Table of contents

Most materials phenomena are manifestations of processes that are operative over a vast range of length and time scales. A complete understanding of the behaviour of materials thereby requires theoretical and computational tools that span the atomic-scale detail of first-principles methods and the more coarse-grained description provided by continuum equations. Recent efforts have focused on combining traditional methodologies—density functional theory, molecular dynamics, Monte Carlo methods and continuum descriptions—within a unified multiscale framework. This review covers the techniques that have been developed to model various aspects of materials behaviour with the ultimate aim of systematically coupling the atomistic to the continuum descriptions. The approaches described typically have been motivated by particular applications but can often be applied in wider contexts. The self-assembly of quantum dot ensembles will be used as a case study for the issues that arise and the methods used for all nanostructures. Although quantum dots can be obtained with all the standard growth methods and for a variety of material systems, their appearance is a quite selective process, involving the competition between equilibrium and kinetic effects, and the interplay between atomistic and long-range interactions. Most theoretical models have addressed particular aspects of the ordering kinetics of quantum dot ensembles, with far fewer attempts at a comprehensive synthesis of this inherently multiscale phenomenon. We conclude with an assessment of the current status of multiscale modelling strategies and highlight the main outstanding issues.

Bound electron–hole pairs—excitons—are light Bose particles with a mass comparable to or smaller than that of the free electron. Since the quantum degeneracy temperature scales inversely with the mass, it is anticipated that Bose–Einstein condensation of an exciton gas can be achieved at temperatures of about 1 K, orders of magnitude larger than the micro-Kelvin temperatures employed in atomic condensation. High quantum degeneracy temperatures and the possibility to control exciton density by laser photoexcitation make cold excitons a model system for studies of collective states and many-body phenomena in a system of cold bosons. Experimentally, an exciton temperature well below 1 K is achieved in a gas of indirect excitons in coupled quantum-well semiconductor heterostructures. Here, we overview phenomena in the cold exciton gases: condensation, pattern formation, and macroscopically ordered exciton states.

The objective of this article is to review, in relation to photovoltaic applications, the current status of crystalline silicon ribbon technologies as an alternative to technologies based on wafers originating from ingots. Increased wafer demand, the foreseeable silicon feedstock shortage, and the need for a substantial module cost reduction are the main issues that must be faced in the booming photovoltaic market. Ribbon technologies make excellent use of silicon, as wafers are crystallized directly from the melt at the desired thickness and no kerf losses occur. Therefore, they offer a high potential for significantly reducing photovoltaic electricity costs as compared to technology based on wafers cut from ingots. However, the defect structure present in the ribbon silicon wafers can limit material quality and cell efficiency.

We will review the most successful of the ribbon techniques already used in large scale production or currently in the pilot demonstration phase, with special emphasis on the defects incorporated during crystal growth. Because of the inhomogeneous distribution of defects, mapped characterization techniques have to be applied. Al and P gettering studies give an insight into the complex interaction of defects in the multicrystalline materials as the gettering efficiency is influenced by the state of the chemical bonding of the metal atoms. The most important technique for improvement of carrier lifetimes is hydrogenation, whose kinetics are strongly influenced by oxygen and carbon concentrations present in the material. The best cell efficiencies for laboratory-type (17%–18%; cell area: 4 cm²) as well as industrial-type (15%–16%; cell area: ) ribbon silicon solar cells are in the same range as for standard wafers cut from ingots. A substantial cost reduction therefore seems achievable, although the most promising techniques need to be improved.

The specific heat of the quasi-one-dimensional organic conductor pyrene hexafluoroarsenate ((Py)₇(Py)₄(AsF₆)₄·4CH₂Cl₂) was measured between 10 and 250 K using the method of continuous heating. The contribution of the lattice to the specific heat was modelled using the Debye model and Einstein terms. The variation of the specific heat at the structural phase transition that is accompanied by 60° rotation of one of the stacked pyrene molecules could be reproduced well using a cooperation model including the formation of domains, and the surprisingly high transition entropy was explained qualitatively.

The effect of biaxial strain on the solubility of the common donor arsenic and acceptor boron is calculated using spin-polarized local density functional theory. The change in solubility with strain is considered in terms of contributions from the change in total energy and Fermi energy with strain. The solubility of boron is found to be enhanced by compressive biaxial strain due to a reduction in the total energy of the small substitutional impurity and an increase in the Fermi energy for compressive strain. The solubility of arsenic is shown to be enhanced by tensile strain and this is due entirely to the change in Fermi energy. For boron as well as arsenic the change in Fermi energy with strain is shown to make the dominant contribution.

Molecular dynamics simulations are performed at 6543 and 3310 K to investigate how the viscosity and self-diffusion coefficient scale with system size in the liquid BKS silica system. We find that at high temperature the finite-size effect on shear viscosity is negligible. However, the size effect on the diffusion coefficient still exists and scales linearly with 1/N^1/3, where N is the total number of particles in the system. At low temperature, the size effect on the viscosity becomes stronger than that on diffusion, and the logarithm of the viscosity and the diffusion coefficient scale linearly with 1/N. These results are consistent with previous theoretical developments, and demonstrate that the finite-size effect should be considered in both high- and low-temperature molecular dynamics simulations of liquid silica.

The results of photoemission study of icosahedral single-grain ZnMgEr quasicrystals are presented. Synchrotron radiation photoemission measurements were performed on in situ cleaved samples at 10⁻¹⁰ mbar pressure and low, 140–150 K, temperature. The valence band photoemission spectra measured reveal a simple-metal type valence band of i-ZnMgEr with a distinct Fermi edge cutoff and a spectral feature at 0.7 eV below ε_F. Analysis of the PE spectra shows that the spectral feature observed corresponds to the van Hove singularities in the density of states, which are due to intersections of the Fermi surface with the 222100 and 311111 Bragg planes.

We demonstrate how the Rashba spin–orbit coupling in semiconductor heterostructures can produce and control a spin-polarized current without ferromagnetic leads. The key idea is to use spin-double refraction of an electronic beam with a nonzero incidence angle. A region where the spin–orbit coupling is present separates the source and the drain without spin–orbit coupling. We show how the transmission and the beam spin polarization critically depend on the incidence angle. The transmission halves when the incidence angle is greater than a limit angle and a significant spin polarization appears. On increasing the spin–orbit coupling one can obtain the modulation of the intensity and of the spin polarization of the output electronic current when the input current is unpolarized. Our analysis shows the possibility of realizing a spin-field-effect transistor based on the propagation of only one mode with the region with spin–orbit coupling, whereas the original Datta and Das device (1990 Appl. Phys. Lett.56 665) uses the spin precession that originates from the interference between two modes with orthogonal spin.

The efficient use of ferroelectric thin films in radio-frequency agile devices faces several limitations. One of them is imposed by the dielectric losses which are usually above 1%, i.e. above the threshold as set by the electronic industry. Following the same route as for bulk ceramics, we have processed composite stacks made of BST/SiO₂ multilayers using radio-frequency magnetron sputtering. Doing so, we were able to repeatedly achieve dielectric losses well below 0.5% while keeping a high dielectric susceptibility and a suitable tunability. All of these improvements have been observed at low frequencies (f<10 MHz) and the transfer to the targeted frequency range (f>1 GHz) is currently in progress.

The electronic structure of CeTe₂ has been investigated by performing high-resolution photoemission spectroscopy (PES) measurements on high-quality stoichiometric single crystals of CeTe₂. The Ce 3d XAS measurement indicates that the Ce ions are trivalent. The Ce 4f PES spectrum reveals two-peak structures with the Ce 4f bulk hybridization peak located around 0.8 eV below E_F. The high-resolution, high-energy, bulk-sensitive PES study of CeTe₂ provides evidence for a metallic density of states near E_F. The carriers near E_F consist mainly of Ce 5d and Te(1) 5p states, while the contribution from the Ce 4f and Te(2) 5p states is negligible. This finding agrees with the CDW instability in the Te(1) sheets.

Two mesoscopic SQUID rings which are far from each other are considered. A source of two-mode nonclassical microwaves irradiates the two rings with correlated photons. The Josephson currents are in this case quantum mechanical operators, and their expectation values with respect to the density matrix of the microwaves yield the experimentally observed currents. Classically correlated (separable) and quantum mechanically correlated (entangled) microwaves are considered, and their effect on the Josephson currents is quantified. Results for two different examples that involve microwaves in number states and coherent states are derived. It is shown that the quantum statistics of the tunnelling electron pairs through the Josephson junctions in the two rings are correlated.

The as-grown magnetic domain patterns of epitaxial single-crystalline Co/FeMn bilayers and Co/Cu/FeMn trilayers were investigated by magnetic circular dichroism domain imaging using a photoelectron emission microscope. Small domains were observed when Co was deposited directly on top of antiferromagnetic FeMn films. On inserting a Cu spacer layer between the ferromagnetic Co layer and the antiferromagnetic FeMn layer, the as-grown domain size increases continuously with increasing Cu layer thickness, which is attributed to the decrease of the interlayer exchange coupling between the Co and FeMn layers. Domain images of the Co layer acquired after applying different external magnetic fields confirm the relation between as-grown domain size and interface coupling to the antiferromagnetic FeMn layer across the non-magnetic Cu spacer layer. It is found that a field of 44 Oe is sufficient to annihilate most small domains in the area where Co and FeMn are coupled through the Cu wedge, while it has no effect on the domain configuration of a Co/FeMn bilayer. By analysing the dependence of the average domain size of the Co layer on the Cu thickness in terms of interface coupling between the Co and FeMn layers across the Cu layer, the apparent estimated coupling energy was found to be different for different Co thickness. This is ascribed to kinetic barriers hindering the formation of larger domains from smaller ones.

Here we present a sensitive spectroscopic method, which allows monitoring of the kinetics of creation of intermolecular bonds during the gelation process. We report the time evolution of the fluorescence spectra and the ultrasound velocity observed in two gelling materials with different kinds of intermolecular bonds, tetramethoxisilane solution and aqueous gelatin solution, during the sol–gel transition. Our results provide experimental evidence of a universal power law, resulting from the percolation theory, which connects the fluorescence yield and the ultrasound velocity. The values of the universal critical exponent a present in this law, found by us experimentally for all materials investigated, appear to be close to the value a = 0.625 predicted by percolation theory.

Pure and Nd-modified Bi₄Ti₃O₁₂ ceramics are prepared using the conventional solid state reaction method and their dielectric properties and mechanical properties are investigated. This shows that the activation energy of oxygen vacancies is enhanced whereas the concentration of oxygen vacancies is reduced when Bi³⁺ ions are partially substituted by Nd³⁺ ions. The Cole–Cole fitting to the dielectric loss reveals a strong correlation among oxygen vacancies, which is found to be proportional to the concentration of oxygen vacancies. The strong correlation reduces the activation energy of oxygen vacancies efficiently. Therefore, we conclude that the enhancement of activation energy originates from the diluted oxygen vacancy concentration and that the diluted oxygen vacancy concentration is the basic aspect of the excellent fatigue resistance in Nd-modified Bi₄Ti₃O₁₂ materials.

The magnetic properties of four compounds in the series YBaCo_4−xZn_xO₇ (x = 0, 1, 2, 3) were investigated. For all compositions magnetic transitions were observed (T_f) over the temperature range 66–3 K observed with ac susceptibility and dc magnetometry. Futhermore, all ac measurements proved to be frequency-dependent: T_f increases with an increase in frequency. The real part of the magnetic susceptibility (χ') was, in all four cases, accompanied by an energy loss in the magnetic coupling, indicated as contributions to the imaginary part (χ''). The maximum χ'' appeared just below the maximum χ'. Using the Arrhenius law, the Vogel–Fulcher law and the power law, it was possible to conclude that the compounds should be defined as spin-glass-like materials. The dc magnetizations clearly show differences between field-cooled and zero-field-cooled measurements. None of the compounds exhibited any metamagnetic property and, using a new data analysis method, a possible saturation field could be calculated for YBaCo₄O₇. Relaxation measurements on YBaCo₃ZnO₇ indicate that the system has no ageing effects. The magnetic properties can be described as having two connected magnetic substructures represented as dimensionalities: axial (1D) and in-plane (2D). This was concluded by comparing the magnetic properties with structural details.

Results of frequency-dependent ferromagnetic resonance (FMR) measurements are presented for thin Fe–Zr–N nanocrystalline films with random magnetocrystalline anisotropy and induced uniaxial anisotropy. The study is done by changing the composition, the grain size and the magnitude of the induced anisotropy. We show that the magnetization dynamics is strongly influenced by the structural parameters of our samples. Although the frequency-dependent spectra can be analysed on the basis of the Landau–Lifshitz equation, an extra field H_shift has to be introduced in order to have agreement between the experiment and calculations. This extra field does not depend on the saturation magnetization and increases significantly when the grain size decreases from 10 to 2 nm. In addition, we observe a nonlinear decrease of the frequency linewidth with the applied dc field. After discussing various existing models we conclude that H_shift originates from variations in the magnitudeof the magnetization, related with the nanocrystalline structure.

Intercalated Ni_xTiSe₂ compounds with Ni concentration up to x = 0.5 have been investigated using x-ray diffraction, magnetic susceptibility and specific heat measurements. A set of peculiarities caused by the intercalation, such as a significant decrease of the gap between Se–Ti–Se trilayers and depletion of the low-frequency phonons due to the stiffening of the lattice as well as an enhancement of the density of electronic states at low Ni content (x < 0.33), is attributed to hybridization of the Ni 3d states with TiSe₂ bands and formation of covalent-like links between Se–Ti–Se trilayers via the inserted Ni ions. Ni_xTiSe₂ compounds with intercalant content up to x = 0.5 show a paramagnetic behaviour unlike M_xTiSe₂ compounds intercalated by other 3d metals (M = Cr, Mn, Fe, Co). The Ni magnetic moment in Ni_xTiSe₂ is suggested to be of an itinerant nature. A pronounced correlation between the change of the lattice parameter c₀ and the value of the effective magnetic moment, which is observed in Ni_xTiSe₂ as well in M_xTiSe₂ systems intercalated by other 3d metals, indicates that the magnetic moment on the inserted atom is controlled by the hybridization degree of M 3d states with TiSe₂ bands.

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