Table of contents

The pressure dependence of crystallization in Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5bulk metallic glass has been studied by means of high-pressure annealing close to and beyond the glass transition temperature, using differential scanning calorimetry and x-ray diffraction. The results reveal that high pressure markedly decreases the crystallization temperature of the glass. It is deduced that high-pressure annealing can promote crystal nucleation by decreasing the work of formation of the critical nucleus, but it also restrains crystal growth by decreasing the diffusion velocity of atoms.

The recently discovered superconductor MgCNi₃ crystallizes in the classical cubic perovskite structure. The electronic structures of the MgCNi_3-xM_x (M = Co, Cu; x = 0, 0.5, 1.0) system have been calculated using the self-consistent tight-binding linear muffin-tin orbital method. The calculations showed that the Fermi level for MgCNi₃ is located in the slope descending from a sharp peak originating from d states of Ni atoms. Electron (Cu) and hole (Co) doping of MgCNi₃ reconstructs its band structure but does not lead to magnetic order.

Using a combination of photoelectron diffraction and scanning tunnelling microscopy the Cu(100)c(2×2)-N surface is shown to undergo a symmetry-lowering reconstruction with a strong (0.34 Å) periodic distortion of the outermost Cu layer perpendicular to the surface. The rumpling reconstruction is shown to account for N-N attractions which create c(2×2) islands and the systematic behaviour of inter-island boundary widths in the mesoscopic self-organization of this surface.

We report the first measurements of the reflectance anisotropy (RA) spectrum from a body-centred cubic transition metal surface, W(110), and provide a detailed interpretation based upon surface electronic structure calculations. The RA profile exhibits a pronounced resonance feature centred at 3.4 eV which decreases upon oxygen adsorption, indicating a surface effect. Calculating the surface dielectric function within a joint density of states approximation, we identify the important transitions: these are between occupied surface states with a large p-component to unoccupied d-states, with the RA arising from the different contributions of p_x and p_y states. These surface states have previously been measured using angle-resolved photoemission. The calculated resonance feature occurs at an energy of 2.9 eV, and the difference with experiment is attributed to self-energy effects.

We review how semiconductor surfaces can be modified by laser irradiation. We deal solely with phenomena induced by electronic excitation, and compare the processes induced by laser irradiation with those induced by electrons from scanning tunnelling microscope tips. The ejection of host atoms from perfect surface sites, or of hydrogen from hydrogen-terminated surfaces, takes place either by single excitation for high-energy photons or with multiple excitation for low-energy photons, even though the quantum yield for ejection by single excitation is extremely low. The atoms neighbouring defects on surfaces are ejected with higher probabilities than those on perfect sites. There appear to be anti-bonding states, embedded in the continuum, such that excitation to them can induce atomic ejection. When the photon energy is too small, multiple photoexcitation or dense excitation is needed to reach these anti-bonding states. Possible applications of laser surface treatments are discussed.

Deuterium thermal desorption experiments were performed on hexagonal (h-BN) and cubic (c-BN) boron nitride thin films deposited on (100) oriented Si substrate by radio frequency (rf) magnetron sputtering. BN samples were deuterated by thermal annealing at 673 K in a 10^-4 mbar D₂ atmosphere for 100 min. No differences were observed between c-BN and h-BN thin film samples in the effusion process, which occurred through a thermal activated heterogeneous first order kinetics. The activation energy for desorption exhibited a Gaussian distribution peaked at 2.28±0.01 eV with 0.18±0.02 eV semidispersion. This result indicated the breaking of the N-D and B-D chemical bonds, probably located at the grain boundaries of the nanocrystalline material, as the rate limiting step in the effusion kinetics. When the deuteration of the BN thin film samples occurred by 20 keV D₂ ion implantation, deuterium effusion is controlled by the migration of deuterium atoms to the sample surface through grain boundaries path and is characterized by 0.52±0.03 eV activation energy.

The multiple-scattering cluster (MSC) method has been used to calculate the sulphur 1s near-edge x-ray absorption fine structure of SO₂ adsorbed on Ni(111). Our investigation confirms the flat-lying adsorption structure of SO₂/Ni(111) and shows for the first time that the fcc threefold hollow site is the most preferable adsorption site. It has been shown that the O-S intramolecular bond length is elongated by 0.07 Å, the ∠OSO bond angle is reduced by 10° after adsorption and the adsorption height is 2.0±0.2 Å. Furthermore, R-factor analysis exhibits a local minimum in the vicinity of the fcc threefold hollow site, which is partly in agreement with the experimental result.

The morphology generated on Ag(110) and Ag(001) by 1 keV Ar⁺ sputtering at normal incidence has been studied by scanning tunnelling microscopy as a function of the substrate temperature T_S and the ion flux Φ. Since ion sputtering is a non-equilibrium process in which erosion competes with diffusion in determining the surface evolution, these macroscopic parameters can be used to tune the final surface morphology. Flat or rough surfaces as well as periodic structures have been observed on both substrates. On Ag(110), ion sputtering at 230 K and 320 K produces two well defined ripple patterns whose wave vectors are parallel to ⟨10⟩ and ⟨001⟩ respectively, while on Ag(001) a periodic pattern of square islands has been observed over a wide range of substrate temperatures (240 K⩽T_S<440 K). Similar to the results reported in growth experiments, a flux increase produces a surface evolution qualitatively comparable with that obtained by lowering the temperature. The results are discussed in terms of a continuum model for the ion sputtering process.

The formation of dendrite and fractal patterns on the surface of 350 keV bismuth-ion-implanted LiNbO₃ samples is presented. The dimensions of the dendrite and fractal patterns are 1.86±0.04 and 1.84±0.02, respectively. For two different growth forms in our system we give some interesting experimental data. According to their morphologies, the two kinds of structure are considered to be diffusion-limit-aggregation-like patterns and dense branching morphologies, respectively. A possible origin of the dendrite and fractal patterns is also presented in a preliminary manner.

Colossal negative magnetoresistance is found over a wide range of temperatures below the Curie point T_C≈240 K in an epitaxial La_0.35Nd_0.35Sr_0.3MnO₃ film on a single-crystal (001)ZrO₂(Y₂O₃) wafer substrate. Isotherms of the magnetoresistance of this film reveal that its absolute value increases with the field, abruptly in the technical magnetization range and almost linearly in stronger fields. For three epitaxial films of the same composition on (001)LaAlO₃, (001)SrTiO₃, and (001)MgO substrates, colossal magnetoresistance only occurred near T_C≈240 K and at T<T_C it increased weakly, almost linearly with the field. In the film on ZrO₂(Y₂O₃) substrate the electrical resistivity was almost 1.5 orders of magnitude higher than that in the other three films. It is shown that this increase is attributable to the electrical resistance of the interfaces between microregions having four types of crystallographic orientation, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunnelling of polarized carriers across these interfaces which coincide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for all four samples, which is only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thickness. The colossal magnetoresistance near T_C and the low-temperature magnetoresistance in fields exceeding the technical saturation level can be attributed to the existence of strong s-d exchange which is responsible for a steep drop in the mobility of the carriers (holes) and their partial localization at levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from these levels.

Barium ferrite films have been grown by pulsed laser deposition on oxidized Si (100) with and without ZnO buffer layers. The films grown directly on SiO₂/Si exhibit a granular structure with two easy axes (in-plane and perpendicular) and strong magnetic coupling. It was found that a ZnO underlayer promotes the orientation of the c-axis perpendicular to the film plane. Additionally, good separation between the grains and a low exchange coupling can be achieved in BaFe₁₂O₁₉/ZnO/SiO₂/Si films. Grain isolation and magnetic de-coupling are believed to be caused by the interdiffusion between the film and the ZnO buffer layer, which gives birth to a new non-magnetic phase (ZnFe₂O₄).

Silicon and silicon-germanium wire-like crystals grown by self-organizing processes have been studied by high-resolution transmission electron microscopy, scanning tunnelling microscopy, and ion and Auger spectroscopy. It is shown for the first time that such a crystal consists of a central part of bulk material and a nanoporous envelope and that these crystals represent the smallest Si-based heterostructures currently known. Possible reasons for such a structure developing for wire-like crystals are discussed, taking into account size effects in the growth process. The surprising thing is that they reveal Auger spectra particular to the atomically clean surfaces which do not vary upon storage in the atmosphere for a few years. It is also shown that the morphology and thickness of the envelope may be controlled by the growth conditions; therefore this technology may be applied in engineering new nanoelectronics materials.

We have studied the effects of gamma-ray irradiation on the Fourier transform infrared grazing incidence reflection-absorption spectra of GaN films grown on α-Al₂O₃ (0001) substrates using metal-organic chemical vapour deposition at atmospheric pressure. After irradiation, the spectral intensity increased markedly and a series of new absorption features appeared, among which those at 3440, 3355, 3174, 3088, 2819, 2712, 2100, 1426 and 894 cm^-1 are discussed and identified. The reason why gamma-ray irradiation can enhance the intensity and give rise to these new features is also discussed.

Positron beam analysis (PBA) and transmission electron microscopy (TEM) were applied to study structural transformations in cold-rolled Fe_0.94Ni_0.04Ti_0.02 foils, which were subjected to different thermal treatments in an atmosphere of a gas mixture of NH₃ + H₂ (nitriding). Positrons proved to be sensitive probes for the microstructure evolution and formation of nitride precipitates. The nitriding of the samples in the α-region (αN) of the Lehrer diagram for the Fe-N system produced a large decrease of the central part of the Doppler broadened annihilation γ-peak (S-parameter) and an increase of the contribution to the wings of the peak (W-parameter). The effect, ascribed to replacing of vacancy type positron traps by nitride-related traps, was much more pronounced for the α-nitrided samples than for samples annealed in vacuum at the same temperature. A reduction of the αN samples by annealing in H₂ atmosphere brings the S-parameter back to a higher value. Further nitriding of αN samples in the γ'-region (αN + γ'N) of the Lehrer diagram increases S and lowers the W-parameter compared with the αN samples. The changes in S- and W-parameters are interpreted on the basis of the evolution of microstructure of the films during the processing.

The effects of some metals (they include Fe, Ni, Cu, Zn and can be regarded as `honorary' transition metals; here these metals are referred to as `TM') and Ce on the microstructures of Al-TM-Ce amorphous alloys have been studied by means of x-ray diffraction and differential scanning calorimetry experiments. The prepeak positions of Al₉₀Fe₅Ce₅, Al₉₀Ni₅Ce₅, Al₉₀Cu₅Ce₅ and Al₈₃Zn₁₀Ce₇ amorphous alloys shift to smaller angles with increasing atomic number of the TM. The Al-Zn-Ce amorphous alloy was first obtained by adding Ce to improve the compound-forming tendency and prevent phase separation. The crystallization temperatures and crystallization activation energies of Al₉₀Fe₅Ce₅, Al₉₀Ni₅Ce₅, Al₉₀Cu₅Ce₅ and Al₈₃Zn₁₀Ce₇ amorphous alloys decrease gradually. The intensity of the interaction between atoms, W, determined by the atomic radius and the electronegativity, can qualitatively reflect the compound-forming tendency and the effects of elements on the glass formability. According to increasing W-value, we can arrange the TM elements in the following order: Mg, Mn, Zn, Cr, Cu, Ni, Fe. The glass formability and the compound-forming tendency of Al-TM-Ce alloys also increase in this order.

The influence of point defects on the dielectric susceptibility as observed on Sr_1-xCa_xTiO₃ with x⩽0.007 is described within the framework of two microscopic models. While the `soft impurity oscillator model' readily explains the induced lattice softening at low concentrations, an adequate description of the ferroelectric instability and nonlinear dielectric behaviour requires dominance of a `mode coupling model' at higher concentrations. It involves the soft-mode and tunnelling ions, whose appearance is explained within a percolation approach.

CeRu₄Sb₁₂ with the filled skutterudite structure exhibits a novel non-Fermi-liquid (NFL) behaviour in the low-temperature specific heat and electrical resistivity at ambient pressure. We investigated the effect of La substitution and magnetic field on the NFL state and found that it can be easily destroyed either by the application of a magnetic field or by a small degree of La substitution. In magnetic fields, C/T becomes strongly depressed and nearly constant below 1 K and ρ(T) turns to following a T²-law, implying that the Fermi-liquid state recovers in high magnetic fields. The type of critical fluctuation is discussed on the basis of the results of magnetic susceptibility, electrical resistivity and specific heat measurements.

In this paper we present a survey on the structure and equation of state for some silicon clathrates and their carbon analogues, as obtained by means of ab initio calculations within the Hartree-Fock approximation. We restrict our consideration to type-I clathrates, namely M_x(Si, C)₄₆, with M = Na, Ba. The insertion of guest species into the carbon clathrate cages promotes a significant increase in the host volume, thus reducing the bulk modulus for these compounds. In spite of that, the estimated hardness for C₄₆, of about 61 GPa, constitutes an exceptionally large value for a structure with such an open framework. The issue of electronic charge transference from the guest species to the host framework and the stability of carbon and silicon clathrates relative to the diamond phase are also discussed.

We solve the linearized Boltzmann equation for a degenerate quasi-two-dimensional electron system confined to a triangular quantum well at a III nitride heterojunction and interacting only with the polar optical phonons. The method of solution makes use of a ladder technique, and employs the Fang-Howard approximation for the description of the confined electrons. The variations of the effective-momentum relaxation time with the electron energy, and of the mobility with electron density are presented for a GaN/AlN heterojunction.

The structure of a single tilted flux line in an anisotropic (d + s)-wave superconductor has been analysed within the Ginzburg-Landau theory generalized for two complex order parameter components. The angular dependence of the lower critical field H_c1 and the vortex structure have been studied for different orientations of an applied magnetic field with respect to the crystal axes. In the limit of weak anisotropy (which can be treated within the Ginzburg-Landau theory with an anisotropic mass tensor), the arrangement of s-wave unit vortices induced in the core is shown to strongly depend on the field orientation. For a large-anisotropy limit (i.e. for layered Josephson coupled systems) the fourfold anisotropy of H_c1 is shown to be dominated by the contributions which come from the Josephson string regions.

Optical absorption measurements were performed on irradiated diamonds containing different concentrations of nitrogen defects. Analysis of the results suggests that the single substitutional nitrogen centres trap vacancies about eight times more efficiently than the substitutional nitrogen pairs. In irradiated diamond crystals with a low impurity content, as well as in synthetic diamond films, an undocumented IR spectrum is observed and ascribed to intrinsic defects.

Measurements of x-ray diffraction pattern and Mössbauer spectra show that β-FeSi₂ is produced in an Fe₃₀Si₇₀ powder sample by low energy ball milling, and its proportion increases step by step as the milling time increases until the milling time reaches 60 h; after that this solid reaction reaches saturation. Within the final products, there is over 90% β-FeSi₂ and a little Fe and Si remnants. The remnant Fe exists in two forms: one is powder and the second is enveloped by β-FeSi₂. When using hydrochloric acid to treat the as-milled sample, the first form of Fe is removed but the second form of Fe still remains. When the sample is annealed at 600 °C, the proportion of β-FeSi₂ is close to 99% in the final products.

We study the kinetics of the L1₀ ordering in face-centred alloys at early stages of phase transformations, when the tetragonal distortion of the L1₀ phase is still insignificant as regards the microstructural evolution, and compare it with the kinetics of the L1₂ ordering. We use the earlier-described kinetic cluster-field method to simulate A1→L1₀ and A1→L1₂ transformations after a quench of an alloy from the disordered A1 phase to the single-phase L1₀ or L1₂ state for a number of alloy models with both short-range and extended-range interactions. The simulations reveal a number of peculiar features in both transient microstructures and transformation kinetics - in particular, a sharp dependence of the microstructural evolution and the structure of antiphase boundaries under the L1₀ ordering on the composition of the alloy; the importance for both the L1₀ and L1₂ ordering of a peculiar mechanism of domain growth via the fusion of different in-phase domains; the presence in transient microstructures under the L1₀ ordering of some specific long-living configurations, quadruple junctions of four different antiphase domains, as well as of peculiar kinetic processes of `splitting' of certain antiphase boundaries into two others; and other features.

The idea of quantum computation is the most promising recent development in the high-tech domain, while experimental realization of a quantum computer poses a formidable challenge. Among the proposed models especially attractive are semiconductor based nuclear spin quantum computers (S-NSQCs), where nuclear spins are used as quantum bistable elements, `qubits', coupled to the electron spin and orbital dynamics. We propose here a scheme for implementation of basic elements for S-NSQCs which are realizable within achievements of the modern nanotechnology. These elements are expected to be based on a nuclear-spin-controlled isotopically engineered Si/SiGe heterojunction, because in these semiconductors one can vary the abundance of nuclear spins by engineering the isotopic composition. A specific device is suggested, which allows one to model the processes of recording, reading and information transfer on a quantum level using the technique of electrical detection of the magnetic state of nuclear spins. Improvement of this technique for a semiconductor system with a relatively small number of nuclei might be applied to the manipulation of nuclear spin `qubits' in the future S-NSQCs.