Table of contents

The author presents a new algorithm for computing Lindhard sums in susceptibility calculations. Based on the joint density of states (JDOS) introduced by him some time ago, his algorithm gives better control of convergence by decoupling the problems related to the mesh in k space and the mesh used for integrating over the energies. His method is an extension of the usual tetrahedron method for densities of states. He finds that, rather than complicating the problem, the presence of matrix elements helps in the computation of the JDOS. He uses the method to calculate the JDOS and the unenhanced susceptibilities for a FCC model band structure. Since it is based on JDOS the method can be easily adapted to calculate the t-matrix which characterizes off diagonal long-range order. He also outlines how he uses the method to calculate susceptibilities for first principles band structures.

There is an abnormal enhancement of the Neel temperature (T_N approximately=21 K) and spin-disorder resistivity beyond that expected on the basis of de Gennes scaling in PrCu₂Si₂. While the electric quadrupole alignment is known to be the driving force for the moderate breakdown of this scaling in homologous heavy rare-earth members, such a mechanism is not applicable to PrCu₂Si₂. Contrary to the behaviour in heavy rare earths, there is no distinct anomaly in the temperature-dependent lattice constants in this compound as if the electric quadrupole moment of Pr is quenched. Thus, this compound is shown to exhibit novel magnetic and quadrupolar behaviour.

In a recent experiment, the broadening of a Ni tracer marker in a Ni substrate was measured under 300 keV Ni⁺ bombardment. The authors analyse these data with the help of Monte Carlo simulations. Only around a quarter of the measured broadening is due to collisional mixing in the collision cascade set up by the bombarding ion. The remaining amount can be explained in the framework of phenomenological models of thermal spike mixing. A dynamic simulation code gives quantitative agreement with the tracer marker distribution measured after broadening.

The magnetoresistance of a device fabricated from a bulk InGaAs layer on an InP substrate has been studied. The device exhibited both 2D conduction at the InP-InGaAs interface and 3D conduction in a doped InGaAs layer at the surface. The authors have observed negative magnetoresistance arising from the quenching of weak localization in the device. The effect is observed in both transverse and parallel fields, although that in the parallel field is of smaller magnitude than that in the transverse field. It is proposed that the structure contains a two-dimensional electron gas (2DEG) in parallel with a bulk conducting region. The temperature dependence of the negative magnetoresistance has been investigated and phase breaking times have been calculated by fitting the experimental data to the formula given by Hikami et al. (1980) for the 2D effect. A comparison is made with theoretical electron inelastic scattering times, and conclusions as to the nature of scattering in InGaAs structures are drawn.

A study has been carried out of the effect of interface scattering on the magnitude of the giant magnetoresistance (MR) of MBE-grown Co/Cu superlattices. The authors find that increasing interface scattering by annealing the Co/Cu superlattice leads to a progressive decrease in the magnitude of the MR. In contrast to their results, it was recently reported that annealing increases the MR for Fe/Cr superlattices. An explanation is presented in terms of the spin dependence of interface scattering which accounts both for their data for the Co/Cu system, as well as for the opposite results obtained for the Fe/Cr system.

A measurement of the total positron backscattering probabilities using a magnetically guided positron beam is reported. Backscattering yields were measured as a function of the incident positron energy in the range 2-30 keV and the atomic number Z of the target (6<or=Z<or=79). Absolute backscattering probabilities extracted from the annihilation count rates at the target are compared with doubly differential measurements at 35 keV. The total backscattering ratios from these two measurements agree with the experimental uncertainties. The experimental backscattering probabilities compare well with Monte Carlo simulations of keV positron slowing in solids.

A calorimetric study of low-coverage (0.13-0.36 layer) Ne films adsorbed on graphite foam explores the broadening of melting peaks in the vicinity of the two-dimensional triple point. Preliminary analysis confirms earlier indications that premelting is due to edge melting of long strips of film that decorate linear defects and steps on the substrate. A new model that incorporates these substrate effects and a new method of analysis that considers coverage-dependent trends have been developed to fit the data. The fitting indicates that additional substrate effects complicate the analysis; these are discussed, and it is concluded that adsorbate size effects are relatively unimportant, but that heterogeneity in the substrate binding energy may augment the edge melting process.

A structure for ordered silicon-tin systems on a silicon substrate, based on the known structure of a monolayer of tin on the (111) face of silicon, is proposed. The corresponding electronic structure of this system is modelled using the local density approximation to the density functional theory of the electron gas in the Kohn-Sham formulation with ab initio pseudopotentials taken for the electron-ion interaction. The total energy is minimized including relaxation of all the ionic positions, and the single electron states then evaluated. It is found that the overall energy gap in the band structure decreases rapidly with increasing tin concentration, and the direct gap is only 0.6 eV larger than the indirect gap for a tin concentration of 0.17 (a silicon concentration of 0.83). Effective masses of the electrons and the holes are evaluated at a tin concentration of 0.17. The lighter holes and electrons are found to be of similar mass to those in silicon. However, heavy electrons and holes are also found in the layer structure.

The problem of currents induced in the scanning tunnelling microscope due to incident laser radiation is analysed. The authors uses a light-binding description of the microscope tunnelling junction, and the interaction with the laser field is taken into account by an effective time-dependent coupling between tip and sample. The currents generated at frequencies that are linear combinations of the incident frequencies are then obtained with the help of a nonequilibrium Green's function formalism. Particular attention is paid to the rectification and difference frequency generation effects. In this work they extend previous results for the rectified current to the case where a direct bias is applied in addition to the laser field. It is shown that in the limit of very low frequencies the dynamical response may be deduced from the static characteristic curve. In order to compare with recent experimental data, they perform model calculations for a graphite sample and study the induced photocurrent as a function of bias voltage, tip-sample distance and photon energy. The limitations of the adiabatic approximation are discussed. Finally, they present results for the rectified current in the presence of an adsorbed molecule with a characteristic vibrational mode. The contribution from inelastic processes is obtained to the lowest order in the electron-phonon coupling. It is shown that the onset of inelastic tunnelling should be reflected as a singularity in the rectified current as a function of both bias voltage and photon energy.

A new quantum mechanical method for calculating the miniband structure of lateral surface superlattices in a perpendicular magnetic field is developed. The Schrodinger equation is solved via expansion in a basis that is well adapted both to the translational symmetry of the problem and the magnetic field. The approach is quite similar to the nearly-free-electron approximation in the zero-field case and is capable of dealing with arbitrary potential shapes.

Epitaxial, boron-doped diamond films were grown by hot-filament-assisted chemical vapour deposition (CVD) on (100) and (110) natural diamond substrates. Resistivity measurements for 10 K<T<500 K showed a clear transition from band to hopping conduction upon lowering of temperature. In the band conduction regime, the (100) films had higher conductivity than the (110) samples. The reverse was found in the hopping regime. This is explained by the difference in crystal growth mechanisms, leading to higher boron concentrations and lower carrier mobilities for (110) samples than for (100) oriented films. Hall effect measurements were performed for the most lightly doped (100) film at a boron level of 2.7*10¹⁸ cm^-3 in the band conduction region up to 750 K. A mobility maximum of mu _H=590 cm² V^-1 s^-1 at 295 K was found, and the compensation ratio was determined to be smaller than 0.02. Some preliminary values for the Hall effective mass of valence band holes are given.

The mixed oxide (1-x)V₂O₅+xMoO₃ (x ⩽ 0.3) has been synthesized by melting the oxides, and its structures and vibrational properties are investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy, Fourier transform infrared (IR) spectroscopy and Raman scattering. XRD revealed that the solid solutions of V₂O₅ with MoO₃ are well formed except that the second phase V₉Mo₆O₄₀ appeared at x ⩾ 0.1. It turned out that the unit-cell dimension c of the solid solutions (orthorhombic) initially decreases whereas a and b increase with increasing x, and that all the lattice parameters tend to remain unchanged above x ≃ 0.1. The IR band due to the V-O stretching vibration at 1028 cm^-1 shifted downwards, diffused and decreased in intensity with increasing x; similar changes are observed in the Raman spectra of the mixed oxide. It is shown that the V valence transition from V⁵⁺ to V⁴⁺ occurs as V is replaced by Mo. The V⁴⁺ to V⁵⁺ ratios obtained by deconvoluting the V 2p_3/2 line showed some qualitative agreement with the prediction for x < 0.1 based on no Mo valence transition and the V⁵⁺ to V⁴⁺ transition as V is replaced by Mo. It is concluded that the V valence transition is responsible for the vibrational property changes of the mixed oxide.

The authors consider periodic structures made of spheres embedded in a host material with a different dielectric function. They show how to calculate the reflection and transmission of electromagnetic waves by a slab of the material parallel to a given crystallographic plane. The method of calculation is based on a doubling-layer scheme which obtains the reflection and transmission matrix elements for the multilayer from those of a single layer. The reflection and transmission characteristics of the slab are related to the complex band structure of the photon field associated with the given crystallographic plane of the corresponding infinite crystal, which is introduced in the manner of the low-energy electron diffraction theory. They present numerical results which demonstrate the applicability of the method to real systems of current interest and point out some interesting physics which arose from their calculations. They show in particular that the nondegenerate bands of the photon field at the centre of the surface Brillouin zone do not couple to the incident radiation, leading to total reflection at normal incidence.

The temperature dependence of the far-infrared reflectivity spectra of the potassium ammonium dihydrogenphosphate mixed crystal system, K_1-x(NH₄)_xH₂PO₄, is reported for three selected ammonium concentrations, x=0.01, 0.3 and 0.97, over a wide temperature range, 7-250 K. A phase transition is observed in mixed crystals for x=0.01 and 0.97. The occurrence of a glassy state in the mixed crystal for x=0.3 is shown by the spectroscopic study, no ferro- or antiferroelectric order appears upon cooling to liquid helium temperature, and the onset of freezing at about 120 K is manifested by the low-frequency dielectric dispersion observed below 10 cm^-1.

Using embedded quantum cluster models, the authors have studied the mechanism of V_K-centre 60 degrees reorientation in KCl and LiCl, and have considered the possibility that both one-centre 'dielectric' polarons and 'molecular' polarons (V_K centres) coexist in KCl. These studies have shown that lattice polarization lowers the activation energy for V_K reorientation since the V_K centre strongly polarizes near the barrier point. The calculated adiabatic barrier for V_K reorientation is 0.63 eV, in reasonable agreement with the available experimental data. From the self-trapping energy calculations and the analysis of the structure and mechanism of the diffusion of the one-centre 'dielectric' polaron, it is apparent that it transforms adiabatically into a V_K centre without an energy barrier.

For pt.I, see ibid, vol.4, p.7417 (1992). An adequate theoretical basis is developed and presented for a nonstationary technique for the experimental detection of reactions in insulating crystals controlled by tunnelling recombination of mobile defects (hole centres) with electron centres. The effect of finite hop lengths of point defects in a crystalline lattice is carefully analysed. This method is based on measuring the inertial increase or decrease in the recombination luminescence intensity after a stepwise change of temperature (defect mobility). Theoretical analysis of the transient process observed experimentally for V_K centres in KCl:Tl crystals argues for their motion through 60 degrees rotations with the frequency factor S_t=4*10¹³ s^-1, very close to that obtained earlier in the optical dichroism studies.

To investigate the kinetics of recovery of antisite disorder in the A15 compound Nb₃Au, isochronal and isothermal annealing procedures at intermediate temperatures (between 833 and 1043 K) were performed on grains quenched from 1355(1) K. From these experiments, the vacancy migration enthalpy, and its pre-exponential factor were determined as 0.39 eV, and 2.7*10² s, respectively. The atomic-migration enthalpy and its pre-exponential factor were found to be 1.43 eV and 7.1*10^-2 s, respectively. These values are compared with data for the A15 compound V₃Ga, and for Ca₃Rh₄Sn₁₃ which is a superconductor with an A15 substructure, and with data on ball-milled Nb₃Au.

The authors investigate the construction of norm-conserving 'soft-core' pseudopotentials with improved convergence properties of the plane-wave and perturbation expansions. The key factor is found to be the kinetic energy of the valence pseudo-orbitals. The total kinetic energy controls the convergence of the perturbation expansion of the total energy, the kinetic energy contained in the Fourier components beyond a certain cut-off limits the convergence of the plane-wave expansion. The simultaneous optimization of both expansions allows them to use the same pseudopotential in a rapidly convergent total-energy calculation for the crystalline phases, and in the calculation of interatomic forces to be used in atomistic simulations of the disordered phases.

The author studies integer S>1 spin chains. He extends the Kennedy-Tasaki nonlocal unitary transformation for S=1 to arbitrary integer S. He shows the main results of Kennedy and Tasaki (1992) are maintained for S>1: Heisenberg-type Hamiltonians are transformed to Hamiltonians of nearest-neighbour interactions with Z₂*Z₂ symmetry, and the den Nijs-Rommelse string observables are transformed to the ferromagnetic correlation observables. He asserts that in general values of integer S there exist several phases with the hidden Z₂*Z₂ symmetry breaking. The den Nijs-Rommelse string order parameters, which measure the hidden Z₂*Z₂ symmetry breaking, are calculated explicitly for several variants of the VBS-type states. In the standard VBS state, the hidden Z₂*Z₂ symmetry breaks down when S is odd but remains unbroken when S is even. His results for partially dimerized VBS states suggest that the hidden Z₂*Z₂ symmetry breaking can be used to detect the successive dimerization transitions predicted by Affleck and Haldane (1987). Some new anisotropic VBS-type states are investigated. The result suggests that there are successive phase transitions when he increases the uniaxial anisotropy in a Heisenberg-type model. Other new VBS-type states with long-range order are considered, and their relevance to the phase diagram of the Heisenberg XXZ model and the magnetization process of antiferromagnets is investigated. He introduces an extended string order parameter which possesses a characteristic behavior in the partially dimerized VBS states.

La(Fe_xAl_1-x)₁₃ (0.80<or=x<or=0.95) amorphous alloys have been prepared by high-rate DC sputtering to investigate the thermal expansion and elastic properties. A large thermal expansion anomaly has been observed in a wide temperature range. This anomaly is retained, even at high temperatures well above T_c. Such a peculiar phenomenon is associated with a large spontaneous volume magnetostriction. A significant Delta E effect has also been found in the temperature and field dependences of Young's modulus. The anomalous behaviour of Young's modulus can be explained in terms of the softening of elastic modulus caused by the anomalous thermal expansion. From these results, it is concluded that the anomalies in the thermal expansion and in Young's modulus for the La(Fe_xAl_1-x)₁₃ amorphous alloys are caused by a large spontaneous volume magnetostriction associated with the large temperature variation in amplitude of the local moment.

The primary properties acquirable by a crystal upon undergoing a displacive structural phase transition (DSPT) and/or magnetic and/or electric transition are defined with the aid of a Gibbs free-energy expansion, and a microscopic group-theoretical analysis of them is carried out. The properties are, apart from those which define the transitions, piezomagnetism, piezoelectricity, magnetoelectricity and piezomagnetoelectricity. It is shown that the macroscopic tensors characterizing them are sums of atomic property tensors, and, as a result, a crystal may exhibit spontaneous piezomagnetism and/or piezoelectricity below its DSPT point T_D and/or spontaneous magnetoelectricity below its ferroelectric transition point T_E. A crystal with a magnetic transition temperature T_M<T_D or T_E (or one with T_E<T_D) may therefore be in a weak or secondary magnetic (or electric) state in the temperature region T_M<T<or=T_D or T_E (or T_E<T<or=T_D). Thus it is found, in particular, that a crystal with T_M<T_D or T_E may exhibit macroscopic properties characterized by axial c-tensors at temperatures T<or=T_D or T_E, and not just at T<or=T_M. Among the crystals cited as being capable of going into the secondary magnetic and/or electric state are the transforming A-15 crystals. It is shown that they may exhibit not only the linear magnetoelectric effect, but also spontaneous piezomagnetism and piezoelectricity, and that their transition into the superconducting state and their high critical fields H_c may be connected with these properties. The analysis brings to the fore the limited scope of the Neumann principle.

The Raman scattering technique was used to study lattice vibrations in single crystals of K₄LiH₃(SO₄)₄ from 100 to 295 K covering their 4 to 2 structural phase transitions. All of the Raman modes of K₄LiH₃(SO₄)₄ were found not to be affected by the transition. In the case of Rb₄LiH₃(SO₄)₄, a slight softening of the B mode (at 31 cm^-1) was observed in the paraelastic phase.

The author reports a study of room temperature photoluminescence, electromodulation and Raman scattering experiments on CdS_xSe_1-x quantum dots of various sizes (18 AA to 108 AA) embedded in a glass matrix. The Raman experiment confirms that all the samples have the same semiconductor composition. Electromodulation is used to identify the quantum confinement peak position. The observed peaks of photoluminescence and electromodulation spectra are not at the same position and the difference is dependent on the particle size. An explanation in terms of a bound exciton is given to interpret the photoluminescence spectra.