The approaches to get lifetimes of the hexagonal metals have been performed making use of the face centred cubic (f.c.c.) structure, instead of the real hexagonal one. Although some structural reasons allow this approximation, it is expected to fail when free volume is created in the solid. In this work, we have used the real hexagonal geometry to study these metals. For the cases of bulk, mono-, di- and trivacancies no appreciable differences have been observed in the calculated lifetimes. However, when studying clusters of vacancies in the real hexagonal structure and in the f.c.c. one, differences between 3.2% and 7.4% appear in lifetimes between these structures. This fact shows that the approximation of using the f.c.c. structure is not good for studies of extended defects. Positron lifetimes in these metals have also been obtained by means of the local density and general gradient approximations, using for enhancement factors Arponen-Pajanne and Boronski-Nieminen approximations. In alkaline earth and early transition metals the calculations performed with the general gradient approximation do not improve the results obtained using Borónski-Nieminen's formula for the enhancement factor.

The superplasticity phenomenon is considered as a particular example of the structure-mechanical behavior of polycrystals during plastic deformation. Based upon the common principles of the plastic deformation of polycrystal materials the fundamental principle of achieving high plasticity is suggested. This principle is assumed to reflect the basic nature of the superplasticity phenomenon. On the basis of the concept involved the experimental procedure to find out the full set of the optimum conditions for superplastic deformation is developed. The applicability of the method suggested is demonstrated on the example of the intermetallic compound TiAl.

At room temperature (T = 293(2) K) (NH₄)₃H(SO₄)_1.42(SeO₄)_0.58 (NHSSe) M_m = 274.34 g possesses a monoclinic structure with space group Cc. The unit cell parameters are: a = 15.564(6) Å, b = 5.9180(6) Å, c = 10.275(2) Å, β = 102.09(2)°, V = 925.4(4) Åü3, Z = 4, D_x = 1.97 g/cm³. The structure was determined with R = 0.0529 and wR2 = 0.1373 for 826 observed reflections. The Raman and infrared spectra at room temperature were investigated in the frequency ranges 10 to 1300 cm ^{- 1}and 400 to 1600 cm ^{- 1}, respectively. The assignment of most of the bands is given. We note for this compound two kinds of disorder to be expected: a statistical or dynamic disorder of the acidic proton O-H-O hydrogen bond and another one which is connected with a reorientational motion of NH⁺₄ ions.

The calibration constants that link the peak-shift measured with X-rays to the macroscopic stress acting on the material are termed X-ray elastic constants. These terms contain both material parameters such as the elastic stiffness or compliance terms and configurational parameters caused by the Miller indices of the diffracting plane of the corresponding crystal structure. The configurational parameters can cause significant variations on the X-ray elastic constants. In this paper a new theoretical model is proposed for the calculation of the average X-ray elastic constants from single crystal data and its extensions will be compared and discussed with respect to the known Voigt and Reuss model. It uses a modification of the Voigt assumption and takes into account the fact that not all grains in a polycrystalline aggregate contribute to the data obtained by diffraction but only those which are selected by Bragg's law for a given reflection. It is shown that this restriction yields compliances that depend on the crystallographic orientation of the diffracting crystallites. The new model is demonstrated for polycrystalline cubic, hexagonal and tetragonal materials but also can be extended to any crystal structure. It is called constraint Voigt (CV) model.

Different from one-dimensional systems, nesting in two-dimensional (2D) systems is not perfect but some 2D systems still have Peierls instability and hidden nesting. This paper shows that the next-nearest neighbor (NNN) hopping which controls the nesting deviation, heavily suppresses the Peierls instability. There is a critical value for the NNN hopping, beyond which the Peierls instability is destroyed and the hidden nesting is lost. The impact of such change to other phase transitions is discussed.

The structure and properties of absorbed fluorine and chlorine atoms on GaAs surfaces are investigated. The energies of surface species' localized vibrations are determined. Our results show that localized As-Cl and Ga-Cl valence vibrations occur in the 40 and 50 meV regimes, respectively, while As-F and Ga-F vibrations are found in the 90 and 80 meV regimes, respectively. Comparisons are made with experimental high resolution electron energy loss spectroscopy spectra. Our results accord well with a variety of experimental data.

Magnetotransport investigations of quantum interference effects at temperatures down to 0.35 K in a series of highly n-doped MBE-grown ZnSe epitaxy layers with electron densities from 8.2x10¹⁷ to 7.5x10¹⁸ cm ^{- 3} are presented. We observed a negative magnetoresistance in all samples studied. The change of magnetoconductivity Δσ_xx(B) shows a linear dependence on the square root of the magnetic field. The slope of the √B dependence approaches the universal value predicted by weak localization (WL) theory when the temperature is reduced to 0.35 K and the electron density is well on the metallic side of the metal-insulator transition (MIT). The temperature exponent of the estimated phase coherence time τ_ϕ is around unity.

The excitation spectrum, steady-state and transient characteristics of photoconductivity in amorphous thermally deposited chalcogenide AsSe films were studied. Besides the excitation of nonequilibrium holes by optical transitions from the valence band tail to the conduction band an additional channel of hole generation from a donor-like defect state with energy E_d - E_v = 1.5 eV was revealed. The defect is associated with the presence of homopolar As-As bonds. The steady-state and transient photoconductivity characteristics are adequately interpreted in the frame of a model, in which transport and recombination of nonequilibrium holes are controlled by exponentially distributed hole traps with the distribution parameter kT* = 0.039 eV.

We have calculated the effects of the electron-confined LO phonon interactions on the ground bound state of a shallow donor impurity located on the axis of the quantum well wire (QWW) as a function of the size of the QWW. The results indicate that the effect of electron-confined LO phonon coupling on the binding energy increases as the confinement length increases and this effect is much more pronounced than that in quasi-two-dimensional quantum well structures.

Analytical expressions for the transmission coefficient and the resonance condition are derived for the first time for the asymmetrical rectangular double-barrier structures with deep well when a dc bias field is applied. It is found that the unity resonance of the resonant tunneling with unity transmission occurs when both the phase difference condition for resonance (PDCR) and the maximum condition for the peak value (MCPV) hold simultaneously, while the under-unity resonance occurs that the peak value of transmission coefficient at resonance is below unity if only PDCR holds. Wave functions of an electron at the resonance level are calculated and the confining phenomena are confirmed.

The permutation-inversion symmetry group of fullerite C₇₀ in the intermediate phase (at temperatures 270 K < T < 340 K) and the permutation-inversion site symmetry group of a rotating molecule C₇₀ in a nonrigid crystal are generated. Their irreducible representations compatible with the principle of symmetry of wave functions with respect to the permutations of identical nuclei are found. Group-theoretical classification of quantum-mechanical states of a rotating molecule and a crystal in the intermediate phase of fullerite C₇₀ is made. The selection rules for electronic and IR spectra in terms of the irreducible representations of the permutation-inversion group are found.

The problem of calculating the parameters of the Kane model from experimental effective masses is reinvestigated. Conditions for the applicability of the Kane and Luttinger-Kohn models are derived in terms of experimental effective masses. It is found that different definitions of the Luttinger parameters are used within the Kane model. Some of the parameters of the Kane model are very sensitive to small changes of effective masses. The reason for this unexpected instability is analyzed, and its effects on bulk band structures and energy levels from effective mass theory are discussed.

The angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions in Si_xSn_{1 - x} and Ge_xSn_{1 - x} is calculated. We observe that the electron-positron momentum density increases rapidly with increasing Si and Ge content. The computational technique used here is based on the independent-particle model (IPM) coupled with the use of the electron pseudo-wave and the virtual crystal approximation (VCA) which incorporates compositional disorder as an effective potential. We also present the variation of the positron lifetime in these alloys.

Using first principle Pseudo Impurity Theory (PIT) and variational method, the binding energies of group V shallow donors in silicon are estimated incorporating the effect of core squeezing of the substitutional impurity atoms, with an empirical squeezing scheme of the core orbitals. Scaling factors are defined for various core states of the impurity atom to take explicit account of the varying degree of rigidity on squeezing. From the extent of squeezing required to reproduce the experimental binding energies of the donors, the local relaxation due to the size difference between the host and donor atoms is deduced and compared with experimental results and theoretical predictions available in literature. It is demonstrated that the squeezing scheme of the present work combined with the first principle pseudo impurity theory could deduce information about a purely local phenomenon such as lattice distortion reasonably well from the binding energy of the donor impurity.

The possibility of superconducting and insulating instability is examined in the one-dimensional electron system with Coulomb interparticle interaction that is evaluated in the plasmon-pole approximation. The insulating instability appears to dominate strongly over possible superconductive effects. The dependence of its mean-field transition temperature on the band width is presented in graphic form for various degrees of band filling and the free-particle energy spectrum as well as the tight-binding energy spectrum is considered.

The self-consistent theory of Singwi, Tosi, Land and Sjölander (STLS), which accounts for exchange and short-range correlation effects through an effective potential depending on the structure factor, is generalized to the Visscher and Falicov model of the layered electron gas. The complete numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The local-field correction is obtained and compared with expressions from previous works, where the strong interplane coupling between the electrons is neglected. The pair correlation function, the correlation energy and the plasmon dispersion are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the layers. For comparison, the Hubbard approximation and the random-phase approximation are also considered.

New analytical formulas for the electronic stopping power and the energy loss straggling of low velocity extended charge projectiles in the degenerate electron gas are calculated within the dielectric function method. The stopping and straggling effective charges Z_eff of a projectile were analysed. They are found to differ one from the other and to depend on the electron gas density r_s, on the projectile atomic number Z_i and on the projectile degree of ionization ζ. For small Z_i, due to antiscreening Z_eff > 1.

Introducing a two-value hopping, we extended the Ginder-Epstein model to oxidized forms of polyaniline (EB, PNB and chains which present a disordered distribution of quinoid and benzenoid rings). The model reproduced the four and two bands for EB and PNB, respectively. The observed absorptions in the related polymers were explained on the basis of the calculated electronic structures. Disorder in the distribution of the quinoid moities leads to larger band spreading on the energy axis and a reduction of the gaps observed between the different occupied and unoccupied bands. The results showed a disorder-conduction mechanism for polyaniline.

The results of calculation of current in a lateral superlattice by electron scattering on the optical phonons are presented. It is shown that under the action of a high applied electric field (E_x) directed along one of the crystal symmetry axes the spontaneous appearance of a transverse electric field (E_y) is possible. The field E_y as a function of temperature proves to show the features of spontaneous polarization in ferroelectrics. The Curie temperature is defined by the value of E_x.

The magnetoresistivity of a disordered metallic system is derived using the "2K_F-scattering" theory in terms of a generalized Boltzmann equation with magnetic field B included. The behaviour is complex but in the limit of weak fields (which are experimentally quite strong) we find a negative contribution to the relaxation time proportional to B² which gives rise to a positive magnetoconductivity proportional to B². The conductivity in zero field increases with temperature. We discuss these results in the context of measurements for amorphous Ca_zAl_{1 - z} and existing theories of magnetoconductivity. The theory explains the nature of the observed results. It can also be generalised to strong localisation and applied in strong scattering situations.

Within first order perturbation theory the time of electron-ion interaction is assumed to be equal to the time of an electron passing through the crystal region related to one impurity ion. In terms of the relaxation time approximation for nondegenerate semiconductors the mobility μ_i, limited by elastic ion scattering, is shown to be proportional to T/N^2/3; the Hall factor is equal to 1.4. Calculated for various temperatures T the majority carrier mobility dependences on the doping impurity concentration N are in good agreement with known experimental data. It is shown that the Brooks-Herring formula for μ_BH ∝ T^3/2/N overestimates the value of mobility. Comparison of mobility calculation in degenerate crystals with experiments also yields μ_i < μ_BH.

The diffusion of electrons in a disordered metal, by means of the perturbation approach, is considered in the presence of a magnetic field. It is shown that the multiple backward scattering of electrons on the randomly oriented or distributed magnetic moments of atoms significantly reduces the diffusion of electrons. Thus, in the diffusive but nearly localized electron system localized magnetic moments appear and a transition into the ferromagnetic phase occurs. The isotope effect in T*_C, the Curie temperature, is obtained, as first observed in La_{1 - x}(Ca,Sr)_xMnO_3+y experiments by Zhao and coworkers. Both a relatively weak classical magnetic field (ωcτ0 < 1, where ω_c is the cyclotron frequency, and τ₀ is the elastic collision time) and a molecular field, appearing as result of the spin polarization or spontaneous magnetization in the system, are found to be able to completely suppress the interference effect in the diffusion. It is shown that the giant magnetoresistance is extremely large near the mobility edge. The effect of magnetic field on the magnetic susceptibility is also considered. The application of the results to a layered 2D case is discussed as well.

The specific heat anomalies of YBa₂Cu₃O_{7 - δ} superconductors have been theoretically analyzed between 5 and 120 K. The lattice contribution to the specific heat is well estimated from the Debye and Einstein model. We have used the characteristic phonon frequencies deduced from the layered electron gas approach of quasi-two dimensional conducting CuO₂ layers. The deduced results on the behaviour of specific heat (C) with temperature (T) follows the same trend as revealed from experiments. The estimated lattice contribution, when subtracted from the experimental data results in a finite value of electronic specific heat coefficient (γ) in the superconducting state. Furthermore, ΔC/T shows a definite break in the vicinity of T_c when the graph is extrapolated in the range of 70 K ≤ T ≤ 110 K. We have also estimated the density of states at the Fermi level which is consistent with earlier reports. The implications of the above analysis are discussed.

We propose an effective transverse Ising model (TIM) as a means to treat both metallic and high-temperature superconductivity. The model is inspired by features which are present in the Gorter-Casimir two-fluid model of superconductivity, but it encompasses also the formalism used to treat the phase transitions in hydrogen-bonded ferroelectrics. We treat the model in the random-phase approximation, and as a consequence, the superconducting state reveals a structure of a gas of elementary excitations where the energy gap appears in a quite natural way.

We use a method which combines the discretized path-integral representation (DPIR) with the double-chain approximation (DCA) to study the thermodynamic properties of an S = 1 antiferromagnetic Heisenberg chain (AFHC) with both uniaxial and non-uniaxial single-ion anisotropy. The magnetization in the presence of an external magnetic field, the zero-field magnetic susceptibility and the magnetic specific heat of the system are obtained. The effects of the anisotropy on the thermodynamic properties of the AFHC are examined and discussed in details.

A very precise determination has been made of the contribution of the terms μ_Bg_eH · S and μ_BL · H of the Zeeman Hamiltonian to the anisotropy of the magnetic field effect on the ⁴E(G)-level of Mn²⁺ in ZnS. First, detailed excitation spectra from Kramers doublets |(⁴E) Γ₆>, |(⁴E) Γ₇>, and spin quartet |(⁴E) Γ₈> to fundamental Zeeman states |(⁶A₁) SM_S>, have been analyzed up to 5 T for B ∥ [001] and B ∥ [111]. Second, for B = 5 T, T = 2 K, angular variations of transitions |(⁴E) Γ₆>, |(⁴E) Γ₇> and |(⁴E) Γ₈> ---> (⁶A₁)SM_S = - 5/2> have been studied for B rotating around the axis [1-10]. For all orientations of the magnetic field, the experimental and theoretical energy levels as given by the Hamiltonian μ_Bg_eH · S, differ by at most 0.2 cm ^{- 1}. Finally, the very small contribution of the term in μ_BH · L has been studied by using an equivalent operator mimicking the second-order perturbation schemes involving the spin-orbit interaction and the operator L · H.

Spin waves in single thin films and exchange-coupled bilayers of f.c.c. MnTe are analyzed theoretically in the low temperature limit. Long-range antiferromagnetic order of AF-III type is assumed. The spin-wave Hamiltonian includes exchange interactions between nearest and next nearest neighbours, magnetic anisotropy and Zeeman term.

Based on a model of granular spheres with concentric shells embedded in a nonmagnetic metallic matrix, in which the spin-dependent interfacial scattering and the bulk scattering are included, the transport properties in systems of magnetic granules are investigated analytically. It is shown that GMR as well as the resistivities of these systems depend strongly on the interfacial roughness and the size of the granules.

The one-dimensional spin-1 Heisenberg antiferromagnetic chain is studied by a Green's function technique. The excitation spectrum is obtained. The dispersion of excitations for an infinite isotropic chain is numerically calculated. At k = 0 we obtain a gap 2Δ which is equal to 0.826J and at k = π we obtain Δ = 0.41J, which is in very good agreement with the exact numerical results of White and Huse.

Resonant Auger electron spectra were measured for several condensed molecules (Si(CH₃)₃Cl, SiCl₄, PCl₃, C₃H₇SH and CCl₄) around Si-, P-, S-, and Cl-1s edges. It was commonly found that the most frequent decay channel after the excitation was a KL_2,3L_2,3 transition. As predicted by a resonant inelastic scattering theory, the phenomenon known as the Auger resonant Raman effect was observed in this transition. Namely, resonant (spectator) Auger peaks exhibited both linear energy dispersion and width sharpening. Only Si- and Cl-KL_1,3L_2,3 transitions in Si(CH₃)₃Cl were found to exhibit complicated dispersion profiles, probably due to two different excitations in each edge. Intensity plots of the spectator and normal Auger peaks revealed that Si-3p* and Cl-3p* orbitals in this molecule have different natures, i.e., the former and the latter consist mainly of localized and delocalized components, respectively.

The application of Tokuyama-Mori formalism for the description of the emission decay of a specific physical system - Pb²⁺ ions in a potassium halide crystal matrix - is shown. The decay is numerically calculated from subpicosecond up to microsecond time region to study in what extent the results differ from the standardly used Pauli master equation.

A complete analysis of the line shape of the excitonic absorption and photoluminescence lines of the yellow series of Cu₂O is given. A detailed fit to the absorption lines up to the n = 5 exciton according to Toyozawa's theory gives precise values for the excitonic resonance energies, allowing to calculate the excitonic Rydberg. The fit to the photoluminescence lines shows very good agreement with experimental data at different temperatures. At high excitation, the fit to the excitonic absorption lines reveals the fundamental mechanism for bleaching of exciton absorption lines in Cu₂O. Based on these findings, a new all-optical method for the observation of exciton transport in Cu₂O is proposed.

We have examined photoluminescence (PL), IR absorption and Raman spectra of a series of hydrogenated amorphous silicon oxide (a-SiO_x:H, (0 < x < 2)) films fabricated by plasma enhanced chemical vapor deposition (PECVD). Two strong luminescence bands were observed at room temperature, one is a broad envelope comprising a main peak around 670 nm and a shoulder at 835 nm, and the other, peaked around 850 nm, is found only after being annealed up to 1170 °C in N₂ environment. In conjunction with IR and Raman spectra, the origins of the two luminescent bands and their annealing behaviors are discussed on the basis of quantum confinement effects.