Table of contents

A single-crystal X-ray structure analysis of the metastable decagonal phase Al₇₈₍₂₎Mn₂₂₍₂₎, centrosymmetric superspace group P10₅/mmc, has been performed using the n-dimensional (n-D) approach. The structure has been solved by Patterson analysis and subsequent least-squares refinement, both in the 5D description. The final wR=0.144 for 233 independent reflections with I>2 sigma (I) and 18 variables. There are four 5D atoms in the asymmetric unit generating one planar and one strongly puckered quasiperiodic atomic layer in physical space. These layers show close resemblance to the respective planar and puckered layers of the crystalline Al₁₃Fe₄ phase. The 3D decagonal structure is built of periodically stacked packings of six such layers per translation period.

The authors are concerned with new ways of understanding and describing short-range stacking order in layered materials. The X-ray pattern from several layered materials, exemplified by Cu(C₆H₄(OH)COO)₂.(.H₂O)₂ (DSC), consists of alternating discrete Bragg spots and diffuse modulated lines, indicating a large degree of disorder, which is interpreted as being associated with the stacking of the rigid layers. From a detailed study of the shape and widths of the diffuse component, a pseudo-spin model is developed, from which short-range order parameters are derived, amounting to first- and second-nearest neighbour coupling of an Ising model. This approach is contrary to models describing the crystal as consisting of a mixture of ordered and disordered phases. The ordering parameters are sample dependent, but for a given sample they are temperature independent, as concluded from Raman studies between 50 K and room temperature.

Diffuse X-ray scattering lines observed from the layered material Cu(C₆H₄(OH)COO)₂.(H₂O)₂ (DSC) reveal a considerable degree of short-range stacking order, indicating direct correlation between the first few neighbouring layers. The data have been fitted to a 1D, two-parameter Markovian growth model, equivalent to a 1D Ising model involving nearest- and next-nearest-neighbour coupling. The disorder is caused by a combined effect of the one-dimensionality and the two coupling parameters, which are of the same sign. In the growth process the configuration of a given layer is influenced both by the previous layer configuration and the next previous one. Several samples from the same batch were studied, showing a considerable variation of the next-nearest-neighbour correlation parameters, indicating that the growth is very sensitive to the local external conditions (concentrations, impurities, temperature gradients, convection).

The A15 intermetallic compound V₃Ga was subjected to grinding in a high-energy ball mill. X-ray diffraction showed that the material transformed from the A15 phase to the high-temperature BCC solid solution after 40 hours of grinding. This state did not change upon further milling. Differential scanning calorimetry revealed two exothermic peaks with a total heat content of 3.0 kJ mol^-1. After the heating the material again exhibited the A15 structure. The activation energy of the first peak turned out to be the same as the activation energy for reordering of thermally disordered V₃Ga. The results were interpreted in terms of atomic disordering.

The spectral density of Al p valence states has been analysed by electron-induced soft X-ray emission spectroscopy (EXES) at the GaAs/AlAs interface. Changes with respect to the density of Al states of bulk AlAs are discussed in relation to the densities of states calculated for various ideal systems. A mixing between the valence states of AlAs and GaAs and point defects such as cation-cation wrong bonds and As vacancies have been observed at the interface.

Second-order vibronic reduction factors for spin-orbit coupling are derived for orbital triplet systems using the recently derived symmetry-related method. This involves the calculation of oscillator overlaps which are projected out of the cubic vibronic states. Details are presented in the weak coupling limit for T(X)e, T(X)t₂ and T(X)(e+t₂) vibronic systems and in the strong coupling limit for T(X)e and T(X)t₂ systems. The analyses are shown to be in agreement with those obtained by other methods. The discussion is then extended to the calculation of the off-diagonal matrix element between the vibronic T₁ ground state and the A₂ inversion level of a T₁ ion for the T(X)t₂-system using symmetry-adapted vibronic states. The effects of anisotropy in the problems studied are also briefly discussed.

Jensen (1989) showed that the positron annihilation rates in metals can be calculated rather successfully within the local-density approximation (LDA) using the total electron density as the only input. Thus, the division into core, valence, and, in the case of transition metals, the d electrons, is not necessary. Jensen calculated the positron lifetimes on the basis of nonself-consistent electron structures. The author has calculated positron annihilation rates using self-consistent electron structures. The use of self-consistent electron densities makes the LDA lifetimes longer and, especially in the case of simple metals, a better agreement with experimental results is obtained. He presents results for several simple and transition metals and study the trends along the different columns and rows of the periodic table. Moreover, he has calculated the positron annihilation rates also for semiconductors using a slightly different enhancement function. The reasons for the success of the LDA are discussed and the implications to the calculation of lifetimes for positrons trapped at crystal defects are demonstrated by examples.

The confining potential and the wave functions of the oxygen anion in the step on the (001) surface of MgO are calculated from first principles. The confining potential is found strong enough for the O^-2 anion in the step to be stable. The method is described and the calculated potential and wave functions are given.

The authors present some results of an investigation into the quantum oscillations in the resistivity and thermopower of a 2D electron gas at low magnetic fields in the liquid ⁴He temperature range. The thermopower is expected to be controlled by electron-phonon scattering in this temperature range and it should be a useful probe of the different effects due to inelastic intra- and inter-Landau level scattering. They see many new phenomena which can be explained in terms of the transition from the one type of scattering to the other as the temperature and field are changed. They also see some new effects in the resistivity which may also be due to electron-phonon scattering.

Defect energies in two- and three-dimensional classical crystals correlate with the shear modulus mu ; in turn this relates the melting temperature T_m intimately to mu . Thus, a model is first proposed for the shear modulus for two-dimensional Wigner crystals. The melting temperature is then determined from the Kosterlitz-Thouless melting criterion or from an anharmonic instability inherent in the model. The relative positrons of these transitions depend on the model parameters used. The calculation is generalized to include (a) zero-point motion which is dominant in the quantum limit and (b) the effect of a magnetic field. For the high field case, this modelling allows T_m to be plotted versus the Landau-level filling factor nu . The predictions of the model are thereby brought into contact with the experiments of Andrei et al. (1988), and Glattli et al. (1990) which have been interpreted as evidence for a magnetically induced Wigner solid (MIWS) in the electron assembly in a GaAs/AlGaAs heterojunction in strong magnetic fields. The model exhibits of the features observed experimentally.

The voltage biased weak link between two bulk superconductors is discussed using the conventional Josephson pendulum model. A parameter lambda is introduced to characterize the ratio between the electron pair tunnelling energy and the electrostatic energy required to store one electron pair in the link. If lambda is small, the Q-V curve, the relationship between the applied voltage and the charge stored in the link, is in a staircase form. When there are strong pair tunnelling processes, lambda >>1, the Q-V curve becomes a straight line. These results are consistent with experimental findings.

The behaviour of the electrical resistance, AC susceptibility, X-ray powder diffraction and upper critical field H_c2 were investigated for the Bi_1.7Pb_0.3Sr_2-xLa_xCa₂Cu₃O_y system. It was found that the transition temperature and the upper critical field are closely related to the lattice constant c of the 2:2:1:2 phase. From the X-ray diffraction analysis the authors found that the lanthanum first substitutes for calcium and then for strontium as the concentration of La is increased.

By measuring the gradual disappearance of diamagnetism in a single crystal of UPt₃ upon warming through the superconducting transition in a constant magnetic field, the authors have investigated the temperature dependence of a characterstic field H_c1*, closely related to the superconducting lower critical field. H_c1. The amplitude of the applied field ranged from 1 to 60 Oe, for directions along the b- and the c-axis of the hexagonal structure. A distinct change in the slope of H_c1*(T) is observed for both directions, at a temperature which coincides well with the lower transition detected by specific heat within the superconducting regime. It is further found that the initial slope of H_c1* is roughly isotropic, in contrast with the marked anisotropy of dH_c2/dT, and that the Meissner effect is extremely small despite the long electronic mean free path.

The authors investigate the solitonic excitations in the easy-plane compressible ferromagnetic Heisenberg chain using a coherent states formalism. It is shown that the existence of out-of-easy-plane excursions is compulsory for the excitations of the elastic models. The shapes of the solitons are shown to be strongly dependent on the values of the parameters of the models.

The authors have studied the doped antiferromagnet using a generalized t-J model which permits a semi-classical expansion about a magnetically ordered ground state. Of particular interest is the interplay between the spin backflow associated with holes and the coherent spin flow of the ordered magnet. A calculation of the leading-order spin-wave spectrum reveals a strong damping effect of the paramagnetic spin currents which, for small doping, renormalizes long-wavelength spin waves and causes short-wavelength spin waves to decay into particle-hole pair excitations. A correct Goldstone mode structure for the spin-wave spectrum is obtained only when the spin backflow is properly taken into account.

Neutron scattering experiments have been performed on single crystals of NpAs and NpSb. The authors report the critical exponents and a detailed investigation of the complex magnetic phase diagram of NpAs as a function of temperature. The anisotropy ratios in the critical scattering are 2.9(5) and 4.5(10) in NpAs and NpSb, respectively. The incommensurate-to-commensurate transition in NpAs is studied and found to be consistent with the McMillan approach, which is based on a Landau expansion. The 4+, 4- magnetic structure that exists over a narrow temperature range in NpAs is almost a complete square-wave modulation. These results, taken together with those for the isostructural uranium monopnictides, indicate the importance of invoking anisotropic interactions. Experiments provide strong motivation for extending the theories to Np, the element following uranium, and in the trivalent state having the configuration 5f⁴.

The magnetic anomaly of weak unidirectional magnetic anisotropy in (Co_0.8Fe_0.2)₈₀B₂₀ has been studied. The low-field magnetization loops are strongly perturbed in the presence of this type of magnetic anisotropy, This effect is more pronounced in metallic glasses obtained at high quenching rates. The author proposes that the magnetic anisotropy is caused by the presence of two magnetic phases with different local anisotropies and weak magnetic couplings. One of these-the amorphous matrix-would have a very small local magnetic anisotropy, while the other-clusters with nearly crystalline order-would have a high local magnetic anisotropy.

Compositionally modulated (Fe-Si)/Si amorphous films have been obtained by RF sputtering. Varying the thickness d_s of Si layers with a fixed thickness of the Fe-Si layers, it is found that, when d_s is smaller than a critical value, a strong-coupling effect between magnetic layers begins to occur. The results are that the low-hysteresis effect disappears, the saturation magnetization M_s and Curie temperature T_C increase monotonically with decreasing d_s, and perpendicular anisotropy appears. The experiments showed that M_s increases exponentially and T_C increases linearly with decreasing d_s because of the coupling effect.

For pt.I see ibid., vol.2, p.1239 (1990). The temperature dependence of the elastic constants of Fe_100-CNi_C and Fe₆₅(Ni_1-xMn_x)₃₅ alloys has been precisely measured at low temperatures by an ultrasonic pulse-echo technique. An anomalous increase in the longitudinal modulus C_L below around 40 K has been observed for C<or=40 in Fe_100-CNi_C and for x<or=0.15 in Fe₆₅(Ni_1-xMn_x)₃₅. The onset temperature of this anomaly increases with increasing frequency of the ultrasound, indicating a relaxation mechanism. The origin of this anomaly is discussed in connection with other low-temperature anomalies, especially with a rapid fall in the forced volume magnetostriction.

A new technique, involved rapid and repeated sweeping of radio-frequency irradiation through a wide frequency range, has been used to enhance the ENDOR signal of the W7 di-nitrogen centre in diamond. This allowed the determination of the following magnetic hyperfine and electric quadrupole interaction parameters (expressed in MHz) for the two nitrogen atoms: A/sub ///⁽¹⁾=121.39(3), A_{perpendicular to} ⁽¹⁾=86.00(3), PA/sub ///⁽¹⁾=2.55(3), P_{perpendicular to} ⁽¹⁾=1.27(3), A₁⁽²⁾=13.58(5), A₂⁽²⁾=16.01(5), A₃⁽²⁾=14.00(5), P₁⁽²⁾=0.13(1), P₂⁽²⁾=-0.13(1), P₃⁽²⁾=0.000(5). The interactions for A⁽¹⁾ and P⁽¹⁾ are axially symmetrical about 111, but those for A⁽²⁾ and P⁽²⁾ have low symmetry, and their principal axes have no clear relationship with one another or with the crystal structure. Hence, the overall symmetry of the site is established as C₁.

No new experiments are reported in this paper: the paper comprises a re-interpretation of published EPR data of the various di-nitrogen centres in brown diamond, including the new ENDOR results of the authors' previous paper on the W7 centre (see ibid., vol.3, p.3591, 1991). The new evidence about W7 provided by these measurements, particularly the identification of the site symmetry as C_1, together with other data about the centre, has enabled them to propose a definitive model for W7: a puckered ring structure (comprising four C atoms, one N atom and N⁺ ion). This model for the W7 centre is shown to lead, by comparison of the properties of the two centres, to a second neighbour, N-C-N⁺, model for the N1 centre. This model for N1 is shown, in the appendix, to allow new information to be drawn from ³C hyperfine structure in the EPR spectrum of P1, the single substitutional nitrogen. A re-examination of published experimental EPR data of Welbourn (1978) for the N4 centre, showing that the two nitrogen atoms are equivalent, is used to propose a collinear N-C-⁺C-N model for N4. The occurrence of only some of the possible orientations of W7 and N4 centres allowed by the symmetry operations of the N4 centre, are explained in terms of a model of the formation of these two centres by plastic deformation.

Mossbauer spectroscopy on radioactive ¹²⁵I impurities implanted in Si was performed using the 35.5 KeV transition in ^125mTe. Implantation was done at room temperature in Si (100) crystals that were made p⁺-type by co-implantation of B. After annealing at 875 degrees C, the I atoms are somewhat displaced from substitutional sites into the (111) direction, as was shown by previous channelling experiments. Mossbauer experiments were performed as a function of source temperature, with the gamma -ray detector along a (100), (110) or (111) direction of the crystal respectively. The spectra were fitted with a quadrupole doublet, the asymmetry of which varied with detector orientation. The areas of all spectra could be fitted consistently assuming different Debye temperatures for variations parallel and perpendicular to the nearest-neighbour directions. From the sign of the observed electric field gradient the authors conclude that the effect is due to the presence of a nearest-neighbour vacancy, in agreement with the interpretation of the channelling experiments.

Thin Sn-O films were prepared by thermal evaporation of high-purity tin in a low-pressure oxygen atmosphere. The films were monitored by conversion electron Mossbauer spectroscopy (CEMS). Radioactive ¹¹¹In⁺-ions were implanted into the films and perturbed angular correlation (PAC) measurements were carried out in air after different annealings up to 1023 K. Two phases were sufficient to describe the PAC data, which are attributed to SnO and SnO₂. The temperature dependence of these phases was found to agree with CEMS measurements on Sn-O films after similar annealing programmes.

The grand partition function has been used to obtain analytic expressions for the excess free energy of mixing, the activity, the small-angle partial structure factors and the chemical short-range order parameter for weak interacting compound-forming molten alloys. It has also been applied to explain the asymmetry in the properties of mixing of AlMg molten alloys by assuming that the chemical complex Al₃Mg₂ exists in the liquid phase. It is concluded that a reasonable degree of chemical order may exist in the concentration range 0.2<or=C_Al<or=1.0, being greatest at 60 at.% of Al. In the Mg-rich end (C_Mg>or=0.8), however the melt is close to the ideal mixture. The topological order, at least at the long-wavelength limit, has been found to depend only weakly on concentration.

Results of Monte Carlo and molecular-dynamics simulations of Lennard-Jones systems are presented in order to compare various methods of computing interfacial properties of liquid-vapour systems. For the computation of the surface tension gamma a new method is developed, which makes use of the Bennett procedure for calculating free-energy differences. The method is compared with the conventional route to the surface tension via the virial expression. For the temperature derivative of the surface tension, gamma /dT, both a fluctuation equation and the Gibbs adsorption equation are employed. It is found that d gamma /dT is determined more accurately by the absorption equation (through the surface entropy). Results of simulations of binary Lennard-Jones mixtures are also presented. For the argon-krypton system, values of the adsorption of argon at the interface are determined from density profiles, and are compared with values predicted by the adsorption equation. Positive adsorption of argon manifests itself in krypton-rich mixtures as a significant 'bump' in the argon density profile near the interface.

The magnetic susceptibility of liquid-mercury-based alloys containing 5, 8, 15 and 18 at.% Na is investigated. Changes in the paramagnetic contribution of the conduction electrons are discussed in terms of negative excess molar volume. The anomalous variation of the total magnetic susceptibility with the composition is analysed. The occurrence of Hg₄ units appears to be corroborated from the composition dependence of the effective mass ratio m/m*.

A kinetic rate law for a partially conserved order parameter is derived in mean field theory of a continuous order parameter Q: ( delta Q/ delta t)(r)=-(( tau ₁+ tau ₂)/kT)(1-( tau ₂/( tau ₁+ tau ₂)) sinh( xi Del )/ xi Del ) delta G/ delta Q. The 'driving force' delta G/ delta Q is related to the excess Gibbs free energy G, tau ₁ and tau ₂ are time constants and xi is the characteristic length scale of the conservation law. For Fourier components of Q with wavevectors short compared with 1/ xi this rate law agrees with the rate law proposed by Salje in 1988.

The validity of semiclassical transport theory for two-dimensional Fermi and Bose conductors is analysed. It is pointed out that if the high-T_c materials are Bose conductors, this theory breaks down. It is also argued that the regime of 'resistivity saturation' in this case occurs at low temperatures with a resistivity that increases linearly with temperature.

Pure ⁹¹Zr NQR in mineral zircon has been observed at room temperature. The ³/₂-⁵/₂ transition is centered at 6.142 MHz with a linewidth of approximately 70 kHz and a T₁ of 0.57+or-0.07 s.