Table of contents

When irradiated nitrogen-rich natural diamond is heated isothermally, the vacancies are destroyed at two rates. In both processes vacancies migrate to and are trapped at the nitrogen. The faster process coincides with a sharpening of the optical transitions of the vacancy. Both processes have the same activation energy of E_m=2.3+or-0.2 eV. By annealing nitrogen-free diamond, E_m is shown to be the migration energy of the neutral charge state of the vacancy.

The authors numerically study the time-dependent behaviour of domain growth in quenched systems with multiply degenerate ordered states. Based on the phenomenological model equations for such degenerate systems recently proposed by Enomoto et al. the authors perform two-dimensional computer simulations for triply degenerate systems both with a conserved and a non-conserved order parameter, respectively. In particular, they discuss the asymptotic scaling behaviour for the systems and the corresponding growth exponents for the characteristic length scales.

It is shown that the excess low-energy (2-10 meV) density of vibrational states (LEDOS) in glasses leads to an increase of the mean-square atomic displacements (R²) in comparison with the case of crystalline counterparts. The estimations show that due to the excess LEDOS (R²) at the glass temperature, T_g in glass is the same as in its crystalline counterpart at the melting point T_m. The relation between T_g and T_m is expressed through the parameters of the excess LEDOS and is shown to be in good agreement with the experimental rule T_m/T_g approximately=3/2. A criterion of glass-forming ability, based on the important role of the excess LEDOS, is formulated.

Wavelength-modulated reflectivity spectra near the fundamental absorption edge of Cd_1-xCo_xSe (0.01<or=x<or=0.053) solid solutions have been obtained at temperatures between 8 and 250 K. A line-shape analysis of the structures observed allows the author to determine the temperature and composition dependence of interband transition energies. The energy gap varies linearly with composition in the range studied. The temperature variation of the energy gap can be fitted well by the empirical Varshni relation.

Inelastic incoherent neutron scattering spectra of ice Ih and recovered high-pressure ice II, IX, V and VI were measured in the range of energy transfer from 50 to 500 meV at 20 K using the high-energy transfer (HET) spectrometer at ISIS. The excellent resolution and large-Q range of this instrument enable the authors to examine the vibration modes of these ices in great detail. The results clearly show that the low-energy cut-offs of the librational bands shift towards lower energy transfers by amounts ranging from 4 meV for ice IX to 12 meV for ice VI as compared with ice Ih. There is no clear high-energy cut-off for this band for ice V and VI. For the intramolecular stretching modes, the main peak at about 414 meV for ice Ih is shifted towards higher energy transfer for these recovered ices while there is no clear evidence of any shift in the intramolecular bending modes at about 200 meV.

The velocity of sound in liquid Tl-Te alloys has been measured in order to investigate the composition dependence of the compressibility. The isothermal compressibility deduced from the velocity of sound, density and specific heat shows a characteristic cusp at Tl₂Te, which supports stable Tl₂Te association persisting in the liquid. In addition a logarithmic singularity in the adiabatic compressibility has been found at the critical point of the two-melt phase separation between Tl and Tl₂Te.

The fracture of a perfect crystal at a number of temperatures was investigated using molecular dynamics computer simulations in which a crystal was pulled apart at a constant strain rate. At low temperatures the overall stress increased to a maximum after which a series of distinct structural rearrangements each accompanied by a sharp reduction in stress were observed. The material maintained a high degree of crystallinity. At higher temperatures structural rearrangement was more continuous and the material in the region of fracture was more disordered and somewhat liquid-like. At all temperatures failure occurred through the formation of voids which subsequently fused to leave bridges between the nascent surfaces. These bridges stretched and finally broke. In another series of experiments the failure of a bridge which had formed spontaneously between two nearby surfaces was investigated under similar conditions of steadily increasing strain.

The elastic eigenvalues associated with hydrogen density fluctuations near the critical point of a metal-hydrogen system have been evaluated approximately for the case of a thin-disc geometry, with a small but finite thickness.

For pt.I see ibid., vol.4, p.2139 (1992). Ultrasonic velocity measurements were made on Pd-H in the pressure and temperature regime above the critical point of the alpha to beta transition. The results were used to determine the elastic constants for the system and to compute the spinodal temperatures for the macroscopic density fluctuations near the critical point.

A series of displacively modulated mixed crystals Ta_1-xNb_xTe₄ (0<or=x<or=1) exist in which the modulation wavevector q varies as a function of composition. The nature of the compositional dependence of q is studied in greater detail and evidence is presented for a stepwise variation of q with x. Electron diffraction experiments at room temperature reveal that the modulation periodicity remains commensurate until a threshold dopant concentration (x approximately=0.3) is exceeded and then jumps discontinuously to an incommensurate value. Further jumps in q are observed as the dopant concentration is increased. Models of the superstructures of the doped crystals corresponding to plateaux in the dependence of q on x are studied by comparison of the observed and calculated electron diffraction patterns. The results indicate that the mixed crystals may be described as a series of long-period commensurate superstructures. In addition, satellite dark-field microscopy has revealed the presence of defects in the modulation structures of the doped crystals, including antiphase boundaries and discommensuration arrays. The effects of substitutional doping on the phase transitions occurring on heating above room temperature in TaTe₄, and on cooling in NbTe₄, are also examined. The observations are interpreted in terms of the charge-density wave origin of the modulations.

Rate constants for vacancy migration in a Lennard-Jones crystal are evaluated within the frame of the reactive flux correlation function formalism by molecular dynamics simulation. In the authors' calculation a key role is played by a holonomic constraint acting on the reaction coordinate. The definition of a suitable reaction coordinate is a subtle matter as becomes evident when using their techniques. They show how the ambiguities in the definition of the reaction coordinate can be circumvented. The authors' results are two orders of magnitude more precise than previous 'exact' calculations.

The consideration of spin-orbit coupling in spin-polarized electronic structure calculations leads to magnetic anisotropies and reduces the symmetry of the problem. The theory of magnetic groups is required to make full use of the remaining symmetry. A systematic method is presented to treat this problem in the framework of the Green function method. An application to the magnetocrystalline anisotropy energy of Fe fails to converge numerically but reveals where this effect originates as a function of reciprocal space vector and energy.

For single crystals of n-InSb and n-InAs in the extreme quantum limit of the applied magnetic field, the transverse resistivity rho _xx decreases and the longitudinal resistivity rho _zz increases with a logarithmic temperature dependence from 1 K to 50 mK. Since the Hall resistivity rho _xy and conductivity sigma _xy approximately=1/ rho _xy( rho _xx<< rho _xy) are constant in this temperature region, the origin of this effect could lie in the electron-electron interaction in a disordered system. Extrapolation of the data to zero temperature points to a Hall-insulating state.

The authors present a simplified tight-binding-based coherent-potential approximation (CPA) formalism for the host electronic structure of semiconductor alloys (e.g. Ga_1-xAl_xAs). Several schemes to locate the gap level due to point defects are discussed. In particular, they introduce a novel scheme: the deep-level approximation. They also point out that the vacancy results in the CPA are identical with the simpler virtual-crystal approximation. They discuss their results by presenting numerical results in Ga_1-xAl_xAs and GaAs_1-xSb_x.

The magnetic phase diagram is constructed for the amorphous (Co_0.9Zr_0.1)_100-xX_x (X=Al, Si, Cu, Ge and Zr) psuedo-binary alloys in the concentration range 0<or=x<or=45 through the DC and AC magnetization measurements in the range 4-800 K. The present results are compared with the previous data for the amorphous Co-Zr binary alloys. The ferromagnetism of the amorphous Co₉₀Zr₁₀ master alloy is rapidly weakened and disappears at about x=30, regardless of the third elements Al, Si, Cu and Ge. The concentration dependence of the density of states at the Fermi level is deduced by extracting the band structure contribution from the measured linearly temperature-dependent specific heat coefficient. A significant difference in the electronic structure emerges above x=30, depending on whether the element X added to the Co₉₀Zr₁₀ master alloy is a non-transition metal, like Si and Ge, or the transition metal Zr. A rapid increase in the electrical resistivity with increasing x beyond x=20 for the Si and Ge bearing alloys is attributed to a rapidly decreasing density of states at the Fermi level.

The DC conductivity and thermopower of polyaniline derivatives were measured for temperatures ranging from 80 to 345 K. The samples were oxidized to different degrees, fully protonated and further doped with -SO₃H, -COOH and phosphorylated polystyrene additives to concentrations varying from 0 to 100 mol.% of functional group per mole of aniline. The DC conductivity sigma (300 K) ranges from 0.01 to 16 S cm^-1, depending on the degree of oxidation and dopant concentration, while sigma (T)= sigma ₀ exp(-(T₀/T)^0.5), where T₀ is mainly determined by the degree of oxidation. The thermopower, which is of the order of -15 mu V K^-1 at 300 K, changes sign from n to p with increasing temperature at T_r, which varies between 290 and 325 K. This change in sign, which is reversed during cooling, is attributed to the existence of a half-filled polaronic band situated deep in the energy gap of polyaniline.

The adsorption of an atom with one valence electron on a metal surface and the field desorption of this atom are considered in this paper. A very simple one-electron model is shown to describe properly the general features of the processes considered.

The thermoelectric power of a narrow constriction in a two-dimensional electron gas (2DEG) in Al_xGa_1-xAs/GaAs heterojunctions is theoretically investigated in the form of the transverse voltage by using an appropriate model for the recent experimental system. The model gives not only the quantized thermoelectric power obtained theoretically and experimentally but also the oscillating chemical potentials in all the participating 2DEG regions. The temperature dependence of the transverse voltage is also studied focusing on the effect of the thermal broadening of the electron distribution functions.

The ring with N weak links in the magnetic field is examined. The exact form of the partition function of the model was obtained, what permits one to evaluate all the thermodynamic characteristics of the model. The authors concentrated on the heat capacity and particularly on the susceptibility, which exhibits quite remarkable features in the finite-size limit. In addition, the influence of the weak-link strength distribution is explored.

A series of single-phase samples with the composition of Y_1-xCa_xBa₂Cu_3-yCo_yO_6.5+z (0<x<0.25; 0<y<0.25) have been synthesized and studied by X-ray powder diffraction analyses, and measurements of the resistivity, AC susceptibility, oxygen content and X-ray photoelectron spectroscopy (XPS). It is found that the reduction in the superconductivity induced by the substitution of Co for Cu(1) can be counterbalanced, to an extent, by the replacement of Ca for Y. This phenomenon has been explained with the opposite effect of Co doping for Cu(1) from Ca doping for Y, on the mobile hole concentration of the system. The substitution of Ca for Y induces a delocalization effect which releases the holes in the Cu-O chain and transfers them to the CuO₂ sheets, leading to an increase in the mobile hole concentration while the substitution of Co for Cu(1) reduces the mobile hole concentration because of its localization effect on the holes. The XPS measurement for the core level of Cu 2p and O 1s reveals that the oxidation state of the copper changes with the dopant content, and a peak corresponding to the Cu³⁺ oxidation state is observed in some co-doped samples.

Motivated by recent data on the strongly inelastic magnetic neutron scattering spectrum of praseodymium and thulium metal, over a wide range of energy and wave vector kappa transfers, the authors have calculated the corresponding spectrum for both ions and the kappa -dependence of the relevant inelastic structure factors. The model includes the seven lowest energy states and their mixing by the spin-orbit coupling: results differ from previous calculations for the f²/f¹² state and this is attributed to mixed phase conventions for the base states used for the representation of the spin-orbit energy matrix and the magnetic neutron interaction. Comparison of results from the correct model with existing neutron beam data shows satisfactory agreement; remaining discrepancies are attributed to potentially interesting shortcomings of an isolated ion model for rare earth metals. The unintentional exercise of using correct and incorrect wave functions underlines the sensitivity of inelastic structure factors to features of the magnetic wave functions.

Rb₂MnCl₄ was always considered to be an ordinary quasi-two-dimensional uniaxial weakly anisotropic antiferromagnet with the crystallographic c axis as the easy axis. This picture became dubious when a splitting was observed of one of the magnetic modes for an external magnetic field applied perpendicular to the c axis. New magnetic resonance measurements revealed also a splitting of the zero-field antiferromagnetic resonance. All the unexpected phenomena observed in this investigation can be explained in terms of an orthorhombic distortion of Rb₂MnCl₄ in its antiferromagnetic state, taking into account that all samples are of the multi-domain type below the Neel temperature.

The ENDOR spectra of Cr³⁺ ions in KDP have been measured at 4.2 K and the hyperfine interaction with the nearest neighbour protons has been investigated. The results confirm the EPR model for the defect: the Cr³⁺ substitutes for K⁺ with two proton vacancies in the first shell. The relaxation of the lattice ion in the neighbourhood of the impurity is also discussed.

The behaviour of the diffuse phase transition of (Pb_1-xBa_x)(Yb₁2/Nb₁2/)O₃ (0<or=x<or=0.3) ceramics has been investigated with special attention to the phase boundary near x=0.12. The temperature and frequency dependence of the dielectric constant in and loss tan delta have been carefully measured together with heat capacity measurements. Above x=0.12, the phase transition is broadened and frequency dependent, while the AFE-PE phase transition is sharp and in closely follows the Curie-Weiss law above T_N for 0<or=x<0.1. Also, above x=0.12, (1/ in -1/ in _m) is proportional to (T-T_t)ⁿ at T>T_t, where in _m is the peak value of in at T_t and n is an exponent. The value of n is dramatically increased for 0.1<or=x<or=0.14, and then increased linearly with increasing Ba-content (dn/dx=1.78*10^-2 %^-1) for 0.14<or=x<or=0.3. T_N decreases markedly for 0<or=x<0.12, whereas the value of T_t is reduced linearly with increasing x-value (dT_t/dx=-6.61 degrees C %^-1) for 0.14<or=x<or=0.3. Another remarkable feature is an anomalous enhancement of in _m between x=0.1 and x=0.14. Heat capacity measurements have revealed a well-defined anomaly associated with the AFE-PE transition, increasingly broadening with Ba-content for 0<or=x<0.1. For 0.1<or=x<or=0.14, the heat capacity anomaly is extremely broadened and a small deviation from the extrapolated baseline can be seen. In particular, the authors cannot detect any heat capacity anomaly associated with T_t above x=0.14, which is suggested as additional evidence for DPT. These salient features are discussed in the light of the formation of locally ordered polar microdomains within a non-polar matrix and the pseudo-cubic structure above x=0.12.

The effect of DC bias on the dielectric properties and the phase transition behaviour of the (1-x)((Pb_0.7Ba_0.3)(Mg₁3/Nb₂3/)O₃)-xPbTiO₃ ceramic system near the morphotropic phase boundary (0.35<or=x<or=0.5) have been investigated. The phase transition goes through a series of character changes from being a diffuse phase transition at x=0.35 to being a more or less normal phase transition at x=0.5. Between the extreme compositions, the system has shown a rather anomalous phase transition behaviour due to the existence of the morphotropic phase boundary centred around x=0.44. On subjection to a series of external biases, the averaged dielectric constant maximum temperatures of the specimens of intermediate compositions tend to oscillate with respect to the bias field. Such anomalous dielectric behaviour is ascribed to the coexistence of multiple phases in a narrow composition band whose centre of gravity may be assigned as the morphotropic phase boundary.

Light scattering measurements have been performed on single crystals of the solid solution of silver tantalate-niobate in a temperature range extending from 10 K to about 800 K. The experimental results are analysed and discussed by comparison with dielectric and structural data as functions of temperature and relative Nb and Ta concentrations. The specific contributions of each constituent ion to the phase transition sequence and mechanisms are critically discussed.

The Gram-Charlier expansion of a band is an expansion in Hermite polynomials multiplied by exponentials, which converges in the mean to the correct band shape, and its coefficients are linear combinations of moments of the band. It is shown here how this expansion can be used to simulate the vibronic band shapes resulting from various types of Jahn-Teller coupling. This process has the advantage of being at its best when the coupling is strong and the standard methods are difficult.

In contrast to virtually all natural diamonds, synthetic diamonds show a considerable, reversible, change in colour upon heating. This colour change from yellow to reddish brown in the type Ib diamonds is especially noticeable above 500 K and is attributed to the interaction of optical transitions at the substitutional nitrogen impurity centres with lattice vibrations. Optical absorption spectra of homogeneous regions in several crystals were measured as a function of temperature (77-773 K) in the wavelength interval 200-700 nm. The spectra are interpreted in terms of an analytical model for electronic transitions from a deep donor level to a continuous range of levels in the conduction band, where strong electron-phonon coupling is taken into account. Numerical evaluation based on this model yielded a Huang-Rhys factor, S=16+or-1, a phonon energy, h(cross) omega =0.15+or-0.02 eV and the temperature dependence of the nitrogen donor ionization energy. Upon a temperature rise to 750 K the latter showed a decrease of about 0.08 eV.