Publications

2005

• Use of ion beam analysis techniques to characterise iron corrosion under water radiolysis
  
  • Lapuerta S.
  • Moncoffre N.
  • Millard-Pinard N.
  • Mendes E.
  • Corbel C.
  • Crusset D.
  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Elsevier, 2005, 240, pp.288. The aim of this paper is to study the effect of water radiolysis under 12 MeV proton irradiation on the corrosion behaviour of pure iron. Oxygen and hydrogen playing a crucial role during the corrosion process have been specifically investigated. Heavy desaerated water ( enriched at 99.9% in D) was also used to determine the origin of hydrogen at the iron surface. Proton irradiations were performed at the CERI cyclotron in Orleans. Both sides of the Fe foil (respectively in contact with air and with water) were analysed with ion beam techniques: alpha Rutherford backscattering spectrometry was used to profile oxygen, elastic recoil detection analysis has allowed to profile hydrogen. The use of D2O gives evidence that the hydrogen concentration present on the water face could originate from wet air. In addition, in case of the aerated deionised H2O media, it is shown that the irradiation process induces a strong corrosion. Scanning electron microscopy experiments confirm the formation of oxide precipitates
• Effect of external gamma irradiation on dissolution of the spent UO2 fuel matrix
  
  • Jegou C.
  • Muzeau B.
  • Broudic V.
  • Peuget S.
  • Poulesquen A.
  • Roudil D.
  • Corbel C.
  Journal of Nuclear Materials, Elsevier, 2005, 341, pp.62. Leaching experiments were performed on UO2 pellets doped with alpha-emitters (Pu-238/239) and on spent fuel, in the presence of an external gamma irradiation source (A(60)Co = 260 Ci, D gamma = 650 Gy h(-1)). The effects of alpha, beta, gamma radiation, the fuel chemistry and the nature of the cover gas (aerated or Ar + 4%H-2) on water radiolysis and on oxidizing dissolution of the UO2 matrix are quantified and discussed. For the doped UO2 pellets, the nature of the cover gas clearly has a major role in the effect of gamma radiolysis. The uranium dissolution rate in an aerated medium is 83 mg m(-2) d(-1) compared with only 6 mg m-2 d-1 in Ar + 4%H-2. The rate drop is accompanied by a reduction of about four orders of magnitude in the hydrogen peroxide concentrations in the homogeneous solution. The uranium dissolution rates also underestimate the matrix alteration rate because of major precipitation phenomena at the UO2 pellet surface. The presence of studtite in particular was demonstrated in aerated media; this is consistent with the measured H2O2 concentrations (1.2 x 10(-4) mol L-1). For spent fuel, the presence of fission products (Cs and Sr), matrix alteration tracers, allowed us to determine the alteration rates under external gamma irradiation. The fission product release rates were higher by a factor of 5-10 than those of the actinides (80-90% of the actinides precipitated on the surface of the fragments) and also depended to a large extent on the nature of the cover gas. No significant effect of the fuel chemistry compared with UO2 was observed on uranium dissolution and H2O2 production in the presence of the 6 Co source in aerated conditions. Conversely, in Ar + 4%H-2 the fuel self-irradiation field cannot be disregarded since the H2O2 concentrations drop by only three orders of magnitude compared with UO2
• Creation of high energy electronic excitations in inorganic insulators by intense femtosecond laser pulses
  
  • Yatsenko B.N.
  • Bachau H.
  • Belsky A.N.
  • Gaudin J.
  • Geoffroy G.
  • Guizard S.
  • Martin P.
  • Petite G.
  • Philippov A.
  • Vasil'Ev A.N.
  Physica Status Solidi C: Current Topics in Solid State Physics, Wiley, 2005, 2, pp.240. We measured photoemission spectra for a number of insulators (CsI, Diamond, SiO2, CeF3) excited by femtosecond Ti-Sapphire laser pulses at peak intensities, from 0.5 to 6 TW/cm2, which are at least one order of magnitude below the optical breakdown threshold. An intense and pronounced plateau of high energy electrons appears in the photoelectron spectra in this intensity range, which extends up to 30-40 eV at the highest intensities, which we used. The excitation of electrons at high energies is treated in terms of direct interbranch transitions in the conduction band of insulator. These processes are described using calculations based on numerical solution of time dependant Schrödinger equation (TDSE) for Bloch electrons in electric fields
• Anomalous electron spin resonance behavior of single-walled carbon nanotubes
  
  • Salvetat J.P.
  • Feher T.
  • L'Huillier C.
  • Beuneu François
  • Forro L.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 7207, pp.5440. We have studied the electron spin resonance (ESR) of single-walled carbon nanotubes (SWNTs) both in their pristine state and after irradiation with fast electrons in order to detect the signal of conduction electrons. No metallic Pauli contribution was observed in the global spin susceptibility, the ESR signal of the conduction electrons is undetectable because it is broadened by magnetic impurities. We measured a paramagnetic contribution from localized states, with an effective Curie constant decreasing when the temperature increases, following a deactivation law of the type A-B exp(-E-a/k(B)T). This behavior is supposed to be characteristic of semiconducting SWNTs interacting with metallic impurities from the catalyst
• TDDFT from molecules to solids: The role of long-range interactions
  
  • Sottile F.
  • Bruneval F.
  • G. Marinopoulos A.
  • K. Dash L.
  • Botti Silvana
  • Olevano Valério
  • Vast N.
  • Rubio A.
  • Reining L.
  International Journal of Quantum Chemistry, Wiley, 2005, 102, pp.684. Classical Hartree effects contribute substantially to the success of time-dependent density functional theory, especially in finite systems. Moreover, exchange-correlation contributions have an asymptotic Coulomb tail similar to the Hartree term, and turn out to be crucial in describing response properties of solids. In this work, we analyze in detail the role of the long-range part of the Coulomb potential in the dielectric response of finite and infinite systems, and elucidate its importance in distinguishing between optical and electron energy loss spectra (in the long wavelength limit q 0). We illustrate numerically and analytically how the imaginary part of the dielectric function and the loss function coincide for finite systems, and how they start to show differences as the distance between objects in an infinite array is decreased (which simulates the formation of a solid). We discuss calculations for the model case of a set of interacting and noninteracting beryllium atoms, as well as for various realistic systems, ranging from molecules to solids, and for complex systems, such as superlattices, nanotubes, nanowires, and nanoclusters. (10.1002/qua.20486)
  DOI : 10.1002/qua.20486
• Photopolymerization of epoxy coatings containing silica nanoparticles
  
  • Sangermano M.
  • Malucelli G.
  • Amerio E.
  • Priola A.
  • Billi E.
  • Rizza G.
  Progress in Organic Coatings, Elsevier, 2005, 54, pp.134. Silica nanoparticles, were employed to obtain organic-inorganic hybrid coatings via UV curing of an epoxy based system. The influence of the presence Of SiO2 on the rate of polymerization was investigated by real time FT-IR. The silica nanofiller induced both a bulk and a surface modification of UV cured coatings with an increase on T-g, modulus and surface hardness by increasing the amount of silica into the photocurable resin. TEM investigations confirmed that silica filler has a size distribution range between 5 and 50 nm without formation of aggregates. The strong decrease on water uptake in the presence of SiO2 makes these nanocomposites materials particularly interesting for gas-barrier coatings applications
• Investigation of the precipitation kinetics and changes of magnetic anisotropy of iron particles in ion-irradiated silica gel films by means of electron-spin resonance
  
  • Pivin J.C.
  • Esnouf S.
  • Singh F.
  • Avasthi D.K.
  Journal of Applied Physics, American Institute of Physics, 2005, 98, pp.23908. This paper reports the effect of the density of ion-induced electronic excitations on the nucleation of Fe particles in a silicon-based gel and on the magnetic ordering in SiO2:Fe composites with a volume fraction of metal of a few percent derived from the same gel. The nucleation occurs for a critical value of the energy density deposited by ions in electronic excitation. The dependence of the nucleation yield on the ion fluence phi is determined by the product of the electronic energy loss S-e and the number n of ion impacts on a same area a which are necessary for reaching locally this critical energy density. Irradiation-induced changes in the magnetic anisotropy of composite films, previously formed by heat treatment of the same gel, depend more drastically on the order of magnitude of S-e. When the latter is high enough for damaging the matrix, a tilt of the easy magnetization axis is observed
• Analysis of molecular formation in irradiated glasses: a Raman depth profile study
  
  • Ollier N.
  • Boizot B.
  • Reynard B.
  • Ghaleb D.
  • Petite G.
  Journal of Nuclear Materials, Elsevier, 2005, 340, pp.209. Depth profiles experiments have been performed by Raman spectroscopy on three alkali (Na, Li, K) borosilicate glasses irradiated with 1.8 MeV electrons at 1 and 3 GGy. These experiments show that molecular oxigen produced under ß irradiation is concentrated near the glass surface according to a depth depending on the irradiation dose. Moreover, we observed that the polymerisation increase is the same in the entire volume sample. The average Si-O-Si angle decrease under irradiation is also homogenous in the whole irradiated glass volume. From all results, we demonstrate that oxygen migrates up to the glass surface during irradiation without strong irradiation with the glass network. Migration of oxigen and probably alkalis takes through percolation channels with a possible departure of oxygen in some cases
• Concave and Convex photonic Barriers in Gradient Optics
  
  • Shvartsburg Alexandr B.
  • Petite Guillaume
  The European Physical Journal D : Atomic, molecular, optical and plasma physics, EDP Sciences, 2005, 36, pp.111. Propagation and tunneling of light through photonic barriers formed by thin dielectric films with continuous curvilinear distributions of dielectric susceptibility across the film, are considered. Giant heterogeneity-induced dispersion of these films, both convex and concave, and its influence on their reflectivity and transmittivity are visualized by means of exact analytical solutions of Maxwell equations. Depending on the cut-off frequency of the film, governed by the spatial profile of its refractive index, propagation or tunneling of light through such barriers are examined. Subject to the shape of refractive index profile the group velocities of EM waves in these films are shown to be either increased or deccreased as compared with the homogeneous layers; however, these velocities for both propagation and tunneling regimes remain subluminal. The decisive influence of gradient and curvature of photonic barriers on the efficiency of tunneling is examined by means of generalized Fresnel formulae. Saturation of the phase of the wave tunneling through a stack of such films (Hartman effect), is demonstrated. The evanescent modes in lossy barriers and violation of Hartman effect in this case is discussed.
• Anomalous electron spin resonance behavior of single-walled carbon nanotubes
  
  • Beuneu Francois
  • Salvetat J.-P.
  • Fehér T.
  • L'Huillier C.
  • Forró L.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 72, pp.075440. We have studied the electron spin resonance ESR of single-walled carbon nanotubes SWNTs both in their pristine state and after irradiation with fast electrons in order to detect the signal of conduction electrons. No metallic Pauli contribution was observed in the global spin susceptibility, the ESR signal of the conduction electrons is undetectable because it is broadened by magnetic impurities. We measured a paramagnetic contribution from localized states, with an effective Curie constant decreasing when the temperature increases, following a deactivation law of the type A-B exp-Ea /kBT. This behavior is supposed to be characteristic of semiconducting SWNTs interacting with metallic impurities from the catalyst. (10.1103/PhysRevB.72.075440)
  DOI : 10.1103/PhysRevB.72.075440
• Thermodynamics of the vortex liquid in heavy-ion-irradiated superconductors
  
  • van Der Beek Cornelis
  • Konczykowski Marcin
  • Fruchter Luc
  • Brusetti René
  • Klein Thierry
  • Marcus Jacques
  • Marcenat C.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 72 (214504), pp.1-9. It is shown that the large effect of heavy ion irradiation on the thermodynamical properties of the anisotropic superconductor YBa2Cu3O7- extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific-heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic KxBa1-xBiO3 x~0.35. In both compounds, vortex pinning manifests itself as a sharp angular dependence of the equilibrium torque. In YBa2Cu3O7-, pinning by the defects appears at the temperature TCp max of the specific-heat maximum, well above the magnetic irreversibility line TirrH. In isotropic KxBa1-xBiO3, the onset of the pinning-related torque anomaly tracks the onset of the specific-heat anomaly and the irreversibility line. In YBa2Cu3O7-, fluctuations of the amplitude of the order parameter and not vortex line wandering are ultimately responsible for the vanishing of pinning. In KxBa1-xBiO3, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the “Bose-glass” transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered.
• Comment on “Quantum Confinement and Electronic Properties of Silicon Nanowires”
  
  • Bruneval Fabien
  • Botti Silvana
  • Reining Lucia
  Physical Review Letters, American Physical Society, 2005, 94, pp.219701. A Comment on the Letter by Xinyuan Zhao et al., Phys. Rev. Lett. 92, 236805 (2004). The authors of the Letter offer a Reply. (10.1103/PhysRevLett.94.219701)
  DOI : 10.1103/PhysRevLett.94.219701
• Current progress in the magnetism of rare-earth-hydrogen systems
  
  • Vajda P.
  Journal of Alloys and Compounds, Elsevier, 2005, 404-406, pp.131. We are reviewing recent progress obtained in the study of hydrogen interaction with magnetic structures in rare-earth metals, emphasizing the role of the ordering state of the H-sublattice. We shall present in particular: (1) the situation in the solid solutions of the heavy rare earths, alpha*-RHx, and the efforts to relate the special quasi-unidimensional configuration of this phase (charge-density waves) to their modulated magnetism (spin-density waves) via the Fermi surface; (2) an assembly of all magnetic structures determined up to now in the heavy-RE dihydrides, beta-RH(D)(2(+x)), for a tentative comparison of the observed commensurate and/or incommensurate configurations; special mention will be made of the remarkably stable short-range ordered (SRO) magnetism always present in these systems; (3) the need for further (thorough) neutron scattering work on the magnetism in the insulating R-trihydrides, gamma-RH(D)(3(-x)), in order to specify its mechanism in the absence of RKKY exchange interaction
• Ferromagnetic Domain Structure of La0.78Ca0.22MnO3 Single Crystals
  
  • Jung Grzegorz
  • Markovich Vladimir
  • van Der Beek Cornelis Jacominus
  • Mogilyansky Dmitrii
  • Mukovskii Yakov M.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 72, pp.134412. The magneto-optical technique has been employed to observe spontaneous ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystals. With decreasing temperature the regular network of ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range of 70-150 K. The apparent rotation of the domain walls can be understood in terms of the Jahn-Teller deformation of the orthorhombic unit cell, accompanied by additional twinning.
• Magnetic structure of beta-ErD2: Long-range and short-range order from powder neutron diffraction
  
  • Vajda P.
  • André G.
  • Udovic T.J.
  • Erwin R.W.
  • Huang Q. S.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 71, pp.054419. Two parallel neutron powder diffraction investigations were performed on different specimens of ErD2 at the LLB (CEN Saclay) and at NIST for the determination of its magnetic structure. It comprises, below Tn=2.23 K, two coexisting sinusoidally modulated antiferromagnetic (AF) configurations, M' and M'', with the respective incommensurate propagation vectors, k'=(0.275,0.275,0.750), together with its third harmonic 3k', and k''=(0.120,0.120,0.750). Contrary to other heavy rare-earth dideuterides, no commensurate AF configuration could be detected down to 120 mK. On the other hand, similar to earlier observations on HoD2 and TbD2, magnetic short range order (SRO shows up near 1.5 K in ErD2 after the disappearance of certain M'' lines. This SRO seems to correlate with the hexagonal gamma-phase of the trideuterite and suggests an electronic phase separation with memory effects
• Raman and EPR studies of ß-irradiated oxide glasses: The effect of iron concentration
  
  • Olivier F.Y.
  • Boizot B.
  • Ghaleb D.
  • Petite G.
  Journal of Non-Crystalline Solids, Elsevier, 2005, 351, pp.1061. The study of iron concentration on the oxide glass structure has shown evidence of depolymerization of the vitreous network with increasing iron content and especially with Fe2+ amount for all types of glass. For all glass compositions studied, we observe by EPR a blockage of defects creation processes under beta irradiation for glasses doped with more than 1 wt% of Fe2O3. Reduction processes of Fe3+ under irradiation is also detected. Moreover for sodalime glass samples, beta irradiation induces depolymerization, in opposite of what was expected and observed for other glasses. Anyway, adding iron up to 4 wt% Fe2O3 always diminishes the beta irradiation effects. This has been linked to the observed iron reduction and to the ability of iron, maybe coupled with charge trapping processes to consume excitons
• Multiple superconducting phases in heavy fermion superconductors
  
  • Matsuda Y.
  • Izawa K.
  • Watanabe T.
  • Kasahara Y.
  • Nakajima Y.
  • Sakakibara T.
  • van Der Beek C.J.
  • Nohara M.
  • Hanaguri T.
  • Takagi H.
  • Goryo J.
  • Maki K.
  • Thalmeier P.
  • Osaki S.
  • Sugawara H.
  • Sato H.
  • Shishido H.
  • Settai R.
  • Onuki Y.
  Journal of Physics and Chemistry of Solids, Elsevier, 2005, 66, pp.1365. We show that in recently discovered heavy fermion superconductors, quasi two-dimensional CeCoIn5 and skutterudite PrOs4Sb12, multiple superconducting phases with different symmetries manifest themselves below T-c. The ultrasound velocity measurements revealed that in CeCoIn5 the Fulde-Ferrel-Larkin-Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. The angle resolved magnetothermal transport measurements revealed that in PrOs4Sb12 a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. These results open up a new realm for the study of the superconductivity with multiple phases
• Conductance in multiwall carbon nanotubes and semiconductor nanowires
  
  • Dayen J.F.
  • Wade T.L.
  • Konczykowski M.
  • Wegrowe J.E.
  • Hoffer X.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 7207, pp.3402. Electronic transport in an ensemble of multiwall carbon nanotubes and semiconductor nanowires was compared. The nanotubes and nanowires are obtained by template synthesis and are contacted in a current perpendicular to the plane geometry by using different methods. In all cases, the nonohmic behavior of the conductance, the so-called zero-bias anomaly, shows a temperature dependence that scales with the voltage dependence. This robust scaling law describes the conductance G(V,T) by a single coefficient alpha. A universal behavior as a function of alpha is found for all samples. Magnetoconductance measurements furthermore show that the conduction regime is weak localization. The observed behavior can be understood in terms of the Coulomb blockade theory, providing that a single tunnel barrier is present. This hypothetical tunnel barrier would have a resistance of the order of 2500 Omega and a typical energy of about 40 meV for all samples
• Nucleation and growth of single wall carbon nanotubes
  
  • Beuneu Francois
  Solid State Communications, Elsevier, 2005, 136, pp.462. The nucleation and growth of single wall carbon nanotubes from a carbon-saturated catalytic particle surrounded by a single sheet of graphene is described qualitatively by using a very restricted number of elementary processes, namely Stone-Wales defects and carbon bi-interstitials. Energies of the different configurations are estimated by using a Tersoff energy minimization scheme. Such a description is compatible with a broad variety of size or helicity of the tubes. Several mechanisms of growth of the embryos are considered: one of them is made more favourable when the tubes embryos are arranged in an hexagonal network in the graphene plane. All the proposed mechanisms can be indefinitely repeated for the growth of the nanotubes. (10.1016/j.ssc.2005.09.007)
  DOI : 10.1016/j.ssc.2005.09.007
• Energy dependence of the exchange-correlation kernel of time-dependent density functional theory: A simple model for solids
  
  • Botti Silvana
  • Fourreau A.
  • Nguyen F.
  • Renault Y.-O.
  • Sottile F.
  • Reining L.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 72, pp.125203. Time-dependent density functional theory faces an important problem when it comes to extended systems: The long-range component of the exchange-correlation kernel fxc is completely absent from local density or generalized gradient approximations, but it is believed to be present in the “exact” fxc . Several attempts have been made to solve this issue, the simplest of them being the use of a model static long-range kernel of the form −αstatic∕q2 . In this paper, we propose and motivate a dynamical extension of this model of the form −(α+βω2)∕q2 . The dynamical model is then used to calculate the dielectric function of a large variety of semiconductors and insulators. The absorption spectra of large gap insulators are remarkably improved with respect to calculations where the kernel is taken to be static. This approach is valid also for energies in the range of plasmons, and hence it yields, e.g., good electron energy loss spectra. Finally, we present some simple theoretical arguments that relate the parameters of the model to physical quantities, like the dielectric constant and the plasmon frequency. (10.1103/PhysRevB.72.125203)
  DOI : 10.1103/PhysRevB.72.125203
• Growth dynamics of hydrogenated silicon nanoparticles under realistic conditions of a plasma reactor
  
  • Vach H.
  • Brulin Q.
  • Chaâbane N.
  • Novikova T.
  • Roca I Cabarrocas Pere
  • Kalache B.
  • Hassouni K.
  • Botti Silvana
  • Reining Lucia
  Computational Materials Science, Elsevier, 2005, 35, pp.216-222. We present results of an extensive numerical study that was motivated by the experimental problem to understand under which conditions SinHm nanoparticles deposited by plasma enhanced chemical vapor deposition (PECVD) take an amorphous or a crystalline structure. A crystalline structure of those particles is crucial, for example, for the electrical properties and lifetime of polymorphous solar cells. First, we use a fluid dynamics model to characterize the experimentally employed silane plasma. The resulting relative densities for all plasma radicals, their temperatures, and their collision interval times are then used as input data for detailed semiempirical quantum molecular dynamics simulations. As a result the growth dynamics of nanometric hydrogenated silicon SinHm clusters is simulated starting out from the collision of individual SiHx radicals under the plasma conditions derived above. We demonstrate how the details of the plasma determine the amorphous or crystalline character of the forming nanoparticles. Finally, we show a preliminary absorption spectrum based on ab initio time-dependent DFT calculations for a crystalline Si10H16 cluster to demonstrate the possibility to monitor the cluster growth in situ. (10.1016/j.commatsci.2004.07.010)
  DOI : 10.1016/j.commatsci.2004.07.010
• Deuterium trapping in sintered polished UO2 disks
  
  • Labrim H.
  • Barthe M.F.
  • Sauvage T.
  • Desgardin P.
  • Blondiaux G.
  • Corbel C.
  • Miserque F.
  • Piron J.P.
  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Elsevier, 2005, 240, pp.434. In this work we present deuterium analysis performed using the H-2(He-3, alpha)H-1 Nuclear Reaction Analysis method. Different sets of sintered polished uranium oxide disks have been investigated before and after annealing at 400 degrees C in ArD2 atmosphere. Heterogeneous lateral deuterium distributions have been found for all the ArD2 annealed UO2 disks. The average deuterium amount increases after annealing at 400 degrees C in ArD2 and decreases after a second annealing at 400 degrees C in vacuum whatever is the set of sintered polished uranium oxide disks
• Relationship between structure, segregation and electrical activity in grain boundaries
  
  • Lamzatouar A.
  • Palais Olivier
  • B. M. Hardouin Duparc O.
  • Thibault J.
  • Charaï A.
  Journal of Materials Science, Springer Verlag, 2005, 40, pp.3163-3167. Using the contactless microwave phase-shift technique (μ W-PS) and High Resolution Transmission Electron Microscopy (HRTEM), we show that the twist and mixed parts of a < 110 > Σ = 51(θ = 16.10∘) grain boundary in germanium (Ge) are electrically active. We also show that we can passivate the electrically active grain boundaries by sulfur segregation which has been studied by energy filtering HRTEM. Atomistic simulations show that the most favorable places for this segregation are the high energy sites of grain boundary
• The planar-to-tubular structural transition in boron clusters from optical absorption
  
  • A. L. Marques Miguel
  • Botti Silvana
  The Journal of Chemical Physics, American Institute of Physics, 2005, 123, pp.014310. The optical response of the lowest-energy isomers of the B20 family is calculated using time-dependent density-functional theory within a real-space, real-time scheme. Significant differences are found among the absorption spectra of the clusters studied. We show that these differences can be easily related to changes in the overall geometry. Optical spectroscopy is thus an efficient tool to characterize the planar-to-tubular structural transition, known to be present in these boron-based systems. (10.1063/1.1948386)
  DOI : 10.1063/1.1948386
• Photoconductivité et photoémission de diamant(s) sous irradiation XUV femtoseconde
  
  • Gaudin J.
  • Geoffroy G.
  • Guizard S.
  • Esnouf S.
  • Olevano Valério
  • Petite G.
  • Klimentov S.M.
  • Pivovarov P.A.
  • Garnov S.V.
  • Carre B.
  • Martin P.
  • Belsky A.
  Journal de Physique IV Proceedings, EDP Sciences, 2005, 127, pp.131-138. En fonction de l'ordre de l'harmonique, on constate que le signal de PC augmente tout d'abord (harmoniques 9 à 13) puis diminue au delà. Si l'augmentation s'interprète aisément comme résultant de phénomènes de multiplication par collisions inélastiques, la diminution ultérieure n'a pas pour le moment d'explication. Les mesures de spectre de photoémission suggèrent un effet important de la relaxation par émission de plasmons. Enfin, nous avons réalisé le premier calcul ab-initio de la durée de vie des porteurs tenant compte des interactions électron-électron, à l'aide d'une approche de théorie quantique à plusieurs corps de type GW. Au voisinage du gap, on observe un comportement proche de celui d'un liquide de Fermi. A plus haute énergie on observe des déviations à ce comportement, provenant d'effets de structure de bande d'une part, et d'excitations de plasmons d'autre part