A M Stoneham

Making tracks: electronic excitation roles in forming swift heavy ion tracks

Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10xa0keVxa0nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T(c) oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self-trapped exciton model of Itoh and Stoneham (1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self-trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser-induced time-resolved x-ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials.

Small polarons in real crystals: concepts and problems

Much of small polaron theory is based on highly idealized models, often essentially a continuum description with a single vibrational frequency. These models ignore much of the wealth of experimental data, which find interpretation in many atomistic simulations. The authors review here a range of properties of small polarons in real, rather than model, systems. The phenomena fall into three main classes: (i) the mechanisms and dynamics of self-trapping of polarons; (ii) static properties-the relative energies of large and small polarons, the optical transitions expected, their effect on positions of other ions and on lattice vibrations, their population in thermal equilibrium, and so on; (iii) small polaron hopping and diffusion. The authors discuss the key concepts and methods of calculation of polarons, and explore the properties of self-trapped-holes and excitons in ionic crystals, and those of an excess electron in liquid water.

Ionic and electronic processes in quartz:Mechanisms of thermoluminescence and optically stimulated luminescence

We suggest a model of the atomic and electronic processes responsible for the so-called 110 and 325u200a°C thermoluminescence (TL) peaks, including predose behavior, and for the room temperature optically stimulated luminescence (OSL) of quartz. Our model is based on defects and defect processes typical of those known from many previous studies of quartz. It explains the experimental observations that the two TL peaks and OSL are correlated with respect to the effects of thermal annealing and photoexcitation after irradiation. The model indicates that the energy for the two TL peaks and OSL is all stored by the same defect pairs. These defect pairs comprise [AlO4]− and [X/M+]+ generated by a radiolytic reaction [AlO4/M+]0→[AlO4]−+M+. Here [AlO4]− is an Al impurity center substituting a Si atom, M+ is an alkali ion, [X/M+] is M+ stabilized by a defect denoted by X and [AlO4/M+]0 is an [AlO4]− center charge compensated by M+. Even though the 110 and 325u200a°C TL peaks and OSL arise from the same defect pairs, they...

Optically driven silicon-based quantum gates with potential for high-temperature operation

We propose a new approach to constructing gates for quantum information processing, exploiting the properties of impurities in silicon. Quantum information, embodied in electron spins bound to deep donors, is coupled via optically induced electronic excitation. Gates are manipulated by magnetic fields and optical light pulses; individual gates are addressed by exploiting spatial and spectroscopic selectivity. Such quantum gates do not rely on small energy scales for operation, so might function at or near room temperature. We show the scheme can produce the classes of gates necessary to construct a universal quantum computer.

The structure and motion of the self-interstitial in diamond

We have made self-consistent semi-empirical molecular orbital calculations for various possible self-interstitial geometries in diamond, both with and without lattice distortion. Total energies are obtained, not merely the sum of one-electron eigenvalues. The results show that the (100) split interstitial has the lowest formation energy, not the cubic, hexagonal or bond-centred forms favour previously. n nThe nature of the interstitial does not support the local heating model of enhanced diffusion in the presence of recombination or ionisation. A Bourgoin-Corbett mechanism involving negative hexagonal and neutral split interstitials is possible, but the apparent stability of the negative hexagonal interstitial may be an artefact of the calculation. We suggest a local excitation model is appropriate in fourfold-coordinated semiconductors.

Understanding electron flow in conducting polymer films : injection, mobility, recombination and mesostructure

We survey the current state of models for electronic processes in conducting polymer devices, especially light-emitting diodes. We pay special attention to several processes that have been somewhat neglected in the previous literature: charge injection from electrodes into a polymer sample, mobility of charge-or energy-carrying defects within a single molecule and (more briefly) transfer of carriers between molecules and the interaction between the charge transport and the mesostructure of the polymer. Within all these areas substantial progress has been made in recent years in elucidating the important physics, but further progress is needed to make quantitative contact with experiment.

Stability of electronic states of the vacancy in diamond

The vacancy in diamond is a fundamental defect which has been studied theoretically and experimentally for forty years. However, although early theories (Coulson C A and Kearsley M J 1957 Proc. R. Soc. A 241 433) were extremely successful in explaining the nature of the ground state of the neutral defect and the Jahn - Teller distortion expected (Lannoo M and Stoneham A M 1968 J. Phys. Chem. Solids 29 1987), there are still several questions which have not been answered satisfactorily. In particular, the many-electron effects and configuration interaction are vital. They determine not only the order of electronic levels in the vacancy, but also the best-known optical transition, GR1, which cannot be expressed in terms of one-electron levels alone. We bring together much of the detailed recent experimental data on the different charge states and excited states of the vacancy to build up a simple empirical model of the defect. We show that the stability of the states and their photoconductivity, or lack of it, can be reproduced. We can predict that other states of the neutral vacancy, observable by EPR, lie very close above the ground state, and another high-energy optical transition might be detectable.

The energies of point defects near metal/oxide interfaces

We demonstrate a simple but accurate method for the atomistic modeling of metal/oxide interfaces, even when these are complicated by charged defects and space charge in the oxide. Thus we calculate the structure and energies of Ag/MgO interfaces, in the presence of point defects in the oxide, using well‐established computer simulation techniques. Our approach, which is complementary to other methods such as local density approximation calculations, requires very modest computer power. The major terms in the interaction between the oxide and the metal can be decomposed into the short‐range interaction between the ions and the metal cores, the energy required to embed the ions in the jellium, and the image interactions between the ionic charges and the metal. The short‐wavelength fluctuations in the induced charge distribution were eliminated in order to represent the finite Fermi wave vector of a real metal. Our predictions are in good accord with observed wetting angles; agreement with other calculations ...

Innovative materials for fusion power plant structures: separating functions

Fusion reactors create extreme conditions for structures close to the plasma. It seems unlikely that materials currently being considered can meet all performance requirements under such conditions. We explore the possibility of separating functionality in composite structures to overcome this barrier. To this end, several suggestions of directions are made for the search for such materials. In particular, we note some of the new materials that have become available only in the last two decades. Those discussed include the use of diamondlike carbon coatings, nano-structured materials, layered structures, stacked structures, and viscous coatings, including more complex carbon composite materials. Materials modelling will be an important component in the search for viable materials. However, the extreme conditions and the nature of the radiation damage demand extensions both to molecular dynamics and to the much-used Norgett–Robinson–Torrens model. We identify some of the relevant condensed matter challenges for modellin ga nd materials testing in the fusion context, including the relevance of spallation source neutron testing to fusion materials evaluation.

The predose effect in thermoluminescent dosimetry

We present a model of the predose effect of thermoluminescence in crystalline quartz in terms of known impurities and defect processes. It involves the recombination-induced dissociation of an aluminium–alkali complex [Al M + ] and the reaction of an alkali [M + ] with an activator [X], possibly Ge, to make the former an efficient electron trap [X M + ]. This excitation-induced ionic process is a new feature, in addition to the simple carrier redistribution processes usually assumed, and is consistent with a number of experiments. The test dose detects [X M + ], which is present in a concentration within the dynamic range of thermoluminescence measurements. Our model enables us to understand the enhanced sensitivity of the predose method, which has been widely used in archaeological and accident dosimetry, and to link it to established impurity and defect centres in α-quartz.