Table of contents

Physical mechanisms of intercalation of semiconductors are determined, establishing the main laws of intercalation, and the physical nature of new phenomena and effects induced by intercalation are explained; general statements for obtaining intercalates with characteristics assigned in advance are also developed.

Unusual magnetoresistance behaviour has been observed in mesoscopic disordered aluminium samples. Large resistance fluctuations were found, with peaks and troughs implying re-entrant superconductivity, occurring as a function of magnetic field, at high field and . The results are consistent with a recently proposed phase diagram, and the apparent re-entrance is attributed to the overlap of a connected superconducting filament with all the measurement probes. The results suggest that microscopic superconducting regions persist in fields of above 1 T in disordered aluminium at mK temperatures.

The quantum Hall effect is a typical realization of topological effects in condensed matter physics. In this article, some of the topological aspects of the quantum Hall effect are reviewed. For the lattice fermions, the Hall conductance of the system is expressed in terms of two different topological invariants. One is the famous TKNN integer which is related to the bulk state. The other is the winding number of the edge state on the complex-energy surface which is generally a high-genus Riemann surface. We will describe them in detail.

Therefore we have two topological expressions for the Hall conductance. Actually these two expressions give the same integer, although they look quite different. This means that one can explain the quantum Hall effect by using either the edge states or the bulk states, that is, .

We present in this paper a statistical model of the growth of amorphous and crystalline structures which is applied here for the investigation of binary glasses. Having defined local configurations (referred to as singlets in what follows), and the ways in which they can assemble together, we establish the statistics of doublets and triplets produced by agglomeration. We obtain fairly good predictions for the glass transition temperature , the composition of mixtures of glass and crystalline phases at x = 0.5, and the and phases for greater values of x. The model predicts also the ratios of typical microstructures and other interesting data, among which are the cooling rates that are necessary to form the glass.

The NQR frequency and the spin - lattice and spin - spin relaxation times were measured as functions of temperature, in order to study the structural phase transitions in deuterated ammonium hexachloroplumbate(IV) . It was shown that the phase transition at 81 K is of a second-order type with an overdamped soft mode, while the deuteration-induced transition at 38 K is of first order. Three NQR frequencies (17.99, 17.86 and 16.35 MHz at about 15 K) were observed below 38 K suggesting an orthorhombic symmetry in the deuteration-induced low-temperature phase. The averaged value of these frequencies almost coincides with the frequency extrapolated from that (17.348 MHz at 39 K) above 38 K. The splitting (about 1.6 MHz) of the frequency below 38 K is quite large compared with that (0.45 MHz) of , which shows a similar phase transition induced by deuteration at 27.5 K. This indicates a large distortion of the crystal lattice in the present compound.

We have numerically solved the equation of motion for a single vortex in a resistively shunted Josephson junction array. The vortex velocity (v), the damping coefficient and the dynamical barrier for the cell-to-cell vortex motion are studied. In particular, we have focused our attention on their dependence on the bias current , the penetration depth of the magnetic field , the vortex position (x), and the extension. The results obtained can be described in terms of the motion of a particle subjected to a potential , the analytical form of which is discussed as a function of the array parameters. Under certain circumstances, the injection of one vortex into the array may unleash a recursive process of vortex/antivortex creation that extends to the whole array. This gives rise to the formation of a stable dynamical state: the AVM (alternate-vortex motion), where vortices and antivortices move along alternate rows of plaquettes.

Although the crystal structures of the Bi-based high- superconductor, which is distinguished by incommensurate modulations, were solved several years ago, their implications for superconductivity are as yet uncertain. This indicates that the deviation from the average structure also should be considered; this may play an important role in the superconductivity. For this purpose the contour maps of the scattering intensity in the plane for single crystals have been observed by both ordinary x-ray and synchrotron radiation. The characteristic feature was obtained in the experiments; that is, the intensity distribution around both the Bragg reflection and its satellites show a strong asymmetry. Both of them are elongated along the c axis, especially the satellites, connecting each other and almost forming a rod shape, which reveals the origin of the presence of the incompatible satellite reflections. This indicates that the structure has a strong two-dimensional feature, which is coincident with the two-dimensional characteristics of physical transportation. Our observation also confirms the previous results that the modulated wavevector is .

The temperature dependence of the magnetic susceptibility of FeSi is calculated over a wide temperature range on the basis of spin-fluctuation theory applied to a model of an exchange-enhanced semiconductor. We can reproduce the low-temperature activation-type behaviour of the susceptibility as well as the Curie - Weiss behaviour observed at high temperature. We also give an explanation for the behaviour of the observed wave-vector- and frequency-dependent neutron scattering intensities.

The structural and magnetic phase transitions of Fe - I boracite have been studied by means of optical examinations of the domain structure and by measurements of the spontaneous birefringence using polarized-light microscopy. These phase transitions have also been characterized by dielectric and spontaneous polarization studies, realized on ferroelastic/ferroelectric single-domain states, which were obtained by electric field poling along and directions, with simultaneous visual control of the domain state. In the trigonal phase, an electric field of one polarity produced a monodomain with the spontaneous polarization and the optical axis perpendicular to the plane, while that of the reversed polarity gave rise to domains with the three other preferential directions of , non-collinear with E, and to a polarization of , indicating the non--reversibility of along . The magnetic phase transition resulted in anomalies both in the dielectric permittivity (up to 2 MHz) and in the spontaneous polarization at the Curie temperature . This suggests an interaction between the magnetic moment and the electric polarization, consistent with the ferroelectric/ferroelastic/ferromagnetic coupling effect previously found for the m phase.

Starting from the basic constitutive equation that describes the magnetic viscosity of a ferromagnetic material, under the single assumption of a constant external field H, a connection is shown between the different expressions used to determine experimentally the fluctuation field . The simplest method uses the relation . If is invariant during the viscous decay of the magnetization, the relation may also be employed. The relaxation curves obtained at different fields, in this case, superimpose onto a single curve on renormalizing the time. An alternative treatment that considers explicitly the demagnetizing field is also presented. The theory is then applied to magneto-optic thin films, where two activation mechanisms are involved, assuming the absence of dispersion in the energy barriers, and also to the common case of relaxation by a single activation mechanism in the presence of a dispersion of the energy barriers. In both situations, it is shown that the fluctuation field may vary in strength during magnetic reversal. A method of classification of the hard ferromagnetic materials, through experimental means, is suggested.

Mössbauer and saturation magnetization studies have been carried out at room temperature on and permanent magnetic materials with x = 0.0, 0.5, 1.0, 1.5 and 2.0. A simple charge transfer from the rare-earth sublattice to the 3d band of iron accounts for the observed increase in the saturation magnetization and hyperfine field with the substitution of Y for Er. This can also be explained on the basis of the reduction in the strength of the Er - Fe negative exchange interaction and an increase in the Fe - Fe exchange interaction strength due to lattice expansion on Y substitution. The decrease in the saturation magnetization and the hyperfine field when Fe is partially replaced by Si is attributed to the reduction in the number of Fe neighbours and also to the reduction in the Fe magnetic moment due to an effective charge transfer from Si to the Fe 3d band. The Fe sublattice magnetization is found to be slightly affected by the type of rare-earth ion in the rare-earth sublattice.

A study on the mixed crystals of formula (1>x>0) has been carried out, using thermal analyses and x-ray diffraction on powder and single-crystal samples at different temperatures. It is shown that mixed crystals with have a transition from the paraelectric to the ferroelectric phase; decreases as the value of x increases. The orientation of the sulphate ion with respect to the crystallographic axes depends on the value of x. Mixed crystals with the paraelectric - ferroelectric transition turn around the b axis when the temperature decreases in the paraelectric phase, while in mixed crystals without a transition the orientation of the sulphate ion does not alter when the temperature varies. The transition is displacive, increasing the value of the parameter a without a soft mode. This is deduced from a study of the crystal structures, the translational and libration tensors and a density map of the sum of the bond valences.

The paper presents a comparison of x-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of and Si wafers. Whereas for the P-Si processed from Si the presence of some amorphous silicon is detected, the x-ray emission spectra of porous Si prepared from Si display a higher content of . For spark-processed Si the x-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10 000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the centre of the processed area is close to that of silicon dioxide, and depends slightly on the emission maximum. Specifically, green-photoluminescing sp-Si shows a higher degree of oxidation than the blue-luminescing specimen. However, the depth of oxidation consistently decreases for areas with weak photoluminescence and without photoluminescence. Possible origins for the photoluminescence are discussed.