First principles calculations in iron: structure and mobility of defect clusters and defect complexes for kinetic modelling

Predictive simulations of the defect population evolution in materials under or after irradiation can be performed in a multi-scale approach, where the atomistic properties of defects are determined by electronic structure calculations based on the Density Functional Theory and used as input for kinetic simulations covering macroscopic time and length scales. Recent advances obtained in iron are presented. The determination of the 3D migration of self-interstitial atoms instead of a fast one-dimensional glide induced an overall revision of the widely accepted picture of radiation damage predicted by previously existing empirical potentials. A coupled ab initio and mesoscopic kinetic Monte Carlo simulation provided strong evidence to clarify controversial interpretations of electrical resistivity recovery experiments concerning the mobility of vacancies, self-interstitial atoms, and their clusters. The results on the dissolution and migration properties of helium in α-Fe were used to parameterize Rate Theory models and new inter-atomic potentials, which improved the understanding of fusion reactor materials behavior. Finally, the effects of carbon, present in all steels as the principal hardening element, are also shown. To cite this article: C.C. Fu, F. Willaime, C. R. Physique 9 (2008).     

The non-Arrhenius migration of interstitial defects in bcc transition metals

Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).     

Fissile core and Tritium-Breeding Blanket: structural materials and their requirements

High radiation resistant structural materials for fusion and fission nuclear power plants are a key issue for the development of both types of reactors. Selection criteria, elements of metallurgy of the selected materials, and the major issues as they are revealed by the results of the present development programmes, are presented. At low temperature (300 °C) ferritic/martensitic steels are suffering from He-embrittlement, associated with possible hardening due to α/α^′ unmixing. The kinetics of hardening and embrittlement versus dose, especially saturation with dose, are still open key issues, difficult to settle on the basis of a purely experimental programme. Important progress is still to be made in mastering the initial microstructure, inclusion cleanness and joining techniques of oxide dispersion strengthened steels for higher heat resistance. Physics modeling as presented in this issue should promote guidance to the understanding of the mechanisms involved, provide solutions to master the initial microstructure and phase stability, and mitigate the in-service property degradation. To cite this article: J.-L. Boutard et al., C. R. Physique 9 (2008).     

The Fe–Cr system: atomistic modelling of thermodynamics and kinetics of phase transformations

To understand the behaviour of irradiated defects and kinetic pathways of micro-structural evolution in Fe–Cr alloys, we use a combination of density functional theory with statistical approaches involving cluster expansions and Monte Carlo simulations. A lowest negative mixing enthalpy is found at 6.25% Cr that is consistent with our DFT prediction of an ordered Fe₁₅Cr structure. At 50% Cr, it is found that the predicted enthalpy of formation is 4 times smaller than that calculated by the CPA approach. Thermodynamic and precipitation properties are then discussed in term of segregation between the Fe₁₅Cr and α^′-Cr phases and of vacancy-mediated kMC simulation. To cite this article: D. Nguyen-Manh et al., C. R. Physique 9 (2008).     

A lightning initiation mechanism: application to a thunderstorm electrification model

The use of numerical models has greatly increased our understanding of the electrical and microphysical process within electrified clouds. We use the University of Washington, 1.5-dimensional thunderstorm model to examine the effects of including a runaway electron based lightning initiation mechanism. We find that this mechanism can significantly alter the electrification history of modeled storms and produce vertical electric field profiles that are very similar to those of observed storms. To cite this article: R. Solomon et al., C. R. Physique 3 (2002) 1325–1333.     

Radiation aging of nonmetallic materials: specific aspects

Under irradiation, all materials experience various forms of structural evolution, from the simplest, associated with point defect creation and accumulation, to complex phase changes, either towards equilibrium or nonequilibrium structures. In nonmetallic ceramics the same processes are known or probable; however, the nature of bonding, partly ionic and partly covalent, as well as the complexity associated with the long range character of the Coulomb interaction, have long posed great difficulties in defect and aging studies under irradiation. Our aim here is to review the current state of knowledge, stressing the specific characteristics of nonmetallic materials, from primary defect creation to collective behavior, with respect to both experimental facts as well as to modeling perspectives. Given the broad field covered, we will illustrate the problem by choosing a few model materials, mostly oxides, in which the whole spectrum of phenomena has been handled. We will begin with threshold energy studies, then go to microstructure formation and evolution, radiation enhanced diffusion results, and lastly to phase changes. To cite this article: Y. Limoge, C. R. Physique 9 (2008).     

Supersymmetric backgrounds from generalized Calabi–Yau manifolds

We show that the supersymmetry transformations for type II string theories on six-manifolds can be written as differential conditions on a pair of pure spinors, the exponentiated Kähler form e^iJ and the holomorphic form Ω. The equations are explicitly symmetric under exchange of the two pure spinors and a choice of even or odd-rank RR field. This is mirror symmetry for manifolds with torsion. Moreover, RR fluxes affect only one of the two equations: e^iJ is closed under the action of the twisted exterior derivative in IIA theory, and similarly Ω is closed in IIB. This means that supersymmetric SU(3)-structure manifolds are always complex in IIB while they are twisted symplectic in IIA. Modulo a different action of the B-field, these are all generalized Calabi–Yau manifolds, as defined by Hitchin. To cite this article: M. Graña et al., C. R. Physique 5 (2004).

Résumé

On montre que les transformations de supersymétrie pour les théories des cordes de type II peuvent être traduites dans des équations différentielles pour une paire de spineurs purs, l'exponentiel de la forme de Kähler e^iJ et la forme holomorphe Ω. Ces équations sont symétriques sous l'échange des deux spineurs purs et des formes de RR de rang pair ou impair. Cette propriété est la symétrie miroir pour les variétés avec torsion. On voit aussi que les fluxes de RR entrent seulement dans une des deux équations : e^iJ est fermé sous l'action de la dérivée extérieure « twisted » dans la corde de type IIA, et de la même manière Ω est fermé en type IIB. Cela implique que les variétés supersymétriques de structure SU(3) sont toujours complexes en type IIB ou bien symplectiques « twisted » en IIA. Ces variétés sont donc des variétés des Calabi–Yau généralisées selon la définition de Hitchin, mais avec une action du champ B différente. Pour citer cet article : M. Graña et al., C. R. Physique 5 (2004).     

QWIP detectors and thermal imagers

Standard GaAs/AlGaAs QWIPs (Quantum Well Infrared Photodetector) are now well established for long wave infrared (LWIR) detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The realization of large FPAs (up to 640×480) drawing on the standard III–V technological process has already been demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIP technology has been growing up over the last ten years and now reaches an undeniable level of maturity. As with all quantum detectors, the thermal current, particularly in the LWIR range, limits the operating temperature of QWIPs. It is very crucial to achieve an operating temperature as high as possible and at least above 77 K in order to reduce volume and power consumption and to improve the reliability of the detection module. This thermal current offset has three detrimental effects: noise increase, storage capacitor saturation and high sensitivity of FPAs to fluctuations in operating temperature. For LWIR FPAs, large cryocoolers are required, which means volume and power consumption unsuitable for handheld systems. The understanding of detection mechanisms has led us to design and realize high performance ‘standard’ QWIPs working near 77 K. Furthermore, a new in situ skimmed architecture accommodating this offset has already been demonstrated. In this paper we summarize the contribution of THALES Research & Technology to this progress. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the potential of our QWIPs through features of Thales Optronique's products for third thermal imager generation. To cite this article: E. Costard et al., C. R. Physique 4 (2003).     

Helium and point defect accumulation: (i) microstructure and mechanical behaviour

Ferritic/martensitic (F/M) steels are good candidate structural materials for the future fusion reactors and spallation sources. However, irradiation of steels is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature (DBTT) and reduction of fracture toughness and creep resistance starting at low doses. Helium (He), produced by transmutation during the irradiation, also impacts mechanical properties. Numerous experimental and theoretical studies on the evolution of the microstructure of steels under irradiation have been conducted until now. We review the effect of irradiation-induced point defects and in particular of He on the mechanical properties of F/M steels. To cite this article: R. Schäublin et al., C. R. Physique 9 (2008).     

Hard X-ray PhotoEmission Spectroscopy of strongly correlated systems

Hard X-ray PhotoEmission Spectroscopy (HAXPES) is a new tool for the study of bulk electronic properties of solids using synchrotron radiation. We review recent achievements of HAXPES, with particular reference to the VOLPE project, showing that high energy resolution and bulk sensitivity can be obtained at kinetic energies of 6–8 keV. We present also the results of recent studies on strongly correlated materials, such as vanadium sesquioxide and bilayered manganites, revealing the presence of different screening properties in the bulk with respect to the surface. We discuss the relevant experimental features of the metal–insulator transition in these materials. To cite this article: G. Panaccione et al., C. R. Physique 9 (2008).     

Nucleation problems in metallurgy of the solid state: recent developments and open questions

Nucleation processes play a key role in the microstructure evolution of metallic alloys during thermomechanical treatments. These processes can involve phase transformations (such as precipitation) and structural instabilities (such as recrystallisation). Although the word ‘nucleation’ is used in both cases, the situation is profoundly different for precipitation and for recrystallisation on which this article is focussed. In the case of precipitation, species are conserved and the underlying physics is stochastic fluctuations, allowing the apparition of critical germs of the new phase. In the case of recrystallisation, the underlying physical phenomenon is the progressive growth of subgrain structures leading to an unstable configuration, allowing a dislocation free grain to grow at the expense of a dislocated one. The two cases require different types of modelling which are presented in the article. To cite this article: Y. Bréchet, G. Martin, C. R. Physique 7 (2006).     

D-brane models and flux supersymmetry breaking

We describe recent work on constructing four-dimensional string models with moduli stabilized by field strength fluxes and chiral gauge sectors close to the Standard Model from D-brane configurations. We discuss how the interplay of both ingredients relates to phenomenological issues, in particular the appearance of soft terms on the D-brane gauge sector induce from non-supersymmetric flux backgrounds. To cite this article: A.M. Uranga, C. R. Physique 5 (2004).

Résumé

Nous décrivons des travaux récents de constructions de modèles de cordes à 4 dimensions ayant des modules stabilisés par des flux, ainsi que des secteurs chiraux de jauge proches du modèle standard, réalisés à partir de configurations de D-branes. Nous expliquons comment les liens entre les différents ingrédients relient des aspects phénoménologiques, en particulier l'apparition de termes doux dans le secteur de jauge des D-branes à partir de champs de fonds non-supersymétriques avec flux. Pour citer cet article : A.M. Uranga, C. R. Physique 5 (2004).     

AC transport in carbon-based devices: challenges and perspectives

Time-dependent fields are a valuable tool to control fundamental quantum phenomena in highly coherent low dimensional electron systems. Carbon nanotubes and graphene are a promising ground for these studies. Here we offer a brief overview of driven electronic transport in carbon-based materials with the main focus on carbon nanotubes. Recent results predicting control of the current and noise in nanotube based Fabry–Pérot devices are highlighted. To cite this article: L.E.F. Foa Torres, G. Cuniberti, C. R. Physique 10 (2009).     

D-branes from matrix factorizations

B-type D-branes can be obtained from matrix factorizations of the Landau–Ginzburg superpotential. We here review this promising approach to learning about the spacetime superpotential of Calabi–Yau compactifications. We discuss the grading of the D-branes, and present applications in two examples: the two-dimensional torus, and the quintic. To cite this article: K. Hori, J. Walcher, C. R. Physique 5 (2004).

Résumé

Les D-branes de type B peuvent être décrites à partir de factorisations matricielles du super-potentiel de Landau–Ginzburg. On revoit ici cette approche prometteuse pour étudier le super-potentiel en espace-temps de compactifications de Calabi–Yau. On discute la graduation des D-branes, et présente deux exemples : le tore en deux dimensions, ainsi que la quintique. Pour citer cet article : K. Hori, J. Walcher, C. R. Physique 5 (2004).     

De la microstructure du polymère à ses propriétés rhéologiquesFrom the microstructure of polymers towards their rheological properties

We extend the previous work by Benallal et al. on the relationship between structure and rheological properties of linear polymer melts. The aim of this paper is to quantify the effect of the chemical structure on the viscoelastic properties. We show that these properties are governed by the monomer dimensions and the interaction energy. To cite this article: A. Allal et al., C. R. Physique 3 (2002) 1451–1458.     

Un coronographe interférentiel achromatique coaxial

We present a new type of stellar interfero-coronagraph, the ‘CIAXE’, which is a variant of the ‘AIC’, the Achromatic Interfero-Coronagraph. The CIAXE is characterized by a very simple, compact and fully coaxial optical combination. Indeed, contrarily to the classical AIC which has a Michelson interferometer structure, the CIAXE delivers its output beam on the same axis as the input beam. This will ease its insertion in the focal instrumentation of existing telescopes or next generation ones. Such a device could be a step forward in the field of instrumental search for exoplanets. To cite this article: J. Gay et al., C. R. Physique 6 (2005).     

Semiclassical relativistic strings in and long scalar operators in SYM theory

We review recent progress in quantitative checking of AdS/CFT duality in the sector of ‘semiclassical’ string states dual to ‘long’ scalar N=4 super Yang–Mills operators. In particular, we describe the effective action approach, in which the same sigma model type action describing coherent states is shown to emerge from the AdS₅×S⁵ string action and from an integrable spin chain Hamiltonian representing the SYM dilatation operator. To cite this article: A.A. Tseytlin, C. R. Physique 5 (2004).

Résumé

Nous passons en revue les progrès récents sur les vérifications quantitatives de la dualité AdS/CFT dans le régime où les états « semiclassiques » de cordes sont du aux « longs » opérateurs scalaires de la théorie de super Yang–Mills N=4. En particulier, nous décrivons l'approche effective, dans laquelle le modèle sigma décrivant les états cohérents est montré émerger de l'action de la corde sur AdS₅×S⁵ et de l'Hamiltonien d'une chaîne de spin intégrable représentant l'opérateur de dilatation en SYM. Pour citer cet article : A.A. Tseytlin, C. R. Physique 5 (2004).     

X-rays and matter – the basic interactions

In this introductory article we attempt to provide the theoretical basis for developing the interaction between X-rays and matter, so that one can unravel properties of matter by interpretation of X-ray experiments on samples. We emphasize that we are dealing with the basics, which means that we shall limit ourselves to a discussion of the interaction of an X-ray photon with an isolated atom, or rather with a single electron in a Hartree–Fock atom. Subsequent articles in this issue deal with more complicated – and interesting – forms of matter encompassing many atoms or molecules. To cite this article: J. Als-Nielsen, C. R. Physique 9 (2008).     

Higher-loop integrability in gauge theory

The dilatation operator measures scaling dimensions of local operator in a conformal field theory. Algebraic methods of constructing the dilatation operator in four-dimensional N=4 gauge theory are reviewed. These led to the discovery of novel integrable spin chain models in the planar limit. Making use of Bethe ansätze a superficial discrepancy in the AdS/CFT correspondence was found, we discuss this issue and give a possible resolution. To cite this article: N. Beisert, C. R. Physique 5 (2004).

Résumé

L'opérateur de dilatation mesure les dimensions d'échelles des opérateurs locaux des théories conformes des champs. Nous passons en revue les méthodes algébriques de construction de l'opérateur de dilatation pour la théorie de jauge N=4 en quatre dimensions. Ceci nous a conduit à découvrir, dans la limite planaire, de nouveaux modèles intégrables de chaînes de spin. En utilisant l'ansätze de Bethe une incompatibilité avec la correspondance AdS/CFT fut découverte, nous discutons ce problème et une résolution possible. Pour citer cet article : N. Beisert, C. R. Physique 5 (2004).