Table of contents

Undoped and tin-doped samples of silicon containing oxygen have been irradiated with 2 MeV electrons at room temperature and subsequently annealed. It is verified that (Sn-V) complexes dissociate at a temperature of about 160 degrees C. A comparative study was made of the rates of removal of stress-induced dichroism of the 9 mu m infrared absorption band due to oxygen in the two types of material as a result of similar electron irradiation. The rate in the tin-doped material is lower than that in undoped material by a factor of six, and there is a corresponding reduction in the generation rate of (O-V) complexes. Irradiation of similar samples held at 200 degrees C produced the same results for undoped material but the rate in tin-doped samples was then almost as high as in the former samples. It is deduced that (O-V) complexes capture silicon self-interstitials to regenerate isolated oxygen interstitial impurities and that the dichroism is lost in the overall process of complex formation and destruction.

The method of 'generalised transfer matrices' previously used to find the average of any power of the transmission coefficient, (t^x) is applied to finding ( mod t mod ^y) in the cases y=0, -2, -4 . . . and Re y>1. These cases include all the integer moments of the conductance, mod t mod ², and its reciprocal, the resistance. The conductance is the more physically important quantity and is found to have a complicated dependence on chain length. The formulae are valid for any distribution of disorder and all lengths of sample. Limiting cases and numerical results are presented. An anomaly in the energy dependence of the conductance of a binary alloy is discovered.

A calculation is done to determine the critical volume fraction of percolation and the range of temperature for which hopping conduction follows the law log sigma approximately T^-12/. It is shown that the critical volume fraction V_c does not have a universal value, but varies as the system is changed. The theoretical exponential law for the DC conductivity shows a good agreement with the experimental law when the metallic concentration is high.

Two recent publications by Ricco and co-workers (1970, 1975) have drawn attention to important consequences that can arise when electrons tunnel through barriers which contain resonant states. The authors reason that there is then the possibility of close to 100% transmission for electrons which are on resonance. Their arguments are based on time-independent solutions of Schrodinger's equation. In this paper their discussion is supplemented by examining the time-dependent solution of a simplified version of their model. In particular an expression is found for the time it takes for an incident pulse to build up on penetrating into the well, and this is combined with an examination of the effect of the Fermi-Dirac statistics for electrons (orthogonal one-electron wavefunctions), both of which are relevant to the question of whether, in an actual experimental arrangement, these high transmissions are to be expected.

The dynamics of the longitudinal spin fluctuations of the two-sublattice three-dimensional Ising model in a transverse field is studied using a continued-fraction expansion for coupled operators.

Closed non-linear equations are derived through mode-coupling approximations for the correlation functions and susceptibilities for the Edwards-Anderson model of a classical Heisenberg system with nearest-neighbour random exchange couplings. These equations, which exhibit full dynamical stability, lead to a self-consistent treatment of spin-density propagation and spin-current relaxation. They are found to describe a transition from a paramagnetic to a spin glass phase, which is viewed as one where the dynamics changes from ergodic to a non-ergodic behaviour. Near the critical temperature T_c the low-frequency correlation functions are obtained as dynamical scaling laws governed by a critical frequency omega _c, which slows down proportionally to epsilon ², with epsilon =(T-T_c)/T_c. Approaching the critical point from the paramagnetic side the diffusivity vanishes linearly with epsilon . The susceptibility exhibits a symmetric cusp. The drastic changes of the spin dynamics at the transition point show up most clearly for the spin-current spectra. The numerical solution of the equations is discussed.

From a variational principle the authors derive a closed set of equations for the spin glass mean-field theory in terms of a local magnetisation on long timescales and a distribution of gradually frozen local fields. These equations are useful for deriving general relations and they find that the frozen field obeys a Langevin equation. Numerical solutions of the equations are obtained at T=0. The local field distribution shows a linear behaviour for small fields.

A new type of evidence is put forward that resolves a long-standing dilemma concerning the assignment of the Gd³⁺ and Eu²⁺ EPR spectra in glasses to a single site or to several sites of different symmetries. The comparative analysis of Gd³⁺ EPR spectra in YAG:Sc crystals and in oxide glasses demonstrates that all characteristic features in glass spectra are due to a single site of lower-than-axial symmetry with distributed values of the spin-Hamiltonian parameters.

From a thorough NMR study of the ⁵⁵Mn hyperfine field distribution in Mn₅(Ge_1-xSi_x)₃ alloys the authors shown that the transition from the ferromagnetic Mn₅Ge₃ to the antiferromagnetic Mn₅Si₃ occurs inhomogeneously in three steps. At the early stage of the substitution of Si for Ge (x<or=0.4) the moment of some Mn atoms nearly vanishes when a critical number ( approximately 2) of Si atoms is reached in their neighbourhood; at this stage such Mn atoms recover their full moment when a low external field is applied announcing at a local scale the metamagnetic transition observed for x>or approximately=0.8. Then for 0.5<or=x<or=0.8 antiferromagnetic domains build up in the samples and ferromagnetic domains progressively shrink with increasing Si content; this is associated with a strong change of the magnetic domain walls behaviour. Finally for x>or=0.8 bulk antiferromagnetism occurs but a metamagnetic transition is still observed for H_ext>30 kOe.

A theory is presented of the relaxation, by any combination of thermally activated hindered classical reorientations, of the longitudinal ²D nuclear magnetism in the 'slow motion' regime. The calculation deals principally with molecular groups in which the deuterons are located at the apices (sites) of a regular tetrahedron and deals with both 'symmetric' and 'asymmetric' mechanisms. In the former all jump probabilities P_kl(t) from site k to site l(k not=l) on the molecule are equal, no matter how long the associated jump times are, and it is concluded that in this case the relaxation is always exponential (in the absence of other relaxation mechanisms). For an 'asymmetric' mechanism, the P_kl(t) are not all equivalent and more than one non-zero time constant is necessary to describe the evolution of the jump probabilities. In this case, the relaxation is exponential only so long as the temperature is high enough to ensure that all the time constants involved are very short compared with the 'observation time' t (t=0 is assumed to correspond to the turn-off of a pi -pulse). The mechanism C₃+C₃' is examined in detail and shown to yield, at high temperatures, an exponential decay with a T₁ given by standard (BWR) theory and at low temperatures a sharply non-exponential decay. Numerical plots of the relaxation are presented and their general features are discussed in detail.

The phase transition in the system MnTiF₆.6D₂O has been studied by investigating the variation in the laser Raman spectra of this system with temperature for various polarisations. The band corresponding to the nu _s(TiF₆) mode is at 614 cm^-1 and at room temperature it appears strongly for the ZZ and XX polarisations but very weakly for cross-polarisations. A sharp change in the Raman intensity of this band has been observed for YZ polarisation at 200+or-3K during the cooling cycle and at 228+or-3K during the heating cycle. This shows that there is structural phase transition in the system at this temperature. Asymmetry in the shape of the spectrum has been observed in the region 220-360 cm^-1. This has been analysed assuming coupling between the nu _as(Mn-O) and delta (TiF₆) modes present in this region. The analysis shows that the frequency of the mode nu _as(Mn-O) increases while that of the mode delta (TiF₆) decreases with the increase of temperature and a sharp change in each of frequencies occurs at the transition temperature. The D₂O molecules are equivalent at room temperature whereas at low temperature as many as three different types of D₂O molecules can be predicted from the observed spectrum.

The Raman spectra of various forms of ice I_h, II, III, IX, V and VI of H₂O, D₂O, H₂¹⁸O and small percentages of HOD in both D₂O and H₂O have been recorded in their true regions of stability. The high-pressure low-temperature Raman cell used for this work is described. The influence of pressure on the OH and OD stretching modes and the lattice vibrational frequencies is discussed. It is estimated that the pressure compresses the O-O bond lengths on average about 5 pm GPa^-1 in the I_h, II, III and V types of ice. An attempt is made to correlate the available spectroscopic and crystallographic data for the various forms of ice in order to make assignments of the Raman- and infrared-active lattice vibrational spectra.

Trap level spectroscopic investigations have been carried out on LiF doped (separately) with uranium and americium using EPR, TSL and TSL-spectral techniques. These studies, apart from obtaining trap parameters-trap depths and frequency factors-helped in elucidating the processes that occur in electron and hole trapping and their subsequent recombination on thermal stimulation. In the uranium-doped sample the electron trapping by U⁶⁺ complexes (UO_6-)^6- and (UO₅F)^5- at (LiF_6-)^5- sites and subsequent electron-hole recombination at U⁶⁺ has been identified as the predominant mode of TSL. In ²⁴¹Am-doped sample americium has been found to be stabilised in the divalent state. Whereas in the high-temperature region F-centre production and Am²⁺ to or from Am³⁺ were found to be the dominant modes of electron and hole trapping, respectively, in the region below room temperature Am²⁺ to or from Am⁺ and production of U₃ centres (hydrogen atoms at F^- sites) were found to be the dominant modes of electron and hole trapping, respectively. In both high-temperature and sub-room-temperature regions electron-hole recombination results in the creation of electronically excited Am²⁺ resulting in the luminescence of Am²⁺ on thermal stimulation.

For pt.I see ibid., vol.17, p.5851 (1984). Trap-level spectroscopic studies of SrSO₄ phosphor doped with ²⁴¹Am were conducted using ESR and thermally stimulated luminescence (TSL) techniques. The formation of electron and hole traps due to internal alpha -irradiation (from the decay of ²⁴¹Am) and external gamma -irradiation were distinguished by carrying out the studies on pure SrSO₄ and also on SrSO₄ doped with europium (lanthanide analogue of Am). The ESR studies on these samples have shown that the paramagnetic radicals SO₄^-, SO₃^-, O₃^-, S₅⁺, SH^2- were stabilised in SrSO₄ under different dopant and irradiation conditions. The radical SH^2- is detected for the first time. From the ESR studies of thermal stabilities of radical ions produced and their correlation with TSL peaks, the role of paramagnetic molecular species in the electron and hole trapping and thermally stimulated free radical reactions were identified and the corresponding activation energies and frequency factors were determined.

Using the continuous or field-theoretical model the authors have calculated stress tensor correlation functions and elastic moduli for a system containing an interface. The shortcomings of that approach as well as its possible applications are discussed.

The techniques of time-of-flight neutron diffraction and high-resolution X-ray diffraction have been used to show that the second-order transition on cooling proustite (Ag₃AsS₃) through approximately 61K is to an incommensurate phase, with a further transition to a commensurate phase at approximately 49K.

For original paper see ibid., vol.17, L419 (1984). Recently Tao (1984b) has given results of a perturbation description of the anomalous quantum Hall systems, which seem to cast doubt on some of Laughlin's (1983) insights. This doubt is the result of a misleading presentation of results and is not warranted. The minimum possible Coulomb interaction energy of a two-dimensional electron gas (2DEG) is the energy of the Wigner crystal, E_WC. E_WC is therefore a lower bound on the contribution of the electron-electron interaction to the ground-state energy, E_GS. In a magnetic field the kinetic energy basis is discrete and is made up of Landau levels. If for a filling factor nu <1 the contribution of all except the lowest Landau level can be neglected, then the kinetic energy is constant. The ground-state energy is just the minimum interaction energy evaluated subject to the restriction that electrons occupy states in the lowest Landau level only. This energy is of course still higher than E_WC (Yoshioka and Lee 1983). Tao (1984b) has given ground-state energies measured from h omega _c/2 per electron which, for nu =¹/₄, ¹/₅, ¹/₆ are lower than this absolute lower bound. The author shows that the result of Tao's perturbation theory is clearly not an upper bound and therefore should not be compared as such with Laughlin's variational result for any value of nu . Unfortunately Tao makes this comparison.

There is considerable difference between the changes in the diamagnetic and dielectric susceptibilities of semiconductors between their crystalline and amorphous phases. The authors use a bond orbital model, recently introduced by them, to explain the changes in the dielectric susceptibility of a model two-dimensional semiconductor with threefold coordination.

The neutron scattering cross section due to the longitudinal fluctuations above and below the Curie temperature in a crystal of palladium 10% iron has been identified with the aid of neutron-spin polarisation analysis. The cross section is observed to be quasielastic, with an intensity which is a maximum at T_c, and which diminishes with increasing wavevector, increasing applied field, and decreasing temperature below T_c.

Studies of photoinduced absorption of crystalline toluene sulphonate polydiacetylene (PDA-TS) and 1-hydroxyhexa-2,4-diyne polydiacetylene (PDA-1OH) show absorption peaks near 1.35 eV. This absorption is compared with photoinduced absorption in polyacetylene and polythiophene and its origin is discussed.