Table of contents

Exact solutions are obtained for the configuration-averaged single-site Green function and the density of eigenvalues for biased diffusion in a one-dimensional bond percolation model. An explicit scaling form is derived for the Green function at low frequencies.

In this comment on the work of S. Clough, (see ibid., vol.18, p.L1, 1985), the author derives explicit expressions for the asymmetric motion induced in a tunnelling particle following an impulse, which was suggested to exist by Clough. He comments on the erroneous quantum mechanics of Clough's treatment that stemmed from his consideration of angular variables and time dependence of his Hamiltonian. He points out that subsuming the lattice vibrations (in thermally assisted tunnelling) into a random time-dependent force is inconsistent with the principle of detailed balance.

It is shown that the recently proposed density-functional theory of freezing predicts a thermodynamically stable hard-sphere solid up to the density of crystal close packing. The behaviour is contrasted with the unphysical remelting of the hard-sphere solid just after freezing found in an alternative theory. Some possible reasons for this discrepancy are pointed out.

Molecular dynamics simulations of supercooled and glassy states of binary 'soft sphere' mixtures are reported. The transition to the amorphous solid phase appears to be smooth and takes place at a roughly concentration-independent value of an effective coupling parameter derived from a simple 'one-fluid' model.

The electron mobility at 77K of heavily doped and compensated gallium arsenide is calculate assuming the predominant scattering comes from a smooth random potential due to a large number of charged impurities with correlated impurity distribution. The nonparabolicity of a conduction band is taken into account at large electron concentrations. A set of samples with given compensation ratios and electron concentrations has been used for experimental verification of the model which explains well the experimental mobilities.

Using long-channel GaAs MESFET structures doped in excess of the Mott criterion, the authors have measured the inelastic length in a 2DEG by two methods, the parallel and perpendicular magnetoresistance. The low-field data were fitted using the theories of Altshuler and Aronov (1981) and of Hikami et al. (1980). The two techniques were found to give good agreement. However, high parallel magnetic field data show a conductivity correction going as delta sigma approximately=B¹2/ln T, a result not predicted by existing theories. The authors present a model for this effect which gives good agreement with experiment.

A three-dimensional Ising model with competing interactions up to third-neighbour layers is analysed using low-temperature series expansions and mean field theory. New sequences of phases springing from multiphase lines are found and the crossover to the behaviour of the ANNNI model is studied. Results are related to experimental findings on alloys, ferroelectrics, and polytypes.

From magnetic susceptibility and hysteresis loop measurements on the cubic perovskite Sr₂FeNbO₆, it is shown that a spin glass transition does exist in this compound which has a non-frustrated lattice, and only antiferromagnetic interactions. This experimental results allows the authors to conclude that lattice frustration is not an essential feature for the spin glass transition in antiferromagnetic insulators. Moreover, it is argued that short-range atomic correlations, which modify the magnetic phase diagrams of the diluted compounds, make this spin glass transition possible.

The model proposed recently by Brezini (1983) to calculate the average probability and the average size of the localisation domain for an electron being localised at a given site in a disordered Cayley tree is extended to the case of a uniform distribution for site energies. Thus, numerical results are presented in the limit of weak disorder. Particular attention is paid to the states near the mobility edge and critical exponents are deduced for the localisation length.

X-ray measurements on the glass state of Rb_1-x(NH₄)_xH₂PO₄ have been made for x=0.78, 0.75, 0.72, 0.68 and 0.49. The results show short-range antiferroelectric ordering for x>0.70 which disappears on warming above 60K. There is also evidence for a tendency to order with a wave-vector, q=( zeta , 0, 0), with zeta approximately 0.35a*. The tendency increases with decreasing x and largely disappears on warming above 160K.

A Green function approach to dopant redistribution calculation of pulsed-laser annealing of Si was formulated. The method included an evaporation loss mechanism for the near-surface dopants. A simpler but less accurate version of the above method was also employed. Both versions were employed in fitting the measured boron and arsenic redistributed profiles due to single-pulse and ten-pulse laser irradiations. The experimentally known maximum melt depths and regrowth velocity in these cases allowed the performance of more vigorous checks on the accuracy of the computation model. With the more accurate version, all computed profiles were in good agreement with the measured data with D₁=4.0*10^-4 cm²s^-1, H=8.8 cm s^-1 for boron and D₁=3.2*10^-4 cm² s^-1, H=3.2 cm s^-1 for arsenic, D₁ being the dopant diffusion coefficient in liquid Si and H the evaporation loss parameter. The simplified version of the computation methods is somewhat faster in terms of computing time. However, it was found to be adequate only for calculating dopant redistribution due to single-pulse laser irradiation.

The phonon dispersion relations have been measured for the (00 zeta ), ( zeta zeta 0) and ( zeta zeta zeta ) directions for uranium sulphide at 300K by neutron inelastic scattering. In the ( zeta zeta zeta ) direction the LA and TA branches are nearly degenerate in frequency and in the ( zeta zeta 0) direction the LA and TA₂ branches are also nearly degenerate. The optical branches exhibit little dispersion. Neither the rigid-ion nor the shell model is capable of giving a satisfactory description of the lattice vibrations. The high-frequency compressional sound velocities are found to be lower than those determined at low frequencies by ultrasound, whereas the high-frequency shear velocities are higher.

AC susceptibility measurements have been performed between 19 Hz and 50 kHz on the insulating spin glass compound CsNiFeF₆. Very large well marked maxima have been found for the in-phase chi ' and out-of-phase chi " susceptibilities. A slight frequency dependence has been observed. The temperatures T_m of the maxima of chi ' follow a Vogel-Fulcher law with (T_m(10)-T₀)/T_m(10)=0.081. The high-temperature side of the chi "-curves is almost independent of the frequency. The Mossbauer experiments for the same sample show that a local spin polarisation occurs at a temperature close to the point where the chi "-curves drop to zero. The analysis of the Mossbauer spectrum at 1.4K shows that the magnetic moments of the Fe ions are not randomly oriented with respect to the local axes of the electric field gradient.

In order to explain ferrielectric behaviour, a phenomenological model given in terms of the polarisations of two non-equivalent sublattices is proposed. The free energy for a second-order phase transition is G=¹/₂f₁P₁²+¹/₂f₂P₂²+gP₁P₂+¹/ 4h(P₁⁴+P₂⁴) (G>0, h>0), where f₁-(T-T₁)/C₁ and f₂=(T-T₂)/C₂; this is the same form as that for the antiferroelectrics given by Kittel (1951) other than for the temperature dependences of f₁ and f₂. The dielectric susceptibility shows a crossover from that of an antiferroelectric at high temperatures to that of a ferroelectric with decreasing temperature down to T_c. The calculated results for the small spontaneous polarisation, the reduction of the Curie-Weiss constant and the unusual E-P curves are discussed in connection with experimental data on ammonium sulphate.

The orientational glass (KBr)_1-x(KCN)_x has been studied by 'adiabatic' specific heat measurements at temperatures between 2 and 50K. No specific heat anomaly was detectable. At low temperatures and in crystals with higher CN^- concentrations (x=0.25 and x=0.53) the molecular system is effectively frozen-in. Only a small fraction of CN^- molecules can equilibrate even on the timescale of these long-time measurements.

Physisorbed 2D O₂ shows saturation magnetisation even within magnetic fields whose magnetic energy mu .B is small compared with the thermal energy of the molecules k*T. This is demonstrated by detection of large magnetocaloric effects, which are for the first time reported for a 2D magnetic systems. The saturation magnetisation at low fields proves that Bloch's spin-wave theory for two dimensions is applicable to 2D physisorbed O₂ and gives evidence for the relevance of this theory from a system which is in a much more ideal manner an approximation to 2D than the evaporated magnetic films studied up to now. In analogy to Koringa's work about nuclear-spin-lattice relaxation in the framework of nuclear magnetic cooling the authors call the observed spin-lattice non-equilibrium phenomenon the '2D Korringa effect'.

Measurements of extremal cross-section areas and of effective masses of the Fermi surface of TiN using the de Haas-van Alphen effect are presented. The data are in very good agreement with the results of detailed calculations for the shape of the Fermi surface.

A unified self-consistent theory of the mutual influence of the electronic and spin subsystems in the s-f model approximation for ferromagnetic semiconductors is developed. The calculations are based on the novel approach of the two-time Green function method. It consists in the introduction of irreducible Green functions (IGF) and the derivation of the exact Dyson equation and the exact self-energy operator. It is shown that the IGF method gives a unified and natural approach for the calculation of the elementary excitation spectrum and damping. The full electronic and magnetic quasiparticle spectra of the s-f model are derived by taking explicitly into account magnon-magnon, electron-magnon and electron-electron scattering processes. The recent Babcenco and Cottam results (1981) follow from this theory in the lowest-order approximation.

A memory-function-projection-operator technique is used to obtain the resistivity of a two-dimensional electron gas under a strong magnetic field. It is shown that in the dilute limit of impurities the resistivity can be expressed in terms of an averaged force-force correlation function.

The collapse of the dissipationless current flow observed in the GaAs-Al_xGa_1-xAs heterostructures at high currents is explained as due to the spontaneous emission of phonons. The dissipative transfer of electrons along the electric field accompanied by the phonon emission starts when their drift velocity exceeds the speed of sound provided that both initial occupied and final empty states have energies equal to the Fermi energy. With the Fermi energy inside the gap between the Landau sub-bands the transitions involve only the localised and extended states which form the energy spectrum of the inhomogeneous sample in this energy interval.

The existence of long-range antiferromagnetic ordering of triangular type has been determined in UCl₃ by neutron diffraction measurements below T_N=a6.5K. At 4.5K the ordered magnetic moment per U³⁺ ion amounts to 1.9(1) mu _B, which is similar to that for UI₃. At temperatures below T_t=2.5K another magnetic phase transition occurs; all superstructure peaks disappear. The exact nature of this transition cannot be determined unequivocally from the present data. The magnetic properties of UCl₃ may be interpreted on the basis of uranium chains along the c axis with predominant antiferromagnetic interchain exchange coupling and weak interactions between different chains. The interchain interaction causes the development of three-dimensional long-range ordering below T_N. The crystal-field and the exchange interactions have been examined by means of neutron spectroscopy measurements performed for UCl₃ and the magnetically dilute samples U_xLa_1-xCl₃(x=0.1, 0.8) which confirm the one-dimensional nature of this compound.

An analysis is presented of the effects of small random distortions on the electron paramagnetic resonance of Re⁴⁺ in (NH₄)₂PtCl₆. It is shown how the large intensities of 'forbidden' transitions, the observed asymmetric lineshapes and significant shifts of the experimentally measured line positions from those predicted by the usual spin Hamiltonian can all be understood in terms of a single strain-related parameter sigma ₂G₄₄=0.016 cm^-1. The line shifts lead to an apparent dependence of the spin Hamiltonian parameters on the measurement frequency. Similar effects, if not recognised could be a significant source of error in many other EPR measurements.

The optical-RF double-resonance technique has been used to study nuclear magnetic resonance in the ground electronic state of Eu³⁺ in LiYF₄. This is the second host (YAlO₃ being the first) for which such measurements have been made. The quadrupole interaction parameter P for YAlO₃ and LiYF₄ are found to be quite different. This demonstrates a substantial contribution to the quadrupole interaction from the surrounding ions. To a good approximation, mod P_Lat/P_4f mod has a host-independent value of about three for Eu³⁺. This provides the first clear experimental test for the Edmonds (1963), and Blok and Shirley (1966) approach to Eu³⁺ quadrupole effects. From the low-magnetic-field studies the authors find the Eu³⁺ nuclear magnetic moment in a plane normal to the S₄-axis quenched by 94%. This is the largest quenching observed so far. They have also observed Delta I_z=0 RF transitions of intensity comparable with the intensity of the Delta I_z=+or-1 transitions for this nearly axially symmetric host ( eta =0.048).

Magnetically dilute frozen aqueous solutions of Fe³⁺ have been studied by the use of Mossbauer spectroscopy at different temperatures. Below the glass transition temperature, T_g, the spectra are influenced by spin-lattice relaxation processes which are identified as Raman processes induced by acoustic phonons. Above T_g several effects are observed. The f-factor decreases rapidly, the Mossbauer absorption lines become broadened because of diffusional motion, and an anomalous rapid increase in the spin-lattice relaxation frequency is observed. It is interesting that all of the these three effects can be characterised by the same characteristics temperature T_- approximately=2100K. This suggests that the new type of spin-lattice relaxation, present above T_g, is related to the molecular dynamics of the (Fe(H₂O)₆)³⁺ complexes in the supercooled liquid.

Muffin-tin orbital methods in electronic structure theory evaluate the Hamiltonian and overlap matrix elements by breaking up the integrals into contributions from non-overlapping cells about each atomic centre. The simplest choice of cells is to take maximally non-overlapping spheres about each atom enclosed in a large sphere, and one then has a single 'cell' for the interstitial region. The authors describe a fast computational procedure for calculating the cell projection operator theta (r) for this configuration.

The K beta ^III,IV, and K beta ^VII,VIII X-ray satellite groups have been measured with high resolution in Na, Mg and Al metals as well as in Na₂O. The spectra interpreted in terms of KLⁿ to LⁿV(n=1, 2) transitions, are analysed in detail using experimental initial multiplet energies from the KLⁿ to Lⁿ⁺¹ X-ray transitions. The final-state energies are taken from optical spectra presuming fully screened Lⁿ core holes. The results show this assumption to be justified. The K beta ^VII,VIII band is here reported for the first time for Na, and also in Mg and Al it has a peak not found earlier. In metals these spectra allow new testing possibilities for the final-state rule. In Na₂O the K beta ^III,IV band is a clear duplet of K beta ₀ cross transitions but the K beta ^VII,VIII band spreads out to a diffuse hump due to interaction between the sodium L² holes and the localised oxygen valence orbitals. The K²Lⁿ to KLⁿ⁺¹ (n=0, 1)hypersatellite spectra overlapping with the K beta ^VII,VIII bands in aluminium were separated from the latter and the energies of the multiplet transitions were determined and found to agree with theoretical energies calculated using the multiconfiguration Dirac-Fock method.