Table of contents

The first stage in the anneal of interstitial boron below room temperature in Czochralski-grown Si (Cz-Si) is the formation of the interstitial boron–oxygen (B_iO_i) defect. First principles modelling show that this defect has a structure similar to the interstitial carbon–oxygen complex. However, whereas the latter defect has been characterized by local vibrational mode infra-red spectroscopy, there is no information on the local vibrational modes of B_iO_i even though the defect is known to be a dominant interstitial boron defect in irradiated Cz-Si. Here, we carry out density functional calculations to determine its vibrational modes and respective isotope shifts, concluding that it possesses six local vibrational modes. As in the case of C_iO_i, we find an oxygen-related vibrational mode with frequency far below the 1136 cm⁻¹ of the oxygen interstitial, characteristic of the three-fold coordinated oxygen.

We analyse the magnetization, magnetic torque and susceptibility data of La_2−xSr_xCu^16,18O₄ and YBa₂^63,65Cu₃O_7−δ near T_c in terms of the universal 3D-XY scaling relations. It is shown that the isotope effect on T_c mirrors that on the anisotropy γ. Invoking the generic behaviour of γ, the doping dependence of the isotope effects on the critical properties, including T_c, correlation lengths and magnetic penetration depths, are traced back to a change of the mobile carrier concentration.

We succeeded in growing a high-quality single crystal of NpRhGa₅ by the Ga-flux method and observed the de Haas–van Alphen oscillation in the antiferromagnetic state. Four kinds of nearly cylindrical Fermi surfaces, which correspond to main Fermi surfaces, were clearly detected. These quasi-two-dimensional Fermi surfaces are formed in the flat antiferromagnetic Brillouin zone and are well explained on the basis of spin- and orbital-polarized LAPW energy band calculations. The cyclotron masses are moderately enhanced, ranging from 8.1 to 11.7 m₀, which are approximately four times larger than the corresponding band masses. This is the first case where the 5f-itinerant band model is applicable to a neptunium magnetic compound.

Diblock copolymers in the melt state form a variety of mesoscopically ordered morphologies, depending on their composition and on temperature. They can thus serve as a model system for studying the dynamics in different ordered morphologies with the disordered state as a reference state. In this review, the methods for studying the dynamics are compiled. The dynamics in the disordered, the lamellar, the gyroid, the hexagonal and the body-centred cubic phase are described with particular emphasis on the polymer self-diffusion as well as on the collective dynamics. The dimensionality of the morphology has a strong influence on the dynamics: the copolymer diffusion along the interfaces is anisotropic, in contrast to the isotropic disordered phase, and collective motions such as undulations of lamellar interfaces or fluctuation of the micellar distance in the body-centred cubic state become possible.

In this article some results regarding film growth considered as a stochastic process of dots are reviewed. The central concept of the theory described in the initial part of the article is the evaluation of the exclusion probability, i.e. the probability that no dots are found in a given region of the surface. This is reviewed to a certain extent for both correlated and uncorrelated dots and, moreover, for distinguishable classes of dots. This theoretical framework allows one to tackle the nucleation and growth of films ruled by diffusion of adspecies. In this specific instance the theory has been employed for computing the coverage dependent characteristic times for monomer capture from islands and for island collision, in the case of impingement and/or coalescence mechanisms. The ultimate aim is to model, by means of rate equations, the kinetics of film formation over the whole range of coverage.

The local atomic structure of CrPt₃(111) thin films grown by molecular beam epitaxy at different temperatures (400 and 850 °C) has been investigated by polarized x-ray absorption spectroscopy (XAS) at the chromium K edge. Previous x-ray diffraction and magnetic studies have revealed that films grown at 850 °C present the L1₂ chemically long range ordered structure and are ferrimagnetic with the occurrence of perpendicular magnetic anisotropy (PMA). In contrast, films grown at 400 °C, which are non-magnetic, do not show any indication of L1₂-type long range order. From the polarization dependence of XAS, different chemical orders in the film plane and perpendicular to it are reported for the non-magnetic film: Cr absorbers are surrounded by only Pt atoms in the film plane while out of the film plane Cr nearest neighbours are observed. The higher deposition temperature of the ferrimagnetic film (850 °C) leads to a more isotropic structure reducing the Cr–Cr out-of-plane correlations, as expected from the L1₂ ordered structure. These findings show that ferrimagnetism in CrPt₃ films is strongly sensitive to the Cr–Cr correlations. Furthermore, the remaining in-plane tensile deformation found in the ferrimagnetic film is too small for yielding the observed PMA. It is suggested that the strong PMA originates from the ferromagnetic coupling between Cr second-nearest neighbours lined up along the directions and which develop an orbital moment parallel to the spin moment as found by relativistic energy band calculations.

This study was undertaken to probe any thickness dependence of the magnetic ordering in Ho film samples of thickness 1000 monolayers (ML), 100 ML and 50 ML. The study was carried out on the antiferromagnetic helical magnetic phase in the temperature range . The magnetism was studied by x-ray magnetic scattering using the resonant process at the Ho L_III absorption edge, 8.071 keV. It was found that the magnetic modulation wavevector, τ, of the Ho helical structure changed systematically with temperature over the range . No evidence of a change in τ with changing layer thickness was seen. The value of the Néel temperature T_N decreased from 130 K for the 1000 ML sample to 110 K for the 50 ML sample. The critical exponent β of the transition between the helical to paramagnetic state is measured as β≈0.5 for the 1000 and 100 ML samples compared to a literature value of β = 0.39 for bulk. For the 50 ML film, β = 0.28 ± 0.05. The measured values of β are discussed in terms of strain and reduced dimensionality.

Sb_1−x(SiO₂)_x granular films were prepared by the co-sputtering method with the volume fraction of SiO₂, x, ranging from 0 (i.e. pure Sb) to about 30%. Systematic electronic transport studies, including resistivity, magnetoresistance, Hall effect and Seebeck effect, were carried out against the temperature, magnetic field and volume fraction x of SiO₂. With the gradual increase of the SiO₂ content, the mean grain size of the Sb decreases, and eventually the film becomes amorphous, as illustrated by the changes of the x-ray diffraction patterns. The temperature coefficient of resistivity also changes its sign from positive to negative, indicating a semimetal–semiconductor or insulator transition. Magnetoresistance studies using the weak localization theory revealed that the electron dephasing time follows approximately a T⁻² law. This behaviour indicates that the electron–phonon (e–ph) scattering still dominates the electron dephasing processes in these granular systems with a fair number of SiO₂ inclusions. The Hall coefficient decreases monotonically with temperature and with the volume fraction of SiO₂. The giant Hall effect is absent in these granular films. Finally, an interesting but rather complicated behaviour of the Seebeck coefficient versus temperature was observed when the volume content of the SiO₂ exceeded 18%.

We present a many-body computational technique for simulating interacting electrons in non-parabolic semiconductor bands. The technique uses an imaginary time propagator for a non-parabolic electron band that is described by an energy dependent effective mass. This derivation exploits a mathematical analogy between the kinetic energy with effective mass corrected to first order in energy and the relativistic kinetic energy. The propagator can be used in ground state and finite temperature quantum Monte Carlo (QMC) algorithms. We give a demonstration of this path integral QMC technique applied to interacting electrons in a self-assembled InGaAs quantum dot.

We have investigated several of the most stable cluster assembled phases of X₈C₁₂ and YX₇C₁₂ (X, Y =  B, N, Si) using pseudopotential plane-wave density functional theory. We have found that many of these were stable, conductive and retained the integrity of the cluster cage, along with some of its bonding characteristics. A large range of metastable molecular solids can be produced in this way. Some of these new covalent conductors and one semiconductor were considered good candidates for high T_c superconductivity and may constitute an improvement over the recently discovered fcc-C₂₂ superconducting molecular solid.

The crystal structure of A-site deficient La₄Mg₃W₃O₁₈ perovskite has been solved by x-ray powder diffraction in combination with group theoretical analysis. Above 700 K, the crystal structure is orthorhombic (space group Ibam; 2a_p × 4a_p × 2a_p type superstructure) and presents a sequence of [LaO]–[Mg_1/2W_1/2O₂]–[La_1/3O]^'–[Mg_1/2W_1/2O₂]–[LaO]– [Mg_1/2W_1/2O₂]–[La_1/3O]^''–[Mg_1/2W_1/2O₂] layers stacked along the b axis. The lanthanum ions and the vacancies in the [La_1/3O]^' and [La_1/3O]^'' layers are ordered and form rows along the c axis. A half-period shift along the a direction between these layers leads to a quadrupling of the primitive perovskite unit cell in the b direction. The ordering of the vacancies in the lanthanum poor layers is connected with the ionic ordering between Mg²⁺ and W⁶⁺ in the neighbouring [Mg_1/2W_1/2O₂] blocks. Around 700 K, due to an anti-phase rotation of the octahedra, a continuous phase transition mediated by the Γ₂⁺ irreducible representation from orthorhombic (Ibam, a⁰b⁰c⁰) to monoclinic (C 2/m, a⁰b⁰c⁻) symmetry takes place.

The occupancy of C₂ and S₆ sites by Eu³⁺ ions in nanocrystalline powders and microcrystalline ceramics of Lu₂O₃ prepared from these powders was studied by Mössbauer spectroscopy. It was proved that in nanopowders prepared by vigorous combustion syntheses the occupancy of the C₂ and S₆ sites by Eu³⁺ is almost random, with only a very limited preference for the former. For ceramic specimens formed at 1750 °C within a few hours, it was found that Eu³⁺ ions strongly prefer to enter the Lu₂O₃ host at the C₂ site. We concluded that thermodynamically the non-centrosymmetric site C₂ is preferred to the S₆ site by Eu³⁺. However, if the formation of crystallites is very fast, like in combustion syntheses, the Eu³⁺ ions are entrapped into the two sites nearly randomly.

A hybrid reverse Monte Carlo (HRMC) algorithm, which incorporates both experimental and energy based constraints, is applied to investigate the microstructure of two disordered carbons of vastly different densities and bonding. We have developed a novel liquid quench procedure which in combination with the HRMC algorithm accurately describes the structure of these solids. Atomic networks generated by this approach are consistent with experimental and ab initio results and the method has been shown to overcome common difficulties associated with alternative approaches for modelling these complex systems. This procedure produces realistic large scale atomic structures which give a detailed picture of the structure of these solids.

The experimental structure factors of evaporated and ion-implanted amorphous silicon have been modelled by reverse Monte Carlo modelling. A detailed comparison, in terms of the pair correlation function and the distribution (of the cosines) of bond angles, is reported here for the two materials. It is found that for an acceptable reproduction of the measured structure factors the evaporated models must contain more 'small' bond angles, of the order of 75°, than their necessary abundance in the implanted models.

The results of an x-ray diffraction energy-dispersive study on solid Cd under high-pressure conditions at room temperature are presented. The trends of the lattice parameters and of the axial ratio c/a as a function of pressure show that an anomalous slope change, previously discussed in connection with an electronic topological transition (ETT), is still observed using a different pressure transmitting medium for relative volume compression V/V₀∼0.86. A detailed study of the peak positions shows that some Bragg reflections, in particular those related to the length of the c axis, are shifted with respect to the calculated positions for a typical hcp structure. This anomalous behaviour, observed for pressures above 4 GPa, is briefly discussed in terms of possible oriented lattice strains and non-hydrostatic effects.

We report the results of high-pressure Raman scattering studies of the cubic and monoclinic polymorphs of tetracyanoethylene (TCNE). The evolution of the Raman spectrum at high pressures suggests that the cubic form is stable up to about 8 GPa. Subsequent pressurization leads to a gradual loss of transparency, and the sample becomes opaque to visible light above 14 GPa. In the monoclinic samples, qualitative changes are observed in the Raman spectrum above 3.6 GPa which indicate a subtle phase transition around this pressure. These changes are reversible when the pressure is reduced from peak values of about 4.5 GPa. At still higher pressures, the sample progressively becomes black, similar to what is observed in cubic TCNE. The Raman spectrum of the sample above 7 GPa is indicative of polymerization of TCNE. The spectrum of the pressure cycled opaque phase shows broad features characteristic of an amorphous phase, which is understood as being due to random cross-linking of TCNE in the pressure-reducing cycle.

The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystal have been measured simultaneously for various electric fields with a conduction calorimeter. The specific heat presents a strong anomaly, but these simultaneous measurements allow us to evaluate the latent heat accurately. The latent heat decreases with field, and the value of the critical electric field—that where the latent heat disappears—is estimated to be (0.44 ± 0.03) kV cm⁻¹. Incidentally, we have measured simultaneously the dielectric permittivity, which suggests that latent heat is developed as domains are growing.

We report a study on the roughening process in wrinkly metal film (aluminium) for thicknesses ranging from 35 to 1000 nm. The spatial and temporal scaling behaviours have been investigated by using atomic force microscopy. We show that fast diffusion is a part of the buckling process on a viscoelastic substrate due to Grinfeld-type instability. Power spectral density analysis reveals that the roughness exponent α is ∼0.85 for all thicknesses. This value is consistent with the fact that fast diffusion is the underlying process. The films exhibit slow temporal evolution, i.e. W∼t⁰ (β = 0.15 ± 0.02,Z = 5.66 ± 0.5), as the film thickness increases. The wavelength or correlation length also changes as ξ = t^{0.19 ± 0.04} (Z = 5.26 ± 1) with the thickness. Deposition through a 400 µm × 1 cm (35 nm thickness) window shows a very organized wrinkled pattern with chain-like island attachment parallel to a surface with a smaller length scale (400 µm). We propose a model explaining why deposited atoms would move parallel to a surface with a shorter length scale to create ordered chain-like structures.

We investigate the Peierls transition in the one-dimensional Peierls–Hubbard model at half-filling in the adiabatic approximation for the lattice. Depending on the value of the electron–lattice coupling constant g the equilibrium dimerization can be either enhanced or suppressed by the Hubbard interaction U. Applying second-order perturbation theory we determine the critical value g_c = 0.689 348 below which the Hubbard interaction enhances the dimerization.

We present high-resolution valence band and core level spectra of silver for photoelectron kinetic energies up to 8 keV. At these kinetic energies we estimate a surface contribution of less than 3%. Taking advantage of the favourable sp/d relative cross-sections, a comparison with the calculated density of states is presented. We observe an increasing photoemission intensity when approaching the Fermi level, which we assign to a free-electron-like character in the 5p-band, whereas the principal s-like contribution is located at the bottom of the d-band. The difference between measured and calculated values of the sp/d cross-section ratio is discussed.

We present a small polaron hopping model for interpreting the strong temperature (T) dependence of the electrical conductivity, σ, observed at high (h) temperatures along DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. Percolation-theoretical considerations lead to analytical expressions for the high temperature multiphonon-assisted small polaron hopping conductivity, the hopping distance and their temperature dependence. The experimental data for lambda phage DNA (λ-DNA) and poly(dA)–poly(dT) DNA follow nicely the theoretically predicted behaviour (lnσ^h∼T^−2/3). Moreover, our model leads to realistic values of the maximum hopping distances, supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbours of the DNA molecular 'wire'. The low temperature case is also investigated.

A new effect called 'charge flipping of spin carriers' is proposed that can be realized in π-conjugated polymeric molecules, where the charge of a spin carrier can be reversed by photoexcitation. The physical mechanism of electron spin resonance, namely spin flipping of charge carriers, is just the complementary effect to charge flipping of spin carriers. The latter phenomenon shows promise for the design of organic optical spin valves.

The Hall effect and electrical resistivity are measured on slightly Cu-rich epitaxial CuGaSe₂ films in the temperature range 15–300 K. The temperature dependence of the Hall coefficient is described by the two-band model with holes in both the valence and defect bands, as manifested by a maximum in the Hall coefficient. The model can be used to separate holes in the valence band and the defect band, allowing the determination of the activation energies and concentrations of the acceptors, and the concentration of the compensating donors.

We have developed a method for simulating multiple electron scattering in a vacuum barrier using real-space single-electron wavefunctions for the separate surfaces. The Green functions in the vacuum barriers are calculated to first order in the Dyson series. We find that the zero-order current is equal to the usual Bardeen approach only in the limit of zero bias and derive the modifications in the finite bias regime. We also derive a first-principles formulation for the energy of interaction between the two surfaces, and show that it is proportional to the tunnelling current. With this method the tunnelling current can in principle be computed to any order in the Dyson expansion.

The magnetic-field dependence of many-body states in quantum dots can be tailored by controlling the mixing of various angular momenta. In lateral quantum dots—defined electrostatically in a two-dimensional electron gas—this mixing can be accomplished by introducing anisotropies in the confinement potential, thereby explicitly breaking rotational symmetry. Mixing can be severe enough to violate Hund's rules, even at zero magnetic field. We illustrate the principle through calculations of states and spectra of four-electron droplets (p shell) with long-range Coulomb repulsions and confined in anisotropic potentials. Our results show that the Hilbert space in these nanostructures can be engineered to the particular application domain.

The time dependence of flux patterns obtained on an untwinned Y Ba₂Cu₃O_7−δ single crystal showing the 'meandering instability' is observed at T = 65 K using magneto-optical imaging. When applying a reversed external field to a remanent state, along the front of invading antiflux, macrovortices or droplets of flux are formed, and eventually separate from the flux front in a spiral-like motion. The time-dependent behaviour of these macrovortices is investigated in detail.

We report on anisotropic magnetoresistance (AMR) and Hall effect measurements along the and [001] directions in a (110)-oriented La_2/3Ca_1/3MnO₃ thin film. While the electrical resistivity and the ordinary Hall coefficient are smaller for I along [001], evidencing some anisotropy of the Fermi surface, both the AMR and the anomalous Hall coefficient are larger when the current is applied along . Since these two phenomena originate from spin–orbit coupling effects, we state that our results support an anisotropy of the spin–orbit interaction in manganites. The possible origin of this anisotropy is discussed.

We propose a new model for the understanding of the magnetic properties of CuFeO₂, which differs significantly from the generally accepted two-dimensional Ising model. We show that a Heisenberg model with a relatively weak anisotropy gives a much better description of all the magnetic data available for CuFeO₂. The model is self-consistent; it allows one to determine for the first time a set of parameters for the exchange interactions and magnetic anisotropy in this frustrated magnetic system. The model is backed up by single-crystal measurements of susceptibility, magnetization and specific heat as a function of magnetic field and temperature.

We report the results of antiferromagnetic resonance (AFMR) measurements conducted on KCuF₃ at various frequencies from 3.8 to 10.6 GHz at 4.2 K. The resonance linewidth is first found to depend on the frequency, i.e., the lower the frequency the greater the resonance linewidth, no matter whether the AFMR field is forced on the easy axis or uneasy axis. Moreover, a linewidth peak seems to exist for at about 4 GHz. Based on the model of Yamada and Kato (1994 J. Phys. Soc. Japan63 289) and considering the Laudau–Lifshitz damping term, the result of numerical calculation for the resonance linewidth is in good agreement with the data of AFMR experiments.

An electron paramagnetic resonance (EPR) study of the Mn concentration and temperature in the range of 4 K ≤T≤300 K for the dilute semimagnetic semiconductor Zn_1−xMn_xIn₂Se₄ is reported. The EPR spectra were measured for the concentrations 0.01<x<1.0. For low Mn content the spectrum contains multiple resonances at g = 2, g∼3 and g∼5. The behaviour of the g = 2 signal linewidth ΔH_pp as a function of temperature was studied and it was related to the fitting parameters at the critical temperature and the Curie point. The j parameter of the modified expression of Silva was obtained and its role is shown for the particular transition of this system.

The EPR spectra of BaTiO₃+0.04 BaO +0.01 Cr₂O₃ ceramics are studied in three frequency bands (9, 34 and 94 GHz). Three different, axially symmetric EPR spectra assigned to Cr³⁺ ions substituted at Ti sites corresponding to the different distorted octahedra in tetragonal and hexagonal crystal surroundings are observed at room temperature. The temperature dependence of the EPR spectra allowed the unambiguous assignment of the chromium ions to the lattice site in tetragonal and hexagonal modifications. The concentration measurements revealed that only 30% of the chromium is incorporated in the grains as Cr³⁺ ions whereas the remnant is in the EPR-silent tetravalent charge state or in Cr-rich secondary phases segregated at grain boundaries, pores and triple points. The intensity ratio of the two Cr³⁺ spectra in hexagonal BaTiO₃ depends on the preparation conditions.

LaYbO₃ is a highly distorted perovskite with orthorhombic structure. However, its exact space group has been a matter of debate. Both the polar Pna 2₁ and the centrosymmetric Pnma space groups, with Z = 4, have been proposed to describe its structure at room temperature. In this work we present optical spectroscopic investigations by Raman scattering and infrared reflectivity of LaYbO₃ ceramics sintered at 1600 °C. The results allowed us to propose a new centrosymmetric Cmcm space group for this compound, and to show which phonons give the main contributions to its dielectric constant and quality factor at microwave frequencies.

The optical spectra of the BaY₂F₈:Dy³⁺ laser crystal have been investigated in the 5000–30 000 cm⁻¹ range. The Judd–Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra. The calculated radiative lifetime of the ⁴F_9/2 state is 1.48 ms. Decay curves of the visible emission have been measured as a function of the temperature for two different Dy³⁺ concentrations (0.5 and 4.4%). In the case of the diluted crystal the emission profiles are single exponential with decay times consistent with the radiative lifetime. The decay curves of the concentrated crystal are not exponential and they obey the Inokuti–Hirayama model for energy transfer for an electric dipole–dipole interaction in the absence of diffusion among the donors. The emission cross section at 575 nm has been estimated using the integral β–τ method in order to assess the potentialities of this compound as a solid state laser material in the yellow region.

This paper investigates thin films of titanium aluminium nitride grown using a stoichiometric metallic cathode in a cathodic arc vapour deposition system with plasma immersion ion implantation. The effect of various deposition conditions on the stress, microstructure and composition are evaluated. In general, the films were found to be both titanium and nitrogen rich. The application of voltages of 2 kV and greater was found to dramatically reduce the stress in the films. This stress reduction was found to be less pronounced at lower nitrogen flow rates due to a reduction in nitrogen implantation. The microstructure of the films was found to be cubic and at high voltages exhibited preferred orientation with {200} planes parallel to the surface of the film. We employ density functional theory to calculate the {100} and {111} surface energies and elastic constants for cubic titanium aluminium nitride. Using these calculated values, we explain this preferred orientation using a model which minimizes both surface energy and bulk strain energy.