Table of contents

Electron spin dephasing in the photo-excited ³B₁ state of the F₂²⁺ centre in CaO is studied from optically detected spin echoes after laser-selective optical pumping. The results show site-selective homogeneous broadening of the zero-field triplet spin transitions. Local concentration variations of nearby F⁺ centres are held responsible for inhomogeneities in the F₂²⁺-centre optical absorption and triplet-spin dynamics.

The temperature dependence of the low-frequency light scattering spectrum of quartz has been studied in the vicinity of the alpha - beta transition and in the incommensurate phase. New excitations have been observed in the range 0-20 cm^-1.

Zeeman measurements are reported on the 1044 and 1045 meV lines of a Li centre in Si, showing that the lower-energy line involves a spin triplet excited state and the higher-energy line a spin singlet excited state. Uniaxial stress data are analysed to show that the transitions occur at a trigonal centre and have orbital Gamma ₃ excited states with properties very similar to the valence band and conduction band extrema of Si.

The BCS gap is computed within the framework of the renormalisation group. The author finds agreement with the exact calculation obtained from the reduced BCS Hamiltonian.

A microscopic theory of the self-current correlation function is presented by deriving its memory function which consists of two parts. The first is the kinetic term whose time decay is expressed in terms of the velocity autocorrelation function and the self-intermediate scattering function. The second is the potential term which is expressed in terms of the interatomic potential, the radial distribution function, the self-intermediate scattering function and another quantity which describes collective effects in the liquid. For the wavenumber value k=0, this memory function is identical with the memory function of the velocity autocorrelation function which has been derived by Chiakwelu and Gaskell (1976). A preliminary calculation with liquid rubidium shows that the memory function is rapidly damped with increasing values of k. This latter behaviour is presumably due to the strong modulating effect of the self-intermediate scattering function.

The structures of molten MgCl₂ and of molten MnCl₂ have been investigated by applying the technique of neutron diffraction to isotopically enriched samples. For MgCl₂, the three partial structure factors and the three pair correlation functions have been extracted from the experimental data. For MnCl₂, the total structure factor and total pair correlation function were derived. Since manganese has a negative scattering length, a single experiment on natural MnCl₂ was sufficient to extract the Mn-Cl interionic separation and coordination number. It is shown that MnCl₂ and MgCl₂ are structurally similar in the liquid phase. It is shown that the structure of both melts is significantly different from that of molten ZnCl₂.

It is reported that the growth of colloids in NaCl powders is easy as compared with that in single crystals. This ease is attributed to the availability of more nucleation sites in the powders and to the motions of the colour centres being aided by dislocations. It is suggested that the study of colloids in microcrystalline powders provide information on such motions.

For pt.III see ibid., vol.16, no.11, p.2087 (1983). The well-known relations for the total amount W of material that diffuses in time t into or out of a semi-infinite solid when the surface y=0 is held at some constant concentration, are to be multiplied by a factor (1+ epsilon ²U) when the solid contains dislocations. epsilon ² is the volume fraction of material in dislocations. U is calculated for the model previously employed by the authors of a solid containing a regular array of straight dislocations all normal to and ending in y=0 and each represented as a pipe of radius a within which the diffusion coefficient D'>>D, the coefficient in regular crystal. U, an integral, is a function of alpha =a/(Dt)^1/2, of Delta =D'/D, and of the ratio, identical to epsilon / alpha , of the diffusion length (Dt)^1/2 to the half-spacing between dislocations. U is represented graphically as a function of alpha for various values of Delta and epsilon ². An application is made to experimental data on dislocation-enhanced isotope exchange rate measurements of anion diffusion in KBr by Dawson and Barr (1967).

From an experimental determination of the temperature and pressure dependence of the R₂₅ soft-mode frequency in RbCaF₃, KZnF₃ and CsCaF₃, it is shown that the overall temperature-dependent shift of the soft mode is mainly due to the self-energy (the purely anharmonic effect) whereas the quasi-harmonic contribution (the thermal expansion effect) is one order of magnitude lower.

The augmented space formalism of Mookerjee (1973, 1975) is used to develop a cluster CPA formalism for the phonon density of states in random binary alloys. Off-diagonal disorder has been incorporated and a self-consistent medium is generated for the calculations. Unlike earlier work, the Green function does not suffer from analytical difficulties in any concentration range and a correct averaging over random masses is performed. Results for linear chains demonstrate that the self-consistent medium gives superior estimates of the band edges and the structures near them.

The two-point correlation function for a random Ising model at d=4 is obtained using the solutions of the renormalisation-group recursion relations in the range of temperatures where the system exhibits non-asymptotic critical behaviour, the origin of which lies in a particular degeneracy of the RG recursion relations. The effects of lattice compressibility are also studied with two types of boundary conditions, corresponding to constant volume and pressure, respectively. A comparison with pure Ising systems is made.

The authors apply, for the first time, the variational multiple scattering method (VMS) for the band calculation of a tridimensional disordered alloy. They calculated for the quaternary system Ga_1-xIn_xAs_1-yP_y, with 0.605<or=x<or=0.917 and 0.094<or=y<or=0.845, lattice matched to InP. For all the alloy compositions, a unique potential for each element was used, chosen so that reasonable band structures for the pure semiconductors GaAs and InP result. No other adjustment was made. The energy density of states and the alloy bands were calculated for six compositions. The calculation is very fast when compared with similar calculations by the average t-matrix approximation method (ATA). The VMS method predicts, with the above potential choice, two separate inner peaks for the energy density of states, corresponding to the s As and s P bands, which is amenable to experimental verification. The importance of the self-consistent process in determining the degree of P and As p-band mixing is clearly shown.

The electronic structure of a one-dimensional system with an incommensurate modulation of hopping integrals is investigated. The extended or localised character of states is analysed by means of Green functions, the direct calculation of eigenvectors, and the resistance of the system. It is shown that for a larger amplitude of the modulation the electronic structure of the outer bands changes drastically and the states become localised.

The hyperfine interaction at dilute ¹¹¹Cd impurities in vanadium sesquioxide V₂O₃ has been investigated as a function of temperature by time differential perturbed angular correlation measurements. The authors have observed a pure electric quadrupole interaction in the metallic phase (M) and a combined magnetic dipole and electric quadrupole interaction in the antiferromagnetic, insulating phase (I). The electric field gradient (EFG) undergoes a first-order change at the M-I transition at T_t=160K from 8.2*10¹⁷ V cm^-2 in the I phase to 6.3*10¹⁷ V cm^-2 in the M phase, but varies smoothly with temperature in the region of the high-temperature resistivity anomaly. In the metallic phase the EFG increases with increasing temperature. A point-charge lattice-sum calculation suggests that this behaviour results from the thermal variation of symmetry and dimension of the oxygen sublattice. The magnetic hyperfine field at ¹¹¹Cd in the antiferromagnetic phase of V₂O₃ has a saturation value of 15(1) kOe and forms an angle beta =68(2) degrees with the direction of the principal EFG component. The field vanishes at the M-I transition temperature T_t. Its temperature variation follows a Brillouin function with an extrapolated Neel temperature T_N(T_N>T_t), which depends sensitively on the impurity concentration of the sample.

Acoustic plasmon excitations have been proposed as a possible mechanism for superconductivity in systems with 'heavy' and 'light' electrons. The authors formulate a theory of superconductivity in which electron excitations play the role of phonons in the conventional theory. The main qualitatively new aspect of such a theory is the role of exchange. The effect of exchange on the electron-acoustic-plasmon coupling parameter lambda is estimated.

A study is made of the 2D Ising model in the presence of quenched site disorder at low concentrations. The fermion representation of the Ising model is used and a parquet expansion approach is followed as in earlier work on the bond model by the present authors. A ln(ln(1/ mod T-T_c mod )) divergence in the specific heat is found, as in the bond case, demonstrating the expected universality. The authors also discuss the point group symmetry of the fermion representation. Finally the discussion is extended to general defects and it is shown that the specific heat divergence is universal for any type of low-concentration inhomogeneity.

The authors discuss the anisotropic frustrated Ising model on a square lattice in the presence of annealing. The host lattice has bonds of strength+J. Defects of strength -J are introduced with concentrations c₁ along the horizontal (x) direction and c₂ along the vertical (y) direction. The annealed model is considered. It is found that for certain ranges of anisotropy and of defect concentration there exists both a high- and a low-temperature paramagnetic phase. A study of bond-bond correlation functions provides an explanation of the low-temperature phase in terms of highly ordered clusters separated from each other by domain walls of negligible energy. The behaviour is very different from the isotopic case.

A capacitance method is used to measure magnetic-field strains at the temperature 4.2K in the cubic metallic compound MnSi. The anomalous behaviour of the magnetostriction along (001) and (111) for a variety of field directions can be interpreted as resulting from field-induced changes in the helical spin-density wave that exists below 29K. A phenomenological model for the magnetostriction near the phase transition marked by field-induced wavevector and spin reorientations fits the observed behaviour well.

Reports the observation in acetone at low temperature of a number of new phenomena connected with methyl tunnelling motion and field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field dependence of proton spin-lattice relaxation at 4K for fields B₀, B₀/3 and B₀/3 and B₀/3 and B₀/3 with B₀=2.25T. They are due to internal resonances (or level crossings) occurring for n nu ₀= nu _t, (n=1, 2, 3) where nu ₀ is the field-dependent nuclear Larmor frequency and nu _t is the methyl tunnel frequency. This structure is a consequence of the field-cycling technique which involves rapid switches of the magnetic field to enable all measurements of proton magnetisation to be made at a single field 0.6 T. A simple thermodynamic model is described in which there is very rapid thermal equilibration between the nuclear Zeeman and methyl tunnel reservoirs at the fields corresponding to the n=1 and 2 anomalies and the nuclear magnetisation is otherwise unaffected by the rapid field changes. This model successfully simulates the observed spectrum and a variety of other field-cycling experiments.

The defect-induced one-phonon infrared absorption in natural diamond can be divided into a number of overlapping components. The computational method of extracting the spectral shapes of these contributions is described. The A and B components are associated with nitrogen defects. The A defect is a pair of substitutional nitrogen atoms and the B defect is currently considered to be an agglomerate containing four nitrogen atoms. The C spectrum is associated with the singly substituted nitrogen atom and is characteristic of synthetic diamonds. The D spectrum has been observed only in diamonds which also contain B defects, and for this reason is also considered to be a defect which contains nitrogen. Some studies have been carried out on the thermal stability of the A, B and D defects during heat treatment in the range 2100-2800 degrees C under a pressure of 9.5 GPa. A possible nitrogen aggregation sequence is suggested to describe the observations.

The physisorption properties of helium interacting with a series of free-electron-like and noble metals are examined. The approach used is that of Zaremba and Kohn (1977), but the theory is extended to include the departure from asymptotic behaviour of the metal wavefunction tails in the selvedge, the proper placement of the ionic background edge and corrections to the asymptotic van der Waals interaction. Calculated binding energies are lower than those reported by Zaremba and Kohn by 20-30%.