Table of contents

The sequence of incommensurate/commensurate phases occurring in TMATC-Co around room temperature was studied by means of X-ray diffraction. The results changed during the course of the experiment, due to radiation damage caused by the X-ray beam. The changes included a pinning of the modulation wave into components with different wavevectors and a lowering as well as smearing of the normal to incommensurate transition temperature.

The lattice sum Sigma r^-n cos(ar) exp(i alpha .r) has not yet been tackled for an arbitrary value of n. Here the authors obtain quite easily a rapidly convergent expression for this sum, by using a Fourier transform relationship. This result is applied to evaluate the Fourier transform of the Ruderman-Kittel interaction. It is shown that from a special case of the Fourier transform relationship, one may derive the Ewald transformation.

New data on the decay of photocurrent following a 10 ns laser pulse are presented for the ideal one-dimensional semiconductor crystal PDATS. The first carrier trapping event is identified and clearly distinguished from subsequent dispersive trapping and trap release events. The trapping is of one-dimensional form. The characteristic trapping time is 500 ns. For a carrier velocity of 2.2*10³ ms^-1 the traps are 1.1 mm apart on a chain.

A simple method is proposed for calculation of the time taken for a particle to tunnel through a potential barrier. The method is applicable to long or short wave packets. For short wave packets the time is reduced below the results given by the stationary phase method.

Reports experimental measurements by coherent neutron scattering on the glassy phase of cyanoadamantane (T=107K). Acoustic dispersion curves and a well defined libration have been measured along the three symmetry directions of the cubic system. Dispersion curves calculated with a phenomenological model taking into account only the average molecules in their site allow good agreement to be obtained with the experimental ones. By elastic scattering, narrow scattering peaks appear near the X points, of the Brillouin zone. These peaks can be explained by the existence of ordered domains with an antiferroelectric arrangement of the molecular dipoles. The correlation length of these domains is of the order of one cell. As for the plastic phase, there is a damped oscillator on the inelastic scans for the glassy state of cyanoadamantane.

The authors discuss the experimental results on radiation damage of NaCl single crystals in terms of the Jain-Lidiard model. This model has been improved by taking into account the back reaction between F centres and Cl₂ molecules. It appears that with this extended Jain-Lidiard model they can describe quantitatively the experimental data of Jenks and Bopp (1974, 1977). From the analysis they conclude that there should be appreciable dose rate effects, which is in agreement with the experimental results published by Levy et al. (1981). Depending upon the temperature and the total dose these does rate effects may cause very large concentrations of radiation damage. The low dose rate region (10⁴ rad h^-1) is of special interest, because the rocksalt in a nuclear waste repository is subjected to ionising radiation of this intensity for a long period of time. Until now no experimental information on radiation damage in NaCl has been available under these conditions. In the present paper they have calculated the concentrations of radiation damage under the above mentioned conditions within the improved Jain-Lidiard model. It appears that these concentrations may be up to about one order of magnitude larger than the ones observed by Jenks and Bopp, who have carried out their radiation damage experiments at dose rates between 10⁷ and 10⁸ rad h^-1.

Standard group theoretical analysis of phase transitions from the hexagonal parent phase of the 2H-TaSe₂ structure into triply commensurate phases is performed. Possible space-group symmetries and the number of single-domain states are determined. A two-layer model of the 2H-TaSe₂ structure is analysed and the authors show how two-layer states correspond to single-domain states of triply commensurate phases of various space-group symmetries. Orthorhombic distortions of hexagonal single-layer states are briefly discussed.

The Collins model is generalised to a two-dimensional alternative binary system and the Gibbs free energy of the system is calculated. According to the equations obtained which determine the solid-liquid equilibrium curves, three simple types of phase diagrams are calculated. They are the 'cigar-type' phase diagram, the phase diagram with a minimum (or a maximum) and the phase diagram with a eutectic point. The results show that the percentage of 4-atoms obviously has an influence on the phase diagrams. These results are quite analogous to the behaviour of the three-dimensional substances.

The (N(CH₃)₄)₂ ZnCl₄ crystal possesses commensurate-incommensurate phase transitions. To describe them the phenomenological free-energy expansion which contains only the second- and fourth-order invariants, has been derived. The model is based on the assumption of two branches of symmetry T^(k,4) of which one is acoustic and the second is related with spontaneous polarisation of the crystal. The periodic modulation in the crystal is described in terms of harmonics. The minimum set of harmonics which completely defines the commensurate modulation is established. Commensurate phases characterised by the wavevectors k/b*=0, 1/2, 1/3, 2/5 and n/m where n, m are any integers, arise from the second- and fourth-order Umklapp terms usually of higher harmonics. Phases specified by n even and m odd are ferroelectric.

The steady-state thermal conductivity of polydiacetylene single crystals has been measured in the range 50 mK-50K. Below 1K the temperature dependence of the thermal conductivity approaches T³ as expected for single crystal whilst above 1K the conductivity is approximately proportional to T¹2/. The data above 1K have been interpreted by assuming that the acoustic phonons responsible for heat transport are scattered by low-energy optical modes associated with the motion of the massive polymer sidegroups. The five lowest optical modes observed by far-infrared absorption and inelastic neutron scattering are sufficient to explain the observed temperature dependence. A good fit to the data was achieved with each mode having an acoustic-optical coupling matrix element of 5 meV; the associated splitting of the phonon dispersion curves is estimated to be about 0.6 meV.

A variational ground state of samarium monochalcogenides is investigated using a one-dimensional periodic Anderson Hamiltonian. The f orbital on one site is taken to be admixed with d orbitals on neighbouring sites rather than on the same site. This odd-parity form corresponds to a localised limit of Steven's model (1978). A complication associated with the use of non-orthogonal orbitals is dealt with by using a generating-function method. Pressure-volume relations that can reproduce experimental phase changes are obtained by taking into account lattice elastic energies.

The authors present a new analysis of the ¹⁵¹Eu Mossbauer spectra as a function of temperature (320-4.2K) for EuCu₂Si₂ and EuPd₂Si₂. Combining the Mossbauer effect results with complementary X-ray diffraction data and chemical analysis provides new information on the local valence behaviour in these compounds and clarifies the confusion that has existed in the past. It is shown that these new Mossbauer data should be considered in a reevaluation of the L_III X-ray absorption and magnetic susceptibility data, in which the details of the local Eu environment can not be resolved.

The authors obtain the leading quantum corrections in S^-1 to the ground-state energy and to the spin wave spectrum of a Heisenberg helimagnet. Both single and double scattering processes contribute. They find that the helix wavevector Q is not modified and the spin wave spectrum has the correct soft-mode behavior (the Goldstone mode) at k=0 and k=+or-Q as they have explicitly verified for the anisotropic next-nearest-neighbour Heisenberg mode. The Goldstone theorem for helimagnets is also briefly considered.

The 1/n expansion has been used to study a Hamiltonian appropriate to the description of the paramagnetic-to-helical ordering transition in Tb, Dy and Ho. The behaviour of the static susceptibilities close to the coexistence curve has been derived to order 1/n.

The authors investigate, by Monte Carlo simulations, the physical properties of a three-dimensional Ising crystal which is partially frustrated due to disorder in the distribution of the bonds of strength+or-J. They calculate, as a function of the bond disorder, the Edwards-Anderson order parameter, the internal energy and the spatial correlation functions with varying temperature. When the disorder is small, there is a transition from the paramagnetic state to an antiferromagnetic state. As the disorder increases, they find evidence of successive phase transitions with decreasing temperature: transitions from the paramagnetic state to an antiferromagnetic state and then to a re-entrant spin-glass state. When disorder becomes very important there is only a transition from the paramagnetic state to a spin-glass state. The different behaviours of spatial correlation functions in these phases are shown and discussed. A general phase diagram is conjectured.

NH₄AlF₄ undergoes an order-disorder transition in the vicinity of 155K. The low-temperature phase is characterised by a parallel order of NH₄⁺ in (001) layers and an antiparallel order along c. Information about local configurations and about dynamics of precursor-order clusters is obtained from EPR spectra of a Fe³⁺ probe. A model is set up postulating a certain set of local atomic configurations and the permissible jumps between them. The Kubo-Anderson theory is applied to this, and its parameters throughout the temperature range 300-150K are obtained by computer matching with the experimental curves. The parameters are compared with those calculated on the assumption of 2D-ordered domains, and give good agreement above 175K; below that temperature discrepancies begin to appear, which are interpreted as the onset of 3D order. The variation of domain lifetime with temperature fits Ising models: the changeover from a 2D to a 3D regime is a new and interesting feature.

Single crystals of NaCl containing a small concentration of iron in the ferrous state, grown in a reactive atmosphere, exhibit after X-radiation strong ESR signals. Such signals can be observed only in crystals freshly quenched before irradiation. Evidence is presented that the corresponding paramagnetic centres are Fe³⁺ ions situated in tetrahedrally symmetric interstitial positions with all four nearest-neighbour cation sites vacant.

The authors report on the motion of oxygen atoms in some ammonium salts as studied by the measurement of the spin-lattice relaxation time T_{1 rho} , of protons in the rotating frame. The temperature dependence of T_{1 rho} for protons shows extremely large minimum values (T_{1 rho} )_min=1-3 sin NH₄ClO₄, (NH₄)₂-Cr₂O₇, (NH₄)₂S₂O₈ and (NH₄)₂Ce(NO₃)₆ at relatively high temperatures ( approximately=300K). T_{1 rho} has also been measured for protons in partially deuterated ammonium perchlorate and ammonium dichromate. It is found that values of (T_{1 rho} )_min are independent of the concentration of the deuterons. The large minimum values, 1-3 s, or T_{1 rho} are attributed to the reorientations of the anions that modulate the dipolar interaction between the protons in ammonium ions and the natural abundance of ¹⁷O(0.037%) in the anions. The activation energies for the reorientations of the anions ClO₄, CrO₃, NO₃ and SO₃ were also determined.

Measurements of the temperature dependence of the ³⁵Cl nuclear quadrupole resonance frequency, nu _Q(T), in the gamma phase of benzenesulphonyl chloride and the spin-lattice relaxation time, T₁(T), for the three crystalline phases of the same compound were used to study the influence of different crystalline structures on the triggering of semiexternal molecular modes. From a comparative analysis of the effects that the semi-external modes (SEM) have upon nu _Q(T) and T₁(T) it is possible to deduce that there is a weak influence of the crystalline structure on the triggering temperature of SEMs. Furthermore, some properties of molecular torsional and semi-external modes, for each crystalline structure of benzenesulphonyl chloride, are deduced.

Experimental evidence is presented for the occurrence of a nuclear dipolar magnetic ordering of the proton spins in Ca(OH)₂ at positive spin temperature in a high externally applied magnetic field H₀. The ordering was produced by dynamic nuclear polarisation (DNP) and adiabatic demagnetisation in the rotating frame (ADRF). From simultaneous measurement of the entropy and the dipolar energy the authors determined the dipolar temperature of the proton spin system after the ADRF. Furthermore an analysis of the NMR absorption lineshape was made. From this analysis of the NMR lineshape and from measurements of the transverse susceptibility as a function of the dipolar energy and the temperature they conclude that a transverse magnetic ordering with a helical structure is created during the ADRF.

With optical detection the electron spin resonance of the ground state of laser-active F₂⁺ and (F₂⁺)* centres in NaF can be selectively measured. From the partially resolved superhyperfine (SHF) structure it follows that most of the SHF interaction for the F₂⁺ centres is concentrated at the 2 Na⁺ and 4 F^- ions in the plane between the two anion vacancies. The sign and magnitude of the magnetic circular dichroism can be understood in terms of a H₂⁺ model for the centre. The SHF structure of the (F₂⁺)* centres in Ca²⁺-, Mg²⁺- and Mn²⁺-doped NaF differs markedly from that of the F₂⁺ centres. From the analysis of this a model is proposed in which the electron is centred in one anion vacancy, which is next to a cation and anion vacancy. The doubly charged stabilising cation is not in the nearest-neighbour position, but shortens the spin-lattice relaxation time. The excited state is H₂⁺-like with a saddle-point configuration.

Optically detected magnetic resonance experiments have been used to study the broad emission bands from iodine transported mixed crystals of ZnS_xSe_1-x for x=0.85, 0.40, 0.10 and 0.00, for which donor to acceptor recombination involving A-centres is found to occur. The different self-activated emission bands are attributed to A-centres involving different configurations of neighbouring sulphur or selenium ions. The highest-energy band results from recombination at a (V_ZnS₃) A-centre whilst recombination at A-centres in which the zinc vacancy has selenium neighbours results in emissions at lower energies.

Infrared absorption (350-520 cm^-1) due to localised vibrational modes has been studied in heavily Al-doped self-compensated ZnS crystals with the wurtzite structure. Measurements were made in polarised light on crystallographically oriented samples for three different configurations of the vectors k and E of the incident radiation and the c axis of the crystal. Using the polarisation selection rules, the following defects were identified in the absorption spectra obtained: (i) unassociated Al_Zn impurities with C_3v symmetry; (ii) Al_Zn-V_Zn next-nearest-neighbour pairs oriented perpendicularly to the c axis of the crystal, with C₁ symmetry; and (iii) Al_Zn-V_Zn next-nearest-neighbour pairs oriented at an acute angle to the c axis of the crystal, with C_s symmetry. It was found that below 600K most of the Al impurities form pairs with Zn vacancies.