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(R = Pr, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm) has been studied using complex chemical bond theory. The results indicated that with the decreasing of R radius, the ionicities for all considered types of bond decrease. This is in good agreement with the experimental fact that decreases with the decreasing of R radius. with no Ba-site Pr in this calculation is also predicted to be a superconductor. This supports the conclusion obtained by Blackstead et al. The ionicity for each bond obeys the following order: Ba - O > R - O > Cu(2) - O(1) > Cu(2) - O(2,3) > Cu(1) - O(4) Cu(1) - O(1).

We present a numerical method for the evaluation of the distribution of energy barriers between metastable states in many-particle systems with arbitrary interparticle interaction. The method is based on the search for the optimal path between the two given metastable states using the minimization of the corresponding action occurring in the Onsager - Machlup functional for the probability of transition between these two states.

This article is an overview of recent experimental and theoretical work on transport in phase-coherent hybrid nanostructures, with particular emphasis on dc electrical conduction. A summary of multiple-scattering theory and the quasi-classical methods is presented and comparisons between the two are made. Several paradigms of phase-coherent transport are discussed, including zero-bias anomalies, re-entrant and long-range proximity effects, Andreev interferometers and superconductivity-induced conductance suppression.

Deviations from Fourier's law emerge from numerical simulations of various lattices modelling solids. Non-integrability and moreover chaotic motion are considered to be conditions of normal heat conduction. However, these are not sufficient conditions. Using a simple model as an example, we show that non-diffusive transport could occur even in the presence of disorder in the lattice and completely chaotic dynamics. We conclude that diffusive and non-diffusive transport can coexist, while the system is moving along a chaotic trajectory in the phase space.

The polarization-dependent part of the neutron scattering in the triangular-lattice antiferromagnet is studied. This scattering appears in an external magnetic field and is determined by the projection of a chiral fluctuation on the sample magnetization (the dynamical chirality, DC). It is shown that the DC cross section is an odd function of the energy transfer, , and is proportional to the magnetic field up to 10 kOe above . A Be filter is used instead of the analyser for measuring the DC scattering integrated over . It is shown that below the field dependence of this intensity reveals two features related to the acoustic spin waves and to a new type of low-energy magnetic excitation, presumably of topological nature. The temperature dependence of the acoustic spin waves near is in qualitative agreement with dynamical scaling.

The enlargement of the time window achieved by combining Mössbauer investigations ( to ) with conventional dielectric spectroscopy to on poly(vinylferrocene-b-propylene sulphide) allows one to characterize three different relaxation processes. The main process of the glass transition is measured by dielectric spectroscopy and can be well described by the common Vogel-Fulcher-Tammann law, which is typical for cooperative processes. A secondary process is shown by dielectric spectroscopy to be Arrhenius activated, indicating a local process which leads to a broadening of the Mössbauer resonance line. The corresponding times can be determined via line-shape analysis. A third process, the so-called cage process, is responsible for an anomalous decrease of the Debye-Waller factor. Its characteristic times are assumed to be in the picosecond region.

The low-temperature oxidation kinetics of liquid Ga and the liquid alloy has been investigated by means of in situ x-ray specular reflectivity. A combination of angle-dispersive and energy-dispersive measurements was used to obtain a time resolution of 20 minutes with a sealed-tube reflectometer, allowing us to follow the growth of a natural oxide layer for 1 to 170 hours following preparation of a fresh surface. Oxide layer thicknesses between 10 and 30 Å were measured for Ga at and the liquid alloy at , and . The oxidation was found to follow a logarithmic growth law in all cases.

A careful examination of the peak profiles, peak shifts and peak separations of Bragg reflections, which are inherent in the disordering of quasicrystal, is carried out using an x-ray diffraction method for decagonal Al-Pd-Mn single quasicrystals. The full widths at half-maximum (FWHM) of the Bragg reflections along the longitudinal direction (L) have no or -dependence whereas those for both transverse directions, which are perpendicular to the L direction in an aperiodic plane and with a periodic axis, have linear -dependence. Notable peak shifts and separations of Bragg reflections are observed. The absolute values of the shifts are found to be proportional to . The peak shifts and separations are analysed in terms of linear phason strains.

We study the stability and migration mechanism of self-interstitials in Si through first-principles self-consistent pseudopotential calculations. The neutral Si interstitial is lowest in energy at a [110]-split site, with energy barriers of 0.15-0.18 eV for migrating into hexagonal and tetrahedral interstitial sites, while the migration barrier from a hexagonal site to a tetrahedral site is lower, 0.12 eV. These migration barriers are further reduced through successive changes in the charge state at different sites, which allow for the athermal diffusion of interstitials at very low temperatures. The [110]-split geometry is also the most stable structure for negatively charged states, while positively charged self-interstitials have the lowest energy at tetrahedral sites. Apart from the migration barrier, the formation energy of the [110]-split interstitial is estimated to be about 4.19 eV; thus, the resulting activation enthalpy of about 4.25 eV is in good agreement with high-temperature experimental data.

The metal-insulator crossover of the perovskite-type system near x = 0 has been explored through measurements of the x-ray diffraction, electrical resistivity, thermoelectric power, Hall effect, and magnetization. This system ranges from being a highly correlated metal phase for x > 0.05 to being an antiferromagnetic metal-like phase for and then to being a variable-range hopping-like insulator phase for the composition with a bandwidth comparable to the electron correlation. The phase for may be characterized in terms of local electron correlations as well as antiferromagnetic spin fluctuations.

Experimental and theoretical investigations of intermetallic semi-Heusler compounds (CoTiSn, FeTiSb, CoTiSb, NiTiSn, CoNbSn, CoVSb, NiTiSb) and their solid solutions are presented. The physical properties of these systems are found to be mostly determined by the number of valence electrons. Resistivity experiments show that compounds with 18 valence electrons are either semiconductors (CoTiSb, NiTiSn) or semi-metals (CoNbSn). The electronic structure calculations performed on 18-valence-electron systems by the KKR method show nine valence bands below the Fermi level and a gap of order 0.4-0.9 eV. A decrease or increase of the number of valence electrons in CoTiSb, NiTiSn or CoNbSn leads in either case to a metallic state and either ferromagnetic (CoTiSn, CoVSb) or paramagnetic (FeTiSb, NiTiSb) properties. The KKR results concerning 17- and 19-valence-electron systems correspond well with experimental characteristics, except in the case of CoVSb which KKR calculations predict to be a half-metallic ferromagnet, which conflicts with experimental data.

Magnetization and resistivity measurements indicate that semiconductor-metal crossovers occur together with the appearance of ferromagnetism in the and series, for x near 0.4. This behaviour is discussed in the context of the KKR-CPA results.

The existence of non-linear magnetoplasma waves in compensated metals and semi-metals in the presence of a strong magnetic field is predicted. Non-linearity in the case considered is caused by the influence of the magnetic field of the wave on the dynamics of the electrons and holes. The conductivity tensor is calculated neglecting the spatial dispersion and is shown to be in the non-linear regime a differential - with respect to time - operator which is a manifestation of the temporal dispersion effects. The shape of the wave solution obtained is determined by two parameters: the amplitude and the phase velocity V. When the amplitude is small and , where is the Alfvén velocity, the solution transforms into the well-known linear magnetoplasma wave. It is shown that, contrary to the linear case, the non-linear magnetoplasma wave exists when the phase velocity is both less and larger than . It is established that with increase of the velocity and the amplitude being fixed the quasiharmonic wave turns into a series of pulses, the interval between which is growing infinitely. In the aperiodic limit the wave becomes a one-parameter soliton. Its velocity is larger than and depends linearly on . With increase of , when V is fixed, the period of the magnetoplasma wave descends and the wave shape becomes a series of sharp spikes. Thus, when we have transition from a linear wave to an anharmonic one, while when we have a transition from a soliton to a sequence of pulses. Both the soliton and the non-linear periodic wave with have no analogues in the linear case. These electromagnetic waves are essentially non-linear even at small - in comparison with the external magnetic field - amplitudes.

The tight-binding model is considered for a square lattice with filling factor 1/2. The array has the shape of a rectangle with boundary conditions in both directions twisted by and . The components of the twist are associated with two components of the magnetic flux in torus geometry. An analytical expression is obtained for the energy and for the components of the persistent current (PC) at a given flux and temperature. It is shown that at zero temperature the PC density is proportional to the vector potential with a coefficient which does not depend on the size of the system. This happens because the Fermi surface for a square lattice at filling factor 1/2 is flat. Both the energy and the PC are periodic functions of the two flux components with the periods and where , and q and s are integers which depend on the aspect ratio of the rectangle. The magnitude of the PC is the same as for superconductors. Therefore, a 3D system constructed from a macroscopic number of isolated coaxial cylinders at zero temperature is reminiscent of London's superconductor. It exhibits the quantization of trapped flux as well as the Meissner effect. However, all of the phenomena are of a mesoscopic nature. The critical field decays with the effective size of the system, . The magnitude of the PC decays with T as , where t is the hopping amplitude and a is the lattice constant.

We have studied the electronic transport properties of various topological mesoscopic rings which consist of several variable-size chains joined at their ends to form ideal single-channel leads. An enhancement of the transmission coefficient is obtained when the chain lengths are randomly distributed, due to the suppression of interchain interference, and, in the presence of magnetic flux, periodic magnetoconductance oscillations are also shown. A single impurity changes the magneto-oscillation pattern drastically, while for more impurities placed at random the periodic magnetoconductance oscillations survive. Finally, the introduction of a transverse link between the chains destroys the bridge-arc shape of the transmission versus energy curves obtained in the absence of such a link.

A lattice boson model is used to study ordering phenomena in regular 2D arrays of superconductive mesoscopic granules, Josephson junctions and pores filled with superfluid helium. The phase diagram of the system, for when quantum fluctuations of both the phase and the local superfluid density are essential, is analysed both analytically and by the quantum Monte Carlo technique. For the system of strongly interacting bosons it is found that as the boson density is increased the boundary of the ordered superconducting state shifts to lower temperatures and at approaches its limiting position corresponding to negligible relative fluctuations of the moduli of the order parameter (as in an array of `macroscopic' granules). In the region of weak quantum fluctuations of phases, mesoscopic phenomena manifest themselves up to . The mean-field theory and functional integral -expansion results are shown to agree with those of quantum Monte Carlo calculations of the boson Hubbard model and its quasiclassical limit, the quantum XY-model.

We use spin-coherent states as the basis of a time-dependent variational ansatz for a semiclassical description of a large family of Heisenberg models. In addition to following common approaches we also evaluate the square variance of the Hamiltonian in terms of coherent states. This quantity turns out to have a natural interpretation with respect to time-dependent solutions of the equations of motion and allows an estimate of quantum fluctuations in a semiclassical regime to be made. The general results are applied to solitons, instantons and vortices in several one- and two-dimensional models.

We use the variational matrix-product ansatz to study elementary excitations in the ladder with additional diagonal coupling, equivalent to a single chain with alternating exchange and next-nearest-neighbour interaction. In the absence of alternation, the elementary excitation consists of two free particles (`spinons') which are solitons in the dimer order. When the nearest-neighbour exchange alternates, the `spinons' are confined into one S = 1 excitation which is a soliton in the generalized string order. The variational results are found to be in qualitative agreement with the exact-diagonalization data for 24 spins. We argue that such an approach gives a reasonably good description over a wide range of the model parameters.

The differential AC magnetoresistance and differential AC susceptibility of three amorphous FeNi re-entrant spin glasses have been examined as a function of temperature at the so-called paramagnetic-ferromagnetic transition under static magnetic fields up to 70 Oe. It is shown that the static field tends to nullify the effect of the ferromagnetic component and the results indicate the presence of spin-glass freezing in both the magnetoresistance and susceptibility which takes place simultaneously with the establishment of the ferromagnetic phase at the Curie temperature. The composite nature of the transition appears to be a general feature of re-entrant spin glasses.

The specific heat of a layered single crystal, , was measured in the temperature range 0.45 K-6.5 K in zero magnetic field. A -type anomaly found at is associated with a phase transition into the ordered state. The experimental values of the magnetic entropy indicate deviations from two-dimensional behaviour of the magnetic system. Honmura's model of a 2D assembly of coupled S = 1/2 Ising ferromagnetic chains was used for the specific heat data analysis. The estimated value of the intrachain interaction is and that of the interchain interaction is ; these were obtained from the fitting procedure for the ordered phase. The fit for the paramagnetic phase yields and . The comparison of the results of the analysis with theoretical calculations made on the assumption of a pure dipolar character of the magnetic correlations is discussed.

has been studied by means of Mössbauer spectroscopy and x-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant . The iron ions are in ferric states. The temperature dependences of the magnetic hyperfine fields at the nuclei at the tetrahedral (A) and octahedral (B) sites are analysed using the Néel theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of while the intra-sublattice superexchange interactions are ferromagnetic with strengths of and . The Debye temperatures of the tetrahedral and octahedral sites are determined to be and , respectively.

The luminescence spectrum of in has been investigated, especially in the R-line range from 7200 up to 7500 Å. For this, we have studied several chromium-doped crystals with various Li/Nb ratios and different doping concentrations. In addition to the lines which were previously reported, several new peaks were detected in our investigations. In fact, the structure, the shape and the relative intensity of each line are shown to be strongly dependent on the content of doping and on the intrinsic defects related to the non-stoichiometry of the crystal. Among all peaks, we focus our attention on four main lines, which can be separated into two groups, according to their behaviours. The contribution of the first group to the whole spectrum diminishes with increasing Li/Nb ratio so that it vanishes for the stoichiometric composition. The second group of lines, occurring at higher wavelengths, is reported for the first time and is shown to be closely related to the Li/Nb ratio, especially when approaching the stoichiometric composition. These lines exhibit a similar behaviour with increasing Li/Nb ratio or Cr concentration. An explanation of the possible origins of these four peaks is given.

The effects of post-implantation annealing of damage in 6H-SiC caused by ion implantation at two different fluences have been studied by monoenergetic positron Doppler broadening and lifetime techniques. The measurements are supported by new calculations of positron lifetimes in vacancy clusters in SiC. At both fluences two defected layers are identified and characterized by depth and defect type as a function of annealing temperature. The results indicate that it is impossible to remove the radiation damage by annealing at temperatures up to .

Polarized Raman spectra of crystals (x = 0.1 and 0.7) were studied in the temperature range 10-300 K. Thorough site-symmetry analysis of internal and vibrations combined with factor-group analysis based on hexagonal pseudosymmetry allowed us to make an assignment of the observed modes. The spectra of the x = 0.1 sample are compatible with those of pure including the features of the disorder in the paraelectric phase above 220 K (the broad central peak of the relaxator type and high mode damping) and ordering in the ferroelectric phase. The spectra of the x = 0.7 sample show a much smaller disorder at high temperatures (no broad central mode and lower mode damping) due to the lower ammonium content, but clearly confirm the onset of the dynamic dipolar glass transition (short-range correlations inducing the appearance of long-lived dipolar clusters) near 220 K.