Publications

2008

• Anomalous Angular Dependence of the Dynamic Structure Factor near Bragg Reflections: Graphite
  
  • Hambach R.
  • Giorgetti C.
  • Hiraoka N.
  • Cai Y.Q.
  • Sottile F.
  • Marinopoulos A.G.
  • Bechstedt F.
  • Reining Lucia
  Physical Review Letters, American Physical Society, 2008, 101 (26), pp.http://prl.aps.org/abstract/PRL/v101/i26/e266406. The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q,ω). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers. (10.1103/PhysRevLett.101.266406)
  DOI : 10.1103/PhysRevLett.101.266406
• Mesoporous silica films with varying porous volume fraction: direct correlation between ortho-positronium annihilation decay and escape yield into vacuum
  
  • Laszlo Liszkay
  • Corbel Catherine
  • Raboin Lorraine
  • Boilot Jean-Pierre
  • Perez P.
  • Brunet-Bruneau Aline
  • Crivelli Paolo
  • Gendotti Ulisse
  • Rubbia André
  • Ohdaira Toshiyuki
  • Suzuki Ryoichi
  , 2008. The behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction.
• Ab initio GW many-body effects in graphene
  
  • Trevisanutto Paolo E.
  • Giorgetti C.
  • Reining Lucia
  • Ladisa M.
  • Olevano Valerio
  Physical Review Letters, American Physical Society, 2008, 101 (22), pp.226405. We present an $ab\ initio$ many-body GW calculation of the self-energy, the quasiparticle band plot and the spectral functions in free-standing undoped graphene. With respect to other approaches, we numerically take into account the full ionic and electronic structure of real graphene and we introduce electron-electron interaction and correlation effects from first principles. Both non-hermitian and also dynamical components of the self-energy are fully taken into account. With respect to DFT-LDA, the Fermi velocity is substantially renormalized and raised by a 17%, in better agreement with magnetotransport experiments. Furthermore, close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed in ARPES experiments. Our calculations show that the kink is due to low-energy $\pi \to \pi^*$ single-particle excitations and to the $\pi$ plasmon. Finally, the GW self-energy does not open the band gap. (10.1103/PhysRevLett.101.226405)
  DOI : 10.1103/PhysRevLett.101.226405
• Spectroscopic studies of Ti3+ ions speciation inside MgAl2O4 spinels
  
  • Lombard Pierre
  • Boizot Bruno
  • Ollier Nadège
  • Jouini Anis
  • Yoshikawa Akira
  Journal of Crystal Growth, Elsevier, 2008, 311, pp.899-903. An electron paramagnetic resonance (EPR) investigation on Ti-doped MgAl2O4 spinels has been made in order to study the Ti3+ sites. The study we present here concerns the angular dependencies of the Ti3+ EPR lines and the variations of the EPR spectra due to a modificatino of the TiO2 content or of the chemical composition from MgAl2O4 to MgAl4O7 and MgAl6O10. In all the studied samples except one, we observe the presence of both Mn2+ and Ti3+ ions. No correlation was observed between Ti3+ amount and TiO2 content; The titanium ions are located in three different sites: the octahdral B site of the spinel structure; the tetrahedral A site and a last site which remains unclear. On the contrary, the major part of the Mn2+ ions is assumed to be in the tetrahedral A site of the spinel structure and a minor part in the octahedral B site. Our work demonstrates the overall interest of EPR spectroscopy in the study of the paramagnetic optically active ions inside optical materials. (10.1016/j.jcrysgro.2008.09.131)
  DOI : 10.1016/j.jcrysgro.2008.09.131
• Ultra short-time dynamics of radiation damage in fcc metals
  
  • Hayoun Marc
  • Coddens Gerrit
  • Petite Guillaume
  , 2008. We have performed molecular-dynamics simulations of displacement cascades in copper in order to investigate the non-equilibrium ultra-short time damage and to evaluate the possibility of observing it experimentally in-situ (e.g. in a pump/probe laser experiment). The atomic trajectories have been analysed by calculating their X-ray diffraction patterns as a function of time. The results show that an integrated X-ray intensity can indeed be used to evidence the irradiation effects. Even though the number of Frenkel defects is large, the main effect of the irradiation showing up in the X-ray intensities at ultra-short times is an important alteration of the lattice vibrations. On the basis of these results, a new pump/probe set-up is proposed.
• Researches in safety of compacted waste and spent nuclear fuel
  
  • Wasselin-Trupin Virginie
  • Guipponi Claire
  • Millard-Pinard Nathalie
  • Serris Eric
  • Pijolat Michèle
  • Bererd Nicolas
  • Corbel Catherine
  • Barthe Marie-France
  • Leverd Pascal-Claude
  , 2009, pp.367-371. IRSN (the French Institute of Radioprotection and Nuclear Safety) is in charge of reviewing the LL-HLW geological repository project managed by ANDRA (the French National Radioactive Waste Management Agency) in support of the nuclear safety authority (ASN). Amongst other IRSN paies particular attention to the performance of confinement of the nuclear wastes packages, to the release of the radionuclides from the wastes and the associated uncertainties. In order to better assess the degradation process of the nuclear waste packages, IRSN has developed an experimental program in collaboration with laboratories from the university, the ENSMSE (École nationale supérieure des mines de Saint-Étienne) and the CNRS (the French National Center for Scientific Research). The studies concern on the one hand the activated compacted wastes packages and on the other hand the spent fuel.Regarding compacted wastes packages, at the end of the light water reactors operation, the Zircaloy (zirconium alloy) cladding tubes had been activated and oxidised. After processing, these cladding tubes are compacted in a wafer form, placed into a steel container, and then into a concrete over-pack with a view to being disposed of in geological repository. These wastes are mostly composed of activated oxidized metal pieces which also contain traces of fission, activation products and actinides. In the repository, they are exposed to radioactivity in presence of resaturation water. The water radiolysis may accelerate the oxidised metal corrosion processes. By the way, the wastes degradation is mainly due to corrosion process. A fundamental study has thus been defined on the effects of water radiolysis on Zircaloy corrosion. The second study aims at better understanding the influence of the fuel matrix defects on the radiolytic degradation of spent fuel. As a matter of fact, fission products and actinides decay during storage may cause defects of the fuel matrix by changing its composition and microstructure. The study is performed in close collaboration with CNRS laboratory.
• Shear viscosity measurements at the vortex melting transition in confined geometry in optimally doped Bi2Sr2CaCu2O8
  
  • Yang Feng
  • van Der Beek Cornelis Jacominus
  • Konczykowski Marcin
  • Gierlowski Piotr
  • Briatico Javier
  • Bernard Rozenn
  • Li Ming
  • Kes Peter H.
  , 2009, 150, pp.052288. In order to probe the vortex shear viscosity in the vortex liquid phase, we have introduced two types of vortex-confining structures in optimally doped Bi2Sr2CaCu2O8 single crystals. First, walls of strong vortex pinning separated by weakly pinning channels are fashioned by heavy ion irradiation through 25 um-thick Ni masks. Second, a low density of homogeneously distributed amorphous columnar defects is known to impose a polycrystalline structure to the vortex lattice. Resistivity measurements show that the inclusion of confining structures impede vortex flow in the liquid. The resistivity is remarkably well described by the Halperin-Nelson theory for the viscosity due to free two-dimensional vortex lattice dislocations. (10.1088/1742-6596/150/5/052288)
  DOI : 10.1088/1742-6596/150/5/052288
• A vortex solid-to-liquid transition with fully anisotropic scaling
  
  • Andersson Magnus
  • Espinosa-Arronte Beatriz
  • van Der Beek Cornelis Jacominus
  • Nikolaou Marios
  • Lidmar Jack
  • Wallin Mats
  , 2009, 150, pp.052007. The vortex solid-to-liquid transition has been studied in heavy ion irradiated untwinned single crystals of YBa2Cu3O7− with an inclined applied magnetic field. For magnetic fields tilted at angles about 45 away from the columnar defects, we find that the electric resistivity in the vortex liquid regime approaches zero with power laws in the reduced temperature T − Tc that have different exponents in all three spatial directions. Since the symmetry in the problem has been broken in two non-collinear directions by i) the direction of the columnar defects and ii) the direction of the applied magnetic field, our findings give evidence for a new type of critical behavior with fully anisotropic critical exponents. A possible view of the vortex topology for the transition is also suggested. (10.1088/1742-6596/150/5/052007)
  DOI : 10.1088/1742-6596/150/5/052007
• Heating and high frequency nonlinearities in the surface impedance of high Tc superconductors
  
  • Kermorvant Julien
  • van Der Beek Cornelis Jacominus
  • Mage Jean-Claude
  • Marcilhac Bruno
  • Lemaître Yves
  • Bernard Rozenn
  • Briatico Javier
  , 2009, 150 (052107). Using the dielectric resonator method, we have investigated nonlinearities in surface impedance Zs = Rs + jXs of YBa2Cu3O7-delta thin films at 10 GHz as a function of the incident microwave power level and temperature. The use of a rutile dielectric resonator allows us to measure the precise temperature of the films. We conclusively show that the usually observed increase of the surface resistance of YBa2Cu3O7-delta thin film as function of microwave power is due to local heating. (10.1088/1742-6596/150/5/052107)
  DOI : 10.1088/1742-6596/150/5/052107
• Magneto-optical imaging of exotic superconductors
  
  • van Der Beek Cornelis Jacominus
  • Losco Jérôme
  • Konczykowski Marcin
  • Pari Patrick
  • Shibauchi Takasada
  • Shishido Hiroaki
  • Matsuda Yuji
  , 2009, 150, pp.012052. We have constructed a novel compact cryostat for optical measurements at temperatures below 2 K. The desktop cryostat, small enough to be placed under the objective of a standard commercial polarized light microscope, functions in a single shot mode, with a five hour autonomy at 1.5 K. Central to its conception are four charcoal pumps for adsorption and desorption of $^{4}$He contained in a closed circuit, and novel thermal switches allowing for thermalization of the pumps and of the two 1 K pots. The latter are connected to the 1" diameter sample holder through braids. Sample access is immediate, through the simple removal of the optical windows. In this contribution, we shall present first results on magneto-optical imaging of flux penetration in the heavy-fermion superconductor CeCoIn5. (10.1088/1742-6596/150/1/012052)
  DOI : 10.1088/1742-6596/150/1/012052
• Disorder and $c$-axis quasiparticle dynamics in underdoped Bi2Sr2CaCu2O8
  
  • van Der Beek Cornelis Jacominus
  • Spathis Panayotis
  • Yang Feng
  • Colson Sylvain
  • Gierlowski Piotr
  • Shibauchi Takasada
  • Matsuda Yuji
  • Gaifullin Marat
  • Li Ming
  • Kes Peter H.
  , 2009, 150, pp.052277. We present measurements of the Josephson plasma frequency and the in-plane penetration depth of underdoped single crystalline Bi2Sr2CaCu2O8 with varying degrees of disorder introduced by irradiation with 2.3 MeV electrons. Increasing disorder drives T_c down, in agreement with in all model descriptions of high T_c superconductivity. However, the manner in which the JPR frequency, the square of which represents the zero-frequency spectral weight of the c-axis conductivity in the superconducting state, is driven down by disorder depends more strongly on the model description. We show that only the model of impurity assisted quasiparticle hopping in a d-wave superconductor, together with strongly scattering point defects in the superconducting layers, can explain the disorder dependence of the c-axis plasma frequency, the in-plane penetration depth, and T_c consistently. From the data, we extract the energy scale governing nodal quasiparticle excitations, Delta_0 ~ 2.5 k_BT_c. (10.1088/1742-6596/150/5/052277)
  DOI : 10.1088/1742-6596/150/5/052277
• Comparison of ZnO nanostructures grown using pulsed laser deposition, metal organic chemical vapor deposition, and physical vapor transport
  
  • Sandana Eric Vinod
  • J. Rogers D.
  • Teherani Ferechteh Hosseini
  • Mcclintock R.
  • Bayram C.
  • Razeghi M.
  • Drouhin H.-J.
  • Clochard Marie-Claude
  • Sallet V.
  • Garry G.
  , 2009. This article compares the forms and properties of ZnO nanostructures grown on Si(111) and c-plane sapphire (c-Al(2)O(3)) substrates using three different growth processes: metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), and physical vapor transport (PVT). A very wide range of ZnO nanostructures was observed, including nanorods, nanoneedles, nanocombs, and some novel structures resembelling "bevelled" nanowires. PVT gave the widest family of nanostructures. PLD gave dense regular arrays of nanorods with a preferred orientation perpendicular to the substrate plane on both Si and c-Al(2)O(3) substrates, without the use of a catalyst. X-ray diffraction (XRD) studies confirmed that nanostructures grown by PLD were better crystallized and more highly oriented than those grown by PVT and MOCVD. Samples grown on Si showed relatively poor XRD response but lower wavelength emission and narrower linewidths in PL studies
• Reversible switching in phase-change materials
  
  • Welnic W.
  • Wuttig Matthias
  Materials Today, Elsevier, 2008, 11 (6), pp.20-27. Phase-change materials are successfully employed in optical data storage and are becoming a promising candidate for future electronic storage applications. Despite the increasing technological interest, many fundamental properties of these materials remain poorly understood. However, in the last few years the understanding of the material properties of phase-change materials has increased significantly. At the same time, great advances have been achieved in technological applications in electronic as well as optical data storage. We review the latest scientific and technological developments in the field, focusing on the electronic, optical, and kinetic properties of phase-change materials. (10.1016/S1369-7021(08)70118-4)
  DOI : 10.1016/S1369-7021(08)70118-4
• Influence of spatial variations in the lower critical field on the equilibrium field penetration into superconductors
  
  • Avraham Nurit
  • Brandt Ernst Helmut
  • Mikitik Grigorii P
  • Myasoedov Yuri
  • Rappaport Michael
  • Zeldov Eli
  • van Der Beek Cornelis Jacominus
  • Konczykowski Marcin
  • Tamegai Tsuyoshi
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2008, 77, pp.214525. The thermodynamic lower critical field Hc1 in Bi2Sr2CaCu2O8+y crystals, in the presence of columnar defects, was determined using magneto-optical measurements combined with the shaking technique. We observe a downward shift in Hc1 due to columnar defects. Using partially irradiated samples, we show that small local variations in Hc1 dramatically affect the way the field penetrates into superconductors. We present a theoretical model which provides a quantitative fit to the experimental results and explains how the influence of small sample inhomogeneities is enhanced by geometrical effects and leads to significant variations in the local-field distribution and penetration. (10.1103/PhysRevB.77.214525)
  DOI : 10.1103/PhysRevB.77.214525
• Massive Cluster Impact : Mechanisms and future Development
  
  • Della-Negra S.
  • Depaw J.
  • Pautrat M.
  • Rizza G.
  • Dunlop A.
  • Guillermier C.
  • Schweikert E. A.
  , 2008.
• Spectroscopic studies of Ti3+ ions speciation inside MgAl2O4 spinels
  
  • Lombard Pierre
  • Boizot Bruno
  • Ollier Nadège
  • Jouini Anis
  • Yoshikawa Akira
  , 2008, 311 (3), pp.899-903. An electron paramagnetic resonance (EPR) investigation on Ti-doped MgAl2O4 spinels has been made in order to study the Ti3+ sites. The study we present here concerns the angular dependencies of the Ti3+ EPR lines and the variations of the EPR spectra due to a modification of the TiO2 content or of the chemical composition from MgAl2O4 to MgAl4O7 and MgAl6O10. In all the studied samples except one, we observe the presence of both Mn2+ and Ti3+ ions. No correlation was observed between Ti3+ amount and TiO2 content; the titanium ions are located in three different sites: the octahedral B site of the spinel structure; the tetrahedral A site and a last site which remains unclear. On the contrary, the major part of the Mn2+ ions is assumed to be in the tetrahedral A site of the spinel structure and a minor part in the octahedral B site. Our work demonstrates the overall interest of EPR spectroscopy in the study of the paramagnetic optically active ions inside optical materials. (10.1016/j.jcrysgro.2008.09.131)
  DOI : 10.1016/j.jcrysgro.2008.09.131
• Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene
  
  • Kramberger C.
  • Hambach R.
  • Giorgetti C.
  • Rummeli M. H.
  • Knupfer M.
  • Fink J.
  • Buchner B.
  • Reining Lucia
  • Einarsson E.
  • Maruyama S.
  • Sottile F.
  • Hannewald K.
  • Olevano Valerio
  • Marinopoulos A. G.
  • Pichler T.
  Physical Review Letters, American Physical Society, 2008, 100 (19), pp.196803. We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion. (10.1103/PhysRevLett.100.196803)
  DOI : 10.1103/PhysRevLett.100.196803
• Effect of Sm-, Gd- codoping on structural modifications in aluminoborosilicate glasses under beta-irradiation.
  
  • Malchukova Evguenia
  • Boizot Bruno
  • Petite Guillaume
  • Ghaleb Dominique
  Journal of Non-Crystalline Solids, Elsevier, 2008, 354, pp.3592-3596. Two series of Sm-, Gd-codoped aluminoborosilicate glasses with different total rare earth content have been studied in order to examine the codoping effect on the structural modifications of beta-irradiated glasses. The data obtained by Electron Paramagnetic Resonance spectroscopy indicated that relative amount of Gd3+ ions located in network former position reveals non-linear dependence on Sm/Gd ratio. Besides, codoping leads to the evolution of the EPR signal attributed to defects created by irradiation: superhyperfine structure of boron oxygen hole centres EPR line becomes less noticeable and resolved with increase of Gd amount. This fact manifests that Gd3+ ions are mainly diluted in vicinity of the boron network. By Raman spectroscopy, we showed that the structural changes induced by the irradiation also reveal non-linear behaviour with Sm/Gd ratio. In fact, the shift of the Si-O-Si bending vibration modes has a clear minimum for the samples containing equal amount of Sm and Gd (50:50) in both series of the investigated glasses. In contrast, for single doped glass there is no influence of dopant's content on Si-O-Si shift (in case of Gd) or its diminution (in case of Sm) occurs which is explained by the reduction process influence. At the same time, no noticeable effect of codoping on Sm3+ intensity as well as on Sm2+ emission or on Sm reduction process was observed. (10.1016/j.jnoncrysol.2008.03.018)
  DOI : 10.1016/j.jnoncrysol.2008.03.018
• Optimization of contact conditions between iron base alloys and mercury at room temperature
  
  • Medina-Almazan L.
  • Rouchaud J.C.
  • Auger T.
  • Gorse D.
  Journal of Nuclear Materials, Elsevier, 2008, 375 (1), pp.102-112. (10.1016/j.jnucmat.2007.10.012)
  DOI : 10.1016/j.jnucmat.2007.10.012
• Blocking of alkaline migration under ionizing irradiation in Cr-doped oxide glasses
  
  • Boizot B.
  • Olivier F.Y.
  • Petite G.
  • Ghaleb D.
  Nuclear Instruments and Methods in Physics Research, North-Holland ; Elsevier [1981-1984], 2008, 266, pp.2966–2970. Effects of ionizing irradiation on different chromium-doped silicate glass compositions have been studied using EPR and Raman spectroscopies. We observe first reduction processes of Cr3+ and Cr6+ ions correlated with the dose integrated inside the glass. With the doping content in theses glasses, we observe also by EPR spectroscopy a complete blocking of defects production under ionizing radiation for samples doped with more than 1 mol% of Cr2O3. This result is explained by dynamical reversible trapping of the excitons produced during irradiation. Associated to the disappearance of defects produced by irradiation, a decrease of the glass structural changes are also detected by Raman spectroscopy at integrated doses higher than 109 Gy
• Biological functionalisation of nanoporous polymer membranes.
  
  • Cuscito Olivia
  , 2008. Poly (vinylidene fluoride) (beta-PVDF) nanoporous membranes were made by chemical revealing of tracks induced from swift heavy ions irradiation. Pore opening and radiican be varied in a controllable manner with the etching time. Nanopores size in nanometer scale (from 12 nm to 50 nm) appears to be linearly dependent to the etching time. It was then necessary to adapt the characterization tools to these membranes. Consequently, we resorted to the use of structural analysis methods (Scanning Electron Microscopy, Small Angle Neutron Scattering) and developed evaluation methods of the membranes transport properties like gas permeation and ionic diusion. Results obtained conrm the pores opening (break through) and the hydrophobicity of material, which we have modied with hydrophilic molecules. In this precise case, the grafting of acrylic acid was initiated by the radicals still remains after track-etching (called radio-grafting). This key result was obtained by a study of Electron Paramagnetic Resonance. The labelling of introduced chemical functionalities with fluorescent probes was a very effective mean to visualize very few amounts of molecules by confocal microscopy. The radiografting was found specically localized inside etched tracks. The protocol others the possibility to create a double functionality, the one localized inside the nanopores and the other on the surface of membranes. The modication of radio-grafting parameters (the acrylic acid concentration, solvent nature, use of transfer agent) and the chemical properties of the nanopore walls have a direct incidence on the transport properties. Key Words : PVDF-beta, track-etching, nanopores, radicals, radiografting, selective functionalisation, Small Angle Neutron Scattering, Confocal Laser Scanning Microscopy.
• $c$-axis coupling in underdoped Bi2Sr2CaCu2O8 with varying degrees of disorder
  
  • Spathis Panayotis
  • Colson Sylvain
  • Yang Feng
  • van Der Beek Cornelis Jacominus
  • Gierlowski Piotr
  • Shibauchi Takasada
  • Matsuda Yuji
  • Gaifullin Marat
  • Li Ming
  • Kes Peter H.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2008, 77 (5), pp.104503. The dependence of the Josephson Plasma Resonance (JPR) frequency in heavily underdoped Bi2Sr2CaCu2O8+\delta on temperature and controlled pointlike disorder, introduced by high-energy electron irradiation, is cross-correlated and compared to the behavior of the ab-plane penetration depth. It is found that the zero temperature plasma frequency, representative of the superfluid component of the c-axis spectral weight, decreases proportionally with T_c when the disorder is increased. The temperature dependence of the JPR frequency is the same for all disorder levels, including pristine crystals. The reduction of the c-axis superfluid density as function of disorder is accounted for by pair-breaking induced by impurity scattering in the CuO2 planes, rather than by quantum fluctuations of the superconducting phase. The reduction of the c-axis superfluid density as function of temperature follows a T^{2}--law and is accounted for by quasi-particle hopping through impurity induced interlayer states. (10.1103/PhysRevB.77.104503)
  DOI : 10.1103/PhysRevB.77.104503
• DYNAMICS OF ELECTRONS IN GRADIENT NANOSTRUCTURES ( EXACTLY SOLVABLE MODEL) .
  
  • Shvartsburg Alexander
  • Kuzmiak Vladimir
  • Petite Guillaume
  , 2008. A flexible multi-parameter exactly solvable model of potential profile, containing an arbitrary number of continuous smoothly shaped barriers and wells, both equal or unequal, characterized by finite values and continuous profiles of the potential and of its gradient, is presented. We demonstrate an influence of both gradient and curvature of these potentials on the electron transport and spectra of symmetric and asymmetric double-well (DW) potentials. The use of this model is simplified due to one to one correspondence between the algorithms of calculation of the transmittance of convex barriers and energy spectra of concave wells. We have shown that the resonant contrast between maximum and minimum in over-barrier reflectivity of curvilinear barrier exceeds significantly the analogous effect for rectangular barrier with the same height and width. Reflectionless tunneling of electrons below the bottom of gradient nanostructures forming concave potential barriers is considered. The analogy between dynamics of electrons in gradient fields and gradient optics of heterogeneous photonic barriers is illustrated.
• MOCVD growth of ZnO nanostrutures using Au droplets as catalysts - art. no. 68950Z
  
  • E. Sandana V.
  • J. Rogers D.
  • Hosseini Teherani F.
  • Mcclintock R.
  • Razeghi M.
  • Drouhin H.-J.
  • Clochard M.-C.
  • Sallet V.
  • Garry G.
  • Fayoud F.
  , 2008. ZnO nanostructures were synthesised by Metal Organic Chemical Vapor Deposition growth on Si (100) and c-Al2O3 substrates coated with a 5nm thick layer of Au. The An coated substrates were annealed in air prior to deposition of ZnO so as to promote formation of Au nanodroplets. The development of the nanodroplets was studied as a function of annealing duration and temperature. Under optimised conditions, a relatively homogeneous distribution of regular An nanodroplets was obtained. Using the An nanodroplets as a catalyst, MOCVD growth of ZnO nanostructures was studied. Scanning electron microscopy revealed nanostructures with various forms including commonly observed structures such as nanorods, nanoneedles and nanotubes. Some novel nanostructures were also observed, however, which resembled twist pastries and bevelled-multifaceted table legs.
• Etude des effets d'irradiation dans le polytype cubique du carbure de silicium par les techniques spectroscopiques de photoluminescence et de résonance paramagnétique électronique.
  
  • Lefevre Jérémie
  , 2008. Ce travail expérimental a consisté en l'étude des défauts ponctuels induits par une irradiation électronique dans la structure cristallographique cubique du carbure de silicium (SiC) au moyen des techniques spectroscopiques de photoluminescence à basse température (LTPL) et de résonance paramagnétique électronique (RPE). Le premier de ces deux outils de mesures a permis d'estimer l'énergie seujl de déplacement dans le sous-réseau silicium puis d'analyser la stabilité thermique des défauts d'irradiation dans le domaine des basses températures (10 – 300K) puis dans la gamme des hautes températures (300 – 1400 K). Par ailleurs, sur la base d'un modèle théorique récent, cette thèse a corroboré la proposition de l'antisite de silicium isolé pour le centre D! 1 dont la persistance au-delà de la température nominale de fonctionnement des réacteurs nucléaires à fission de génération IV, pour lequel SiC est en partie destiné, se révèle particulièrement problématique. Des mesures entreprises par RPE sous illumination ont enfin permises de détecter un nouveau défaut dans son état métastable de spin S=1, possiblement associé à une configuration d'interstitiel de silicium.