Publications

2015

• Swift heavy ion-beam induced amorphization and recrystallization of yttrium iron garnet
  
  • Costantini Jean-Marc
  • Miro Sandrine
  • Beuneu François
  • Toulemonde Marcel
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2015, 27 (49), pp.496001. Pure and (Ca and Si)-substituted yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers and amorphous films on gadolinium gallium garnet (Gd3Ga5O12, or GGG) single crystal substrates were irradiated by 50 MeV (32)Si and 50 MeV (or 60 MeV) (63)Cu ions for electronic stopping powers larger than the threshold value (~4 MeV μm(-1)) for amorphous track formation in YIG crystals. Conductivity data of crystalline samples in a broad ion fluence range (10(11)-10(16) cm(-2)) are modeled with a set of rate equations corresponding to the amorphization and recrystallization induced in ion tracks by electronic excitations. The data for amorphous layers confirm that a recrystallization process takes place above ~10(14) cm(-2). Cross sections for both processes deduced from this analysis are discussed in comparison to previous determinations with reference to the inelastic thermal-spike model of track formation. Micro-Raman spectroscopy was also used to follow the related structural modifications. Raman spectra show the progressive vanishing and randomization of crystal phonon modes in relation to the ion-induced damage. For crystalline samples irradiated at high fluences (⩾10(14) cm(-2)), only two prominent broad bands remain like for amorphous films, thereby reflecting the phonon density of states of the disordered solid, regardless of samples and irradiation conditions. The main band peaked at ~660 cm(-1) is assigned to vibration modes of randomized bonds in tetrahedral (FeO4) units. (10.1088/0953-8984/27/49/496001)
  DOI : 10.1088/0953-8984/27/49/496001
• Hydrogen generation from irradiated aluminum hydroxydes and oxyhydroxydes
  
  • Kaddissy Josiane A
  • Esnouf Stéphane
  • Cochin F.
  • Saffré Dimitri
  • Renault Jean-Philippe
  , 2015.
• Optical properties of surfaces with supercell ab initio calculations: Local-field effects
  
  • Tancogne-Dejean Nicolas
  • Giorgetti Christine
  • Véniard Valérie
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2015, 92 (24). Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects. Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using the “effective-medium theory” with vacuum and also in terms of interaction between replicas, as the supercell approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) 2×1 surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to nonlinear optics for surfaces. (10.1103/PhysRevB.92.245308)
  DOI : 10.1103/PhysRevB.92.245308
• Angle dependence on the anisotropic magnetoresistance amplitude of a single-contacted Ni nanowire subjected to a thermo-mechanical strain
  
  • Melilli G.
  • Madon B.
  • Wegrowe J.-E.
  • Clochard M.-C.
  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Elsevier, 2015, 365, pp.675-679. The effects of thermoelastic and piezoelectric strain of an active track-etched β-PVDF polymer matrix on an electrodeposited single-contacted Ni nanowire (NW) are investigated at the nanoscale by measuring the change of magnetization (i.e. using the inverse magnetostriction effect). The magnetization state is measured locally by anisotropic magnetoresistance (AMR). The ferromagnetic NW plays thus the role of a mechanical probe that allows the effects of mechanical strain to be characterized and described qualitatively and quantitatively. The inverse magnetostriction was found to be responsible for a quasi-disappearance of the AMR signal for a variation of the order of ΔT ≈ 10 K. In other terms, the variation of the magnetization due to the stress compensates the effect of external magnetic field applied on the NW resistance. The induced stress field in a single Ni NW was found 1000 time higher than the bulk stress field (due to thermal expansion measured on the PVDF). This amplification could be attributed to three nanoscopic effects: (1) a stress mismatch between the Ni NW and the membrane, (2) a non-negligible role of the surface tension on Ni NW Young modulus, and (3) the possibility of non-linear stress–strain law. We investigate here the role of these different contributions using track-etched polymer membranes irradiated at various angles (αirrad) leading to, after electrodeposition, embedded Ni NWs of different orientations. (10.1016/j.nimb.2015.09.055)
  DOI : 10.1016/j.nimb.2015.09.055
• Simulation du Comportement sous Irradiation aux Ions Lourds du Nanocomposite Au-SiO2
  
  • Vu Thi Hai Yen
  , 2015. Les matériaux nanocomposites de type métal-verre sont intéressants en raison de leurs propriétés optiques particulières. La technique de fabrication utilisant les faisceaux d’ions est un outil prometteur pour les nanocomposites. L’objectif de cette thèse était d’étudier d’un point de vue fondamental le comportement sous irradiation de nanoparticules (NPs) d’or enfouies dans une matrice de silice amorphe. Ma contribution réside dans l’interprétation par modélisation et simulation numérique des résultats expérimentaux obtenus auparavant par des chercheurs du Laboratoire des Solides Irradiés. Dans le premier type d’expériences, le système a été irradié par des ions d’or de 4 MeV alors que dans le second type l’irradiation est effectuée avec des ions de krypton de 74 MeV. Dans la première partie, la simulation par Monte Carlo cinétique (KMC) sur réseau rigide a été choisie et développée spécifiquement pour étudier le comportement de NPs sous irradiation à différentes températures. Les simulations ont permis de reproduire qualitativement les résultats expérimentaux à toute température. Nous avons retrouvé en particulier le mûrissement d’Ostwald qui se produit à haute température (T>900 K) et la dissolution des NPs qui se produit à basse température (T<600 K). Un régime de transition est observé entre 600 K et 900 K. La simulation par KMC a permis de mettre en évidence et d’expliquer un effet de taille sur la dissolution de NPs sous irradiation. Elle a également révélé que les modèles balistiques unidirectionnels pouvaient rendre compte de la loi de dissolution. Sur la base de l’approche théorique de Frost et Russel, nous avons alors construit avec succès un modèle analytique unidirectionnel capable de décrire la loi de dissolution. Dans la deuxième partie, le système évolue dans le régime de dépôt d’énergie électronique. Selon la taille de la NP, l’irradiation la vaporise (diamètre<10 nm), la transforme en nanobâtonnet (10-30 nm), en NP à facettes (30-60 nm) ou la déforme légèrement (>60 nm) selon l’axe de l’ion incident. Nous avons imaginé un scénario du mécanisme de la déformation qui repose sur la dilatation thermique de la NP métallique fondue dans la région de la trace de l’ion incident. Sa mise en œuvre, grâce à une nouvelle technique de simulation développée spécialement, a permis d’affiner certains éléments du scénario et d’améliorer la compréhension globale du processus. L’accord qualitatif entre les données expérimentales et les résultats obtenus par simulation valide le mécanisme proposé.
• Dynamical effects in electron spectroscopy
  
  • Zhou Jianqiang Sky
  • Kas J. J.
  • Sponza Lorenzo
  • Reshetnyak Igor
  • Guzzo Matteo
  • Giorgetti Christine
  • Gatti Matteo
  • Sottile Francesco
  • Rehr J.
  • Reining Lucia
  The Journal of Chemical Physics, American Institute of Physics, 2015, 143 (18), pp.184109. One of the big challenges of theoretical condensed-matter physics is the description, understanding, and prediction of the effects of the Coulomb interaction on materials properties. In electronic spectra, the Coulomb interaction causes a renormalization of energies and change of spectral weight. Most importantly, it can lead to new structures, often called satellites. These can be linked to the coupling of excitations, also termed dynamical effects. State-of-the-art methods in the framework of many-body perturbation theory, in particular, the widely used GW approximation, often fail to describe satellite spectra. Instead, approaches based on a picture of electron-boson coupling such as the cumulant expansion are promising for the description of plasmon satellites. In this work, we give a unified derivation of the GW approximation and the cumulant expansion for the one-body Green’s function. Using the example of bulk sodium, we compare the resulting spectral functions both in the valence and in the core region, and we discuss the dispersion of quasi-particles and satellites. We show that self-consistency is crucial to obtain meaningful results, in particular, at large binding energies. Very good agreement with experiment is obtained when the intrinsic spectral function is corrected for extrinsic and interference effects. Finally, we sketch how one can approach the problem in the case of the two-body Green’s function, and we discuss the cancellation of various dynamical effects that occur in that case. (10.1063/1.4934965)
  DOI : 10.1063/1.4934965
• S-wave Superconductivity in Optimally Doped SrTi 1−x Nb x O 3 Unveiled by Electron Irradiation
  
  • Lin Xiao
  • Rischau Willem
  • van Der Beek Cornelis
  • Fauqué Benoît
  • Behnia Kamran
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2015, 92, pp.174504. We report on a study of electric resistivity and magnetic susceptibility measurements in electron irradiated SrTi0.987Nb0.013O3 single crystals. Point-like defects, induced by electron irradiation, lead to an almost threefold enhancement of the residual resistivity, but barely affect the superconducting critical temperature (Tc). The pertinence of Anderson's theorem provides strong evidence for a s-wave superconducting order parameter. Stronger scattering leads to a reduction of the effective coherence length (ξ) and lifts the upper critical field (Hc2), with a characteristic length scale five times larger than electronic mean-free-path. Combined with thermal conductivity data pointing to multiple nodeless gaps, the current results identify optimally doped SrTi1−xNbxO3 as a multi-band s-wave superconductor with unusually long-range electrodynamics. (10.1103/PhysRevB.92.174504)
  DOI : 10.1103/PhysRevB.92.174504
• Inertial Regime of the Magnetization: Nutation resonance Beyond Precession Resonance
  
  • Wegrowe J.-E.
  • Meyer M.
  • Hayoun M.
  • Olive Enrick
  , 2015, pp.143-145.
• Exciton energy-momentum map of hexagonal boron nitride
  
  • Fugallo Giorgia
  • Aramini Matteo
  • Koskelo Jaakko
  • Watanabe Kenji
  • Taniguchi Takashi
  • Hakala Mikko
  • Huotari Simo
  • Gatti Matteo
  • Sottile Francesco
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2015, 92 (16). Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q, as previously reported, but also at large q. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials. (10.1103/PhysRevB.92.165122)
  DOI : 10.1103/PhysRevB.92.165122
• Computing optical properties and photo-emission spectra : a new starting point
  
  • Reshetnyak Igor
  , 2015. When a material is irradiated by particles or light, it responds with the excitation of electrons and nuclei. Because of the Coulomb interaction, this gives rise to interesting many-body effects, that cannot be explained in a single-particle picture. In this thesis we are interested in their contribution to the electronic spectra. In particular we will be looking into excitonic effects. These phenomena are due to excitations that can be described as electron-hole pairs that interact. The Bethe-Salpeter Equation (BSE) for the two-particle Green's function, in an approximation based on the GW approximation to the self-energy, is a well established approach for accounting for excitonic effects in theoretical spectroscopy. However, in its current formulation it is computationally heavy, as its starting point requires the knowledge of the interacting single particle Green's function. Moreover, the existing implementations give access to only the diagonal parts of the microscopic screening function $\varepsilon^{-1}(\mathbf{q},\omega)_{\mathbf{G},\mathbf{G}'}$ and Dynamic Structure Factor $S(\mathbf{q},\omega)_{\mathbf{G},\mathbf{G}'}$, both of which, in their full form, are dense matrices in reciprocal lattice vectors $\mathbf{G}$ and $\mathbf{G}'$. In inhomogeneous systems these off-diagonal elements can be important and, thus, it is highly desirable to be able to describe them. In this work, on the one hand, we try to make the Bethe-Salpeter Equation approach more efficient. To this end we study the possibility of deriving alternative equations for the two-particle Green's function and modifying the standard Bethe-Salpeter Equation. In particular, we use the fact that the shifts of spectral weight induced by the GW correction to the single-particle energies and by the electron-hole interaction cancel at least partially. The idea is to incorporate these cancelation effects, and moreover to use insight from Time-dependent Density Functional Theory, to render our calculations lighter. Furthermore, based on detailed analysis and comparison of different approaches to theoretical spectroscopy we discuss the importance of various ingredients contained in them. On the other hand we extend the Bethe-Salpeter Equation to the off-diagonal elements of the microscopic screening function and Dynamic Structure Factor. This allows us, first of all, to reproduce available Coherent Inelastic X-ray Scattering results and make theoretical prediction for new ones. Second, this gives us the possibility to calculate the induced charge distributions due to excitons when the material is subject to an external perturbation. And, third, we demonstrate the existence of exciton satellites, alongside the plasmon ones, in photo-emission spectra of wide gap insulators.
• Ab initio description of second-harmonic generation from crystal surfaces
  
  • Tancogne-Dejean Nicolas
  , 2015. More than 50 years after the first experimental observation of second-harmonic generation, the theoretical description of second-harmonic generation is still under debate, whereas it is well understood from an experimental point of view. This is the gap that this thesis aims to fill. This work aims to improve the theoretical description and understanding of the generation of second-harmonic from the surfaces of crystalline semiconductors. When applying an external electric field to a dielectric material, electric dipoles are created at a microscopic level. These dipoles are responsible for the apparition, inside the material, of an induced field. The fluctuations of the electric field at a microscopic level, the density fluctuations or any kind of microscopic inhomogeneities must be taken into account when describing the optical properties of a system. These effects are often referred as “local-field effects”. These local-field effects have been widely studied in the past and in particular their effects on the optical properties of bulk materials are now well established. In the case of surfaces, the theoretical description and the numerical simulations are more intricate than for bulk materials. The abrupt change in the electronic density leads to a huge variation of the electric field at the interface with vacuum. As a result, strong effects of the local-field are expected, in particular in the direction perpendicular to the plane of the surface. The goal of this thesis is to quantify how important these effects are for the linear and second-order optical properties of surfaces. A macroscopic theory of second-harmonic generation from crystal surfaces has been developed in order to account for local-field effects. The latter are calculated from first-principles, in the framework of the Time-Dependent Density-Functional Theory (TDDFT). The primary interest is the description of non-linear optical responses of surfaces, but new theoretical tools for improving the description of local-field effects in the case of linear optics have also been developed. The numerical simulations have been focused on the Si(001) surface, and the macroscopic formalism developed during this thesis has been applied to three surface reconstructions, namely the clean Si(001)2x1, the monohydride Si(001)2x1:H and the dihydride Si(001)1x1:2H surfaces. Comparison with available experimental results is also reported.
• Unphysical and physical solutions in many-body theories: from weak to strong correlation
  
  • Stan Adrian
  • Romaniello Pina
  • Rigamonti Santiago
  • Reining Lucia
  • Berger Arjan
  New Journal of Physics, Institute of Physics: Open Access Journals, 2015, 17 (9), pp.093045. Many-body theory is largely based on self-consistent equations that are constructed in terms of the physical quantity of interest itself, for example the density. Therefore, the calculation of important properties such as total energies or photoemission spectra requires the solution of nonlinear equations that have unphysical and physical solutions. In this work we show in which circumstances one runs into an unphysical solution, and we indicate how one can overcome this problem. Moreover, we solve the puzzle of when and why the interacting Green's function does not unambiguously determine the underlying system, given in terms of its potential, or non-interacting Green's function. Our results are general since they originate from the fundamental structure of the equations. The absorption spectrum of lithium fluoride is shown as one illustration, and observations in the literature for some widely used models are explained by our approach. Our findings apply to both the weak and strong-correlation regimes. For the strong-correlation regime we show that one cannot use the expressions that are obtained from standard perturbation theory, and we suggest a different approach that is exact in the limit of strong interaction. (10.1088/1367-2630/17/9/093045)
  DOI : 10.1088/1367-2630/17/9/093045
• Strain-designed strategy to induce and enhance second-harmonic generation in centrosymmetric and noncentrosymmetric materials
  
  • Luppi Eleonora
  • Degoli Elena
  • Bertocchi Matteo
  • Ossicini Stefano
  • Véniard Valérie
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2015, 92 (7). Second-harmonic generation is described by the second-order nonlinear susceptibility χ(2) which, in the electric-dipole approximation, requires a noncentrosymmetric medium. It is very challenging and of high technological interest to search whether it is possible to find a way to break inversion symmetry in centrosymmetric crystals in order to induce second-order nonlinearities. A new intriguing way to observe second-order nonlinear phenomena is strain. Here, we present a detailed analysis of the correlation between the strain and the χ(2) in both centrosymmetric and noncentrosymmetric materials. We considered Si and SiC as test materials and we studied different types of strain (tensile/compressive), in different directions (uniaxial/biaxial) and for different light-polarization directions. We found which is the type of strain necessary in order to induce, tune, and enhance second-harmonic generation in different energy regions for centrosymmetric and noncentrosymmetric materials. (10.1103/PhysRevB.92.075204)
  DOI : 10.1103/PhysRevB.92.075204
• Millijoule femtosecond micro-Bessel beams for ultra-high aspect ratio machining
  
  • Mitra Sambit
  • Chanal Margaux
  • Clady Raphael
  • Mouskeftaras Alexandros
  • Grojo David
  Applied optics, Optical Society of America, 2015, 54 (24), pp.7358-7365. We report on a functional experimental design for Bessel beam generation capable of handling high-energy ultra-short pulses (up to 1.2 mJ per pulse of 50 fs duration). This allows us to deliver intensities exceeding the breakdown threshold for air or any dielectric along controlled micro-filaments with lengths exceeding 4 mm. It represents an unprecedented upscaling in comparison to recent femtosecond Bessel beam micromachining experiments. We produce void microchannels through glass substrates to demonstrate that aspect ratios exceeding 1200: 1 can be achieved by using single high-intensity pulses. This demonstration must lead to new methodologies for deep-drilling and high-speed cutting applications. (C) 2015 Optical Society of America (10.1364/AO.54.007358)
  DOI : 10.1364/AO.54.007358
• Wannier interpolation of the electron-phonon matrix elements in polar semiconductors: Polar-optical coupling in GaAs
  
  • Sjakste J.
  • Vast N.
  • Calandra M.
  • Mauri F.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2015, 92 (5). (10.1103/PhysRevB.92.054307)
  DOI : 10.1103/PhysRevB.92.054307
• Influence of impurities on Cr3+ luminescence properties in Brazilian emerald and alexandrite
  
  • Ollier Nadège
  • Fuchs Y.
  • Cavani Olivier
  • Horn H.A.
  • Rossano Stéphanie
  European Journal of Mineralogy, Copernicus, 2015. Emerald and alexandrite, the chromiferous varieties of beryl, Be3Al2[Si6O18], and chrysoberyl, BeAl2O4, respectively, are, as gems, of high economical and technological interest. The Cr3+ properties in natural beryl and chrysoberyl samples have been studied by mainly photoluminescence technique and electron paramagnetic resonance (EPR) as a function of Cr content (50–11370 ppm) as well as impurities, such as Fe and V. In emeralds, the Cr3+ crystal field value is linked to the Cr content and decreases when Cr increases. In chrysoberyl with low Cr content, the vanishing of Cr3+ emission from Cr3+ ions located on the inversion-symmetry site suggests a competition between Cr and V regarding chemical substitution. The Fe3+ ion substitutes efficiently on the mirror-symmetry site, with a strong impact on the Cr3+ lifetime on this site. (10.1127/ejm/2015/0027-2484)
  DOI : 10.1127/ejm/2015/0027-2484
• Fermi surface symmetry and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)
  
  • Nicolaou Alessandro
  • Gatti Matteo
  • Magnano Elena
  • Le Fèvre Patrick
  • Bondino Federica
  • Bertran François
  • Tejeda Antonio
  • Sauvage-Simkin Michèle
  • Vlad Alina
  • Garreau Yves
  • Coati Alessandro
  • Guérin Nicolas
  • Parmigiani Fulvio
  • Taleb-Ibrahimi Amina
  Physical Review B, American Physical Society, 2015, 92 (8), pp.081110. (10.1103/PhysRevB.92.081110)
  DOI : 10.1103/PhysRevB.92.081110
• Poly(vinylimidazole) radiografted PVDF nanospheres as alternative binder for high temperature PEMFC electrodes
  
  • Galbiati Samuele
  • Coulon Pierre-Eugène
  • G (rizza Giancarlo) Rizza
  • Clochard Marie-Claude
  • Castellino Micaela
  • Sangermano Marco
  • Nayoze Christine
  • Morin Arnaud
  Journal of Power Sources, Elsevier, 2015, pp.117-121. Within the framework of high-temperature polymer fuel cells doped with phosphoric acid, we investigate the replacement of the conventional binder in the catalyst layers by functionalized solid PVDF nanospheres. Aim of this study is to develop and test an innovative binder which might create enhanced electrode porosity and acid distribution. Aqueous suspensions of PVDF nanospheres (d ~ 200 nm) are obtained by radical emulsion polymerization and are functionalized by Vinyl-Imidazole (VI) groups via in situ g-radiation. As a consequence the nanospheres can interact with H3PO4 to obtain proton conductivity. Catalyst inks are prepared mixing the nanospheres with commercial Pt/C electrocatalyst powder, solvents and phosphoric acid. Prototype electrodes are deposited by spraying and preliminary fuel cell tests are carried out at 160 C under dry H2/air. Electrodes with grafted PVDF nanospheres as solid binder are demonstrated and its understanding is in progress. Further improvements are outlined.
• Local magnetic measurements in multi-layered nanowires observed by electron holography
  
  • Reyes Vasquez David Fernando
  • Gatel Christophe
  • Biziere Nicolas
  • L. Wade Travis
  • Warot-Fonrose Bénédicte
  , 2015. In this study, we elaborate multi-layered Co/Cu nanowires by pulsed electrodeposition technique into polycarbonate membranes, with a diameter between 55nm and 80nm. Cobalt and copper maps were obtained by EFTEM to get the position and the average thickness of the layers. Using electron holography we observed the magnetic states in individual cobalt layers for two different configurations.
• Reduced density-matrix functional theory: Correlation and spectroscopy
  
  • Di Sabatino Stefano
  • Berger Arjan
  • Reining Lucia
  • Romaniello Pina
  The Journal of Chemical Physics, American Institute of Physics, 2015, 143 (2), pp.024108. (10.1063/1.4926327)
  DOI : 10.1063/1.4926327
• Magnetic states in Co/Cu multi-layered nano-wires observed by electron holography
  
  • Reyes Vasquez David Fernando
  • Gatel Christophe
  • Biziere Nicolas
  • L. Wade Travis
  • Warot-Fonrose Bénédicte
  , 2015.
• Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation
  
  • Byrne Declan J.
  • Coffey William T.
  • Kalmykov Yuri P.
  • Titov Serguey V.
  • Wegrowe Jean-Eric
  , 2015. Spin-transfer torque (STT) effects on the stationary forced response of nanoscale ferromagnets subject to thermal fluctuations and driven by an ac magnetic field of arbitrary strength and direction are investigated via a generic nanopillar model of a spin-torque device comprising two ferromagnetic strata representing the free and fixed layers and a nonmagnetic conducting spacer all sandwiched between two Ohmic contacts. The STT effects are treated via Brown's magnetic Langevin equation generalized to include the Slonczewski STT term thereby extending the statistical moment method [Y. P. Kalmykov et al., Phys. Rev. B 88, 144406 (2013)] to the forced response of the most general version of the nanopillar model. The dynamic susceptibility, nonlinear frequency-dependent dc magnetization, dynamic hysteresis loops, etc. are then evaluated highlighting STT effects on both the low-frequency thermal relaxation processes and the high-frequency ferromagnetic resonance, etc., demonstrating a pronounced dependence of these on the spin polarization current and facilitating interpretation of STT experiments. (10.1103/PhysRevB.91.174406)
  DOI : 10.1103/PhysRevB.91.174406
• Deviation from the Landau-Lifshitz-Gilbert equation in the inertial regime of the magnetization
  
  • Olive Enrick
  • Lansac Yves
  • Meyer M.
  • Hayoun M.
  • Wegrowe E.
  Journal of Applied Physics, American Institute of Physics, 2015, 117 (21), pp.213904. We investigate in details the inertial dynamics of a uniform magnetization in the ferromagnetic resonance context. Analytical predictions and numerical simulations of the complete equations within the Inertial Landau-Lifshitz-Gilbert (ILLG) model are presented. In addition to the usual precession resonance, the inertial model gives a second resonance peak associated to the nutation dynamics provided that the damping is not too large. The analytical resolution of the equations of motion yields both the precession and nutation angular frequencies. They are function of the inertial dynamics characteristic time τ, the dimensionless damping α, and the static magnetic field H. A scaling function with respect to ατγH is found for the nutation angular frequency, also valid for the precession angular frequency when ατγH ≫ 1. Beyond the direct measurement of the nutation resonance peak, we show that the inertial dynamics of the magnetization has measurable effects on both the width and the angular frequency of the precession resonance peak when varying the applied static field. These predictions could be used to experimentally identify the inertial dynamics of the magnetization proposed in the ILLG model. (10.1063/1.4921908)
  DOI : 10.1063/1.4921908
• Ultrafast filling of an electronic pseudogap in photoexcited (LaS) 1.196 VS 2
  
  • Brouet V
  • Mauchain Julien
  • Papalazarou E
  • Faure Jérôme
  • Marsi M
  • Taleb-Ibrahimi A
  • Le Fèvre P
  • Bertran F
  • Cario Laurent
  • Janod Etienne
  • Corraze Benoît
  • Ta Phuoc V
  • Perfetti L
  , 2015. We investigate by angle and time resolved photoemission spectroscopy the unusual insulating state of strongly distorted triangular V slabs in (LaS)1.196VS2. We show that the electronic structure is dominated by the lowest band of the V t2g manifold, which disperses over 0.7 eV and is nearly filled. Hence, (LaS)1.196VS2 is not a Mott insulator. The spectra are strongly temperature dependent, shifting by 100 meV upon cooling to 50 K. The sudden photoexcitation at 50K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable timescale. We conclude that a very strong electron-phonon coupling makes this state extremely sensitive to small perturbations of the V clusters distortions.
• Nanoselective area growth of high quality thick InGaN/GaN on sacrificial ZnO templates
  
  • Puybaret Renaud
  • Sundaram Suresh
  • Li Xin
  • El Gmili Youssef
  • Pantzas Konstantinos
  • Troadec David
  • Patriarche Gilles
  • Rogers David
  • Teherani Ferechteh Hosseini
  • Sandana Eric Vinod
  • Bove Philippe
  • Voss Paul L
  • Salvestrini Jean-Paul
  • Ougazzaden Abdallah
  , 2015.