Table of contents

The authors report results on photophysical properties of solid films at C₆₀, including optical absorption, photoluminescence, electroabsorption, photoconductivity, and photoinduced absorption. They observe photoconductivity for excitation energies from the UV to the near IR and find that the excitation spectrum follows the optical absorption. The electroabsorption response varies quadratically with electric field, shows oscillations at the band edge, and has an anisotropy of slightly less than three with respect to the light polarization and the direction of the applied electric field. Photoluminescence from excited states shows vibrational fine structure, a strong temperature dependence, and some variation with excitation energy. They see photoinduced absorption attributed to triplet-triplet transitions at 1.7 eV and also a band peaking at 1.3 eV. A further band at 2.33 eV is seen in C₆₀/C₇₀ mixtures (10% C₇₀) and also in degraded C₆₀ samples. They consider that these bands may arise from extrinsic stabilization of photoseparated charges.

The phonon density of states in the monoclinic phase of tetracyanoethylene is investigated at 20 K using inelastic neutron scattering, and found to be in fair agreement with theoretical calculations based on a semi-rigid molecule model and 6-exponential pair potentials. The results provide support for the atom-atom potential model which was used to explain the relative stability of different phases in this solid. The monoclinic crystal structure at 295 K is also refined using powder neutron diffraction data with a higher resolution than in earlier reports. Comparison is made with the molecular geometry in the cubic phase.

A natural way to describe the diamond structure using reference frames having four axes is presented. The symmetry group O_h⁷ of the diamond structure has natural representations in the mathematical spaces that are used. Geometric quantities, operators and fields having physical meaning and an obvious O_h⁷-invariant form are taken into consideration. Their expressions in the usual description are very intricate and difficult to use.

NdCu₂ crystallizes in the orthorhombic CeCu₂ structure. Using temperature dependent X-ray diffraction the variations of the lattice parameters with temperature have been measured and compared with those of YCu₂. A magnetic ordering temperature at T_N=6.5+or-0.1 K and a spin-reorientation temperature (T_R=4.1+or-0.1 K) have been observed in the susceptibility, specific heat and resistivity. Further field-induced magnetic transitions around 1, 3 and 15 T (depending on the measuring temperature) were found in the magnetization and magnetoresistance. A preliminary magnetic phase diagram of NdCu₂ is presented. The crystal field excitation temperatures determined by inelastic neutron scattering experiments are E₁/k_B=34 K, E₂/k_B=58 K, E₃/k_B=84 K and E₄/k_B=164 K. For the determination of the nine crystal field parameters V_l^m the superposition model has been applied. The deduced set of parameters was used to calculate the crystal field influence on the magnetization, specific heat, thermal expansion and electrical resistivity. From an analysis of the X-ray powder diffraction data the lattice constants a=4.3844 AA, b=7.0350 AA and c=7.4186 AA and the atomic positional parameters (y_Cu=0.0506, z_Cu=0.1659, z_Nd=0.5383) inside the unit cell were obtained.

The A15 intermetallic compound Nb₃Au was subjected to grinding in a high-energy ball mill. A phase transformation from the A15 phase to the high-temperature BCC solid solution of gold in niobium was observed after milling. Differential scanning calorimetry revealed an exothermic peak with a heat content of 4.6 kJ mol^-1. The activation energy was 435 kJ mol^-1. The present result is very similar to the previous results on the V₃Ga compound. The energies involved in the order-disorder reactions for these two compounds scale very well. These results, together with similar results obtained by other authors, strongly support the concept of energy storage by ball milling mainly in the form of atomic disorder.

An O^- centre formed upon hole trapping by O^2- in X-irradiated BaFBr is long-lived at room temperature and has been detected by EPR below 30 K. It can also be produced in low concentrations by exposure of BaFBr to sub-bandgap (200-300 nm) radiation. The formation of F(Br^-) centres at a charge-compensating bromide ion vacancies is a corollary of the photoionization of O^2-. High concentrations of oxide have been successfully introduced into this material by doping with anhydrous BaO, including Ba¹⁷O for unambiguous identification of the O^- defect. Oxide addition and the formation of O^- produces a series of optical bands that have been assigned using a combination of optical and magneto-optical techniques. The influence of the material preparation conditions on the concentration of O^2- is discussed and possible mechanisms for the formation of O^- are proposed.

For pt.I see ibid., vol.3, p.9327 (1991). Oxygen contamination introduces hole-trapping centres into barium fluorohalides. In the preceding paper, an O^- centre was reported as the product of the reaction between out-of-plane Br₂^- V_K centres and oxide impurities. This paper describes an ENDOR study of this centre which identifies it as O_F^-, an oxygen ion substituted at a fluoride site. No evidence has been found for the presence of a neighbouring defect (anion vacancy or interstitial cation) remaining from its charge-compensating role for the proposed precursor O_F^2-.

Non-linear ultrasound absorption in solids is mainly attributed to the depinning of dislocations from pinning points. Confirming the validity of this hypothesis the authors introduce a further different nonlinear mechanism associated with the release of dislocation pile-ups in polycrystalline materials. Pile-up dislocation dynamics under the action of ultrasonic stress waves has been computed by numerically solving the equation of motion of a population of interacting dislocations initially confined in a crystal grain with low-angle boundaries. Dissipated power and microscopic dynamic stress-strain cycles have then been computed together with the dependence of local critical resolved shear stress on excitation frequency and amplitude. The dissipated power plots against frequency characteristics show both regular collective dislocation effects and irregular single dislocation effects such as peaks. This last result could make possible quantitative nondestructive inspection of microstructural properties by means of a new interpretation of ultrasound attenuation measurements whenever pile-up dislocations are thought to be present.

Isotope studies of states of atomic hydrogen in silicon are performed. It is shown that H is a bistable impurity: it has an equilibrium site at a tetrahedral interstitial site for H⁺ and H^- and at a bond-centred site for H⁰. It is found that, in the tetrahedral configuration, H has donor and acceptor levels in the gap, located close to E_v+0.3 eV and E_c-0.2 eV, respectively.

The electronic states of a one-dimensional two-band tight-binding model in a uniform electric field, in which the strength of interband coupling is varied, are studied. Eigenenergies, densities of states and wavefunctions are numerically computed for a finite length of the system in order to see the transition from Bloch bands at low electric fields to the eigenstates at high fields explicitly. The results for high electric fields indicate the existence of sets of Stark ladders arising from different bands even when interband coupling is strong.

The electronic properties of the layered compound 2H-TaS₂ and its Li and Sn intercalates 2H-LiTaS₂ and 2H-SnTaS₂ are determined from first principles using the full potential linearized augmented plane wave (LAPW) method. Energy band structures, total and partial densities of states, partial charges and electron densities are presented and the bonding mechanism in these compounds is discussed. Using self-consistent electron densities, the electric field gradients (EFG) at all nuclear positions are determined without further approximations and a detailed explanation of the origin of the EFG is given. Good agreement with experimental EFGs is found. From the contact densities at the Ta nuclei, the isomer shifts and a new interpretation of the Mossbauer data for LiTaS₂ are presented.

The authors present experimental thermopower data for amorphous Nb_100-xSi_x (55>or=x>or=91) in the temperature range 4 K-280 K which extends through the metal/insulator transition. The higher Nb content samples clearly show metallic behaviour having a low temperature 'knee' attributed to electron-phonon mass enhancement. As the Nb concentration is reduced, deviations away from this behaviour are marked, the thermopower becoming largely independent of temperature. Further reduction of the Nb content causes the temperature dependence to return. At low temperatures, close to the metal/insulator transition, the thermopower appears to diverge and does not seem to conform to the universal behaviour described by Enderby and Barnes (1990). This is probably because the density of states is still large at the Fermi energy and there is an appreciable hopping contribution to the conductivity.

The bound-polaron problem in a purely two-dimensional quantum well is studied variationally for the entire range of the electron-phonon coupling constant and the Coulomb binding parameter. The ground-state energy, the average number of virtual phonons around the electron and the size of the polaron are calculated. A comparison made with the corresponding quantities for bulk crystals shows that the polaronic effects are more pronounced in two dimensions. The energies of the first two excited states are obtained and the phonon-induced Lamb shift corrections are computed for several polar materials.

A current-density-functional theory is formulated in terms of the physical gauge-invariant current density instead of the 'paramagnetic' or canonical current density used previously by Vignale and Rasolt (1988). The appropriate energy functional is obtained by a procedure of constrained search. An equivalent Hohenberg-Kohn theorem is also proved and Kohn-Sham equations are derived. The possibility of local approximations is discussed. As a by-product a generalized Ritz inequality for systems in a magnetic field is found.

The authors present calculations of the energies and wavefunctions for the vibrational ground state and over 40 excited states of a hydrogen impurity in niobium. Predictions are also given for the transition intensities measured by neutron scattering. The calculations are based on an interaction model for the niobium-hydrogen system developed previously. The aims of the work are to investigate the nature of the quantum states and to show how comparisons of the energies and transition intensities with experiment can be used to test the interaction model. It is shown that for excitation energies above approximately 0.2 eV, many of the states differ greatly from the harmonic oscillator states often assumed in previous analyses. Agreement of the excitation energies and intensities with experimental data is fairly good, though problems of assignment for higher experimental transitions hamper the comparison. It is suggested further that calculations of the present kind with help resolve these problems.

The author presents a systematic evaluation of transport properties of semiconducting submicron structures as predicted by computer simulations based on the Boltzmann equation. The influence of variations in the relaxation time of the collision process and the geometry of a model n⁺nn⁺-diode are studied. Both ballistic and nonballistic situations are considered and compared to approximating moment models. Already small ballistic effects, showing up as nonuniform features in the high velocity behaviour of the distribution function, cause considerable error in the moment model predictions for the I-V characteristics.

The authors have measured size effects in the electrical resistivity of thin films of potassium below 1 K. These size-effect data appear to be highly anomalous when compared with their previous resistivity data for thin wires of potassium. However, they show that both sets of measurements can be explained within a unified theoretical framework in terms of normal electron-electron scattering and normal electron-phonon scattering. Specific predictions of size effects are made for size ranges not covered by the present experimental data.

The authors present the effect of intergrain interactions, at T=75 K, on the intragranular critical current density and on the susceptibility of a system consisting of small superconducting grains of GdBa₂Cu₃O_{7- delta} embedded in solid epoxy. The critical current density in a single grain is unaffected by the intergrain interactions. They also found that the magnetic screening volume susceptibility, chi , varies linearly with the grain concentration epsilon , of the system. Non-interacting models can be used to describe the magnetic phenomena of such systems. Finally the variation of - chi with epsilon was found to be strong near the first critical field H_Cl and - chi becomes insensitive to epsilon in the neighbourhood of H₀, which the authors call the cut-off field.

The magnetization 'irreversibility line' of (Tl_0.4Pb_0.4Sn_0.2)Sr₂(Ca_0.8Y_0.2)Cu₂O_y was studied. The results showed that, characteristically, it is similar to that of Bi superconducting compounds, although the structure of this superconductor is close to that of 1:2:3 yttrium superconductors.

The XRD analyses for the Bi-Pb-Sr-Ca-Ba-Cu-O system show that the addition of Ba ions is favourable to the formation of the higher-T_c phase (T_c>100 K), and TEM observations show that the partial substitution of Ba ions for Ca ions induces a change in the crystal microstructure of the higher-T_c phase (T_c>100 K); a new modulation mode with a wavelength of 43 AA appears in the electron diffraction pattern of the higher-T_c phase. The EDS results confirm that Ba ions really intercalate into the crystal of the 2:2:2:3 phase. The DC magnetization measurements show that the sample displays a much larger hysteresis than that of a general Pb-doped sample. In addition, a study on the influence of low DC magnetic fields on the real component chi ' and imaginary component chi " of the susceptibility for the Ba-doped sample shows that T_p is related to the applied field H^DC and their relation can be fitted by (1-T_p/T_c) varies as H^0.23 when H^DC<140 G, and (1-T_p/T_c) varies as H^0.65 when H^DC>140 G. These data can be excellently interpreted in terms of Bean's critical-state model.

High-temperature Raman and infrared spectra measurements have been performed on KTiOPO₄ single crystals in a temperature range from room temperature to 1000 K. Special attention is paid to the low-frequency Raman spectra which exhibit an anomalously large quasi-elastic scattering: this appears with an increase in temperature. The Raman spectra were fitted using a response function involving one oscillator and two Debye relaxations. The resulting parameters are analysed and discussed in order to clarify the phase transition mechanism and, more specifically, the influence of the potassium (K) ion on this transition and on the ionic conductivity. The high-temperature infrared reflectivity spectra were fitted for the two polar mode configurations A₁ and B₂ and have been analysed critically within the previously-mentioned framework.

Changes in the infrared and Raman spectra of lithium hydroxide monohydrate indicate that a phase transition occurs at close to 90 kbar upon compression. In particular, there is a doubling of the vibrations associated with the hydroxyl ion and a change in sign of the d nu /dp values for the hydroxyl O-H stretches from positive to negative. These results indicate that the transition involves the formation of weak hydrogen bonds, in which the hydroxyl hydrogens take part, and doubling of the size of the primitive unit cell. A large hysteresis was observed with the reverse transition occurring near 33 kbar.

The structure around lithium ions in solutions of lithium bromide in acetonitrile and water has been studied by neutron diffraction. For this purpose the isotopic first-order difference method has been applied to the lithium ion. For a 0.58 M acetonitrile solution it has been found that the bromide anion enters into the first solvation shell around the lithium ion, whereas in the case of a 1.88 M aqueous solution the first hydration shell of the cation is not disturbed by the anion. The solvation number is approximately 3 in the case of acetonitrile and approximately 4.5 in the case of water.

Neutron scattering techniques have been used to study the structure of amorphous materials. The experiment used both polarized incident and scattered neutron beams and so did not need to assume a collinear magnetic structure. Five different Fe-based amorphous materials were studied and the structures were found to differ. In the case of Fe₈₃B₁₇ the spins are canted in a magnetic field of 2 T about 30 degrees from the applied direction. The transverse components have only very short-range order. Other materials show less canting. Fe-Ni amorphous materials shows a large amount of disorder in the spin directions and in the aligned moment. This suggests that the Ni atoms do carry a magnetic moment but that it may be largely misaligned to the Fe moment. Inelastic measurements of the density of magnetic states show a very different distribution to that expected from a powdered ferromagnet. This may arise from 'hidden' short-wavelength excitations.

In a recent paper (J. Phys. Cond. Matter vol.1, p.2199, 1989), M.C. Payne has analysed the method originally introduced by two of the authors to perform molecular dynamics simulations with interatomic forces derived directly from the electronic ground state (Phys. Rev. Lett. vol.55 p.2471, 1985). However the equations quoted in the paper by M.C. Payne do not correspond entirely to the original procedure. For this reason the authors have decided to write a joint comment in order to outline the differences and explain the consequences.

The pressure effect in the frictional behaviour of a colloidal suspension of over-based calcium carbonate stabilized in pure dodecane has been investigated. With a surface force apparatus, the authors have observed the squeeze effect and the frictional resistance of the suspension using sphere- and plane-geometry on a nanometric scale and for low contact pressure (10⁴ to 10⁶ Pa). With tribometers, which can support heavy loading, they have analysed the high pressure domain (10⁴ to 10⁹ Pa). After a critical pressure, evaluated at 10⁶ Pa, the colloidal film does not flow in the contact of the solids, but forms a compacted 'mattress' sliding on the plane surface, and squeezing a hydrocarbon layer. The mean shear strength of the interface, tau , can then be expressed by a relation of the form tau = tau ₀+ alpha p, where p is the mean contact pressure. The meaning of tau ₀ and alpha is discussed.

Fractal structure of a diffusion-limited aggregation (DLA) type was observed in a Co thin film after 50 keV carbon ion implantation to a dose of 2.5*10¹⁷ cm^-2 at room temperature. The authors simulation showed that the structure and the aggregating process could be well described within the framework of the DLA model with the following observations. The atoms in the matrix not only play the role of randomizing the track of mobile particles but also take part in the aggregation. The number of steps each particle undergoes is very limited whether it contacts the aggregate or not. Moreover, the particles may be randomly launched anywhere. The correlation dimension of the structure was determined to be about 1.82+or-0.01.

Using a variational method the authors have obtained the binding energy and equilibrium distance of a two-dimensional hydrogen molecule (HM), these being much larger and shorter than those of a three-dimensional HM. It is shown that the dimensionality effect on exciton molecules is as important as that of the electron-to-hole ratio of effective masses.

Investigations of the anisotropic photocurrent behaviour, as a function of laser intensity and sample temperature, in thin film polydiacetylene single crystals are reported. These polydiacetylenes, PTS (poly-(2,4-hexadiyne-1,6-diol bis(p-toluenesulphonate))) and BTFP (poly-bis-(4-n-butyl-2,3,5,6-tetra-fluorophenyl) butadiyne), have similar one-dimensional electronic band structures. However, the polarization and temperature-dependent photocurrent have completely different behaviour for PTS and BTFR. These results indicate that photocurrent behaviour in these quasi-one-dimensional semiconductors is strongly influenced by the nature and organization of the side groups.

The author has studied the dynamical behaviour of flux kinks, which appear in layered high-T_c superconductors when the magnetic field is parallel and slightly inclined to the plane of the layers. The author has obtained the equation of motion for flux kink position from the equation of motion for the displacement field of the flux line. Having performed computer simulations of the resulting equation the author discusses the time-dependent behaviour of the system with the initially random distributed kinks.

The authors show how the equation-of-motion method can be used to compute the dielectric function epsilon ( omega ) for a model of amorphous Si. The calculation uses a plane-wave basis but could be adapted to any other convenient basis. In principle, such calculations can be extended to non-linear optical properties.