K) there is an anisotropy in kappa , with the values measured parallel to the extrusion direction up to 5 times greater than those observed in the perpendicular direction. At low temperatures, on the other hand, there is much less variation in kappa and, at helium temperatures, the anisotropy is completely absent. In the direction of the extrusion the thermal resistance of the crystallites is assumed to be negligible, and the variation of kappa with T can be explained on the basis of a recent model for amorphous solids. At low temperatures, where the dominant phonon wavelengths are much greater, the anisotropy of the crystallites is not important, and measurements can be accounted for by the fluctuations in the sound velocity between the crystalline and amorphous regions.

The dynamic structure factor S(q, omega ) appropriate to the interpretation of neutron scattering measurements on ³He is studied within two theories that together cover the range of wavevectors down to the Fermi wavevector k_F. Both theories express the dynamics in terms of the isothermal susceptibility chi (q) and the longitudinal frequency omega _l(q). These quantities are estimated from precise X-ray measurements and a Lennard-Jones potential. An intermediate q theory is used to investigate omega _q in the vicinity of the main peak in the static structure factor. omega _q passes through a minimum at q=1.75 AA^-1 where it has a value of some 0.4 meV. The magnitude, and to a lesser extend the position, of the minimum, is shown to be a sensitive function of the particle potential. Using a Lennard-Jones potential with De Boer and Michels parameters the authors obtain a quantitive interpretation of preliminary neutron scattering data by Scherm et al. (1974). S(q, omega does not show a narrow, intense peak indicative of a collective mode for q approximately k_F.

It is shown that by working in a Hilbert space of arbitrary dimensionality it is possible to use the formulation of the second quantization, and some interesting new formulae are derived.

A new self-consistent band-structure calculation for stoichiometric vanadium carbide is performed by means of the augmented plane wave method using a ratio of atomic sphere radii R_C/R_V=0.645. The results are compared with a similar calculation based on the ratio 0.899. It is demonstrated how the choice of sphere radii affects the potentials, the band structure and the charge distribution. A recommendation for determining the atomic sphere radii is given.

The reflection spectra of RbCl, RbBr and RbI single crystals were investigated for temperatures between 300K and 8K in order to study excitations from the Rb⁺4p level (<or approximately=16 eV) as well as the higher continuum transitions from the valence band (<or approximately=10 eV). The measurements were performed by use of the synchrotron radiation of DESY. The sensitivity for detecting details of the fine structure was increased by simultaneously measuring the wavelength-modulated spectra. The experimental procedure is briefly described. New spectral features have been resolved for the exciton multiplets from the Rb⁺ 4p level. They are discussed in the light of predictions of a recent model for the Rb⁺ 4p excitons based on ligand field theory. The continuum transitions associated with the valence band and the Rb⁺ 4p level show characteristic structure which is compared with calculations of the joint density of states.

Pr-Nd alloys represent an almost ideal model system for a mixture of singlet-doublet ions (S_eff=1) and Kramers' doublet ions (S_eff=¹/₂). Neutron scattering and magnetization measurements have been made on single crystals of Pr-3.0% Nd, Pr-5.5% Nd and Pr-26.3% Nd. These alloys order antiferromagnetically (on the hexagonal sites) at 6.3K, 6.5K and 11.4K, respectively. Measurements of the magnetic-field dependence of the magnetization were also made. The concentration dependence of T_N and the temperature dependence and magnitude of the induced moments can be understood in terms of a simple molecular-field theory. The authors conclude that pure Pr has a close-to-critical ratio between exchange interaction and the crystal field (=0.95). The crystal field acting on the Nd ion is smaller than that acting on Pr. The exchange interactions between Pr-Pr, Nd-Nd, and Pr-Nd are found. The relative magnitudes agree with the de Gennes scaling factor.

The magnetic structures and the magnitudes of the ordered magnetic moments for the hexagonal ternary U₂N₂M-type compounds (where M is P, As, S, Se) have been determined by neutron diffraction. In U₂N₂S and U₂N₂Se the observed magnetic unit cells have the same size as the chemical ones while in U₂N₂P and U₂N₂As the magnetic unit cell is doubled along the c-axis. At 4K the magnetic moments on the uranium ion amount to 1.4, 1.7, 1.6 and 2.3 mu _B for U₂N₂S, U₂N₂P, U₂N₂As and U₂N₂Se, respectively. In all of them the direction of the magnetic moment is along the c-axis.

Using a technique developed earlier by one of the authors, a method is presented for describing the behaviour of spin waves in a ferromagnetic lattice in which the bond linking two neighbouring spins may be either present, with probability c, or absent, with probability p=(1-c). A Green-function technique is used to describe the dynamics of the system and an expression is given for the energy and damping of long-wavelength spin waves in the limit of low defect concentration. Comments are made about the critical concentration of the system and a discussion is given of the relation between the present work and recent theories of conductivity in random systems. Finally the results of a self-consistent calculation of the magnetization of the system for the case of spin one-half are presented.

Two previously unreported EPR spectra have been observed from defects in neutron-irradiated CaO. The first spectrum consists of a single isotropic line with no apparent structure at g=2.0051. The second spectrum is described using spin Hamiltonians with S=¹/₂ and I=¹/₂. At room temperature the spin Hamiltonian has axial symmetry, but at 77K it is orthorhombic. By considering the behaviour of the spectrum at intermediate temperatures it is shown that the higher-temperature spectrum is due to a defect which is moving in the lattice, thereby averaging an otherwise orthorhombic Hamiltonian. The activation energy for this reorientation is found to be approximately 0.13 eV. A detailed model is presented to explain the observed spectra, involving the molecular ion OH^2- at an anion site.

The effective mass of electrons in n-type Pb_1-xSn_xSe is measured as a function of carrier concentration and band gap. The masses obtained are in agreement with the six-band Martinez theory which has already been successfully applied to the band structure of these alloys.

For pt.I see abstr. A79623 of 1974. The theory of nuclear spin relaxation due to dipole-dipole interactions between diffusive species on a crystal lattice has been formulated exactly and applied to a FCC lattice in a previous publication. The results are presented here for BCC and SC lattices and it is found that the differences between the exact theory and the approximate Torrey theory are larger in these lattices than in the FCC lattice.

Measurements of the pure quadrupole resonance frequency of ³⁵Cl in CH₂ClCOOH have been made over a wide range of temperature (77K to 300K). Two resonance lines due to chemically inequivalent sites have been observed throughout the above temperature range.

Low-temperature electron spin resonance spectra exhibiting methyl group hyperfine structure may have weak sidebands (usually unobservable by conventional ESR) shifted from the main spectrum by a frequency equal to the tunnelling rotation frequency of the methyl group. When the magnetic field is adjusted to bring one of the sidebands into a frequency range comparable with the proton Larmor frequency, irradiation of the sideband with a radiofrequency field leads to an observable degree of saturation of the proton magnetic resonance signal. Experiments are described on free radicals in gamma -irradiated 4-methyl-2,6-ditertiarybutylphenol.

A detailed Raman study of phonons in monoclinic Li₂SO₄H₂O and its deuterate is given. The spectrum for two directions of the phonon wave vector at 45 degrees to the y axis from liquid nitrogen to room temperature have been determined and the majority of the lines observed have been identified: although the conventional selection rules are violated because all Raman active phonons are polar. The correlation field splittings of some internal sulphate modes are found to be large, in contrast to results in a recent IR study by Meshitsuka, Takahashi and Higasi (1971). The authors have also determined the Raman spectrum for phonons with wavevector q// and perpendicular to to the y axis. The transverse optic modes are tabulated and classified for 90 and 295K. The angular dispersion of modes propagating in the yz plane has been observed and compared with theory.

Normal-incidence reflectivity spectra of layer-type beta -GaSe crystals have been measured for E perpendicular to c as well as E//c at 78K. A strong anisotropy in the selection rules for interband transitions has been observed and a suitable energy level scheme is proposed. The reflectivity spectra of the mixed polytype epsilon , gamma -GaSe have also been measured for E perpendicular to c at 78K and 300K. These show similar selection rules as in the beta form for the same polarization, indicating that the optical properties of GaSe are basically determined by the symmetry of a single layer, rather than by the symmetry resulting from the different layer stacking.

High-resolution X-ray emission spectroscopy (XES) is used to measure splittings of core p-hole final states of Mn²⁺ ions in MnF₂. It turns out that the Hartree-Fock free-ion approximation, including spin-orbit and crystal-field effects, is inadequate for a proper interpretation of the Mn K alpha _1,2 and K beta ₁ structures. A comparison is made with the corresponding core p-level X-ray photoemission spectra (XPS). The observed 2p XES multiplet splitting is absent in the XPS in spite of narrower linewidths obtained by XPS. The K alpha ₁ and K alpha ₂ lines in MnF₂ are broadened by 1.35 and 0.97 eV, respectively, while at least a 1.3 eV broadening of both 2p XPS levels is reported. The discrepancies between the XES and XPS may indicate importance of non-lifetime contributions to intrinsic XPS linewidths in MnF₂.

For pt.I see abstr. A19014 of 1975. X-ray L-fluorescence spectra of niobium metal and some of its compounds have been studied with the help of a flat crystal. Philips semi-automatic X-ray spectrometer (PW 1220). The emission spectra of niobium compounds are discussed with reference to the corresponding spectra of niobium metal. The change in intensity and shape of the L beta ₂ and L_{gamma 1} emission bands is related to the involvement of the 4d electrons in the chemical bonding.