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The inelastic scattering of high-energy neutrons from a single crystal of beryllium has been measured in order to test the validity of the impulse approximation. Separate measurements were performed, all with the sample at room temperature, with the wave-vector transfer along (100), (110) and (001) using the time-of-flight spectrometer HET at ISIS. For wave-vector transfers >or approximately=35 AA^-1, corresponding to an incident neutron energy of approximately=1 eV, the authors find that the scattering is centred at the recoil energy given by the impulse approximation. Also, in this regime, the width is independent of the orientation of the wave-vector transfer with respect to the crystal axes and has a value consistent with that expected from a harmonic model of beryllium. They show that their experimental criterion for the validity of the impulse approximation is in agreement with a numerical calculation of the asymptotic behaviour of the exact incoherent neutron dynamic structure factor.

The one-dimensional Thue-Morse sequence with a generating matrix of determinant 0 has been suggested to be intermediate between a periodic structure and a standard quasi-periodic structure like the Fibonacci sequence. The renormalisation procedure developed in the authors' previous study of the one-dimensional Fibonacci quasi-crystal is applied here to discuss the electronic spectrum of a tight-binding Thue-Morse aperiodic chain. The resulting integrated electronic density of states indeed shows a structure that is more like that of a periodic chain than a Fibonacci quasi-crystal. They suggest that the analytical procedure can be extended to study one-dimensional quasi-crystals of any other kind, the two-dimensional Penrose lattice as well as three-dimensional real quasi-crystals.

At low temperatures TbVO₄ and DyVO₄ exhibit a structural phase transition from the same high-temperature structure to different low-temperature structures. Owing to symmetry different crystallographic domains are possible. Investigations of single crystals of the mixed system (Tb_x,Dy_1-x)VO₄ with a polarising microscope gave information about domain distribution and distortion type. Specific heat measurements, spectroscopic and birefringence data yielded the transition temperatures. The phase diagram of the mixed system may be interpreted as a superposition of the phase diagram of diluted TbVO₄ and that of diluted DyVO₄. At high concentrations of one of the rare-earth ions the crystals distort like the respective pure substances, whereas at medium concentrations x approximately=0.4 a superposition of the distortion types is observed. The measurements are interpreted in the frame of a mean-field approximation, which is modified by the phenomenological compressible Ising model and by the inclusion of local strains of random distribution.

For pt. I see ibid., vol.2, p.1073 (1990). Measurements of the linear birefringence at different wavelengths lambda in the visible part of the spectrum are presented for the two orthorhombic phases of the mixed Jahn-Teller system (Tb_x,Dy_1-x)VO₄. These two phases have different crystal structures according to different distortion types of the pure substances. For low concentrations x, the crystals distort like DyVO₄, and the measured curves are proportional to each other for different lambda . For high x, however, where the crystals distort like TbVO₄, the measured curves are not proportional to each other and are not proportional to the order parameter (distortion). The measured curves are explained by calculations within the frame of a modified mean-field formalism including higher-order contributions to the birefringence.

For pt. II see ibid., vol.2, p.1097 (1990). Birefringence measurements on the mixed crystal system (Tb_x,Dy_1-x)VO₄ at different wavelengths in the visible part of the spectrum prove the macroscopic superposition of the two distortion types of Jahn-Teller-active TbVO₄ and DyVO₄ for concentrations 0.35<or=x<or=0.45. Because of the superposition of the distortion types the crystals become monoclinic at low temperatures.

A systematic description of the observed phase transitions in bromoform (CHBr₃) is given. From the symmetry and structures of the alpha , beta , gamma phases, the primary order parameters for the transitions are selected. By using induced representations and projection operator techniques, detailed microscopic distortions consistent with the symmetry and structural changes are obtained. To describe the transitions it is necessary to couple representations. Secondary order parameters for the transitions are discussed. Invariant free energy forms containing the primary and secondary order parameters are presented. Using numerical methods, the free energy is minimised and a phase diagram is obtained. Thus both microscopic modes and macroscopic thermodynamics are obtained for the bromoform structural transitions.

A model of the electronic energy spectrum for the rhombohedral III-V-VI₂ compounds (the ternary isoelectronic analogues of the bismuth-type semimetals and the IV-VI semiconductors) has been elaborated on the basis of the genesis of their crystalline structure from the simple cubic lattice as well as the derivation of the band spectrum from the atomic p states. Rhombohedral III-V-VI₂ compounds are shown to be narrow-gap semiconductors, with two conduction and two valence bands located at Gamma - and L points of the Brillouin zone. The band structure of TlBiC₂^VI compounds has been found to be normal, while that of TlSbC₂^VI is inverted.

For direct transitions in angle-resolved photoemission from a single crystal, the final electronic states inside the crystal can be approximately described by a free-electron-like band for energies higher than approximately=30 eV above the Fermi level. The authors report a determination of the dispersion relation for this band in Ag along the (111) direction over a wide photon energy range (40-180 eV), from which the inner potential (average electrostatic potential in the solid within the Hartree approximation) and the energy-dependent electron effective mass are deduced. The effective mass is found to increase from approximately 1.00 m at 54 eV to 1.08 m at 149 eV above the Fermi level (m is the free electron mass). This result is compared with existing many-body model calculations. The present study is also relevant to the problem of surface state resonance in this system discussed previously.

The temperature dependences of the effective mass and the self-energy of the interface polaron are investigated by means of the Lee-Low-Pines variation method. The influence of the electron interaction with both the bulk longitudinal optical (LO) and the surface optical (SO) phonons on the temperature characteristic of the interface polaron is discussed. Numerical calculation, for GaAs-GaSb as an example, illustrates that both the effective mass and the self-energy will decrease with increasing temperature. Near the interface of crystals, the electron-so phonon interaction will play the main role in determining the temperature behaviour of the interface polaron.

The sequence of the low-energy vibronic states of a T* tau ₂ Jahn-Teller system (an orbital triplet electronic state in cubic symmetry linearly coupled to T₂ vibrational modes) corresponds in strong coupling to the number of nodes in the vibrational factor of the Born-Oppenheimer wavefunction on these states. Berry's geometrical phase in the electronic state corresponding to the lowest adiabatic potential energy surface causes the lowest singlet state to have more nodes than the lowest triplet, with the result that the triplet is the vibronic ground state, the opposite of the sequence expected for a particle tunnelling between four equivalent wells. Reduction factors reflecting Berry's phase are evaluated for this triplet ground state. Their behaviour in departing from their limiting values in the strong-coupling limit is shown to be different from that predicted by a widely used asymptotic approximation based on the crude adiabatic approximation.

Metallic Mo/Al multilayers have been successfully fabricated by a magnetron-radiofrequency sputtering system. The modulation wavelength ranged from 640 down to 15 AA. Detailed structural properties were studied using X-ray and electron diffraction methods. These studies indicated that the films are polycrystalline with high structural order, BCC Mo(110) and FCC Al(111) parallel to the films. Model calculations based on paracrystal theory were performed to analyse the X-ray diffraction patterns. In-plane resistivity was measured as a function of the modulation wavelength and temperature. The resistivity did not saturate even at very small wavelength and negative temperature gradients were usual below 60 AA, where enhancements of the superconducting transition temperature were observed.

In a strong magnetic field (flux lattice constant much smaller than penetration depth), vibrations of the vortex lattice are found to decouple from those of the applied field. This suggests that three-dimensional effects (vortex bending) are much more important than in previous theories. A modified Lindemann criterion shows that the vanishing of critical currents in the high-T_c superconductors can be well described by flux lattice melting, with numerical fits yielding estimates for H_c2 which agree with the most recent previous values. The Lindemann criterion is found to be insensitive to the effective-mass anisotropy of the material except at high fields. In analysing the dispersion relation for the combined field-vortex oscillations, Doppler-shifted cyclotron resonance of core electrons is found to provide a roton-like minimum.

Powder neutron diffraction measurements of the magnetic and structural properties of a range of pseudobinary Ce(Fe_1-xM_x)₂ intermetallic compounds, with M=Al, Co, Ru, are reported. In each of these pseudobinaries addition of a small amount of the metallic impurity (M) to CeFe₂ reduces the ferromagnetic ordering temperature and enhances the small antiferromagnetic component which appears at low temperature. At higher impurity concentrations antiferromagnetism is achieved. This antiferromagnetism is accompanied by a cubic to rhombohedral distortion of the crystalline lattice. The antiferromagnetic component of the ordered moment in these compounds is oriented at approximately=18.5 degrees to the antiferromagnetic propagation vector ( tau =(111)). An ordered cerium moment is seen in the ferromagnetic phase of CeFe₂ and Ce(Fe_1-xRu_x)₂. This moment is coupled ferrimagnetically to the iron-sublattice moment and mu _Ce/ mu _Fe approximately=-0.3.

In this paper the magnetic structures of various substituted ferrites are reviewed. Attention is focused on the more usual case of a local canted state. Several systems and detailed magnetic experiments are listed and discussed. Then experimental phase diagrams (tetrahedral-octahedral iron concentrations) are established to distinguish between different magnetic regions: ferrimagnetic order, local canted state or perturbed magnetic order, spin-glass-like phase and in between intermediate regions.

Temperature and field dependences of the elastic constants of Fe₆₅(Ni_1-xMn_x)₃₅ invar alloys have been measured by an ultrasonic pulse-echo technique from 4.2 up to 750 K. All of the elastic moduli, C_L, C₄₄ and C', show softening due to magnetoelastic coupling. The softening of shear moduli, Delta C₄₄ and Delta C', is proportional to the square of the magnetisation. The softening of the bulk modulus is mainly ascribed to an inverse effect of the volume magnetostriction, Delta B_omega . The authors have shown that magnetic ordering gives rise to an increase of the bulk modulus, Delta B_m. The large negative field dependence of shear moduli in the para-process is explained as a result of the increase of magnetisation caused by an external field. The positive field dependence of the bulk modulus for x<0.1 and the negative one for x>0.1 are also explained by taking into account the magnetic contributions to the bulk modulus, Delta B_omega and Delta B_m.

¹H CW and pulsed NMR studies on the hydrides ZrNiAlH_x, with hydrogen concentration x=0.33 and 0.53, and YNiAlH_x, with hydrogen concentration 0.12<or=x<or=1.17, have been performed in the temperature range 150-400 K. In the case of the Zr compounds, only a single resonance line has been observed; the linewidth remains almost unchanged over the whole temperature range studied. A superposition of two lines, one broad and one narrow, has been observed in the case of the Y compounds with x>or=0.75 in the temperature range 220-320 K, whereas only a single line is observed for hydrides of YNiAl with x<or=0.66 at all temperatures studied. The narrow component for the former and the single line for the latter exhibit similar features of motional narrowing due to hydrogen diffusion. Measurements of T₁ of the protons reveal that the relaxation process in YNiAlH_x at low temperatures (below 200 K) and in ZrNiAlH_x at all temperatures is dominated by interactions of ¹H nuclei with the unpaired conduction electrons at the Fermi level. In the high-temperature region, 200-400 K, T₁ for the narrow lines for the Y compounds at all hydrogen concentrations is characteristic of a diffusionally dominated relaxation mechanism. Motion of hydrogen in the broad component is also revealed through relaxation measurements, although it is not clearly observed in the CW experiments. The pressure-composition isotherms, and the fact that below 200 K only a single T₁ is observed for the samples with x>0.75, indicate that YNiAlH_x is a single-phase hydride. Hydrogen atoms occupy two types of interstitial sites in YNiAl, in which the mobility of hydrogen in one of the sites is higher than that in the other. The activation energies for these motions have been estimated to be 0.24 eV/atom and 0.33 eV/atom.

Direct statistical simulation of non-stationary non-equilibrium electronic phenomena in one-dimensional semiconductor structures is carried out to estimate their influence on the field emission from semiconductors in different working modes of needle cathodes. A macroparticles method is used with self-consistent electric field conditions, taking into account the three-dimensional character of the electron scattering. Computation results are given for n-type A_IIIB_V semiconductors.

The author presents a molecular-dynamics calculation of the structure of molten tellurium, based on effective interatomic forces derived from pseudopotential theory. Interatomic distances, coordination numbers, and bond angles are predicted in good agreement with recent neutron diffraction data. The analysis of the local topology shows that in liquid tellurium the atoms from a network of entangled broken chains with an average coordination number of N_c=2.5 just above the melting point. The author shows that the physical mechanism stabilising this structure is the modulation of the random packing of the atoms in the melt by Friedel oscillations in the effective interatomic potential. He argues that this is just a generalised real-space description of the Peierls distortion argument conventionally used to explain the structure of crystalline Se and Te. The role of truly covalent interactions not included in this simple picture is discussed.

The onset of convection in a viscoelastic fluid that obeys the Jeffreys model is investigated. Two boundary conditions have been considered separately: free-free and rigid-rigid. The role played by the retardation time, characteristic of the Jeffreys model, is emphasised. The threshold values of the parameters (critical Rayleigh number, critical wavenumber, onset frequency, etc.) for stationary and oscillatory convection are obtained. The frontier between oscillatory and stationary convection is calculated and the possibility to obtain a codimension-two point is discussed.

Molecular dynamics studies of the temperature dependent properties of copper have been carried out. The constant pressure and constant temperature (NPT) MD method has been used together with the Finnis-Sinclair many-body potential. On the basis only of the room temperature properties the behaviour of solid and liquid copper over wide temperature range have been simulated and compared with experiment.

Medium-resolution ( Delta E approximately=0.1 meV) quasielastic neutron scattering spectra have been measured for partially (CD₃OH) and fully deuterated methanol, (CD₃OD) in the temperature range between 200 and 300 K. The spectra are analysed in terms of a simplified model which takes into account both coherent and incoherent contributions, and includes translational and rotational motions of the molecular frame as well as the rapid reorientation of the CD₃ group. Such a procedure enabled the separation of the translational and rotational contributions to the measured intensity. The apparent lack of saturation of the translational linewidths versus the momentum transfer is finally discussed in terms of a phenomenological stochastic model.

The authors present a non-equilibrium molecular dynamics (NEMD) computation of both direct and cross thermotransport coefficients of a strong coupled binary mixture of positive charges in a negative neutralising background (binary ionic mixture) in the special case in which the charge and mass ratios of the mobile species are the same. They use a modified version of the MacGowan-Evans NEMD algorithm. They find that the direct coefficients qualitatively agree with the predictions of the one-fluid approximation. The authors show that the cross properties depend mainly on the relative interaction energy of the two different species in the mixture, while the range of interaction gives a small correction.

A new first-order phase transition is proposed to manifest itself in rotating ³He-B for the vortex-free state, owing to counterflow. The 'flare-out' texture is found to become spontaneously unstable towards a new 'flare-in' texture, which is suggested to explain the transition, first observed with the use of NMR by Hakonen and Nummila in 1987 and confirmed by Korhonen and co-workers in 1989, to a previously unidentified state characterised by a signal corresponding to beta =90 degrees , i.e., with a vector n perpendicular to H// Omega //z. This transition is also expected to be observable in other experiments, such as ones based on ultrasonic measurements, on the vortex-free state of rotating superfluid ³He-B, a new tenfold splitting of the real squashing mode is predicted to occur for the flare-in texture.

The authors describe a novel system in which low-angle, asymmetric tilt boundaries organise spontaneously under conditions of molecular beam epitaxial growth. In the cases studied, the (1012) surfaces of hexagonal rare earths grow coherently but tilted on the (211) surfaces of BCC transition metals. The main driving forces are interfacial coherency and the relief of long-range epitaxial strain.

Certain types of wavefunctions with quasiperiodic amplitudes, which arise when blocks of two kinds of atoms are repeated in a Fibonacci sequence, are shown to have the same signature as the Bloch wavefunctions in terms of generalised dimensions.

The electronic structure of a one-dimensional binary alloy A₁ tau /B₁ tau /² (with tau being the golden mean) with atoms A and B distributed in a Fibonacci sequence is studied using a tight-binding model, in which variation in both the diagonal and the off-diagonal terms is treated simultaneously. In contrast to the previous diagonal and off-diagonal models, for the 'mixed' model the author finds an extended state. Also in the limiting case of the 'off-diagonal' model the state corresponding to E=0 is an extended state. These extended states are found to be independent of the arrangement of BA and BAA sequences in the Fibonacci chain.

Electrostatic and electrochemical potentials are calculated for a current carrying cylindrical wire that contains a single impurity positioned on its axis. It is found that long-range electric fields are present in the region around the impurity despite the presence of metallic screening and that the direction of the electric field alternates along the axis of the wire.

Hysteresis in the intensity of elastic neutron scattering from the incommensurate Bragg peak in the spiral phase of holmium is studied as temperature is cycled from above the Neel temperature, T_N=132.2 K, to various low temperatures, T₀. The nature of the hysteresis is different according to whether T₀<T_L or T₀>T_L, where T_L approximately=100 K is the lock-in transition temperature at which the magnetic structure is supercommensurate.