Table of contents

A simple theory is presented to explain the observation that heavy metal ferrocyanides and ferricyanides become n- and p-type semiconductors respectively on absorption of trace quantities of water.

The authors examine the free energy barrier associated with critical nuclei in the homogeneous nucleation of discommensurations at the commensurate-incommensurate transition in the charge density wave state of 2H-TaSe₂. The various terms entering the free energy of critical nuclei are estimated within a Landau theory. The parameters in the Landau theory are deduced, as well as possible, from experiment. The energy barrier that results is reasonable, also somewhat too high to account for the homogeneous nucleation rate observed by electron microscopy. The reduction of the energy barrier due to internal strains associated with the orthorhombic domain structure is estimated.

Brillouin scattering measurements were made in the temperature range from 80K to 600K. From the data the velocities of the phonon modes corresponding to the elastic stiffness coefficients C₁₁, C₂₂ and C₃₃ were determined. In the incommensurate phase between T₁=297K and T₂=280K, C₂₂ and more clearly C₃₃ shows an anomalous decrease with decreasing temperature. Between T₁ and T=260K the Brillouin peaks are broadened attaining their maximum widths just above T₂. These effects are interpreted by assuming a coupling between phonons and modulations of the incommensurate phase. The results suggest that the dominant contribution comes from the phase mode-acoustic mode interactions.

The EPR spectra of RbMgBr₃ crystals doped with Mn²⁺, Cr³⁺ and Gd³⁺ indicate that the host lattice undergoes two structural phase transitions between the melting point and 77K. At high temperatures, 738-449K, RbMgBr₃ adopts the hexagonal CsNiCl₃ structure which consists of identical symmetric (MgBr₃^-)_n chains (phase I). At intermediate temperatures, 449-235K, the structure appears to contain two structurally distinct types of (MgBr₃^-)_n chains (phase II). The spectral properties indicate that one type of chain is polar while the other is non-polar (symmetric). At low temperatures, 228-77K, the lattice is no longer hexagonal and seems to consist of a single type of polar (MgBr₃^-)_n chain (phase III). The first phase transformation is quite abrupt while the second transformation takes place over an appreciable temperature range ( approximately 7K).

The authors present the numerical results for the static and dynamical properties of the two-dimensional quantum electron gas interacting with a ln(r) potential, obtained by using the self-consistent theory of Singwi, Tosi, Land and Sjolander (1968). The static structure factor and the pair correlation function are calculated for the coupling parameter r_s=1,2,3 and 4. The polarisation around an impurity charge and the plasmon dispersion have also been obtained. The results are qualitatively similar to the corresponding results for three dimensions but the correlation effects are more pronounced.

Using modified CNDO/2 calculations on small silica fragments, allowing for structure relaxation, the authors have estimated the formation energy of a pair of oppositely charged dangling bonds in vitreous silica to be 3-4 eV.

Thermal conductivity measurements on the Cr²⁺:Al₂O₃ systems are reported. Results are obtained as an applied uniaxial stress is varied and they are analysed in terms of three resonant scattering frequencies. Two of these frequencies are found to vary with stress but one remains unchanged. The stress-dependent measurements are accounted for by the stress Hamiltonian previously derived with the ion-lattice coupling constant V_Eb=43000 cm^-1.

APR experiments on weak n-type samples of chromium-doped GaAs produce a weak set of absorptions consisting of sharp peaks on a broad background when the sample is kept in the dark. A model proposed earlier for substitutional Cr²⁺ ions at Ga sites in which the dynamic Jahn-Teller effect and random strains play a crucial role, has been examined to see whether it can account for the EPR and for this weak APR absorption. Revised electronic and Jahn-Teller parameters are deduced which satisfy the requirements of data from conventional and thermally detected EPR experiments and data deduced from the structure of the zero-phonon line and phonon scattering experiments, while at the same time the model then satisfactorily fits most, but not all, of the APR data. The first-order Ham factor gamma is found to be about 10^-4 and the implications of this value in understanding the properties of Cr²⁺ are also discussed.

The structure and energy of the eigenstates of a 1D Hubbard Hamiltonian with negative U is studied. Solutions for the Lieb-Wu equations (1968) corresponding to U<0 are constructed from those for U>0. It is found that the ground state is continuous, while the ground-state energy, although continuous, is not analytic at the U=0 point. In the excited states, in contrast to the U>0 case, both kinds of excitation are connected with the charge distribution and the state of the spins does not affect the energy explicitly. It is also found that the distribution of parameters can change discontinuously in the excited states as U crosses zero.

The dependence of the conductivity prefactor sigma ₀ on the activation energy E_a in a-Si:H has been determined by measuring the Staebler-Wronski effect (1977, 1980) in undoped and n-doped glow discharge a-Si:H samples and by comparing DC conductivity measurements in differently n-doped samples in defined states. A very exact linear dependence of ln sigma ₀ on E_a as described by the Meyer-Neldel rule is observed, with good quantitative agreement between the parameters for the two cases. On the basis of these results it is proposed that the Meyer-Neldel rule is fundamental in a-Si:H, by which it is meant that it has to be taken into account in any model for the electronic transport properties of this material, and that it should not be explained by a model specific to a-Si:H.

The stability of ferromagnetism is studied in the three-dimensional Ising model, in the presence of a static, spatially random magnetic field. An approximation is introduced in which regions with a given spin direction are assumed not to contain smaller clusters of the reverse magnetisation. A combination of the Peierls argument with a rescaling transformation leads to a lower bound for the magnetisation, which is non-zero at low temperatures, provided the random field is not too strong.

EPR of two new tetragonal centres of U³⁺ in alkali-compensated SrF₂ reported. The g-values are independent of the compensating ion (sodium or potassium) and are mod g/sub /// mod =1.72+or-0.01, mod g_{perpendicular to} mod =2.49+or-0.01 for centre I and mod g/sub /// mod =2.824+or-0.005, mod g_{perpendicular to} mod =1.97+or-0.01 for centre II. They can be deduced from a unique set of cubic parameters mod 2P mod =2.824 and mod 2Q mod =1.72 if weak tetragonal distortions of opposite signs for the two cases are assumed. These parameters give a composition parameter x for the crystal-field potential of the virtual cubic centre and an orbital reduction factor k_J in agreement with the values measured for cubic U³⁺ in CaF₂. The centre II is attributed to a substitutional U³⁺ ion compensated by a substitutional monovalent ion at a (2,0,0) lattice site while the other centre is attributed to a distant compensated substitutional ion, slightly displaced towards a cube face.

The EPR hyperfine structure has been measured for a large variety of ²³⁵U³⁺ centres in CaF₂ and SrF₂ crystals. With these data and with the previously reported results on U³⁺ in LaCl₃, by using an effective relativistic theory, a value of -(0.46+or-0.03) mu _n for the nuclear magnetic moment of ²³⁵U has been estimated.

Antiferromagnetic resonance experiments in the range 10< nu <60 GHz have been performed on a series of quasi-one-dimensional Mn²⁺ compounds with varying anisotropy. Pronounced deviations from classical AFMR theory are observed, in accordance with the experiments at higher frequencies reported by Tuchendler et al. (1981). These deviations show a substantial temperature dependence and seem to be related to the amount of anisotropy between the preferred and next-preferred direction of spin alignment. The experimental evidence will be compared with the results of a renormalised spin wave calculation.

The general theory of the behaviour of the muonium atom of 'finite size' with an electric quadrupole moment in matter is presented. The experimental situation in alpha -quartz is discussed, and the constants of hyperfine and quadrupole interactions are derived.

Resonance Raman scattering in AgCl is investigated at 1.8K for excitation around the indirect exciton absorption edge. The observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 and 12.9 meV, respectively. The weakness of the scattered intensity is explained by a reduction of free-exciton lifetime due to self-trapping. The quantitative analysis of the scattered intensity as a function of exciton kinetic energy reveals that, besides self-trapping as the most important relaxation mechanism, intravalley scattering by long-wavelength LA and LO phonons and intervalley scattering by TA(X) and LA(X) phonons are effective. Relative exciton-phonon matrix elements and scattering probabilities are determined. An estimate is made for the effective mass and total lifetime of the free exciton giving M*=(2.4+or-1.2)m_e and tau _tot=65 ps, respectively.

The authors derive an analytic expression for the magnon line positions of a quasi-one-dimensional antiferromagnet like CsCoCl₃. The results obtained from this expression show excellent agreement with the values obtained from Raman scattering experiments. Some analytical calculations have also been performed to explain the hitherto unexplained experimental observation that once the magnon lines cross the phonon line position at 118 cm^-1 the peaks disappear and cannot be traced.

Absorption and luminescence-excitation measurements show that the H3 vibronic centre in diamond (zero-phonon line at 2.463 eV) has a series of excited states which give rise to absorption lines between 3.2 and 3.6 eV. There is no luminescence produced by transitions from these higher excited states to the ground state. Following absorption in this region the centre relaxes to the lowest excited state and then emits luminescence in the vibronic band associated with the 2.463 eV line.

The authors have measured the metal L_III absorption spectra of layered 4d transition-metal dichalcogenides with the use of a Yohan-type curved-crystal spectrometer. The spectra showed that the metal d orbitals are strongly admixed with the chalcogen p orbitals to form the covalent bonding. The rigid-band model which has been suggested by Wilson and Yoffe (1969) appears to be valid in the first approximation for the unoccupied bands to about 5 eV above the Fermi level, but is not valid for the higher-energy bands. As the atomic number of the chalcogen increases, the bands broaden and overlap each other, probably due to the spin-orbit interaction. The absorption-edge shifts observed can be explained in terms of the energy differences of the bottom of the conduction band or the lowest empty state.