Table of contents

The dynamical characteristics of the Brochard-Leger wall in nematics have been investigated experimentally. The dependences of the width, the moving speed and the relaxation time of the wall on the reduced magnetic field have been measured. The linear dependence of the speed of the wall on the tilt angle of the magnetic field has also been revealed. The experimental results are in good agreement with the theoretical conclusions.

The static structural properties of Mo and MoSi₂ are studies using the linear muffin-tin orbital method. The Ewald correction to the Coulombic interaction between different Wigner-Seitz spheres is included in the total energy calculation. The results are compared with other theoretical calculations and experiments. The elastic constants of Mo are also calculated using the frozen-phonon method.

Ultrasonic attenuation measurements of Y₁Ba₂Cu₃O_7-y have been carried out between 100 and 10 K by using 10-25 MHz longitudinal acoustic waves. It was shown from the experimental results that when T<T_c the attenuation was apparently decreased and the granular superconducting characteristics of the samples were remarkable. It was indicated from the attenuation curves that the superconductivity of Y₁Ba₂Cu₃O_7-y is related to the electron pairing states. By means of introducing a transition volume distribution function f_T, the calculated attenuation curves fitted the experimental ones very well, leading to a result of 2 Delta (0)=3.5k_BT_c. This suggests that the existence of a superconducting energy gap and its temperature dependence should be still the same with the BCS theory, although the mechanism of the electron coupling may be quite different.

Anharmonicity in the lattice vibrations of oxygens in UO₂ has been examined by using EXAFS of the UL_III absorption edge. Whilst the EXAFS-determined U-U distance of 3.89 AA closely matches the crystallographic separation, the U-O distance of 2.43 AA is 0.06 AA longer. This shift is interpreted on the basis of an anharmonic lattice model in which oxygens occupy anti-tetrahedral sites.

Room-temperature low-energy Raman spectra of gel-derived silica glasses are reported for samples annealed at different temperatures. The very low-frequency spectrum is interpreted in terms of localised extra modes coexisting with phonons, and of phonon-fracton crossover. The role of OH^- in determining dynamical discontinuities in the network is discussed.

Self-consistent spin-polarised electronic band structure calculations have been performed for La₂NiO₄ in its nonmagnetic, ferromagnetic and antiferromagnetic states. The principal results are: the band structure of nonmagnetic tetragonal La₂NiO₄ is similar to the isostructural La₂CuO₄, but with two Ni-O (3d(x²-y²)-2p_x,y and 3d(z²)-2(p_z)) anti-bonding bands and also a higher density of states at the Fermi level the ground state is found to be an antiferromagnet with a spin moment of about 0.8 mu _B/Ni; no ferromagnetic solution is obtained for lattice constants within 5% of the experimental value. The electronic properties of the La₂NiO₄ compounds and the nature of the tetragonal-orthorhombic phase transition in both La₂CuO₄ and La₂NiO₄ are discussed in view of the present results.

Using a simple sech(at) form of the relaxation kernel in the Mori-Zwanzig scheme, the authors calculate the dynamical structure factor S(q, omega ) of liquid aluminium. Their calculation predicts a collective excitation peak in S(q omega ) for q<or=1.2AA^-1 and a plateau for 1.5<or=q<or=1.8 AA^-1, in agreement with molecular dynamics data. An overall very satisfactory agreement with the MD data of Ebbsjo and co-workers (1980) has been achieved for all wavenumbers.

The author discusses the possibility of flow of a transverse current driven by phase slip between the edge currents in a narrow quantum Hall sample. The underlying physics and the consequences appear exactly parallel to those of the Josephson tunnel junctions: for instance he can predict the appearance of Shapiro steps in I-V characteristics on application of a high-frequency AC voltage between the two edges, and the existence of a Riedel-type peak in the frequency dependence of the AC etc. The analysis throws fresh light on the quantum interference phenomena currently being discussed extensively in connection with the quantum Hall effect.

The author investigates the effect of an electric field on the conductivity of the low temperature few-phonon-assisted small-polaron hopping regime. The deformation of the surroundings induced by the carrier is taken into account. Comparison is made with other theories.

Two groups of features with different time evolution characteristics can be identified in the low-field microwave absorption signal in ceramic Y₁Ba₂Cu₃O_y. One set of features has its origin in the flux reconfiguration in a magnetically viscous medium while the second set of features arise from the flux quantisation through the random Josephson current network. The periodicity and existence of these features cannot be explained by the usual concepts of randomness in superconducting network models.

It is customary to use the classical approximation for studying magnetic helical configurations. Recently, Harris and Rastelli performed a T-matrix calculation in order to take into account long-wavelength quantum fluctuations exactly in a hexagonal lattice with competing exchange interactions up to third neighbours in the basal plane. Interesting new features such as a first-order phase transition between the helix and ferromagnetic phases were found. Here the authors apply that approach to an axial next-nearest-neighbour Heisenberg model, (ANNNH), the quantum version of the well known ANNNI model of Fisher and Selke (1980). They find that in the ANNNH model, quantum fluctuations do not change the location or the order of the ferro-helix phase transition found in the classical limit (S to infinity ) for any S>¹/₂, even in the extreme 1D limit (linear chain). For S=¹/₂ they cannot produce results by the same method because the coefficient that should, if positive, ensure the stability of the ferromagnetic phase vanishes.

Recent experiments on the antiferromagnetically ordered compound CeAs suggest that the order parameter is almost conserved in this system. The spin-wave spectra display quadratic dispersion which is unusual in an antiferromagnet. The author obtains an exact solution for the spin-wave spectra in an order-parameter-preserving FCC antiferromagnet and explain the origin of the quadratic dispersion. The relevance of these results to the experimental situation is also discussed.

The ⁸⁹Y_NMR shift and relaxation have been measured as a function of temperature and composition in the YBa₂Cu₃O_{7- delta} system. Values are deduced for N(E_F)_s and N(E_F)_d at the yttrium site. The authors find N(E_F)_s<0.007 states eV^-1/atom and 0.13<or=N(E_F)_d<or=0.26 states eV^-1/atom for YBa₂Cu₃O₇ with lower values for N(E_F)_d as delta increases.

The authors have theoretically investigated the transmission of radiation through quasi-periodic multi-layered media, constructed of linear and non-linear dielectric films arranged in a Fibonacci sequence. The self-similar dependence on N in the transmission process is destroyed by the non-linearity introduced. Some non-linear effects different from those for similar periodic systems are also found.

A nonmonotonic temperature dependence of the reflection factor R(T) of 5 keV Ne⁺ ions incident onto polycrystalline nickel at 0 degrees and 30 degrees to a surface normal has been observed. The value of R has been found to increase by a factor of two during the transition to the paramagnetic state in Ni. R(T) fine structure (local minima and maxima) has been found near the Curie point. An explanation of the dependences obtained, based on the data on the variations of the electron state density near the Fermi level, and on the ratio of the magnetisation fluctuation correlation radius to the effective length for scattered ion interactions with the target, is proposed.

The crystal structure has been redetermined using X-ray diffraction data collected from a triply twinned crystal, a=10.8568(8), b=6.2682(5), c=5.0340(7)AA, Cmc2₁ grown using I₂ as the vapour transport agent. The weak orthorhombic superlattice originally observed by convergent-beam electron diffraction, corresponding to the freezing in of a q=¹/₃(1120) Sigma ₆ sub-cell phonon mode, is confirmed, and magnitudes of the atomic displacements from their ideal positions refined. Closer inspection of the data revealed the presence of a q=0, Sigma _6u symmetry, frozen-in-phonon mode, which depending on its sign when added to the q=¹/₃(1120) mode, results in two homometric solutions.

The authors present an extensive series of molecular dynamics simulations of UO₂ in the solid and liquid states, in which they calculate the ionic diffusion coefficients and some of the important thermodynamic quantities. The simulations are based on a rigid-ion model derived from the new shell model potentials of Jackson and co-workers (1986) and make use of recently developed constant-pressure and constant-temperature techniques. The simulations confirm that UO₂ is an oxygen superionic conductor, as suggested by recent neutron scattering experiments. The temperature of the diffuse transition to the superionic regime is in satisfactory agreement with experiment, as is the melting point of the model system. The thermal expansion coefficient, specific heat and bulk modulus for the solid agree well with experiment below about 2500 K but are less satisfactory near the melting point; they suggest that the differences may be due to the effect of electronic excitations. The volume increase on melting and thermodynamic quantities of the liquid are sensitive to details of the inter-ionic potentials and are in only fair agreement with experiment.

The authors have measured the change Delta v in the sound velocity in Sb-doped Ge over a concentration N range 3.1*10¹⁶-1.6*10¹⁸cm^-3 and a temperature range 1.8-40 K. The stress dependence of Delta v has been also measured in a limited concentration range near the critical concentration N_c for the metal-nonmetal transition over a temperature range 2.3-4.2 K. The density of states for N>10¹⁷cm^-3 has been derived from fitting to Delta v under the assumption that the electron-phonon coupling in the impurity band is identical with that in the conduction band. The stress dependence is qualitatively explained using the density of states obtained. The relaxation time in the sound attenuation near N_c is found to be sensitive to the stress. The metal-nonmetal transition in this system occurs in the impurity band.

For pt.II see ibid., vol.18, p.4471 (1985). The authors have further advanced the study of the spin-dependent features of electron correlations by means of an interpolation that treats in a unified way long- and short-range correlations together with their exchange effects. The correlation energy evaluated from the present interpolation is in excellent agreement with that of Ceperley and Alder (1980) throughout the whole region of metallic densities. They have given some criticisms on the present interpolation a la Hubbard and cleared up its application limit. A detailed examination of the contribution from multi-pair excitations beyond the scope of the present interpolation has been given.

The electrons in the half-filled l shell of an atom are separated according to their spin orientation. Electrons in the two different groups (A for spin up and B for spin down) are regarded as distinct particles and are coupled to a resultant orbital angular momentum L. When the numbers of spin-up and spin-down electrons are 2l and 1 respectively, the states mod (L_Al_B)LM_L) coincide with the conventional states mod l^2l+1SM_sLM_L) provided M_s=l-¹/₂ and S=1+or-¹/₂. The former states are used as the basis for calculations involving the two-electron crystal-field operators of the coupled tensorial form Sigma (w_i^(ab)wi^(cd))^(ef)k, where the sum runs over electrons i and j for which i not=j. A revised table of matrix elements is given for Gd³⁺4f⁷⁶P₇2/. New selection rules and matrix proportionalities characterised by the two sets (0,0,0) and (1,1,0) of spin ranks (a,c,e) are established for arbitrary l.

A self-consistent coherent potential approximation calculation of the density of states (DOS) for a two-conduction-band alloy model has been given in the framework of the slave boson mean-field theory. The results show that, with increasing concentration of Kondo impurities, the f-electron DOS near the Fermi level will gradually change from a resonance peak for the dilute system into a coherent f band with a pseudo-gap for the concentrated system. Simultaneously, there appears a narrow valley for the DOS of the conduction electrons in the vicinity of the Fermi level. These results can be used to explain why the low-temperature thermoelectric power of the system changes from negative values to positive values as the system transforms from the Kondo impurity to the Kondo lattice. The authors also demonstrate that the Kondo temperature does not vary with the concentration of the alloy system. This is in agreement with the recent experimental results for heavy-fermion alloys.

Superconductivity in GdBa₂Cu₃O_x has been detected by AC and DC electrical resistance measurements. For both cases the onset temperature for the superconducting transition is 108 K, with a zero-resistance estimate of T_c=100 K for the DC measurement and 99 K for the AC measurement. X-ray diffraction analysis indicates that the sample is mostly single-phase.

For pt. I, see ibid., vol.21, p.1021 (1988). The general dispersion equations for bulk and surface magnetostatic modes in N-layered superlattices derived in the previous paper by the 'transfer matrix' method, are applied here to the cases of N=2 and N=3 different magnetic layers in an elementary unit. The layers are assumed to be magnetised parallel to the film planes and parallel to one another. Explicit forms of the 'transfer matrix' and of the dispersion equations for bulk and surface modes are given for propagation in the Voigt configuration. Numerical examples of the wave spectrum are presented for some superlattices composed of ferro magnetic, antiferromagnetic and nonmagnetic materials.

The magnetic structure of the incommensurate spiral phase of holmium has been studied using elastic neutron scattering for temperatures between 30 and 18K, the temperature region just above the transition to a ferromagnetic cone phase with a wavevector of ¹/₆ c^*. The results show that the structure consists of commensurate regions separated by spin discommensurations or spin slips. In the commensurate regions, pairs of spins (one on each sublattice) deviate by about 10 degrees from each of the easy directions within the plane, and there is evidence for a c axis antiferromagnetic component which changes domain type from one commensurate region to another. The spin slips, which occur on alternate sublattices, have one plane of spins aligned with the planar easy directions, and are found to disturb the commensurate regions for about three planes on either side of each spin slip.

For pt.I see ibid., vol.21, p.4113 (1988). A spin-slip model of the helical antiferromagnetic phase of holmium is seen to provide a good description of both elastic neutron diffraction and ultrasonic velocity measurements close to the commensurate lock in transition (T_c=17.8K). The measurements show that contained within the smooth thermal evolution of the spin-slip structure, there exist 'special' structures possessing an average spin-slip spacing, b, given by b-3n-1 layers. These structures, which occur at temperatures of 97.4, 40.5, 24.5 and 19.8 K, are shown to break the hexagonal symmetry. Selection rules governing the relationship between anomalous elastic constant behaviour and these special structures are derived. At higher temperatures approaching the paramagnetic phase (T_N approximately 133K) the validity of the spin-slip model is seen to be increasingly questionable.

The properties of Ps-like systems in atactic and isotactic polypropylene were investigated by lifetime measurements and by magnetic quenching experiments. The magnetic quenching results are markedly different: in atactic polypropylene they indicate the formation of an electron-positron bound system, 'swollen' in comparison with a Ps atom in free space. The magnetic quenching for isotactic polypropylene is anomalously high at low fields and seems to suggest the existence of two Ps-like centres, related to regions with different degrees of crystallinity.