Table of contents

The θ-term in the action is calculated for a superfluid ³He-A film using the gradient expansion. This term defines the spin and statistics of the particle-like topological objects in the film. The skyrmion, which has spin s = 1/2, is a fermion, while the 2π-disclination and the half-quantum vortex seem to obey parastatistics.

The cross section for creation of scalar, pseudoscalar, vector and axial vector massive particles in high energy collisions of electrons on atoms is obtained.

Spectra of multicharged ions have been analyzed for all stable elements in the period from Z = 37 (Rb) to Z = 50 (Sn), in the region of n = 4, Δn = 0 transitions. The transition arrays (4s² + 4p² + 4s4d)-4s4p, 4s4f and 3d⁹4s, 3d⁹4d-3d⁹4p have been identified in zinc-like and nickel-like ions respectively, with the help of the ab initio relativistic parametric potential method and of the Slater-Condon method with generalized least-squares fits of energy parameters.

The spectrum of five times ionized titanium emitted from a sliding spark and a vacuum spark discharge has been observed using a 5-m grazing incidence spectrograph and a 3-m normal incidence spectrograph. About 450 lines in the region 125 Å to 2315 Å have been identified as combinations between 64 odd levels belonging to the 3s² 3p⁵, 3s3p⁵ 3d, 3s² 3p³ 3d² and 3s² 3p⁴ 4p and 4f configurations and 72 even levels of the 3s3p⁶, 3s² 3p⁴ 3d, 4d, 5d, 4s and 5s configurations. The level structure of the observed configurations has been theoretically interpreted by means of calculations including configuration interactions. The energy parameters determined from least-squares fits to the observed levels are compared with theoretical values obtained from the Cowan computer code.

The spectrum of doubly ionized xenon has been investigated. The study is based on photographic recordings of xenon spectra in the 490-8900 Å range. The number of classified lines has been increased from about 300 to about 1400. The lines have been classified as transitions between 73 even levels belonging to the 5s²5p⁴, 5s²5p³6p, 4f, 5f and 5s⁰5p⁶ configurations, and 83 odd levels belonging to the 5s5p⁵, 5s²5p³6s, 7s, 5d and 6d configurations. In particular, the classifications include most of the Xe III laser lines. The experimentally observed level structures are compared with the results of Hartree-Fock calculations and least-squares fits. A comparison is also made between the results of the present analysis and the published data on the Xe N_4,5 OO Auger spectrum.

Using collinear fast beam cw dye laser modulation spectroscopy accurate lifetimes in ThII have been measured. Together with known branching ratios these lifetimes have allowed the determination of absolute oscillator strengths for ThII. This element is of astrophysical interest as a chronometer for stellar and galaxy evolution, with oscillator strengths playing an important role in synthesis of stellar spectral data.

The non-linear motion of an ideally conducting current-carrying rigid cylinder in a magnetic multipole field is analyzed. It is found that non-linear stability is achieved provided the current in the external conductor rods is sufficiently large. Since the results agree qualitatively with experimental observations of the Extrap z-pinch, it is suggested that the so far unexplained stability of this configuration is due to non-linear effects.

This paper presents a theoretical analysis of high-frequency, diffusion-controlled discharges in which the ionization is provided by direct electron impact with gas atoms within the discharge volume. The theoretical formulation uses the electron Boltzmann equation along with two-moment, hydrodynamic-type equations for the electrons and the ions. The treatment followed covers a wide range of applied field frequencies, ω/2π, from ω ≪ ν_c to ω ≫ ν_c, where ν_c is the electron-neutral collision frequency for momentum transfer; it is, however, limited to situations for which ω ≫ τ⁻¹, where τ is the characteristic electron energy relaxation time by collisions. It is shown that, under steady-state conditions, the maintenance field and the average absorbed power per electron are practically independent of the field configuration and essentially determined by the gas pressure, the configuration and the dimensions of the discharge vessel, and ω, and not by the applied HF power level. Both the maintenance field and the average absorbed power per electron are shown to obey similarity laws as a function of the discharge operating parameters.

Nonlinear dynamics of short wavelength drift Alfvén waves (Ω_e > ω > Ω_i) is studied, and a systematic procedure for the construction of the general travelling solution of the full nonlinear system is presented. The solution has the form of a superpositron of single and multiple vortex forms, and in the special case reduces to a double vortex.

The results of a systematic experimental study of the spontaneous supercooled water-ice transition are here reported. We have measured the mean lifetime and the rate of ice growth of water sample supercooled in capillary tubes down to −34.7°C. From the analysis of the experimental results we have obtained the parameters characterizing the nucleation rate and the solidification kinetics, and some evidences which support a new picture for supercooled water-ice transition which appears to consist of successive processes: the first one (nucleation) is driven by stochastic mechanisms, after which a complex deterministic process produces the growth of the macroscopic solid phase (solidification).

The difference between the K-edges XANES (X-ray Absorption Near Edge Structure) spectra of crystal and amorphous silicon is discussed. We show that the multiple scattering signal gives a large contribution to the total absorption in crystalline silicon in an energy range of about 70eV. The double scattering term probing the triplet distribution function gives the major contribution to the XANES. We show that the difference between the crystalline and amorphous absorption spectra is due mainly to the crystalline multiple scattering signal which is quenched in the amorphous spectrum. The suppression of the signal due to the triplet distribution function in the amorphous silicon is assigned to the large orientational disorder.

The results of experiments aimed to study the behaviour of the metals Ni, Cu and Zr irradiated by 230 MeV Ne ions are described. Great changes in the yield strength Δσ_0.2, as function of irradiation dose are obtained in the fluence range 10¹⁴−5 × 10¹⁵ion/cm². These results are discussed in the framework of a model assuming the interaction between old clusters and newly produced cascades. The data are described by the dependence of the type Δσ = A[1 - exp (– BΦt]^1/2.

Sum rules for the forces on the atoms of semi-infinite crystals (in the Born-Oppenheimer approximation) are derived. They connect the macroscopic stress tensor of the bulk with the forces on the unrelaxed or partially relaxed surface atoms. For a stress free bulk the sum of these forces per unit area has to vanish. Because these forces drive relaxations, the sum rule gives a general understanding of oscillatory multi-layer relaxations and Friedel oscillations.

There exists a broad experimental evidence of transient pattern formation in the Fréedericksz transition for nematics. This is a problem of interest in the general context of pattern selection mechanisms. We present a dynamical theoretical analysis of pattern structuration appropriate to the twist geometry. Our scheme is based on stochastic nematodynamics equations which incorporate fluctuations. By monitoring the time evolution of the wavenumber corresponding to the maximum of the structure factor we are able to predict the time of appearance and the stage of formation of the pattern. This dynamics is studied both in a linear and nonlinear regimes. Finally the case of a periodically modulated Fréedericksz transition is also presented.

The amorphous to crystalline phase transition of Ge in contact with Pb has been extensively investigated in Pb/Ge multilayers. We find that the Ge crystallizes at reduced temperatures in the range of 100-200°C depending on the thickness of the Ge and the Pb. Simultaneously the texture of the Pb improves and the layered structure completely disappears. It is suggested that the basis for the reduction of the crystallization temperature is the enhanced diffusion kinetics at the interface, caused by the adjacent Pb layer.

In the randomly quasi-two-dimensional magnetic system Rb₂Mn_xCr_1-xCl₄ with competing exchange interactions, the spectrum of magnetic excitations was studied by means of neutron inelastic scattering, namely for three samples with different concentrations x: one which shows at low temperatures antiferromagnetic order (x = 0.70), one with ferromagnetic order (x = 0.25) and one with spin glass behaviour (x = 0.59). This investigation has revealed for all three samples dispersionless Ising cluster modes, coexisting in the same frequency range with q-dependent spin wave branches. In the ferromagnetic sample, the spin wave branch extends above and below the frequency range of the Ising cluster modes. In the spin glass sample, ferromagnetic spin waves as well as antiferromagnetic ones have been found, in addition to the Ising cluster modes, at least for q-values where the wavelength is smaller than the two-dimensional correlation-length. Such an excitation spectrum as observed in our sample of a quasi-two-dimensional XY spin glass has not yet been reported for any other spin glass.

For a single plane of dipoles of arbitrary configuration the optical reflection has been calculated, using the screened discrete dipole model. Results differ from continuum approaches. Further the results show that the experimentally observed reflection anisotropy has besides the dominating intrinsic contribution, also a structural one.

A method of treating the screening by valence electrons in a semiconductor which takes account of the lower and more uniform charge density in the regions between atoms is applied to silicon and cadmium sulphide. Improved values of the band gap are obtained and these results along with the densities of states and energy bands in the region of the gap are discussed.

We have measured the Raman spectra of dilute magnetic semiconductors Zn_1-xMn_xS (0.03 ⩽ x ⩽ 0.5). The observed x-dependence of the optical phonon frequencies seems to follow a "one-mode" behavior described by a random element isodisplacement model. Impurity-induced vibrations were also observed for concentrations up to x = 0.5. Luminescence measurements were performed under different temperatures (T = 20-1000 K) and hydrostatic pressures (P = 0-14.3 GPa). The spectra consist of five emission bands whose peak frequencies are found independent of x. A red shift with pressure is observed for the characteristic yellow emission band. A phase transition occurs at pressures which depend critically on Mn concentration.

In this paper it is proven that, in general, the shape-demagnetizing-field transformation breaks, or modifies, the stability of the Preisach diagram PD. The proof is based on the Mayergoyz's theorem. A new theorem which gives the necessary and sufficient conditions for the invariance of such a stability is also proven. Moreover, a rigorous mathematical base for feedback generalizations of the Preisach model PM is given. These results are relevant not only for a proper interpretation of magnetic hysteresis measurements, but also, more generally, for the mathematical modelling of any kind of hysteretic system regardless of the physical origin of hysteresis nonlinearities.

We report the luminescence properties, under high resolution conditions, and the fine structure of a zero phonon line (ZPL) at 735 meV in silicon and the associated phonon side band. We relate this band to a complex defect containing iron.

Analysis of electron energy and spin allows a determination of the characteristic parameters for thermalization and spin relaxation for electrons in the conduction band.

The use of a Al_0.3 Ga_0.7 As sample covered by a thin GaAs overlayer as a convenient source of polarized electrons under He-Ne laser excitation is suggested.

Optical properties of multilayered materials, including synthesized superlattices received special attention in the last few years. To some extent, reflectance and absorption of these materials are adjustable: the polariton structure depends sensitively on the layer geometry.

Though the approach considered here is applicable to any stratified structure, we emphasize on polaritons of semi-infinite superlattices, that is superlattices which exhibit a free surface. In these thin-film structures, the accumulation of interfaces gives rise to peculiar electromagnetic eigenmodes, distributed as continuous frequency bands. The termination of the superlattice at the surface modifies the density of these modes, as compared with the mode density of an infinite, truly periodic superlattice. In particular, one notices the appearance of isolated branches analogous to surface polariton modes of semi-infinite homogeneous materials.

We approach the description of radiative and non-radiative modes, including finite life-time effects, from the point of view of scattering theory, using a Green's function technique. The local density of states of the polariton modes is calculated and provides a complete information on allowed electromagnetic excitations, as a function of wavelength, at any depth in the superlattice. These results will be used to discuss reflectance and attenuated total reflection experiments on superlattices.

The size and magnetic field quantisation of electron motion in narrow 2D systems is investigated. The confining potential is approximated by an irregular set of parabolic quantum wells of various width connected in series. The conductivity is given the form which reflects the magnetic crossover in quantisation under high magnetic fields. It is shown that each 1D subband with the index n, which originates from size quantisation, is continuously transformed into a Landau level with the same index under the influence of magnetic field, i.e., the number of nodes of corresponding eigenfunctions is conserved. Thus, due to the magnetic field, all zero field conductance peaks pertaining to one subband index are grouped into one Landau level. The application of a magnetic field may serve as a useful tool for the determination of the number of 1D subbands in the system, which can otherwise be masked by universal conductance fluctuations.

Optical properties of Li_xInSe are presented as a function of the lithium concentration and temperature. Investigations of the photoluminescence and absorption measurements show that the presence of alternating sheets of lithium induces changes in the electronic band structure. The Raman spectrum does not change appreciatively unless it is modified by the presence of excitons near certain resonance conditions.

Starting from a Landau expansion of the free energy of a system with two order parameters (OP) in an external field we first identify four control parameters which describe the state of the system and allow a comparison between different physical systems. We consider all combinations of the parities of the coupling term with respect to the OP's. A piecewise linear approximation of the system is then worked out to yield the field and temperature dependences of the OP's. We found that depending on the parity of the coupling and on the values of the control parameters the OP's may exhibit several kinds of features both versus temperature and versus field. Many of these features fit the description of "strange" effects reported in the literature.

Niobium nitride (NbN) is one of the leading materials for superconducting thin film applications such as electronic devices and coated r.f. cavities for high energy physics. In the following the fabrication technique of superconducting magnetron-sputtered NbN films is described and the experimental results are discussed.

We have investigated the Scanning Tunneling Microscopy (STM) of graphite with varying tip-to-surface distance. Using an LCAO type approach we showed that at small separations states are localized between the tip and the surface. The energies and the characters of these Tip Induced Localized States (TILS) depend on the height and the lateral position of the tip. These states play a significant role in the tunneling process and influence the STM corrugations predicted from the local density of states. We have developed a current expression, which includes these local interactions, but differes significantly from earlier theories.

In many polycrystalline semiconductors, the current transport is controlled by potential barriers formed at the grain boundaries. We investigate the response of ZnO grain boundaries to high-voltage pulses with risetime of less than 1 μs. The experimental results, in particular the voltage overshoot effect, cannot be explained by the dynamics of electrons (majority carriers) alone. A model which includes holes (generated by hot electrons in the depletion regions) yields an appreciable overshoot when the steady state is close to a current-voltage bistability. In this case, the approach to equilibrium is altered by the delayed response of the barriers to the presence of holes. Further implications of the model are also discussed.

Several extensions of our "minimal" model of valency-mixing Jahn-Teller systems are given and discussed. It is found that a model which increases the "softness" of the defect surrounding (additonal collective T_2g state in the Sm subspace of the system SmB₆: Er³⁺) yields the strongest improvement of the calculated ESR results. Good results are also found for a "double" Jahn-Teller model.

The symmetry classification of the possible superconducting states is made for the high-T_c superconductor YBa₂Cu₃O₇ in assumption that the superconductivity is generated in the nearly tetragonal background of the layers of fivefold-coordinated Cu atoms and that the orthorhombic distortion may be considered as a perturbation. The interaction of the order parameter with the orthorhombic splitting b-a, which gives rise to the orthorhombic distortion anomaly at T_c as well as to the possibility of the additional second-order transition slightly below T_c is discussed.

Dynamics of biological systems is investigated in the framework of spontaneous symmetry breaking. Non equilibrium features are considered taking advantage of the existence of many unitarily inequivalent vacua. Anderson-Higgs-Kibble mechanism is shown to be relevant and electromagnetic fields are shown to propagate as self-trapped filaments. Magnetic flux quantization is derived. Some phenomenological consequences are investigated and discussed.

The magnetic fractions of basaltic rock samples from Nigeria have been studied by Mössbauer spectroscopy, X-ray powder diffraction, and magnetic susceptibility measurements. The relative areas of the magnetically split components in the Mössbauer spectra decrease with increasing temperature. It is shown that this is not because of superparamagnetic relaxation but rather due to inhomogeneous distributions of the iron atoms in the titanomagnetites which lead to a distribution in Curie temperatures. Heating of some samples in air results in an increase in the Curie temperature and an increase in the magnetic susceptibility at room temperature. For other samples the susceptibility had decreased after heating, and the Curie temperature was not affected. Mössbauer and X-ray studies of the heated samples showed a transformation of titanomagnetite into magnetite and titanium-oxides and a partial oxidation of magnetite into hematite. These observations can explain the changes in the magnetic susceptibility. The results suggest that Mössbauer studies of iron-titanium oxides can give information on the cooling history of rock samples.