Publications

2016

• Ge- and Al-related point defects generated by gamma irradiation in nanostructured erbium-doped optical fiber preforms
  
  • León M.
  • Lancry M.
  • Ollier N.
  • Babu B. H.
  • Bigot L.
  • Hamzaoui H. El
  • Savelii I.
  • Pastouret A.
  • Burov E.
  • Trompier F.
  • Poumellec B.
  • Bouazaoui M.
  Journal of Materials Science, Springer Verlag, 2016, 51, pp.10245–10261. Erbium-doped amplifiers (EDFAs) are of special interest for space applications. In this environment, the ionizing radiations decrease the gain of these optical amplifiers, due to the ionization of defects precursors, mainly linked to dopants as Germanium (Ge), Aluminum (Al), or Phosphorus (P). The aim of this work is to study the influence of the Ge and Al relative concentration on the radiation resistance of different nanostructured fiber preforms, manufactured by Modified Chemical Vapor Deposition (MCVD), in which various types of nanoparticles (Er@SiO2-NP, Al2O3-NP, and Er@Al2O3-NP) have been introduced in the silica matrix. The radiation resistance of these fibers has been compared with that of standard MCVD Er-doped preforms. All of them have been characterized by optical absorption and Electronic Paramagnetic Resonance (EPR) spectroscopies before and after irradiation with a total gamma dose of 5.9 kGy. EPR results show that Al-related defects are not observed in fiber preforms with Ge concentrations higher than 4.4 wt%. We also demonstrated that NP technology can limit the formation of Aluminum-Oxygen Hole Centers (AlOHCs), reducing the Radiation-Induced Attenuation at the energy of interest for EDFAs. (10.1007/s10853-016-0253-5)
  DOI : 10.1007/s10853-016-0253-5
• Ion-shaping of embedded gold hollow nanoshells into vertically aligned prolate morphologies
  
  • Dufour Christian
  • Khomrenkov V
  • Coulon Pierre-Eugène
  • Amici Julia
  • Clochard Marie-Claude
  • Monnet Isabelle
  • Grygiel Clara
  • Perruchas Sandrine
  • Ulysse Christian
  • Largeau Ludovic
  • Rizza Giancarlo
  Scientific Reports, Nature Publishing Group, 2016, 6, pp.21116. Ion beam shaping is a novel technique with which one can shape nano-structures that are embedded in a matrix, while simultaneously imposing their orientation in space. In this work, we demonstrate that the ion-shaping technique can be implemented successfully to engineer the morphology of hollow metallic spherical particles embedded within a silica matrix. The outer diameter of these particles ranges between 20 and 60 nm and their shell thickness between 3 and 14 nm. Samples have been irradiated with 74 MeV Kr ions at room temperature and for increasing fluences up to 3.8 × 10$^{14}$ cm$^{−2}$. In parallel, the experimental results have been theoretically simulated by using a three-dimensional code based on the thermal-spike model. These calculations show that the particles undergo a partial melting during the ion impact, and that the amount of molten phase is maximal when the impact is off-center, hitting only one hemisphere of the hollow nano-particle. We suggest a deformation scenario which differs from the one that is generally proposed for solid nano-particles. Finally, these functional materials can be seen as building blocks for the fabrication of nanodevices with really three-dimensional architecture. (10.1038/srep21116)
  DOI : 10.1038/srep21116
• Effects of quantum confinement on excited state properties of SrTiO 3 from ab initio many-body perturbation theory
  
  • Reyes-Lillo Sebastian
  • Rangel Tonatiuh
  • Bruneval Fabien
  • Neaton Jeffrey
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2016, 94 (4). The Ruddlesden-Popper (RP) homologous series Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO$_3$. We use ab initio many-body perturbation theory within the $GW$ approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectrum of Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ for $n=1-5$ and $\infty$. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small $n$ and reach a value of 330~meV for $n=1$, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr$_2$TiO$_4$ ($n=1$) localizes in the 2D plane defined by the TiO$_2$ layer, and explain the origin of its localization. (10.1103/PhysRevB.94.041107)
  DOI : 10.1103/PhysRevB.94.041107
• Spin-transfer torque effects in the magnetization reversal
  
  • Kalmykov Yu. P.
  • Byrne D. J.
  • Coffee W. T.
  • Dowling W. J.
  • Titov S. V.
  • Wegrowe J. -E.
  , 2016.
• Leaching of radio-oxidized poly(ester urethane): Water-soluble molecules characterization
  
  • Fromentin E.
  • Pielawski M.
  • Lebeau D.
  • Esnouf S.
  • Cochin F.
  • Legand S.
  • Ferry M.
  Polymer Degradation and Stability, Elsevier, 2016, 128, pp.172-181. In the context of the geological nuclear waste storage, water-soluble products of an industrial poly(ester urethane) are investigated. This polymer was radio-oxidized up to 10 MGy with gamma rays under air at room temperature. The unirradiated and irradiated materials were hydrolyzed for 28 days at 60 C in pure water (MilliQ water). The solutions were characterized using different analytical tools (total organic carbon analyzer, ionic chromatography, gas chromatography coupled with mass spectrometry and tandem mass spectrometry). From 11 to 30 per cent of the water-soluble molecules present in the solutions have been quantified. Most of molecules identified are esters, alcohols, carboxylic acids, diols and ketones. Mechanisms were deduced from the detected molecules. It was observed that the irradiated material was about 70 times more sensitive to hydrolysis than the unirradiated one. (10.1016/j.polymdegradstab.2016.03.007)
  DOI : 10.1016/j.polymdegradstab.2016.03.007
• Improved ab initio calculation of surface second-harmonic generation from Si(111)( 1 × 1 ):H
  
  • Anderson Sean
  • Tancogne-Dejean Nicolas
  • Mendoza Bernardo
  • Véniard Valérie
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2016, 93 (23). (10.1103/PhysRevB.93.235304)
  DOI : 10.1103/PhysRevB.93.235304
• Evaluating the GW Approximation with CCSD(T) for Charged Excitations Across the Oligoacenes
  
  • Rangel Tonatiuh
  • Hamed Samia
  • Bruneval Fabien
  • Neaton Jeffrey
  Journal of Chemical Theory and Computation, American Chemical Society, 2016, 12 (6), pp.2834-2842. Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technological applications, are widely studied and therefore provide an excellent system for benchmarking theoretical methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approximation relative to new high-quality CCSD(T) reference data for charged excitations of the acenes. We compare GW calculations with a number of hybrid density functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with molecule length increasing from benzene to hexacene. We find that GW calculations with starting points based on an optimally tuned range-separated hybrid (OTRSH) density functional and eigenvalue self-consistency can yield quantitative ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from reference values. Our work paves the way for predictive and cost-effective GW calculations of charged excitations of molecules and identifies certain limitations of current GW methods used in practice for larger molecules. (10.1021/acs.jctc.6b00163)
  DOI : 10.1021/acs.jctc.6b00163
• Théorie de la spectroscopie électronique : au-delà de l'état de l'art
  
  • Zhou Jianqiang
  , 2016. Le sujet de cette thèse se place dans le cadre de la spectroscopie théorique. En particulier, je propose une nouvelle dérivation ab-initio pour trouver des approximations pour la fonction de Green (GF) à un corps. Cette approche conduit à une meilleure description du couplage fermion-plasmon dans le cadre de la théorie des perturbations à plusieurs corps (MBPT), qui peut être utilisée pour étudier la spectroscopie de photoémission directe et inverse.En spectroscopie de photoémission, un échantillon est irradié par des photons et des électrons sont émis. A partir de la différence d'énergie du photon incident et des d'électrons sortant, un grand nombre d'informations sur les propriétés de l'échantillon peut être obtenu, par exemple les structures de bandes ou la durée de vie des excitations. Dans une cadre de particules indépendantes, cette différence d'énergie correspond au niveau d'énergie d'une particule que l'électron émis occupait avant la mesure. Cela conduit à un pic très intense dans le spectre, avec un poids normalisé à un. En réalité, la photoémission n'est pas juste des photons entrants et des électrons indépendants sortants, car l'échantillon est un système à plusieurs corps en interaction. L'interaction de Coulomb et la nature anti-symétrique des fermions donnent lieu aux effets d'échange-corrélation, ce qui rend le problème fondamentalement difficile à résoudre. La description, la compréhension et la prédiction des effets de l'interaction de Coulomb sur les propriétés des matériaux a été, pendant des années, l'un des grands défis de la physique théorique de la matière condensée. Dans le cadre de cette thèse, on peut imaginer que, premièrement, la photoémission crée un trou (à savoir, un électron manquant) dans l'échantillon, ce qui provoque la relaxation de tous les électrons restants. En raison de l'interaction attractive entre les trous chargés positivement et les électrons chargés négativement, les électrons se déplacent vers les trous et créent des ''quasi-particules''. L'interaction effective entre les quasi-particules est l'interaction de Coulomb écrantée dynamiquement. Elle est en général plus faible que l'interaction de Coulomb nue. Par conséquent, la structure de bandes observée est celle de quasi-particules, qui diffère du résultat en particules indépendantes. Deuxièmement, lorsque le trou se propage dans l'échantillon les électrons restants peuvent présenter des oscillations collectives : réponse de la densité à la perturbation. Ce sont des excitations neutres avec une nature approximativement bosonique, parce qu'elles sont constituées par des paires de fermions.Le couplage du trou avec les excitations neutres conduit à des structures supplémentaires dans le spectre de photoémission, appelées satellites. Cela réduit le poids des quasi-particules qui est maintenant fractionnée. Le plus souvent, les satellites dominants sont dus à des plasmons, des oscillations collectives à longue portée, mais on peut aussi observer des transitions ou excitons interbandes ou d'autres satellites qui sont dus à des couplages plus complexes.Cela montre que pour avoir une bonne description de la spectroscopie de photoémission, nous devrions étudier la propagation de particules, ainsi que l'interaction entre les particules et les plasmons ou d'autres excitations. La fonction de Green donne l'amplitude de probabilité de particules se propageant d'un point à un autre. Sa partie imaginaire donne la funtion spectrale qui a un lien direct vers le spectre mesuré dans une expérience de photoémission. Les dérivations et approximations proposées dans cette thèse donnent une nouvelle façon de calculer la fonction de Green, ce qui améliore la description de la spectroscopie de photoémission. En outre, cela permet d'accéder à d'autres grandeurs qui peuvent être obtenues à partir de la fonction de Green à un corps, en particulier les énergies totales.
• Femtosecond laser-induced damage: lessons learned from time-resolved measurements
  
  • Melnikaitis Andrius
  • Mongaudys Balys
  • Smalakys Linas
  • Grigutis Robertas
  • Vaicenavičius Julius
  • Sirutkaitis Valdas
  • Guizard Stéphane
  • Gallais Laurent
  , 2016.
• Manipulating the Topological Interface by Molecular Adsorbates: Adsorption of Co-Phthalocyanine on Bi 2 Se 3
  
  • Caputo Marco
  • Panighel Mirko
  • Lisi Simone
  • Khalil Lama
  • Santo Giovanni Di
  • Papalazarou Evangelos
  • Hruban Andrzej
  • Konczykowski Marcin
  • Krusin-Elbaum Lia
  • Aliev Ziya
  • Babanly Mahammad
  • Otrokov Mikhail
  • Politano Antonio
  • Chulkov Evgueni
  • Arnau Andrés
  • Marinova Vera
  • Das Pranab
  • Fujii Jun
  • Vobornik Ivana
  • Perfetti Luca
  • Mugarza Aitor
  • Goldoni Andrea
  • Marsi Marino
  Nano Letters, American Chemical Society, 2016, 16 (6), pp.3409-3414. Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal–organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer. (10.1021/acs.nanolett.5b02635)
  DOI : 10.1021/acs.nanolett.5b02635
• Supercritical CO2 extraction of molybdenum-ligand complexes from sulfuric solutions
  
  • Hung Laurence
  • Hertz Audrey
  • Hartmann Didier
  • Charton Frédéric
  • Boutin Olivier
  Journal of Supercritical Fluids, Elsevier, 2016, 111, pp.97–103. The development of environmental-friendly process for strategic metal extraction, limiting organic solvent use and effluent production, appears to be quite a challenging purpose. Production of pure molybdenum using supercritical CO2 extraction process, from sulfuric solution obtained by ore or used catalyst leaching, has been evaluated. Two organic ligands, trioctylamine and 2-ethylhexyl 2-ethylhexylphosphonic acid, were studied to extract Mo as metal complexes solubilized in supercritical CO2. Extraction with trioctylamine revealed to be non-efficient due to a lack of Mo-trioctylamine complex solubility in CO2, even if extraction selectivity of molybdenum versus iron impurity was interesting. On the contrary, extraction with 2-ethylhexyl 2-ethylhexylphosphonic acid leads to high Mo collection efficiency (up to 90% in 7 h). However, no selectivity was observed between molybdenum and zirconium. The selectivity of 2-ethylhexyl 2-ethylhexylphosphonic acid towards molybdenum in presence of iron was better but clearly decreased in presence of zirconium. (10.1016/j.supflu.2016.01.017)
  DOI : 10.1016/j.supflu.2016.01.017
• In situ observation of the Yb2+ emission in the radiodarkening process of Yb-doped optical preform
  
  • Ollier Nadège
  • Corbel Catherine
  • Duchez Jean Bernard
  • Cavani Olivier
  • Benabdesselam Mourad
  • Mady Franck
  Optics Letters, Optical Society of America - OSA Publishing, 2016. This Letter relates the clear evidence of Yb2+ formation under 2.5 MeV electron irradiation in optical fiber preforms showing a darkening of the core. We thus detected by in situ photoluminescence measurements the green emission of divalent Yb2+ under the 355 nm excitation. Moreover, we showed the existence of two types of Yb2+ ion species with different stabilities. We demonstrated that the radiodarkening mechanism is based on a pair association of Yb2+ with aluminum oxygen hole center point defects. (10.1364/OL.41.002025)
  DOI : 10.1364/OL.41.002025
• Formation of hot-electron ensembles quasiequilibrated in momentum space by ultrafast momentum scattering of highly excited hot electrons photoinjected into the Γ valley of GaAs
  
  • Tanimura Hiroshi
  • Kanasaki Jun'Ichi
  • Tanimura Katsumi
  • Sjakste Jelena
  • Vast Nathalie
  • Calandra Matteo
  • Mauri Francesco
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2016, 93 (16). We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the Γ valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the Γ valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction. (10.1103/PhysRevB.93.161203)
  DOI : 10.1103/PhysRevB.93.161203
• Atomistic study of hetero-phase, semi-coherent interfaces between immiscible metals : The cases of AgCu and CuW.
  
  • Pontikis Vassilis
  • Baldinozzi Gianguido
  • Bose Abhishek
  • Li Chenyi
  • Luneville Laurence
  • Simeone David
  , 2016.
• Radiation damages in nuclear waste glasses: a « NMR point of view »
  
  • Charpentier Thibault
  • Peuget S.
  • Le Gac A.
  • Boizot Bruno
  • Rountree C.L.
  • Martel L.
  • Somers J.
  , 2016. Borosilicate glasses have been recognized as valuable materials for the conditioning of nuclear wastes. An important issue for their long-term behaviour is radiation effects which may impact their performance and stability. To address these concerns, a fundamental understanding of the origin at the atomic scale of the macroscopic property evolutions must be established. Over the la st decade, magic-angle spinning nuclear magnetic resonance (MAS NMR) has firrrùy established itself as one of the most powerful tool to investigate a glass's structure. It offers several probes of the local structure, nuclei such as 11B, 23Na, 27 Al, 29Si and 17 0, to probe changes either in the glass network or in the alkali distribution. Recently, using external heavy ions irradiation (Xe, Au, Kr) ta simulate alpha decays,[1-3] dramatic changes in the local network structure were evidenced : conversion of tetrahedral B0 4 units into planar trigonal B0 3 units (l1B), appearance of high-coordination aluminum units (AlOs, AI0 6); glass depolymerisation e 9 Si) and changes in the distribution of alkali cations CZ 3 Na). Additionally, the spectra broaden globally which supports the hypothesis of an increased topological disorder after irradiation. AH these structural changes are similar to those observed with increasing the glass temperature or quenching rate and support therefore the model of ballistic disordering fast quenching events which induce a new glassy state with higher fictive temperature. Effects of external electronic (beta) irradiations will be also discussed. If NMR spectra variations show similar trends-but much less pronounced-they are mainly engendered by alkali migration phenomena and formation of molecular oxygen. Until recently, these studies were limited to externally irradiated samples (enabling the different components of irradiation to be dissociated for their precise investigation), but recently, the first MAS-NMR experiments could be performed on radioactive glass es (doped with 244Cm 0.1 % mol.) paving the way for future MAS NMR exarninations of self-irradiation damages in glasses. Experiments were performed at the Joint Research Centre Institute for Transuranium Elements (JRC-ITU) where a commercial NMR spectrometer were integrated with a radioactive glovebox and a MAS commercial probe. Pirst results will be presented. Competitive effects between the recoil nuclei and alpha decays were evidenced and the high resistance of the nuclear waste glasses corroborated.
• Ab initio simulation of extended defects of &-Ti in presence of interstitial atoms H & O
  
  • Liang Liang
  , 2016. ABSTRACT: The aim of this thesis is to study the influence of hydrogen or oxygen solutes on extended defects in alpha titanium by ab initio calculation. Results are divided into three parts. In a first part the octahedral interstitial site of alpha-Ti is found energetically more favorable for a H or an O atom. The presence of H increases the volume while O has the opposite effect. The presence of H slightly decreases the elastic constants of alpha-Ti while O has an opposite effect. In a second part two new SFs are found: 0.57·(c+a) on π2 and 0.215·[1-102] on π1 plane. The second one is related to the low formation energy of the {10-11} twin boundary. A c+a screw dislocation 3-part dissociation mechanism is proposed. However the c+a screw core tends to spread differently according to the initial core position and a complete 3-part dissociation is not found, which may mean that such a dissociation is not easily obtainable from an initially perfect dislocation core. As segregation to SF means a decrease of the SF energy, the presence of O may make the SF formation energetically more difficult, contrary to H case. H strongly segregates to the a screw dislocation core region with segregation energies varing from 0.06 to 0.3 eV while O hardly segregates to it. Both H and O in core sites change the meta-stable gliding prismatic dissociation to π1 plane or a prism-π1 plane mixed configuration. According to our measurements of Peierls energy barriers with H or O in different sites and concentrations, H makes the gliding more difficult, thus increasing the CRSS in prismatic plane, in agreement with experimental measurements. The effect of H is not big enough to induce a cross-slip of the gliding a-screw dislocation to the π1 plane and that screw will prefer to keep on gliding in its same prismatic plane. The Peierls energy barrier is extremely increased when an O is present in the core position, much higher than the barrier for π1 plane glide or a glide in the nearest prismatic plane. A cross-slip could happen in this case. In the last part, three different deformations are applied to TBs. Their structural stabilities depend not only on their intrinsic characters at the atomistic level but also on the deformation mode applied. {10-12}, {11-22} TB structures fail for deformations as low as 1% or 2% along the c-axis. The {11-21} and the {10-11} TBs are much more resistant. The presence of segregated H and O enhances the {10-12} and {11-22} TB limited stability. A twinning disconnection dipole model is proposed which allows the simulation of a TD in a size limited supercell. Segregation energy calculations with the {10-12} TB and its TD validate the model at the TB level and show that H and O should distribute more or less homogeneously to the TD core and the TB, with only a slight preference to the TD core although not at the interstitial sites of the atomic layer related to the disconnection step itself.
• Prediction of thermal conductivity and strategies for heat transport reduction in bismuth : an ab initio study .
  
  • Markov Maksim
  , 2016. This work is devoted to the theoretical investigation of the heat conduction in bulk bismuth and the possible strategies for its reduction. Thermal properties of Bi are extremely interesting because of its low thermal conductivity that makes this material suitable for the thermal management applications. Moreover, bismuth is an excellent model substance for the study of thermoelectricity and bismuth-based compounds such as Bi2 Te3 and Bi2 Se3 which are typical thermoelectric materials used in industrial applications.In collaboration with L. Paulatto (IMPMC), G. Fugallo (Ecole Polytechnique), F. Mauri(IMPMC) and M. Lazzeri (IMPMC) I have applied the recently developed advanced methods of the solution of the Boltzmann transport equation (BTE) and of the phonon-phonon matrix elements calculation to describe thermal transport in bismuth. I have obtained the temperature dependence of the lattice thermal conductivity which is in excellent agreement with experiment. Moreover I am able to predict the lattice thermal conductivity (LTC) at temperatures at which it has not been measured. I have found that most of heat is carried by the acoustic phonons. However, the optical phonons were shown to play an important role by modulating the magnitude of the acoustic-optical phonon interaction (AOPI) and thus the value of the lattice thermal conductivity. Furthermore, I have shown that the available experimental data for the lattice thermal conductivity for polycrystalline thin-films are remarkably explained by my calculations, which enables me to predict the effect of the LTC size reduction for various temperatures and nanostructure shapes and sizes.The methods I use contain no empirical fitting parameters and give a direct insight into the microscopic mechanisms determining the transport and anharmonic properties of the materials. This allows me to analyze the anharmonic broadening that is inversely proportional to the phonon lifetime, for the various phonon modes along the high symmetry directions in the Brillouin zone and show what are the major scattering channels for coalescence/decays of phonons that govern the thermal transport in Bi.
• Stable room-temperature ferromagnetic phase at the FeRh(100) surface
  
  • Pressacco Federico
  • Uhlίř Vojtěch
  • Gatti Matteo
  • Bendounan Azzedine
  • Fullerton Eric E
  • Sirotti Fausto
  Scientific Reports, Nature Publishing Group, 2016, 6 (1). Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at ~400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We find that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that support the experimental findings. (10.1038/srep22383)
  DOI : 10.1038/srep22383
• Quantum Thermal Bath for Path Integral Molecular Dynamics Simulation
  
  • Brieuc Fabien
  • Dammak Hichem
  • Hayoun Marc
  Journal of Chemical Theory and Computation, American Chemical Society, 2016, 12 (3), pp.1351 - 1359. The quantum thermal bath (QTB) method has been recently developed to account for the quantum nature of the nuclei by using standard molecular dynamics (MD) simulation. QTB-MD is an efficient but approximate method for dealing with strongly anharmonic systems, while path integral molecular dynamics (PIMD) gives exact results in a huge amount of computation time. The QTB and PIMD methods have been combined in order to improve the PIMD convergence or correct the failures of the QTB-MD technique. A new power spectral density of the random force within the QTB has therefore been developed. A modified centroid-virial estimator of the kinetic energy, especially adapted to QTB-PIMD, has also been proposed. The method is applied to selected systems: a one-dimensional double well system, a ferroelectric phase transition, and the position distribution of an hydrogen atom in a fuel cell material. The advantage of the QTB-PIMD method is its ability to give exact results with a more reasonable computation time for strongly anharmonic systems. † (10.1021/acs.jctc.5b01146)
  DOI : 10.1021/acs.jctc.5b01146
• On magnetic monopoles, the anomalous g-factor of the electron and the spin-orbit coupling in the Dirac Theory
  
  • Coddens Gerrit
  , 2016. We discuss the algebra and the interpretation of the anomalous Zeeman effect and the spin-orbit coupling within the Dirac theory. Whereas the algebra for the anomalous Zeeman effect is impeccable and therefore in excellent agreement with experiment, the physical interpretation of that algebra uses images that are based on macroscopic intuition but do not correspond to the meaning of this algebra. The interpretation violates the Lorentz symmetry. We therefore reconsider the interpretation to see if we can render it consistent also with the symmetry. The results confirm clearly that the traditional physical interpretation of the anomalous Zeeman effect is not correct. We give an alternative intuitive description of the meaning of this effect, which respects the symmetry and is exact. It can be summarized by stating that a magnetic field makes any charged particle spin. This is even true for charged particles " without spin ". Particles " with spin " acquire additional spin in a magnetic field. This additional spin must be combined algebraically with the pre-existing spin. We show also that the traditional discussion about magnetic monopoles confuses two issues, viz. the symmetry of the Maxwell equations and the quantization of charge. These two issues define each a different concept of magnetic monopole. They cannot be merged together into a unique all-encompassing issue. We also generalize the minimal substitution for a charged particle, and provide some intuition for the magnetic vector potential. We finally explore the algebra of the spin-orbit coupling, which turns out to be badly wrong. The traditional theory that is claimed to reproduce the Thomas half is based on a number of errors. An error-free application of the Dirac theory cannot account for the Thomas precession, because it only accounts for the instantaneous local boosts, not for the rotational component of the Lorentz transformation. This runs contrary to established beliefs, but can be understood in terms of the Berry phase on a path through the Lorentz group manifold. These results clearly reveal the limitations of the prevailing working philosophy to " shut up and calculate ".
• Ab initio electronic stopping power of protons in bulk materials
  
  • Shukri Abdullah Atef
  • Bruneval Fabien
  • Reining Lucia
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2016, 93 (3). The electronic stopping power is a crucial quantity for ion irradiation: it governs the deposited heat, the damage profile, and the implantation depth. Whereas experimental data are readily available for elemental solids, the data are much more scarce for compounds. Here we develop a fully ab initio computational scheme based on linear response time-dependent density-functional theory to predict the random electronic stopping power (RESP) of materials without any empirical fitting. We show that the calculated RESP compares well with experimental data, when at full convergence, with the inclusion of the core states and of the exchange correlation. We evaluate the unexpectedly limited magnitude of the nonlinear terms in the RESP by comparing with other approaches based on the time propagation of time-dependent density-functional theory. Finally, we check the validity of a few empirical rules of thumbs that are commonly used to estimate the electronic stopping power. (10.1103/PhysRevB.93.035128)
  DOI : 10.1103/PhysRevB.93.035128
• Origin of the Degradation of Triple Junction Solar Cells at low Temperature
  
  • Park Seonyong
  • Bourgoin Jacques C.
  • Cavani Olivier
  • Khorenko Victor
  • Baur Carsten
  • Boizot Bruno
  , 2017, 16. (10.1051/e3sconf/20171604004)
  DOI : 10.1051/e3sconf/20171604004
• Surface and bulk electron irradiation effects in simple and complex glasses
  
  • Mir Anamul H.
  • Boizot B.
  • Charpentier T.
  • Gennisson M.
  • Odorico Michaël
  • Podor Renaud
  • Jégou C.
  • Bouffard S.
  • Peuget S.
  Journal of Non-Crystalline Solids, Elsevier, 2016, 453, pp.141 - 149. (10.1016/j.jnoncrysol.2016.10.009)
  DOI : 10.1016/j.jnoncrysol.2016.10.009
• The Magnetic Monopole and the Separation between Fast and Slow Magnetic Degrees of Freedom
  
  • Wegrowe J-E
  • Olive Enrick
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2016, 28 (10), pp.106001. The Landau\textendashLifshitz\textendashGilbert (LLG) equation that describes the dynamics of a macroscopic magnetic moment finds its limit of validity at very short times. The reason for this limit is well understood in terms of separation of the characteristic time scales between slow degrees of freedom (the magnetization) and fast degrees of freedom. The fast degrees of freedom are introduced as the variation of the angular momentum responsible for the inertia. In order to study the effect of the fast degrees of freedom on the precession, we calculate the geometric phase of the magnetization (i.e. the Hannay angle) and the corresponding magnetic monopole. In the case of the pure precession (the slow manifold), a simple expression of the magnetic monopole is given as a function of the slowness parameter, i.e. as a function of the ratio of the slow over the fast characteristic times. (10.1088/0953-8984/28/10/106001)
  DOI : 10.1088/0953-8984/28/10/106001
• Interpretation of monoclinic hafnia valence electron energy-loss spectra by time-dependent density functional theory
  
  • Hung L.
  • Guedj C.
  • Bernier N.
  • Blaise P.
  • Olevano V.
  • Sottile F.
  Physical Review B, American Physical Society, 2016, 93 (16), pp.165105. We present the valence electron energy-loss spectrum and the dielectric function of monoclinic hafnia (m-HfO$_2$) obtained from time-dependent density-functional theory (TDDFT) predictions and compared to energy-filtered spectroscopic imaging measurements in a high-resolution transmission-electron microscope. Fermi's Golden Rule density-functional theory (DFT) calculations can capture the qualitative features of the energy-loss spectrum, but we find that TDDFT, which accounts for local-field effects, provides nearly quantitative agreement with experiment. Using the DFT density of states and TDDFT dielectric functions, we characterize the excitations that result in the m-HfO$_2$ energy loss spectrum. The sole plasmon occurs between 13-16 eV, although the peaks $\sim$28 and above 40 eV are also due to collective excitations. We furthermore elaborate on the first-principles techniques used, their accuracy, and remaining discrepancies among spectra. More specifically, we assess the influence of Hf semicore electrons (5$p$ and 4$f$) on the energy-loss spectrum, and find that the inclusion of transitions from the 4$f$ band damps the energy-loss intensity in the region above 13 eV. We study the impact of many-body effects in a DFT framework using the adiabatic local-density approximation (ALDA) exchange-correlation kernel, as well as from a many-body perspective using a $GW$-derived electronic structure to account for self-energy corrections. These results demonstrate some cancellation of errors between self-energy and excitonic effects, even for excitations from the Hf $4f$ shell. We also simulate the dispersion with increasing momentum transfer for plasmon and collective excitation peaks. (10.1103/PhysRevB.93.165105)
  DOI : 10.1103/PhysRevB.93.165105