Publications

2018

• Ab initio study of electronic surfaces states and plasmons of gold : role of the spin-orbit coupling and surface geometry.
  
  • Motornyi Oleksandr
  , 2018. The PhD thesis is devoted to the ab initio study of surface plasmons and surface states offlat and vicinal surfaces of Au through the simulation of electron energy loss (EEL) spectraby means of the density functional theory (DFT) and the time-dependent density func-tional perturbation theory (TDDFPT). The influence of the spin-orbit coupling (SOC)and of the surface geometry has been investigated. In bulk Au I have studied the effect ofthe inclusion of semi-core electrons on the EEL spectrum at q = 0 and the plasmon peakposition and intensity. In particular, I have shown that in order to reproduce the EELspectrum on a wide frequency range (0-60 eV) it is important to account for semi-coreelectrons in the pseudopotential although they can be frozen in the core in studies of thelow energy part of the spectrum (below 20 eV). I have made methodological developmentsfor TDDFPT with SOC in the ultrasoft pseudopotential scheme that led to the practicalimplementation of SOC in the Liouville-Lanczos and Sternheimer approaches. I have thensuccessfully applied these approaches that allowed me to model systems with hundreds ofatoms. I have revisited the plasmonic excitations in bulk Au, pointing out that, in partic-ular, one can observe traces of an unscreened s-like bulk plasmon in the EEL spectrum atq = 0 calculated without SOC. I have also demonstrated that SOC has a small but notice-able effect on the Au EEL spectrum and plasmon peak, mainly modifying the unscreeneds-like plasmon peak and thus bringing the calculated spectrum into a better agreementwith experimental results at q = 0. Moreover I have observed that the dispersion ofthe acoustic surface plasmon (ASP) on the Au(111) surface is slightly modified by SOC,because the ASP comes from the surface state that itself is modified by SOC through theRashba splitting. To investigate the effect of geometry I have studied the vicinal (322),(455) and (788) surfaces of Au. In particular I have performed the theoretical study of thesurface states, analyzing the evolution of the Shockley surface state from the flat Au(111)surface towards the surfaces with terraces of different width. I have shown the surfaceresonance-to-surface state transition from (322) to (455) and (788) surfaces. I have shownalso the transition from the average-surface-modulated to the terrace-modulated statefrom (322) to (455) and (788) surfaces, as well as the transition from the extended 2Dstate to the quasi-1D state confined within the terrace. These results are in agreementwith experiments and results obtained with the Kronig-Penney periodic potential model.I have performed the EEL spectrum calculations for the Au(455) surface which I havemodeled with a 5 nm sized slab separated from its periodic neighbors by 5 nm of vacuum.I have identified signatures of the ASP in these spectra, showing that indeed, for the caseof the transferred electron wavevector momentum perpendicular to the step, the ASPdispersion is not changed with respect to the ASP dispersion of the Au(111) surface forq < 0.125 Å −1 . For bigger values of q, however, the ASP peak has a lower energy com-pared to the ASP peak of the Au(111) surface, showing signs of the ASP confinement, andsuggesting that two types of the ASP could occur: an intra(sub)band plasmon, similarto the Au(111) surface plasmon, and an inter(sub)band plasmon, characteristic of thisvicinal surface.
• Band Gap Renormalization, Carrier Multiplication, and Stark Broadening in Photoexcited Black Phosphorus
  
  • Chen Zhesheng
  • Dong Jingwei
  • Papalazarou Evangelos
  • Marsi Marino
  • Giorgetti Christine
  • Zhang Zailan
  • Tian Bingbing
  • Rueff Jean-Pascal
  • Taleb-Ibrahimi Amina
  • Perfetti Luca
  Nano Letters, American Chemical Society, 2018, 19 (1), pp.488-493. We investigate black phosphorus by time- and angle-resolved photoelectron spectroscopy. The electrons excited by 1.57 eV photons relax down to a conduction band minimum within 1 ps. Despite the low band gap value, no relevant amount of carrier multiplication could be detected at an excitation density 3–6 × 1019 cm–3. In the thermalized state, the band gap renormalization is negligible up to a photoexcitation density that fills the conduction band by 150 meV. Astonishingly, a Stark broadening of the valence band takes place at an early delay time. We argue that electrons and holes with a high excess energy lead to inhomogeneous screening of near surface fields. As a consequence, the chemical potential is no longer pinned in a narrow impurity band. (10.1021/acs.nanolett.8b04344)
  DOI : 10.1021/acs.nanolett.8b04344
• Breakdown of Herring's processes in cubic semiconductors for sub-terahertz longitudinal acoustic phonons
  
  • Markov Maxime
  • Sjakste Jelena
  • Vast Nathalie
  • Legrand Romain
  • Perrin Bernard
  • Paulatto Lorenzo
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2018. In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy ω in the sub-THz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes as Herring's mechanism, which dominates at low frequencies, saturates and disappears. The plateau is shown to be determined by the phononic final-state phase-space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2-1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors. (10.1103/PhysRevB.98.245201)
  DOI : 10.1103/PhysRevB.98.245201
• Design numérique de métamatériaux pour des applications photovoltaïques
  
  • Iagupov Ilia
  , 2018. The purpose of the thesis was to simulate the absorption spectrum of meta-materials for photovoltaic applications. By meta-material, we mean an assembly of nanometric size objects at mesoscopic distance. The underlying idea is that by adjusting the size of the nano-object and the geometric arrangement, one could tune the absorption edge. To calculate these quantities, I used state-of-the art formalism, namely ab-initio methods.The first step of the work has been dedicated to the calculation of the absorption of an isolated object (slab of silicon, graphene, hBN). In the framework of periodic codes, one uses a supercell with vacuum to isolate the object, and a method has been developed previously in the Theoretical spectroscopy group at LSI, to provide results independent of vacuum. It is called “Selectd-G” method, and was successfully applied to silicon surfaces. For an isolated slab, a modified expression of the reciprocal space Coulomb potential, called “slab potential”, must be used. To validate the use of the slab potential on the microscopic dielectric matrix, I have simulated Electron Energy Loss spectra for slabs of few graphene layers, and successfully reproduced available experimental data. This has also offered the possibility to study the plasmon dispersion of a single graphene layer, and discuss the nature of electronic excitations in the system (intraband transitions or 2D-plasmon).The second step has been dedicated to the study of the absorption spectrum of an array of interacting slabs. Since it has been evidenced that the supercell formalism acts as an effective medium theory with vacuum, with the spurious effect of having spectra dependent on the size of the supercell, I have reversed the procedure to extract the spectrum of the interacting slab, "cured" from the vacuum problem. First, the feasibility has been demonstrated on slabs of hBN, as their semi-conducting characteristics with a the large gap prevent numerical instabilities. Then, it has allowed us to understand the reason why the absorption of the interacting slab of silicon appears at lower energy than its bulk counterpart: it is due to the presence of surface states in the gap of the bulk band structure. Nevertheless, the difference with the isolated slab must be further investigated.The third part has been dedicated to the study of materials currently used or candidates for photovoltaic applications: InP and InSe. I have first studied the band structures of bulk InP and InSe. To correct for the underestimation of the band gap in the local density approximation (LDA), I have used GW corrections and the Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation functional. The absorption spectrum for bulk InP has been calculated by means of the solution of the Bethe-Salpeter equation to correctly account for the excitonic effects. As expected, the experimental macroscopic function is well reproduced. Since the calculation is numerically demanding, I have also compared the results with the much lighter calculation using TDDFT where I used the long range kernel to mimic the excitonic effects. For bulk InSe, I have calculated the HSE corrections for the eigenvalues and obtained a good agreement with the experimental band gap. The spectrum obtained within TDDFT, with the long range kernel, gives satisfying results. We have started the calculations for slabs of these two materials. Thick slabs of InP and InSe have been considered and a 2x2 reconstruction have been performed for the InP slab to recover the semi-conducting surface. The LDA band structures and absorption spectra have been calculated. Then, such large systems being out of range of HSE corrections calculations, the study has been focused on much thiner slabs in the case of InSe.
• Nuclear quantum effects in molecular dynamics simulations
  
  • Dammak H.
  • Hayoun M.
  • Brieuc F
  • Geneste G.
  Journal of Physics: Conference Series, IOP Science, 2018, 1136, pp.012014. To take into account nuclear quantum effects on the dynamics of atoms, the path integral molecular dynamics (PIMD) method used since 1980s is based on the formalism developed by R. P. Feynman. However, the huge computation time required for the PIMD reduces its range of applicability. Another drawback is the requirement of additional techniques to access time correlation functions (ring polymer MD or centroid MD). We developed an alternative technique based on a quantum thermal bath (QTB) which reduces the computation time by a factor of ~20. The QTB approach consists in a classical Langevin dynamics in which the white noise random force is replaced by a Gaussian random force having the power spectral density given by the quantum fluctuation-dissipation theorem. The method has yielded satisfactory results for weakly anharmonic systems: the quantum harmonic oscillator, the heat capacity of a MgO crystal, and isotope effects in 7 LiH and 7 LiD. Unfortunately, the QTB is subject to the problem of zero-point energy leakage (ZPEL) in highly anharmonic systems, which is inherent in the use of classical mechanics. Indeed, a part of the energy of the high-frequency modes is transferred to the low-frequency modes leading to a wrong energy distribution. We have shown that in order to reduce or even eliminate ZPEL, it is sufficient to increase the value of the frictional coefficient. Another way to solve the ZPEL problem is to combine the QTB and PIMD techniques. It requires the modification of the power spectral density of the random force within the QTB. This combination can also be seen as a way to speed up the PIMD. (10.1088/1742-6596/1136/1/012014)
  DOI : 10.1088/1742-6596/1136/1/012014
• Drapeaux piezolélectriques de nouveaux générateurs à la frontière entre physqiue et mécanique
  
  • Michelin Sébastien
  • Clochard Marie-Claude
  La jaune et la rouge [revue mensuelle de la société amicale des anciens élèves de l'Ecole Polytechnique], Association des anciens élèves de l'Ecole Polytechnique, 2018 (740), pp.38-41.
• Using controlled disorder to probe the interplay between charge order and superconductivity in NbSe$_2$
  
  • Cho Kyuil E
  • Kończykowski M.
  • Teknowijoyo S.
  • Tanatar A.
  • Guss J.
  • Gartin P.
  • Wilde J.
  • Kreyssig A.
  • Mc Queeney R.
  • Goldman A.
  • Mishra V.
  • Hirschfeld P.
  • Prozorov R.
  Nature Communications, Nature Publishing Group, 2018, 9, pp.2796. The interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe$_2$ is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, T$_c$, varies non-monotonically, whereas the CDW transition temperature, T$_{CDW}$, monotonically decreases and becomes unresolvable above a critical irradiation dose where T$_c$ drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing. (10.1038/s41467-018-05153-0)
  DOI : 10.1038/s41467-018-05153-0
• Croissance en une seule étape de nanotubes de carbone verticalement alignés sur des feuilles d'aluminium
  
  • Reynaud Cécile
  • Pinault Mathieu
  • Mayne-L'Hermite Martine
  • Nassoy Fabien
  • Descarpentries Jéremie
  • Vignal Thomas
  • Banet Philippe
  • Aubert Pierre-Henri
  • Coulon Pierre-Eugène
  , 2018. Les tapis de nanotubes de carbone verticalement alignés (VACNT) sont des matériaux aux propriétés structurales, électriques et thermiques très intéressantes pour de nombreuses applications. En particulier, leur croissance directe sur des feuilles ’aluminium est recherchée pour l’élaboration d’électrodes à faible résistance de contact applicables au domaine du stockage de l’énergie. Le développement industriel de ce type de produit passe par la mise au point d’un procédé de synthèse en continu, simple, peu couteux et transposable à grande échelle. La méthode de choix pour la synthèse de VACNT de haute qualité est le dépôt chimique en phase vapeur catalytique (CCVD). Plus précisément, le CCVD assisté par aérosol est un procédé à pression atmosphérique en une seule étape où le réacteur est alimenté en continu et simultanément par les précurseurs du carbone et du catalyseur métallique, et plus simple à mettre en œuvre à l'échelle industrielle que le procédé CCVD classique où les particules catalytiques sont préformées sur le substrat [Castro et al. Carbon 2013;61:585–94]. Il permet d'obtenir des tapis de VACNT de fortes épaisseur et vitesse de croissance à température modérée (> 750 °C) sur de nombreux substrats, y compris des substrats métalliques [Delmas et al. Nanotechnology 2012;23:105604]. Cependant, pour la croissance sur substrat en aluminium, il faut abaisser la température de synthèse sous son point de fusion (660 °C), ce qui a un impact sur la vitesse de croissance. Dans la littérature, le meilleur résultat obtenu avec le procédé en une seule étape est de l’ordre de 1µm/min seulement [Arcila-Velez et al. Nano Energy 2014;8:9–16]. Ici, nous montrons qu’il est possible d’améliorer considérablement les performances du procédé CCVD en une seule étape à basse température sur des feuilles d’Al de qualité industrielle et d’en faire le développement à grande échelle. Des tapis de VACNT jusqu'à 200 $\mu$m d'épaisseur sont obtenus à 615°C avec du ferrocène comme précurseur de catalyseur et de l'acétylène comme source de carbone. Les tapis sont propres (pureté 99.5%), bien alignés et denses (>10$^{11}$CNTs/cm$^2$) avec un diamètre de nanotube entre 8 et 12nm. L'analyse de l'interface VACNT/Al par microscopie électronique à transmission couplée à la dispersion d'énergie des rayons X permet de localiser les nanoparticules catalytiques à base de fer à la surface du substrat même après des synthèses de longue durée. Les mesures de voltampérométrie cyclique révèlent une accessibilité totale de la surface développée par les VACNT à l'électrolyte à base de liquides ioniques.
• Influence of the cement composition on the radiolytic behavior under irradiation
  
  • Acher Loren
  • Dannoux-Papin Adeline
  • Haas Jérémy
  • Courtial Mireille
  • de Noirfontaine Marie-Noëlle
  • Dunstetter Frédéric
  • Gorse-Pomonti Dominique
  • Tusseau-Nenez Sandrine
  , 2018. Cement based materials are used for the conditioning of Low and Intermediate Level Wastes (LILW) due to their low cost, their ease of making and their ability to immobilize radioelements. Nevertheless, hydrogen gas is produced radiolytically under irradiation. For the safety of nuclear waste disposals, it is important to limit as much as possible the hydrogen gas release. A comparison of the behavior of Portland and Ciment Fondu® cements under irradiation taking into account both real cement pastes and synthetic hydrates was performed to find out the role of the cement composition on the radiolytic behavior under irradiation. First, γ-irradiations were performed using a 60Co source (dose rate: 0.17 - 0.25 Gy.s-1, dose: up to 500×103 Gy). Thermogravimetric analysis measurements were performed in order to determine the amount and the type of water involved. H2 gas production was measured by gas chromatography. Regardless of the water to cement ratio (W/C) chosen (0.2, 0.4 and 0.6), it is shown that Ciment Fondu® pastes produce less H2 under irradiation. In agreement with a recent study by Kadissy et al. [1], it is shown that the amount of gas produced by portlandite and gibbsite, which are respectively the constitutive hydrates of Portland and Ciment Fondu ® cements, strongly depends on the nature of the hydrate. Secondly, portlandite and gibbsite were irradiated by electrons up to 270 MGy and 3.5 GGy using the LSI SIRIUS accelerator platform. X-Ray Diffraction analyses were performed before and after irradiation in order to investigate the structural damage. Despite some observed disorder, results show a good structural stability for both hydrates under irradiation.
• Variations on the “exact factorization” theme
  
  • Gonze Xavier
  • Zhou Jianqiang Sky
  • Reining Lucia
  The European Physical Journal B: Condensed Matter and Complex Systems, Springer-Verlag, 2018, 91 (10). In a series of publications, Hardy Gross and co-workers have highlighted the interest of an “exact factorization” approach to the interacting electron-nuclei problem, be it time-independent or time-dependent. In this approach, an effective potential governs the dynamics of the nuclei such that the resulting N-body nuclear density is in principle exact. This contrasts with the more usual adiabatic approach, where the effective potential leads to an approximate nuclear density. Inspired by discussions with Hardy, we explore the factorization idea for arbitrary many-body Hamiltonians, generalizing the electron-nuclei case, with a focus on the static case. While the exact equations do not lead to any practical advantage, they are illuminating, and may therefore constitute a suitable starting point for approximations. In particular, we find that unitary transformations that diagonalize the coupling term for one of the sub-systems make exact factorization appealing. The algorithms by which the equations for the separate subsystems can be solved in the time-independent case are also explored. We illustrate our discussions using the two-site Holstein model and the quantum Rabi model. Two factorization schemes are possible: one where the boson field feels a potential determined by the electrons, and the reverse exact factorization, where the electrons feel a potential determined by the bosons; both are explored in this work. A comparison with a self-energy approach is also presented. (10.1140/epjb/e2018-90278-2)
  DOI : 10.1140/epjb/e2018-90278-2
• An X-ray powder diffraction study of damage produced in Ca(OH)2 and Mg(OH)2 by electron irradiation using the 2.5 MeV SIRIUS accelerator
  
  • de Noirfontaine Marie-Noëlle
  • Acher Loren
  • Courtial Mireille
  • Dunstetter Frederic
  • Gorse-Pomonti Dominique
  Journal of Nuclear Materials, Elsevier, 2018, 509, pp.78-93. Two isomorphous hydrous minerals, Mg(OH)2 and Ca(OH)2, were exposed to the 2.5 MeV electron beam of the SIRIUS accelerator platform. Both compounds remain stable under the beam up to high doses, in the range of 3–3.5 GGy. No decomposition is observed. But contrary to earlier statements, a net difference of reactivity between them is highlighted as a result of the present X-ray powder diffraction study: i) a significant dilatation is observed along the c - axis, more significant in brucite than in portlandite as already reported during thermal decomposition studies, ii) but a contraction is here revealed in the basal plane of brucite, along the a – axis, while a slight dilatation is still being observed in portlandite. Contraction in the basal plane seems a specific feature of electron irradiation only once previously observed by TEM in brucite. Moreover for brucite, the decreasing intensities of Bragg lines together with the appearance of a diffuse scattering over the whole angular range is compatible with the appearance of some static structural disorder induced by electron irradiation. Finally electron irradiation leads to a significant reduction in crystallite size with increasing dose, by a factor of 2 at the intermediate dose of 310 MGy in brucite, while for a comparable effect to occur an absorbed dose of 3.5 GGy should be attained in portlandite. (10.1016/j.jnucmat.2018.06.019)
  DOI : 10.1016/j.jnucmat.2018.06.019
• Hot electron relaxation dynamics in semiconductors: assessing the strength of the electron–phonon coupling from the theoretical and experimental viewpoints
  
  • Sjakste J.
  • Tanimura K
  • Barbarino G
  • Perfetti L.
  • Vast N.
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2018, 30 (35), pp.353001. The rapid development of the computational methods based on density functional theory, on the one hand, and of time-, energy-, and momentum-resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot-electron relaxation dynamics, and, in particular, into the role of the electron–phonon coupling. Instead of focusing on the development of a particular method, theoretical or experimental, this review aims to treat the progress in the understanding of the electron–phonon coupling which can be gained from both, on the basis of recently obtained results. We start by defining several regimes of hot electron relaxation via electron–phonon coupling, with respect to the electron excitation energy. We distinguish between energy and momentum relaxation of hot electrons, and summarize, for several semiconductors of the IV and III–V groups, the orders of magnitude of different relaxation times in different regimes, on the basis of known experimental and numerical data. Momentum relaxation times of hot electrons become very short around 1 eV above the bottom of the conduction band, and such ultrafast relaxation mechanisms are measurable only in the most recent pump-probe experiments. Then, we give an overview of the recent progress in the experimental techniques allowing to obtain detailed information on the hot-electron relaxation dynamics, with the main focus on time-, energy-, and momentum-resolved photoemission experiments. The particularities of the experimental approach developed by one of us, which allows to capture time-, energy-, and momentum-resolved hot-electron distributions, as well as to measure momentum relaxation times of the order of 10 fs, are discussed. We further discuss the main advances in the calculation of the electron–phonon scattering times from first principles over the past ten years, in semiconducting materials. Ab initio techniques and efficient interpolation methods provide the possibility to calculate electron–phonon scattering times with high precision at reasonable numerical cost. We highlight the methods of analysis of the obtained numerical results, which allow to give insight into the details of the electron–phonon scattering mechanisms. Finally, we discuss the concept of hot electron ensemble which has been proposed recently to describe the hot-electron relaxation dynamics in GaAs, the applicability of this concept to other materials, and its limitations. We also mention some open problems. (10.1088/1361-648X/aad487)
  DOI : 10.1088/1361-648X/aad487
• Optical gap and optically active intragap defects in cubic BN
  
  • Tararan Anna
  • Di Sabatino Stefano
  • Gatti Matteo
  • Taniguchi Takashi
  • Watanabe Kenji
  • Reining Lucia
  • Tizei Luiz
  • Kociak Mathieu
  • Zobelli Alberto
  Physical Review B, American Physical Society, 2018, 98 (9), pp.094106. We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV. Further theoretical analysis in the framework of the Bethe-Salpeter equation of many-body perturbation theory supports this result. The spatial localization of defect-related emissions has been investigated using nanometric resolved cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a c-BN powder, we have promoted phase transitions in nanometric domains which have been detected by the appearance of specific hexagonal-phase signatures in both EELS and CL spectra. A high number of intragap optically active centers are known in c-BN, but the literature is rather scattered and hence has been summarized here. For several emission lines we have obtained nano-CL maps which show emission spot sizes as small as few tens of nanometers. Finally, by coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have addressed individual spots in order to identify the possible presence of single-photon sources. The observed CL bunching effect is compatible with a limited set of single-photon emitters and it permits obtaining emission lifetimes of the order of the nanosecond. (10.1103/PhysRevB.98.094106)
  DOI : 10.1103/PhysRevB.98.094106
• Spectroscopy of the Hubbard dimer: the spectral potential
  
  • Vanzini Marco
  • Reining Lucia
  • Gatti Matteo
  The European Physical Journal B: Condensed Matter and Complex Systems, Springer-Verlag, 2018, 91 (8). (10.1140/epjb/e2018-90277-3)
  DOI : 10.1140/epjb/e2018-90277-3
• Single crystal growth, optical absorption and luminescence properties under VUV-UV synchrotron excitation of type III Ce$^{3+}$:KGd(PO$_3$)$_4$, a promising scintillator material
  
  • Adell Irina
  • Solé Rosa Maria
  • Pujol Maria Cinta
  • Lancry Matthieu
  • Ollier Nadège
  • Aguiló Magdalena
  • Díaz Francesc
  Scientific Reports, Nature Publishing Group, 2018, 8, pp.11002. Scintillator materials have gained great interest for many applications, among which the medical applications stand out. Nowadays, the research is focused on finding new scintillator materials with properties that suit the needs of each application. In particular, for medical diagnosis a fast and intense response under high-energy radiation excitation is of great importance. Here, type III Ce$^{3+}$:KGd(PO$_3$)$_4$ single crystals with high crystalline quality are grown and optically characterized as a new promising scintillator material. The $4f→5d$ electronic transitions of Ce$^{3+}$ are identified by optical absorption. The optical absorption cross section of Ce$^{3+}$ for the electronic transition from the $^2$F$_{5/2}$ to the $5d_1$ level is 370 × 10$^{−20}$ cm$^2$. The luminescence of KGd$_{0.996}$ Ce$_{0.004}$ (PO$_3$)$_4$ crystal by exciting the $5d$ levels of Ce$^{3+}$ with VUV-UV synchrotron radiation shows down-shifting properties with strong emissions at 322 and 342 nm from the $5d_1$ to $^2$F$_{5/2}$ and $^2$F$_{7/2}$ levels of Ce$^{3+}$ with a short decay time of ~16 ns, which is very suitable for scintillator applications. Moreover, these intense emissions are also observed when Gd$^{3+}$ is excited since an energy transfer from Gd$^{3+}$ to Ce$^{3+}$ exists. (10.1038/s41598-018-29372-z)
  DOI : 10.1038/s41598-018-29372-z
• Coherent light sources with spin-polarized current
  
  • Fördös Tibor
  , 2018. Spin-lasers are semiconductor devices in which the radiative recombination processes involving spin-polarized carriers result in an emission of circularly polarized photons. Nevertheless, additional linear in-plane anisotropies in the cavity generally lead in preferential linearly-polarized laser emission and to possible coupling between modes. In this thesis, a general method for the modeling of semiconductor laser such as vertical-(external)-cavity surface-emitting laser containing multiple quantum wells and involving anisotropies that may reveal i) a local linear birefringence due to the strain field at the surface or ii) a birefringence in quantum wells (QWs) due to phase amplitude coupling originating from the reduction of the biaxial D2d to the C2v symmetry group at the III-V ternary semiconductor interfaces. A novel scattering S-matrix recursive method is implemented using a gain tensor derived analytically from the Maxwell-Bloch equations. It enables to model the properties of the emission (threshold, polarization, mode splitting) from the laser with multiple quantum well active zones by searching for the resonant eigenmodes of the cavity. The method is demonstrated on real laser structures and is used for the extraction of optical permittivity tensors of surface strain and quantum wells in agreement with experiments. The method is generalized to find the laser eigenmodes in the most general case of circular polarized pumps (unbalance between the spin-up and spin-down channels) and linear gain dichroism. In addition, the measurement of full 4x4 Mueller matrix for multiple angles of incidence and in-plane azimuthal angles has been used for extraction of optical permittivity tensors of surface strained layers and quantum wells. Such spectral dependence of optical tensor elements are crucial for modeling of spin-laser eigenmodes, resonance conditions, and also for understanding of sources of structure anisotropies.
• Irradiation effect in triple junction solar cells for spatial applications
  
  • Park Seonyong
  , 2018. This thesis is the result of work on the irradiation effect of lattice matched GaInP/GaAs/Ge triple junction (TJ) solar cells in LILT conditions. Initiated by needs of the understanding of EOL performances of the solar cells in JUICE mission, we have found very peculiar phenomena which are not supposed to occur if it was irradiated at room temperature. First, a bottom component cell exhibited a larger drop of Isc at a lower temperature, which potentially proposes a current limiting by the bottom sub-cell in the TJ structure. A temperature dependence of RF(Isc) recovery by an isochronal annealing and the orientation dependence of Isc degradation of the bottom component cell have implied that its degradation mechanism could be related to defect clusters formed along proton tracks, acting like insulating (non active) area for minority carriers. Second, we have observed in general larger degradation of FF and Pmax from electron irradiated TJ cells compared to proton irradiated ones. This distinct difference has originated especially from the top and bottom sub-cells due to the occurrence of excess dark current. This additional current in dark seems to be related to the indirect tunneling effect by defects induced by electron irradiation. Furthermore, EOL FF and Pmax appeared to be more and more spread from cell to cell as the electron fluence increased. A displacement damage dose (DDD) approach was applied to 1 and 2 MeV electron and proton irradiated TJ cells and its component cells. It turned out that 2 MeV electrons induced greater degradation than others for all parameters (Isc, Voc, FF, Pmax). The middle component cell showed almost a perfect match of DDD between electron and proton irradiated cells in LILT condition, indicating that the final defects produced by electron and proton irradiations are perhaps the same. TJ and its top component cell showed less degradation on Voc under the electron irradiation compared to the proton irradiation. For the Ge bottom component cell, the electron irradiation induced much larger downgrading of Voc, FF and Pmax compared to the proton irradiation. To improve the radiation hardness of the cells by reducing the excess dark current, it would be worth to decrease the doping concentration of junctions to reduce the creation of secondary defects related to impurities.
• Optical properties of chlorine-and oxygen-related defects in SiO$_2$ glass and optical fibers
  
  • Skuja Linards
  • Ollier N.
  , 2018, pp.BM2A.1. Photoinduced processes involving chlorine molecules in synthetic silica were studied. Interstitial Cl$_2$ in SiO$_2$ are prevented from VIS/UV photolysis by cage effect. However, they react with oxygen interstitials yielding photosensitive ClClO molecules absorbing at 264nm.
• Etude des mécanismes fondamentaux d'interaction entre impulsions laser ultra-brèves et matériaux diélectriques
  
  • Bilde Allan
  , 2018. L'interaction entre impulsions lasers ultra-brèves et matériaux diélectriques est un sujet d'étude en constant renouvellement, motivé aussi bien par la naissance d'une multitude d'applications (micro-usinage laser, opérations de la cornée, ...) que par ses aspects fondamentaux (génération d'harmoniques d'ordre élevé, électronique au cycle optique, ...). Pourtant, les mécanismes sous-jacents à cette interaction sont encore mal compris, en particulier d'un point de vue quantitatif. En effet, une bonne partie des phénomènes ont lieu durant le passage de l'impulsion laser, c'est-à-dire pendant quelques dizaines de femtosecondes. De surcroît, les éclairements impliqués dans l'interaction sont élevés : quelques dizaines de TW/cm^2. La compréhension et la quantification des processus élémentaires ayant lieu durant l'interaction requiert donc de concevoir des expériences sur mesure ainsi que d'effectuer un travail approfondi de modélisation. Dans ce manuscrit sont présentées deux techniques expérimentales complémentaires utilisées durant la thèse : l'interférométrie fréquentielle et la spectroscopie d'absorption résolues en temps. Ces deux techniques sont employées pour étudier les processus électroniques d'excitation et de relaxation dans trois matériaux distincts : le quartz, le saphir et l'oxyde de magnésium. La quasi-intégralité des résultats expérimentaux sont analysés à l'aide d'un modèle en équation de taux multiples (MRE) permettant de discriminer l'importance relative de chacun des processus pris en compte. Tout d'abord, une preuve expérimentale directe de l'existence de l'ionisation par impact dans le quartz est présentée. Ces résultats ont été obtenus par l'exposition de l'échantillon à une série de deux impulsions, ce qui permet de moduler indépendamment la densité et la température du plasma. Les résultats expérimentaux sont reproduits avec succès par le modèle MRE intégrant l'ionisation multiphotonique, le chauffage des porteurs photo-excités ainsi que l'ionisation par impact comme processus d'excitation. Nous nous concentrons ensuite sur une seconde série de résultats concernant la relaxation du saphir après excitation induite par laser. Un nouveau mécanisme de relaxation est proposé et testé par la modélisation pour tenter d'expliquer la dynamique de ce processus. Ce mécanisme implique la formation d'excitons auto-piégés puis leur recombinaison. Enfin, le modèle MRE est appliqué à la détection de seuils d'ablation dans les trois solides. Le choix du critère physique déterminant ce seuil, sujet soumis à d'intenses débats dans la littérature, est alors discuté.
• Ab initio study of Silver chloride: Becquerel’s photochromatic photography
  
  • Lorin Arnaud
  , 2018.
• Evaluation of III-V/Si Multi-Junction Solar Cells Potential for Space
  
  • Cariou Romain
  • Medjoubi Karim
  • Vauche Laura
  • Veinberg-Vidal Elias
  • Park Seonyong
  • Lefèvre Jérémie
  • Baudrit Mathieu
  • Voarino Philippe
  • Mur Pierre
  • Boizot Bruno
  , 2018, pp.3335-3338. Hybrid III-V/Si multi-junction solar cells, which combine efficient III-V top cells with a silicon bottom cell, the most wide-spread photovoltaic material, offer great opportunities. Recent advances in this field have shown that mechanically stacked or wafer bonded III-V/Si can reach similar AM1.5g efficiencies than the standard GaInP/GaAs/Ge space triplejunction solar cells. In this study, we review the potential of this new hybrid III-V/Si cell technology for space applications. The first experimental results of wafer bonded 2-terminal GaInP/GaAs/Si solar cell electron irradiation will be presented. The BOL and EOL (1 MeV electrons) performances will be characterized under AM0 spectrum, and the radiation tolerance will be compared with single-junction silicon and standard Ge based triple-junction. (10.1109/PVSC.2018.8547773)
  DOI : 10.1109/PVSC.2018.8547773
• Ab initio study of Silver chloride: Becquerel's photochromatic photography
  
  • Lorin Arnaud
  , 2018.
• Dielectric properties of graphene/ MoS 2 heterostructures from ab initio calculations and electron energy-loss experiments
  
  • Mohn Michael
  • Hambach Ralf
  • Wachsmuth Philipp
  • Giorgetti Christine
  • Kaiser Ute
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2018, 97 (23). High-energy electronic excitations of graphene and MoS2 heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/MoS2/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack. (10.1103/PhysRevB.97.235410)
  DOI : 10.1103/PhysRevB.97.235410
• Localized Plasmonic Resonances of Prolate Nanoparticles in a Symmetric Environment: Experimental Verification of the Accuracy of Numerical and Analytical Models
  
  • Kobylko Mathias
  • Coulon Pierre-Eugène
  • Slablab Abdallah
  • Fafin Alexandre
  • Cardin Julien
  • Dufour Christian
  • Losquin Arthur
  • Kociak Mathieu K
  • Monnet Isabelle
  • Mailly Dominique
  • Lafosse Xavier
  • Ulysse Christian
  • Garcia-Caurel Enric
  • Rizza Giancarlo
  Physical Review Applied, American Physical Society, 2018, 9 (6). We study the evolution of the surface-plasmon resonances of individual ion-beam-shaped prolate gold nanoparticles embedded in a dielectric SiO 2 environment by electron-energy-loss spectroscopy mapping in a scanning transmission electron microscope. The controlled symmetric dielectric environment obtained through the ion-beam-shaping method allows a direct quantitative comparison with numerical results obtained through simulations (auxiliary differential-equation finite-difference time-domain and boundary-element method) and with theoretical results obtained through analytical models (quasistatic model for prolate nanoellipsoids and waveguide model for infinite one-dimensional plasmonic waveguides), with which our experimental results are in very good agreement. We confirm the accuracy of state-of-the-art numerical tools and analytical theories that establish ion-beam shaping as a very promising method to design metal-dielectric nanocomposites with well-predicted optical properties, and with many possible applications in surface-enhanced Raman spectroscopy and second-harmonic generation, as well as in conventional applications of metamaterials like negative refraction, superimaging, and invisibility cloaking. (10.1103/PhysRevApplied.9.064038)
  DOI : 10.1103/PhysRevApplied.9.064038
• Ultrafast electron dynamics reveal the high potential of InSe for hot-carrier optoelectronics
  
  • Chen Zhesheng
  • Giorgetti Christine
  • Sjakste Jelena
  • Cabouat Raphael
  • Véniard Valérie
  • Zhang Zailan
  • Taleb-Ibrahimi Amina
  • Papalazarou Evangelos
  • Marsi Marino
  • Shukla Abhay
  • Peretti Jacques
  • Perfetti Luca
  Physical Review B, American Physical Society, 2018, 97 (24). We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the ¯¯¯¯M valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the ¯¯¯¯M valley could travel through a multilayer flake of InSe with a lateral size of 1 μm. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides. (10.1103/PhysRevB.97.241201)
  DOI : 10.1103/PhysRevB.97.241201