Table of contents

C₆₀ molecules were chemically coupled on porous silicon by means of a kind of silane coupling agent, (CH₃O)₃Si(CH₂)₃NH₂. After annealing in N₂ at 900^oC, three photoluminescence (PL) peaks were observed, at 384, 440, and 465 nm, under an excitation with the 310 nm line of a Xe lamp. The 440 and 465 nm PL peaks were attributed to optical transitions in hydrogenated amorphous silicon carbide (a-Si_1−xC_x:H). Upon excitation at 345 nm, three PL peaks appear, at 384, 400, and 480 nm. Spectral analyses suggest that radiative recombination of carriers occurs in the states at the surface of the β-SiC nanocrystal formed during annealing, whereas their photogeneration takes place in the β-SiC core. After further annealing at 1100^oC, two new PL peaks were observed, at 417 and 436 nm. Spectral examinations reveal that the two blue PL peaks arise from excess Si defect centres at the β-SiC surface, while the photoexcited carriers still come from the β-SiC core. Our experiments and results provide a good understanding of light-emitting mechanisms related to SiC materials.

An analytical result has been obtained for the value of the dynamical charge necessary for calculations of oxygen vacancy kinetics in dielectric perovskite-type crystals. It is shown, by using the Berry phase analysis, that this charge equals the nominal charge of the vacancy: for example, for the doubly charged state (no electrons associated with the vacancy), it is 2e where e is the electron charge; a neutral vacancy (two electrons in the vacancy) has zero average charge.

Injecting charge into dangling bond wires on Si(001) has been shown to induce polarons, which are weakly coupled to the underlying bulk phonons. We present elevated-temperature tight-binding molecular dynamics simulations designed to obtain a diffusion barrier for the diffusive motion of these polarons. The results indicate that diffusion of the polarons would be observable at room temperature, and that the polarons remain localized even at high temperatures.

An efficient method for doping a transition metal oxide (TMO) with hole carriers is presented: photocarrier injection (PCI) in an oxide heterostructure. It is shown that an insulating vanadium dioxide (VO₂) film is rendered metallic under light irradiation by PCI from an n-type titanium dioxide (TiO₂) substrate doped with Nb. Consequently, a large photoconductivity, which is exceptional for TMOs, is found in the VO₂/TiO₂:Nb heterostructure. We propose an electronic band structure where photoinduced holes created in TiO₂:Nb can be transferred into the filled V 3d band via the low-lying O 2p band of VO₂.

This review is intended as an introduction to mesomagnetism, with an emphasis on what the defining length scales and their origins are. It includes a brief introduction to the mathematics of domains and domain walls before examining the domain patterns and their stability in 1D and 2D confined magnetic structures. This is followed by an investigation of the effects of size and temperature on confined magnetic structures. Then, the relationship between mesomagnetism and the developing field of spin electronics is discussed. In particular, the various types of magnetoresistance, with an emphasis on the theory and applications of giant magnetoresistance and tunnelling magnetoresistance, are studied. Single electronics are briefly examined before concluding with an outlook on future developments in mesomagnetism.

We consider the effect of photoinduced optical anisotropy (POA) in azopolymers. Using a unified approach to the kinetics of photo-reorientation we discuss the assumptions underlying the known theoretical models of POA and formulate a tractable phenomenological model in terms of angular redistribution probabilities and order parameter correlation functions. The model takes into account biaxiality effects and long-term stability of POA in azopolymers. It predicts that under certain conditions two different mechanisms, photo-orientation and photoselection, will dominate POA depending on the wavelength of pumping light. By using available experimental data, we employ the model to compute dependences of principal absorption coefficients on the illumination time. Our calculations clearly indicate the different regimes of POA and the numerical results are found to be in good agreement with the experimental data.

The acoustic velocity and absorption have been measured in a nitroethane–isooctane mixture at critical composition in the frequency range 2–40 MHz and for temperatures of 31–43^oC. The experimental data have been analysed in terms of the Ferrell–Bhattacharjee dynamic scaling theory with the appropriate crossover corrections and recently developed Folk–Moser renormalization group theory. The shear viscosity of the studied mixture has been also measured to enable this analysis. A good agreement with theoretical predictions has been obtained.

The temperature dependence of the spontaneous polarization across the phase transition from the fluid smectic C* to the hexatic smectic I* phase of a chiral liquid crystal was investigated with respect to confinement effects, due to closely spaced substrates. A strong thickness dependence of the polarization was observed in both the SmC* and SmI* phases, which cannot be interpreted either by polarization screening due to ionic impurities, or by the polarization self-screening model. A possible explanation is given in terms of non-switching interface regions close to the substrates, caused by strong anchoring conditions. It is demonstrated that the occurrence of bond-orientational order does not affect the anchoring behaviour of the liquid crystal. Confinement effects were observed for decreasing cell gap below 4 μm, while the first order character of the SmC*–SmI* phase transition is retained.

Interactions between two like-charged macroions in the presence of their counterions confined in a cylindrical cell have been investigated by Monte Carlo simulations. The mean force and the potential of mean force (pmf) as a function of the macroion separation have been determined at conditions at which short-range attractions become significant. As the electrostatic coupling is increased, the nature of the mean force changes from being purely repulsive to being attractive at short separations and finally attractive at all separations. At a sufficiently large electrostatic coupling, the attractive mean force is promoted by an increase of the macroion charge to counterion charge ratio and by an increase of the macroion volume fraction (assuming a not too concentrated solution). At the onset of an attractive force, nearly all counterions are localized at the macroion surfaces, but the counterions still display a two-dimensional fluid structure. The pmfs determined from the cylindrical cell model agreed qualitatively very well with those obtained from simulations of corresponding fluids, suggesting that higher-order macroion correlations are of less importance. Finally, it is argued that the attractive component of the mean force originates from spatial correlations between counterions residing near different macroions.

The recent experiments on processes in cold Rydberg gases produced by laser excitation have so far only a weak relation to Rydberg matter (RM) and its formation, despite some attempts to make such a connection. The problems of forming RM from cold Rydberg gases are severe and due to at least five obvious factors: the condensation energy, the non-circular states formed, the spatial and temporal incoherence of the electronic motion, the low density of Rydberg states formed and the too large interionic distance. Evidence on why these factors are important is given. It is furthermore not obvious that formation of RM would have been detected in the laser excitation experiments since no suitable diagnostics methods were employed. The methods of forming RM using condensation in the boundary layer at surfaces are discussed for comparison. Better designed experiments seem necessary to achieve true condensation to RM in cold Rydberg gases, since large differences exist between Bose–Einstein and Rydberg condensation.

It is shown that a vortex can be continuously created in a Bose–Einstein condensate with hyperfine spin F = 2 in a Ioffe–Pritchard trap by reversing the axial magnetic field adiabatically. It may be speculated that the condensate cannot be confined in the trap since the weak-field seeking state makes transitions to the neutral and the strong-field seeking states due to the degeneracy of these states along the vortex axis when the axial field vanishes. We have solved the Gross–Pitaevskii equation numerically with given external magnetic fields to show that this is not the case. It is shown that a considerable fraction of the condensate remains in the trap even when the axial field is reversed rather slowly. This scenario is also analysed in the presence of an optical plug along the vortex axis. Then the condensate remains within the F_z = 2 manifold, with respect to the local magnetic field, throughout the formation of a vortex and hence the loss of atoms does not take place.

Second-harmonic generation (SHG) has been observed in optically poled tellurite glasses doped with V₂O₅. The optical poling has been performed by using the fundamental wave at the wavelength of 1064 nm and the second-harmonic wave at 532 nm from a Nd:YAG pulsed laser. The second-harmonic intensity of 15Nb₂O₅·85TeO₂ glasses containing V₂O₅ is larger by two orders of magnitude than that of tellurite glass of the same composition without V₂O₅. Electron spin resonance spectra indicate that vanadium ions are present as either V⁴⁺ or V⁵⁺, and that the relative amount of V⁴⁺ is decreased when the glass is irradiated with second-harmonic waves of a Nd:YLF laser (523 nm) whose wavelength lies in the tail of the intense absorption mainly ascribable to charge transfer in the V⁵⁺–O²⁻ bond. We propose a model in which a positive hole and an electron, generated by photo-oxidation of V⁴⁺ into V⁵⁺ due to two-step photon absorption at 532 nm, bring about a periodic arrangement of electric dipoles and/or periodic alternation of an internal dc electric field, resulting in optically induced SHG in the present tellurite glasses doped with V₂O₅.

The structure of GaSb layers grown epitaxially on GaAs(001) substrates has been studied by high-resolution x-ray diffraction. The samples with thicknesses between 95 and 2845 Å were grown in Oxford by metal–organic vapour phase epitaxy. The large lattice mismatch is largely relaxed by two intersecting regular arrays of 90^o dislocations along the [110] and [10 ] directions. The density of the dislocations changes with the thickness but the dislocations remain localized at the interface and both the GaSb layers and GaAs substrates are distorted. The spacing between the dislocations in both the [110] and [10 ] directions decreases while the relaxation increases as the layer thickness increases. We have found the spacing of the dislocations, relaxation and residual in-plane strain along the [110] and [10 ] directions and showed a strong asymmetry in dislocation spacing and relaxation between the two directions. This is because adjacent {110} planes in the zincblende structure are not equivalent as discussed by Abrahams and Buiocchi. Due to this asymmetry the structure of the GaSb layers is orthorhombic. Scattering from the thinner, ≲ 450 Å, layers shows that the GaSb grows in islands first and their size and distribution is different along the [110] and [10 ] directions. The 60^o dislocations in this system only appear in thick, ≳ 1200 Å, layers and their density is significantly higher along the [110] direction. The results show the power of high-resolution x-ray diffraction for studying non-destructively growth, relaxation and dislocation structure of thin films and for providing detailed quantitative information about the distortions caused by misfit dislocations in epitaxial layers.

Both double-electron excitation (DEE) and atomic XAFS (AXAFS) contributions are shown to be present in L₂₃ x-ray absorption spectra of Pt foil and Na₂Pt(OH)₆. In the Fourier transform of the spectra, the DEE feature appears at lower R values than the AXAFS feature. A background subtraction procedure is developed that leaves the DEE contribution in the background and the AXAFS in the oscillatory part of the spectrum. The separation of the DEE and the AXAFS contributions is possible only when using a cubic spline function routine in which the smoothing parameter can be continuously adjusted.

The local structure of Nb₃Ge intermetallic superconductor has been studied by Ge K-edge absorption spectroscopy. Extended x-ray absorption fine structure (EXAFS) experiments show two Ge–Nb distances. In addition to the crystallographic distance of ∼2.87 Å, there exists a second Ge–Nb distance, shorter than the first by ∼0.2 Å, assigned to a phase with short-range symmetry related to local displacements in the Nb–Nb chains. The x-ray absorption near-edge structure (XANES) spectrum has been simulated by full multiple-scattering calculations considering the local displacements determined by the EXAFS analysis. The XANES spectrum has been well reproduced by considering a cluster of 99 atoms within a radius of about 7 Å from the central Ge atom and introducing determined local displacements.

We present studies of the structural and magnetic properties of core–shell iron–iron oxide nanoparticles. α-Fe nanoparticles were fabricated by sputtering and subsequently covered with a protective nanocrystalline oxide shell consisting of either maghaemite (γ-Fe₂O₃) or partially oxidized magnetite (Fe₃O₄). We observed that the nanoparticles were stable against further oxidation, and Mössbauer spectroscopy at high applied magnetic fields and low temperatures revealed a stable form of partly oxidized magnetite. The nanocrystalline structure of the oxide shell results in strong canting of the spin structure in the oxide shell, which thereby modifies the magnetic properties of the core–shell nanoparticles.

The crystal structure of the anion-deficient perovskite Ca_2.5Sr_0.5GaMn₂O₈ has been studied at 290 and 5 K by neutron diffraction (290 K; space group Pcm2₁, a = 5.4294(1), b = 11.3722(3), c = 5.2983(1)Å). The vacant oxide sites order to create a structure in which perovskite bilayers consisting of MnO₆ octahedra are isolated from each other along [010] by a single layer of GaO₄ tetrahedra. At 5 K the material is antiferromagnetic with an ordered magnetic moment of 3.09(1) μ_B per Mn cation. Magnetic susceptibility measurements suggest that short-range magnetic ordering within the bilayers occurs above 200 K, and muon spin relaxation data show that the transition to long-range magnetic order occurs between 150 and 125 K. The resistivity of Ca_2.5Sr_0.5GaMn₂O₈ decreases by an order of magnitude at 125 K, and ∼50% magnetoresistance is seen in a field of 80 kOe at 110 K.

The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag₂CdI₄, Ag₂ZnI₄ and Ag₃SnI₅ have been investigated by powder diffraction and impedance spectroscopy techniques. ε-Ag₂CdI₄ adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2), 447(3) and 532(4) K, consistent with the sequence of transitions ε-Ag₂CdI ₄ → β-Ag₂CdI ₄ + β-AgI + CdI₂ → α-AgI + CdI₂ → α-Ag₂CdI₄. Hexagonal β-Ag₂CdI₄ is metastable at ambient temperature. The ambient-temperature β phase of Ag₂ZnI₄ is orthorhombic and the structures of β-Ag₂CdI₄ and β-Ag₂ZnI₄ can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ) phases of AgI. On heating Ag₂ZnI₄, there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag₂ZnI₄ into α-AgI + ZnI₂, followed by the appearance of superionic α-Ag₂ZnI₄ at the higher temperature. The hexagonal crystal structure of α-Ag₂ZnI₄ is a dynamically disordered counterpart to the β modification. Ag₃SnI₅ is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19Ω⁻¹ cm⁻¹ at 420 K) due to dynamic disorder of the Ag⁺ and Sn²⁺ within a cubic close packed I⁻ sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI–MI₂ compounds is discussed, with reference to recently published studies of Ag₄PbI₆ and Ag₂HgI₄ and new data for the temperature dependence of the ionic conductivity of the latter compound.

The interaction between two spheres with the use of a macroscopic self-consistent approach allowing one to rigorously describe multiple scattering in near-field optics is considered. The implementation of this approach leads to the concept of the effective susceptibility of the scattering system that explicitly relates the incident field to the self-consistent field. We demonstrate that the approach developed can be advantageously used to determine the self-consistent field in systems with simple geometry. Considering two interacting spherical scatterers we show that the self-consistent fields inside the spheres, calculated with and without taking into account multiple scattering, differ noticeably at a resonance.

Tin phosphate glasses, of general formula xSnO · (1 − x)P₂ O₅ (0.3 < x < 0.8), have been prepared by conventional melt-quench techniques and their structures studied using  ³¹P and  ¹¹⁹Sn nuclear magnetic resonance. The distribution of [PO₄] Qⁿ species changes with composition in accordance with the simple binary model, and the changes in chemical shift can be explained by the redistribution of electron charge from the P = O double bond. Sn(II) is found to occupy a highly asymmetric site, typical of a sterically active lone pair of electrons. The ¹¹⁹Sn parameters of the chemical shift tensor change systematically with x, reflecting the change in local environment from one where the next nearest neighbours are predominantly Q² phosphorus to one where they are predominantly Q⁰ phosphorus.

The heat capacity, nuclear quadrupole resonance (NQR) and x-ray diffraction of (Cs_1−xRb_x)₂ZnI₄ single crystals have been measured, for x = 0, 0.001, 0.005, 0.01, 0.025 and 0.05. The normal to incommensurate (N–Inc) phase transition at T_I, the incommensurate to commensurate (Inc–C) lock-in transition at T_L and the structural commensurate monoclinic to triclinic transition at T_LT, observed in the parent compound (x = 0), takes place for x = 0, 0.001, 0.005 and 0.01. For x = 0.025 only T_I and T_L are detected, while for x = 0.05 no transitions were observable. The values of T_I and T_L increase with x while T_LT decreases and disappears at the concentration x = 0.025. The effect of defects, besides modifying the transition temperatures, is that of broadening and lowering the heat capacity anomaly at the lock-in transition until its total quenching for x = 0.05. No observable hysteresis is detected in this transition. NQR and x-ray diffraction data show the Inc–C transition up to the highest concentration. We conclude that this phenomenology is caused by weak interaction of the incommensurate modulation with point defects even in the region close to the Inc–C transition.

A density functional plane-wave pseudopotential method is used to study various Σ = 5 (001) twist boundary models for strontium titanate. Results concerning the atom-level geometries and electronic structures are reported. The structures have varying SrO/TiO₂ ratios and their relative stabilities are discussed in terms of the SrO chemical potential. A twist boundary containing a Sr–O pair of vacancies is found to be exceptionally stable and have a low volume expansion and is a possible candidate for showing impurity segregation.

An in situ observation of the α-Fe₂O₃-to-Fe₃O₄ transformation has been performed using a dual-ion-beam accelerator interfaced with a transmission electron microscope (TEM). During the hydrogen-ion implantation of α-Fe₂O₃, transformation into the new phase (γ-Fe₂O₃ or Fe₃O₄) was observed. It was also found that the orientation relationship between α-Fe₂O₃ and the new phase (γ-Fe₂O₃ or Fe₃O₄) satisfies the Shoji–Nishiyama relationship, in agreement with previous experiments. It was also found that the nearest interatomic distance does not vary by the implantation until the re-stacked phase appears, although when the re-stacked phase is formed, the lattice expansion is observed in the transformed (re-stacked) phase. Judging from these results, we have concluded that the α-Fe₂O₃ to Fe₃O₄ transformation is induced during the hydrogen ion implantation of α-Fe₂O₃.

Using two-dimensional macroscopic Wigner islands consisting of electrostatically interacting charged balls with millimetric size, we have investigated the behaviour of a finite number of particles confined in a mesoscopic elliptic frame. In particular, we have experimentally checked the influence of the asymmetry of the confinement potential on the ground state configuration. The results obtained are compared with published numerical results.

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magnetic semiconductor quantum dot (QD), with the Coulomb interaction and the sp–d exchange interaction included. The MP energy decreases rapidly with increasing magnetic field at low magnetic field and saturates at high magnetic field for small QDs, and the dependences of the MP energy on magnetic field are quite different for different QD radii due to the different carrier-induced magnetic fields B_MP. The competition between the sp–d exchange interaction and the band gap shrinkage results in there being a maximum exhibited by the MP energy with increasing temperature. Our numerical results are in good agreement with experiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, Forchel A, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

We solve the problem of infinite-barrier lens-shaped quantum dots (QDs) in parabolic rotational coordinates exactly. The solutions are obtained directly using the Frobenius method and also by first transforming the separated differential equations into the Whittaker equation. We have obtained a new relation connecting the Bessel wavefunctions to the Whittaker functions. Results are given for both symmetrical and asymmetrical QDs. Studies of the energy spectra at constant volume were also performed; it is found that the shape dependence is very different from those found in previous studies of QDs with ellipsoidal and elliptic shapes.

The W27 centre has been characterized by means of electron spin resonance in natural diamond. The centre exhibits spin S = 1, a large spin–spin coupling constant D = 99 mT, and a complex hyperfine interaction structure interpreted as originating from interaction of an S = 1 electronic system with five nitrogen atoms: two of these sites are equivalent and are located near the S = 1 electrons; three others are nearly equivalent and more remote. The centre is suggested to include a divacancy, where one vacancy, bound to two nitrogen atoms and one carbon atom, has trapped an extra electron, while the second vacancy is bound to three substitutional nitrogen atoms.

Photoluminescence (PL) spectra of TlInSeS layered single crystals were investigated in the wavelength region 460–800 nm and in the temperature range 10–65 K. We observed one wide PL band centred at 584 nm (2.122 eV) at T = 10 K and an excitation intensity of 7.5 W cm⁻². We have also studied the variation of the PL intensity versus excitation laser intensity in the range from 0.023 to 7.5 W cm⁻². The red shift of this band with increasing temperature and blue shift with increasing laser excitation intensity was observed. The PL was found to be due to radiative transitions from the moderately deep donor level located at 0.243 eV below the bottom of the conduction band to the shallow acceptor level at 0.023 eV located above the top of the valence band. The proposed energy-level diagram permits us to interpret the recombination processes in TlInSeS layered single crystals.

Raman investigation of isostructural polycrystalline M^II Ga ₂ (S, Se)₄ compounds with M^II = Pb, Sr, Eu, Yb and Ca was performed. We observed that the effect of M^II atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the M^II cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates M^II Ga ₂S₄ exhibit nearly the same phonon energies: about 280 cm⁻¹ (∼35 meV) and 360 cm⁻¹ (∼45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic M^II Ga ₂Se₄ compounds present phonon energy at about 185 cm⁻¹ (∼23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.

Positron lifetime spectroscopy combined with a 50 V AC applied square-wave bias has been used to probe the electric field in the 2 μm region next to an Au/SI-GaAs contact. The electric fields spatially and time averaged over the reverse half-period at different temperatures (255–295 K) and pulsing periods (∼1 ms–10 ks) have been obtained using a numerical differentiation technique. It is found that within the temperature range studied, the electric field first increases as a function of time to a maximum of about 10 kV cm⁻¹ and then decreases, before finally saturating at the DC value of 7 kV cm⁻¹. The increase of the electric field with time is attributed as previously to the electron emission from the EL2 defect. The subsequent electric field decrease is associated with the onset of a thermally activated process with an energy barrier of 1.03 ± 0.15 eV. This process, which causes the neutralization of the space charge region, may be tentatively attributed to the recombination centres EL5 or EL6.