Table of contents

We study the nonequilibrium transport properties of a normal - superconductor - normal structure, focusing on the effect of adding an impurity in the superconducting region. Current conservation requires the superfluid velocity to be nonzero, causing a distortion of the quasiparticle dispersion relation within the superconductor. For weakly reflecting interfaces we find a regime of intermediate voltages in which Andreev transmission is the only permitted mechanism for quasiparticles to enter the superconductor. Impurities in the superconductor can only cause Andreev reflection of these quasiparticles and thus cannot degrade the current. At higher voltages, a state of gapless superconductivity develops which is sensitive to the presence of impurities.

The forces on individual electrons in an unscreened nondegenerate electron fluid, due to electron density fluctuations, have been calculated using Monte Carlo simulations and determined experimentally over a broad range of the plasma parameter . The experimental results are obtained from the magnetoconductivity measured for electrons on liquid helium below 1 K for T. The magnitude and density dependence of are explained by the many-electron theory of magnetotransport. The internal electric fields found from the experiments are in excellent agreement with the simulations.

X-ray diffraction measurements are made for urea adducts with n-eicosane to investigate the arrangement of n-eicosane molecules in the channels formed by urea molecules. Extra Bragg reflections have been found on the layers with the -coordinate corresponding to twice the chain length of the n-eicosane molecule in both the low- and high-temperature phases. This kind of Bragg reflection has not been observed for other n-alkanes in urea adducts. On the basis of the intensity distribution of the extra Bragg reflections, it is found that the n-eicosane molecules are arranged in such a way that the orientations of zigzag planes of longitudinally neighbouring molecules are anti-parallel to each other.

Neutron diffraction measurements have been performed to study the nature of magnetic ordering in the compounds HoNiBC and ErNiBC. HoNiBC exhibits antiferromagnetic ordering of the Ho moments with a simple commensurate magnetic structure below K. The magnetic unit cell, doubled along the c-direction with respect to the chemical unit cell, consists of ferromagnetic planes antiferromagnetically coupled along the c-axis. On the other hand, the Er moments in ErNiBC show ferromagnetic ordering with a Curie temperature of 4.5 K. In contrast to the case for the structurally related superconducting counterparts ( and ) no modulated magnetic structures were observed for any temperatures above 1.4 K.

Although it is some five years since fullerenes were extracted in macroscopic quantities from the black, superficially amorphous sooty deposits produced by a carbon arc under helium, little is known in detail about the structure of the deposit or its electrical and magnetic properties. Here we provide evidence that this deposit, known as fullerene soot, is composed of defective networks of carbon atoms which do not have all valencies satisfied. We have studied these soots, before and after thermal annealing, using x-ray and electron diffraction, electron spin-resonance (ESR) spectroscopy, infra-red transmission and measurements of electrical conductivity. We find that localized states associated with such dangling bonds are removed from the soot on annealing and this process is accompanied by an ordering transition which modifies the electrical and magnetic properties. The fullerene soot particles appear to be encapsulated aggregates of highly defective carbon `onions'. Such metastable defective networks undergo a subtle ordering processes upon heat treatment which is accompanied by a rise in the electrical conductivity and a loss of paramagnetism due to the elimination of unsatisfied carbon atom valencies. Electrical conductivity and infra-red transmission measurements indicate that the centre of these `onions' is graphitic, with metallic properties. The temperature dependence of the electrical conductivity suggests that charge transport in both annealed and unannealed materials occurs by tunnelling between metallic islands in the sample. The ESR linewidth, arising from the spin centres in fullerene soots, is not significantly changed by exposure to oxygen. This suggests that the free radical centres in fullerene soots are extremely efficiently isolated from the atmosphere - presumably by encapsulation. This behaviour contrasts with that of amorphous carbons prepared by thermal decomposition of organic materials (chars). The ESR g-factors of the fullerene soots are lower than those of chars, which suggests that the radicals in fullerene soots have strong sigma character due to unsatisfied -type valencies. In this paper, a plausible structure and associated annealing mechanism for the fullerene soot is presented based on these experimental observations.

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Starting from a classical Hamiltonian for nonhomogeneous elastic media, a procedure is developed for acoustic phonon quantization in resonators as well as linear and planar waveguides. The formalism is illustrated in an example of acoustic phonon modes in a buried cylindrical waveguide. The deformation potential Hamiltonian for electron - acoustic phonon interaction is also obtained.

The absolute double-differential Compton scattering cross section for high-energy x-rays has been measured for K-shell electrons of copper using the coincidence technique. Three different momentum-transfer values have been used which are all in the interesting range where the role of the electron binding energy is important. The results have been compared with the existing calculations emphasizing the role of the impulse approximation. The impulse approximation is found to work surprisingly well and the small deviations are explained via more detailed calculations.

For the first time we present the results of an analysis of the energy distribution function of 2D excitons for fixed nonequilibrium phonon spectra. The calculations are based on the kinetic equation for the interacting 2D exciton and 3D acoustic phonon gases. For the low-density exciton gas in GaAs/AlGaAs quantum wells (QW), we find a strong dependence of the exciton energy distribution on the QW width and nonequilibrium phonon spectrum.

The ground-state energy of an acoustical bipolaron in three dimensions (3D) is studied within the Feynman path integral method. The acoustical polaron exhibits a transition from the quasi-free state to the self-trapped state. Depending on the value of the Debye cut-off in phonon wavevector space this transition can be continuous or discontinuous. If two electrons are present a second transition becomes possible: the transition to the bipolaron state in which the two electrons are self-trapped around the same position. The corresponding phase diagram is calculated as function of the cut-off , the electron - phonon coupling strength , and the repulsion (U) between the electrons. We found that: (i) the bipolaron transition is a first-order transition; (ii) it is also possible between two polarons which are in the quasi-free state, i.e. are not self-trapped; and (iii) the effective particle mass increases substantially at the transition point.

In the framework of the tight-binding model, the effects of longitudinal as well as transverse coupling between polyacetylene chains with different lengths and the electron - electron interactions on excitations have been studied. A new spinless charged polaronic excitation was obtained due to the transverse interchain coupling, which is different from the general magnetic polaron obtained in the perfect one-dimensional model. Due to its end positioning, a polaron is more easily stimulated energetically in a longer chain than in a shorter one, and it can hop to the longer chains nearby via either transverse or longitudinal interchain coupling. In addition, the transverse coupling decreases the energy of creation of a polaron. Considering the strong electron - electron interactions and the end positioning of chains in actual materials, we concluded that the one-dimensional model of conducting polymers is fairly appropriate in certain circumstances.

A semiconductor nanostructure represented by the cylindrical electrostatic -potential subjected to magnetic fields is considered theoretically. The possibility of particle penetration into the region outside the ring for a finite opacity leads to modification of the energy spectrum and the associated azimuthal currents compared to the results for the quantum ring. In particular, when the Aharonov - Bohm whisker is introduced at the origin the currents do not tend to zero in the limit of vanishing magnetic flux. This is attributed to the change of the topology of the structure after introducing the non-zero Aharonov - Bohm flux. This feature diminishes when more realistic distributions of the magnetic field are considered. For uniform magnetic fields anticrossings of the energy levels are observed as a function of the magnetic index and their role in determining the quantum currents is investigated for a wide range of the potential strength. Similarities and differences between the rectangular and cylindrical geometry are discussed.

Electronic structures of d electrons under magnetic fields for the transition-metal compounds with the cubic Laves phase structure are calculated in the self-consistent tight-binding approximation. The induced moments as a function of magnetic field are obtained for compounds with various numbers of d electrons, and , on the constituent transition-metal elements A and B. Ferromagnetic and paramagnetic phases are obtained on the plane of and , by performing calculations at more than 50 points on this plane. In a narrow range along the boundary between the two phases a metamagnetic phase is found. The critical fields of the metamagnetic transition are calculated in this range. The calculated results are qualitatively in good agreement with the observed values for , and .

Annealed amorphous alloys have been investigated by means of AC magnetic susceptibility measurement and x-ray diffraction analysis. The amorphous (x = 15, 16, 20, 25 and 30) alloys annealed at 523 K for 180 min exhibit re-entrant spin-glass behaviour, while a direct transition from a paramagnetic to a spin-glass phase occurs in the amorphous alloys with x = 7.5, 10 and 33.3. For the amorphous alloy, the Curie temperature and the magnetic moment increase, but the spin freezing temperature decreases on annealing. The magnetic phase diagram for the amorphous Y - Fe alloy system in the annealed state is similar to that for many other amorphous Fe-based alloy systems in the as-prepared state. Increases in the atomic distance of Fe - Fe pairs and in the coordination number of the nearest-neighbour Fe atoms on annealing have been observed. These structural changes explain the changes in the magnetic properties for annealed amorphous alloys.

The density functional formalism of Hohenberg, Kohn and Sham can be applied only to the calculation of the ground-state properties of the system. In general, the energy of the electronic excitations can be obtained from the self-energy operator , which requires a much larger computational effort. Recently, Fritsche has proposed a generalization of the density functional theory to excited states, that has been applied to the calculation of the band gaps of solid rare gases, alkali halides, diamond and silicon. We show that Fritsche's theory is incorrect since it is in strong disagreement with the Kohn - Sham theory when the ground state properties are considered.

The two exchange - correlation potentials that derive from a variation of the exchange - correlation energy for full and averaged electron - electron coupling strengths, respectively, are shown to be identical, which invalidates the conclusions of M Biagini presented in the preceding comment.

Unfortunately figures 1 and 2 in the printed version of this article showed the dependence of the remanence and coercivity, respectively, on the weight composition x for composite magnets (Sm-Fe-N)_1-xFe_x rather than the dependence on the atomic composition x for (Sm-Fe-N)_100-xFe_x, as stated in the captions. The correct figures are given below.

Figure 1. Dependence of the remanence on x for isotropic composite magnets (Sm-Fe-N)_100-xFe_x prepared by directly mixing micrometre Sm - Fe - N grains with nanometre Fe particles and micrometre Fe particles, respectively.

Figure 2. Dependence of the coercivity on x for isotropic composite magnets (Sm-Fe-N)_100-xFe_x prepared by directly mixing micrometre Sm - Fe - N grains with nanometre Fe particles and micrometre Fe particles, respectively.

Unfortunately the word `fluids' was replaced by the word `fields' in the title of the printed version of this article. The correct title, as used in the contents list of the issue, is shown above.