Table of contents

A theoretical approach is developed for reconstructing phonon dispersion in Si nanocrystals. It is found that for Si nanocrystals with size larger than 2 nm, smeared phonon dispersion could be established. The evolution of nanocrystal phonons into bulk ones is studied. Frequency shifts of bulk-like phonons versus nanocrystal size are calculated.

We investigate critical wetting transitions for fluids adsorbed in wedge-like geometries where the substrate height varies as a power law, z(x, y) ∼ |x|^γ, in one direction. As γ is increased from 0 to 1 the substrate shape is smoothly changed from a planar wall to a linear wedge. The continuous wetting and filling transitions pertinent to these limiting geometries are known to have distinct phase boundaries and critical singularities. We predict that the intermediate critical wetting behaviour occurring for 0 < γ < 1 falls into one of three possible regimes depending on the values of γ, p and q. The unbinding behaviour is characterized by a high degree of non-universality, strongly anisotropic correlations and enhanced interfacial roughness. The shift in phase boundary and emergence of universal critical behaviour in the linear wedge limit are discussed in detail.

The magnetization and magnetoresistance (MR) were measured for GdMn₂Ge₂, which shows a first-order transition from a low-temperature ferrimagnetic (Fi) state to a high-temperature antiferromagnetic (AF) state at T₁ = 95 K. The field-induced transition from the Fi state to the high-field state was observed below T₁. The transition field decreases with increasing temperature and it falls to zero at T₁ for H ⊥ c, while the transition disappears above 90 K for H ∥ c. A large positive MR jump was observed associated with the field-induced transition for H ⊥ c. These results suggest that this may be the transition from the Fi to AF state caused by a magnetic field.

Between 0.2 and 2000 MHz, sonic absorption spectra of solutions of albumin in water and in aqueous phosphate buffer systems have been measured at different temperatures (15-35 °C), pH values (1.8-12.3), and protein concentrations (0.2-4% w/w). Broad absorption spectra, indicating the presence of various relaxation processes, have been found. Depending on the albumin concentration, the spectra can be analytically represented by a sum of the asymptotic high-frequency contribution and up to four Debye-type relaxation terms. Other relaxation models, including one based on a continuous distribution of relaxation times, also appropriately describe the measured spectra. Low-frequency relaxation processes with relaxation times between about 30 and 300 ns appear to be due to cooperative conformational changes of the polypeptide chain. High-frequency processes with relaxation times between 0.3 and 3 ns seem to reflect elementary reactions of the albumin molecule that are associated with changes in the hydration properties.

We study the scaling properties of polymer macromolecules in media with defects that are correlated or belong to some porous or spongelike structure. To this end, we use the model of self-avoiding walks on a randomly diluted lattice with long-range-correlated quenched disorder. We apply the field-theoretical renormalization group refined by the resummation technique and perform calculations directly in three dimensions up to the two-loop approximation. The scaling behaviour of polymers in media with long-range-correlated disorder is found to be governed by a new set of exponents.

According to experimental data, irradiation of water droplets accelerates the evaporation process. In this paper, we propose a model that considers the influence of non-equilibrium molecular vibrations created by the radiation on the evaporation rate. The excited molecules transform their vibration energy into energy of translational motion while colliding with the droplet surface and increase the effective evaporation rate. The theoretical results are in qualitative agreement with the experimental data.

The non-Markovian collision integral is obtained on the base of the Kadanoff–Baym equations for Green functions in a form with allowance for small retardation effects. The collisional relaxation times and damping width of the giant isovector dipole resonances in nuclear matter are calculated. For an infinite Fermi liquid the dependence of the relaxation times on the collective vibration frequency and the temperature corresponds to the Landau prescription.

Biological channels are protein molecules that contain a hole and that allow selected ions to pass through a membrane. They provide the mechanism for the control of many physiological functions. For example, a calcium channel selects calcium ions and controls the beating of a heart. We report our Monte Carlo results on model calcium channels. Because of the presence of negatively charged glutamate side chains in the channel, the channel selects calcium ions over sodium ions because of the ability of the divalent calcium ions to deliver twice the charge while occupying the same volume as the monovalent ions in the narrow channel.

Ionic conductivity data and structural relaxation times for poly(phenyl glycidyl ether)-co-formaldehyde (PPGE) are compared for the supercooled liquid under a wide range of temperatures and pressures. Conformance to the fractional Debye–Stokes–Einstein (fDSE) relation was observed under all conditions. The fDSE exponent, = 0.81, provides an estimate of the relative magnitude of the activation volumes for conductivity and dielectric relaxation. The smaller activation volume for the former suggests less free volume is necessary to accommodate ion diffusion in the PPGE than that required for dielectric relaxation. We also fit the combined temperature and pressure dependences of both the conductivity and relaxation times to the Avramov equation.

This work presents the spectroscopic characterization of Tm³⁺ doped gallium lanthanum sulfide (GaLaS) chalcogenide glass through absorption, fluorescence and lifetime measurements of excited³H₄ and³F₄ states, and a study of Tm³⁺:Tm³⁺ energy transfer. The cross relaxation ³ H ₄,³ H ₆ →³ F ₄,³ F ₄ responsible for the pumping of level³F₄ and the laser transition at 1800 nm (³ F ₄ →³ H ₆), as well as the energy migration ³ H ₄,³ H ₆ →³ H ₄,³ H ₆ processes are studied in terms of the microscopic parameters of energy transfer C_da and C_dd obtained by the Kushida model of multipolar interactions and by a rate equation treatment of the dynamics of levels³F₄ and³H₄. From this treatment it was possible to simulate level³F₄ temporal evolution curves for different Tm³⁺ concentrations, leading to results that are in excellent agreement with experimental ones. All the samples studied in the work present positive optical gain coefficients for excitation densities higher than 12 kW cm⁻² indicating the potentiality of GaLaS:Tm³⁺ glass as a mid-infrared laser active medium.

The mixing rules for the permittivity and permeability of composites are known to depend greatly on the microscopic structure of the composite. This dependence can be quantified in terms of Bergman's spectral function. In this paper, the spectral function of actual magnetic composites is reconstructed from their measured microwave constitutive parameters.

The samples under study are composed of carbonyl iron or Fe–Cr–Al alloy powders embedded in a paraffin wax matrix. The permittivity and permeability of the samples is measured in the 0.1–10 GHz frequency band. The proposed approach to process the measured data allows the spectral function of the composite and frequency dependence of intrinsic permeability of inclusions to be derived. The obtained results are in agreement with available tabulated data.

A magnetic structure of SmFe₂ with a Laves phase crystalline structure was investigated by means of white-x-ray magnetic diffraction. The orbital and the spin magnetic form factors of Sm and Fe ions have been obtained as a function of momentum transfer. Their extrapolation to zero momentum transfer, according to the dipole approximation based on an atomic model, has given the values of the spin and the orbital magnetic moments of Sm ions as −2.54 +/- 0.55μ_B and 2.79 +/- 0.61μ_B, respectively. This result confirms that the small magnetic moment of Sm ions arises from antiparallel coupling between the spin and the orbital magnetic moments. The spin and the orbital magnetic moments of Fe ions have been also estimated, as 1.13 +/- 0.25μ_B and 0.19 +/- 0.04μ_B, respectively.

Tight binding molecular dynamics is used to predict the structure and the total energy of the most relevant intrinsic point defects in C54 and C49 TiSi₂. The comparison between the relative formation energies of point defects of the two phases in contact with a Si substrate suggests that the metastable C49 form has a higher concentration of point defects. In particular, we point out that Si vacancies and (010) stacking faults should be quite common in the C49 structure. This issue could be important in explaining the kinetic advantage of the latter phase in film growth by solid state reaction.

A mechanism of interaction of dislocations with point defects is suggested on the basis of a study of deformation-stimulated luminescence (DL) from irradiated KCl. DL is considered to be a result of destruction of hole colour centres by dislocations. The centres destroyed by dislocations are responsible for irradiation-induced strengthening of alkali halides.

In this paper, a comprehensive diffusion kinetics theory is formulated to describe seamlessly tracer and chemical diffusion in antistructurally disordered B2 intermetallics showing positive and negative deviations from stoichiometry. The theory is based around unit processes consisting of six-jump cycles that can be assisted by intrinsic and extrinsic antistructural atoms of either atomic species. The Ising alloy model is used to illustrate the formalism, but the formalism can be adapted to other models. Expressions are developed for the tracer diffusion coefficients, the phenomenological coefficients, the intrinsic diffusion coefficients, the interdiffusion coefficient and the various correlation factor components. Results for the tracer and collective correlation factors and the vacancy wind factor (in interdiffusion) are in excellent agreement with results from Monte Carlo computer simulations based around single vacancy jumps.

The results of a plane-wave pseudopotential study on the mechanical and electronic properties of twelve III–V zinc-blende (ZB) and wurtzite (WZ) semiconductors under pressure are presented. The lattice parameters, bulk moduli B₀, energy band types, band-gaps E_g^Γ at the Γ point, and pressure dependences of E_g^Γ are investigated in detail. Our results show that the E_g^Γ – P relations can be classified into two distinct types for these ZB and WZ phases. A transformation from one type of E_g^Γ – P relation to the other type is found to occur in some WZ phases. Linear relationships between the bulk moduli and the inverse of unit-cell volumes at P = 0 are also found for the ZB and WZ phases.

The structural and electronic properties of crystalline GeSe₂ are studied using ab initio density functional theory. GeSe₂ exists in a variety of phases; here eight phases, six previously synthesized and two hypothetical, are investigated within the local density approximation and the generalized gradient approximation (GGA). The results show that the GGA correctly describes the energetics of the GeSe₂ even with variations in charge density of layer GeSe₂ structures. The stability of the phases studied is discussed, and the possible phase transitions under pressure are predicted and compared to experiment. The electronic structure of GeSe₂ confirms, for the first time, that the high-pressure CdI₂-type phase is metallic which may produce the semiconductor to metal transition in the GeSe₂ system under pressure.

The vertical magnetoresistance was explored in short-period doped GaAs/AlAs superlattices. The effect of the vertical longitudinal magnetoresistance expected to occur in the case of the open Fermi surface was found. The analysis of the possible position of the Fermi level allowed us to distinguish the type of the lowest miniband—between Γ and X origins. Our results indicate that the lowest miniband is formed by the X_L electron states of GaAs and AlAs. The result agrees with the theoretical predictions and shows that the residual stress does not significantly influence the miniband structure of the short-period superlattices studied.

Polymer-embedded La_2/3Sr_1/3MnO₃ (LSMO) composites, (LSMO)_{1 −x}(PPP)_x (PPP is polyparaphenylene, and x is the weight fraction of PPP), were prepared by mixing pre-prepared LSMO and PPP powders. Thermogravimetric analysis, x-ray diffraction, and infrared spectra show that the composites are stable when calcined at 400^oC. Significant enhancement in the magnetoresistance (MR) effect over a wide temperature range is observed for the composites compared with the parent LSMO. The MR enhancement reaches its maximum at x = 0.2. The magnetotransport is mainly attributed to intergrain spin-polarized tunnelling. We argue that the introduction of PPP gives rise to magnetic disorder and hence an enhanced tunnelling effect, which is responsible for the MR enhancement.

The ν = 2/5 state is spin unpolarized at weak magnetic field and fully polarized at strong field. At intermediate field, a plateau of half the maximal polarization is observed. We study this phenomenon in the framework of composite fermion (CF) theory. Due to the mixing of the CF Landau levels (LLs), the unidirectional charge/spin density wave state of CFs is lower in energy than the Wigner crystal. This means that transport anisotropy, similar to that for electrons in higher Landau levels at half-filling, may appear in this fractional quantum Hall state when the external magnetic field is in an appropriate range. When the magnetic field is tilted, the easy transport direction is perpendicular to the direction of the in-plane field. Varying the partial filling factor of CF LLs from 0 to 1, we find that the energy minimum occurs in the vicinity of a half.

The temperature dependence of the electronic heat capacity C(T) was calculated for a mesoscopically disordered s-wave superconductor treated as a spatial ensemble of domains with continuously varying superconducting properties. The domains are assumed to have sizes L > ξ, where ξ is the coherence length. Each domain is characterized by a certain critical temperature T_c0 in the range [0, T_c ]. The averaging over a broad superconducting gap distribution leads to ⟨C(T)⟩ ∝ T² for low T, whereas the specific heat anomaly at T_c is substantially smeared. For narrow gap distributions there exists an intermediate-T range, where the curve ⟨C(T)⟩ can be well approximated by an exponential Bardeen–Cooper–Schrieffer-like dependence with the effective gap smaller than the weak-coupling value. The results are applicable in the general case of inhomogeneous superconductors including, e.g., electron-doped and hole-doped cuprates. The C(T) data for MgB₂, where multiple gaps are observed, are discussed in more detail.

We study the superfluid properties of short-coherence-length superconductors described by the extended Hubbard model with on-site repulsive and intersite attractive interaction. We have analysed the superfluid stiffness and thermodynamic properties of the model versus electron concentration and pairing interaction on a two-dimensional square lattice with nearest- and next-nearest-neighbour hopping. The effects of phase fluctuations on anisotropic superconductivity of extended s-and d_{x² −y²}-wave type were examined within the Kosterlitz–Thouless (KT) scenario. The KT critical temperatures were determined and compared with those from the BCS–Hartree–Fock approximation. The Uemura-type plots, i.e. the critical temperature versus zero-temperature phase stiffness, were obtained for both extended s- and d-wave pairings. The states with mixed (s^∗ and d_{x² −y²}) symmetry, and in particular the time-reversal breaking state of s + id symmetry, were analysed and the phase diagrams taking into account these states are presented. We also briefly discuss the crossover from BCS to local pair superconductivity for the d_{x² −y²}-wave pairing. A comparison of the theoretical results with the experimental data, in particular those of T_c versus λ⁻² (0), for high-T_c cuprate superconductors is made.

The in-plane optical conductivity of high-T_c cuprates in the superconducting (SC) and normal states is studied on the basis of the slave-boson mean-field approach to the 2D t–t^'–J model and the antiferromagnetic spin fluctuation correction in the framework of the renormalized random-phase approximation. The mid-infrared contribution to the conductivity in the normal state and the peak/dip/hump feature of the spectra in the SC state are reproduced, and are shown to be caused by coupling to spin fluctuations, in particular by coupling to the resonance mode of the spin fluctuations.

In singly doped samples of La_1.85Sr_0.15Cu_1−xFe_xO₄ and doubly doped samples of La_{1.85 −x}Sr_{0.15 +x}Cu_1−xFe_xO₄, Fe ions substitute for Cu²⁺ at the valence of 3+. The depression of T_c is less sharp in doubly doped samples than in singly doped samples due to the compensation of carriers. We consider that doping with Fe severely disturbs the alignment of Cu (3d_{x² −y²}) and O(2p_σ) orbitals and leads to the formation of CuO₆ clusters. The charge carriers in these CuO₆ clusters lose their itineracy and show localized behaviour. The decrease in T_c caused by Fe doping is due to the localization of carriers but not to the existence of an impurity moment.

The diluted mixed spin Ising system consisting of spin-1/2 and spin-1 with a random field is studied by the use of the finite cluster approximation within the framework of a single-site cluster theory. The state equations are derived using a probability distribution method based on the use of Van der Waerden identities. In this approach, the complete phase diagrams are investigated for the simple cubic lattice, when the random field is bimodally and trimodally distributed. The internal energy, specific heat and susceptibility are also calculated for both kinds of probability distributions.

We present the results of our measurements of magnetization, heat capacity, resistivity and magnetoresistance on the Ce-based intermetallic compound CeRhSn₂. The magnetic transition, as reported earlier, is confirmed by the presence of well defined anomalies in magnetization, resistivity and specific heat. The Ce ions are in a crystal field doublet ground state and undergo a magnetic transition below ∼4 K. The magnetic contribution to the resistivity is similar to that of a Kondo lattice system which orders magnetically. The magnetoresistance of the compound measured at 2 K in a field of 10 T is negative and large in magnitude (−41%).

The magnetic susceptibility and specific heat of single crystals of KTm(MoO₄)₂ in the temperature interval 0.1–300 K are investigated. It is shown that the ground state of the rare-earth magnetic system of the Tm³⁺ ions can be represented by two close-lying singlet levels (a non-Kramers quasidoublet with M_J = | +/- 6⟩) separated by a rather large energy interval from the excited levels. The effect of the 'freezing' of energy levels on magnetic susceptibility and the Schottky anomaly on magnetic specific heat have been observed in the low-temperature region T < 2.5 K. These results show that this magnetic system can be considered as a simple two-level quantum system over a wide temperature range.

The crystal structure and magnetic and electronic transport properties of the ternary intermediate compound Pr₅Si₂Ge₂ were investigated by means of x-ray powder diffraction, magnetic and electrical resistance measurements. The compound has two polymorphs: α-Pr₅Si₂Ge₂ crystallizing in a monoclinic structure with space group P 112₁/a , for the as-cast sample, and β-Pr₅Si₂Ge₂ crystallizing in a tetragonal structure with space group P 4₁ 2₁ 2, for the sample annealed at 1273 K. The average interatomic distances of magnetic Pr atoms and the amounts of covalent (Si, Ge)–(Si, Ge) bond pairs in these two phases are different, which may be responsible for their different magnetic properties. The Curie temperatures T_C of α-Pr₅Si₂Ge₂ and β-Pr₅Si₂Ge₂ are 38 and 50 K, respectively. An irreversibility between zero-field-cooling and field-cooling magnetization curves is observed. The zero-field resistance of these two phases exhibits an anomaly near T_C. On the basis of the results of magnetic and electrical resistance measurements, it is inferred that both of these phases exhibit spin-glass-like behaviour. The magnetic properties and structure dependence of the Pr₅Si₂Ge₂ can be elucidated well using the Ruderman–Kittel–Kasuya–Yosida model.

The solid-state 'amorphization' reaction produced by the mechanical alloying (MA) of the equiatomic alloy Fe₅₀Re₅₀ has been studied using ⁵⁷ Fe Mössbauer spectrometry plus x-ray and neutron diffraction. The percentages of iron atoms in the magnetic phases present in the reaction have been obtained from the Mössbauer spectra and the consumption of the parent elements derived from the reduction in intensity of their Bragg peaks in the diffraction patterns. The reaction in the Fe₅₀Re₅₀ alloy appears to be 2–3 times slower than the reaction in the related Fe₅₈Ta₄₂ alloy studied by us previously. A possible reason for this is the presence of an intermediate phase in the reaction revealed by the hyperfine structure at ⁵⁷ Fe. This is identified as an iron-rich solid solution by the absence of any new Bragg peaks in the diffraction patterns. Although a small fraction of the rhenium remains unconsumed at the end of the reaction, the FeRe alloy obtained after 24 h of MA has a genuine amorphous structure, whose reduced radial distribution function G(r) is sensitive to the topological order present.

We report the values of some thermal and electrical properties of candelilla wax (from Euphorbia cerifera). The open-cell photoacoustic technique and another photothermic technique—based on the measurement of the temperature decay of a heated sample—were employed to obtain the thermal diffusivity (α_s = 0.026 +/- 0.000 95 cm² s⁻¹) as well as the thermal conductivity (k = 2.132 +/- 0.16 W mK⁻¹) of this wax. The Kelvin null method was used to measure the dark decay of the surface potential of the sample after a corona discharge, giving a resistivity of ρ_e = 5.98 +/- 0.19 × 10¹⁷ Ω cm.

Utilizing the quantum statistical operator algebra technique, we derive a new absorption coefficient formula for the optical transition in a system of electrons interacting with longitudinal optical phonons in a quantum well. The electron and phonon distribution functions are included in the lineshape function. So we can explain the phonon absorptions and emissions in an organized way for all the electron transition processes. The numerical results show that the width decreases with the well width and increases with the temperature.

We report here photoluminescence of CdS nanoparticles suspended in vacuum. The CdS nanoparticles, whose size distribution was bimodal having peaks at 11.3 and 43.1 nm, were produced by the gas-evaporation method and the nanoparticle beams were formed within the rarefied inert-gas flow. The band-edge emission and surface luminescence of the CdS nanoparticles, whose surfaces were completely free from adsorption, reaction or contamination with their surroundings, were observed. As the excitation intensity increased, the band-edge emission shifted to lower energy and the surface state luminescence shifted to higher energy. These energy shifts in the photoluminescence spectra as functions of excitation intensity were interpreted as an internal temperature rise of the CdS nanoparticles suspended in vacuum, and the internal temperature of the CdS nanoparticles was evaluated by means of photoluminescence. It was found that the nanoparticles suspended in vacuum were heated by much weaker laser irradiation than the bulk surface due to the large surface-to-volume ratio.

Positron distributions and lifetimes in carbon-nanotube bundles have been calculated using a combination of the superposed-neutral-atom model and the finite-difference method (SNA-FD) as well as with an ab initio method. The electron–positron correlation was described with the local density approximation (LDA) or the generalized gradient approximation (GGA). For the LDA, the SNA-FD and ab initio calculations give similar results. Positrons are predominantly distributed in interstitial regions for smaller-size nanotubes, while they are distributed inside nanotubes for larger sizes. The estimated positron lifetime ranges from 250 to 480 ps as a function of the nanotube diameter. In contrast to this, for the GGA, the SNA-FD and ab initio calculations give quite different results concerning the positron distribution and lifetime.

We report here on the nature of the twinning associated with the tetragonal to orthorhombic I (T/OI) phase transition in YBCO, for temperatures between 500 and 600^ºC. In situ neutron diffraction measurements show the coexistence of the T and split OI peaks in the vicinity of the transition. However, by direct numerical simulation, we show that this is to be expected from the twinned OI structure when the separation of adjacent twinning planes and the orthorhombicity both approach zero. It is demonstrated that the observed hkl-dependent peak broadening is consistent with this interpretation. A comparison of the measured and simulated peak shapes enables us to make an approximate estimate of the actual twin widths for a given orthorhombicity.

We present results of magnetization measurements showing that the magnetic response of the antiferromagnetic state of SmMn₂Ge₂ depends on the path used in the field (H)–temperature (T) phase space to reach this state. A distinct signature of metastability is observed in this antiferromagnetic state when obtained via field-cooling/field-warming paths.