Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering

We report a Raman scattering investigation of columnar BiFeO3-CoFe2O4 (BFO-CFO) epitaxial thin film nanostructures, where BFO pillars are embedded in a CFO matrix. The feasibility of a strain analysis is illustrated through an investigation of two nanostructures with different BFO-CFO ratios. We show that the CFO matrix presents the same strain state in both nanostructures, while the strain state of the BFO pillars depends on the BFO/CFO ratio with an increasing tensile strain along the out-of-plane direction with decreasing BFO content. Our results demonstrate that Raman scattering allows monitoring strain states in complex 3D multiferroic pillar/matrix composites.Comment: revised version submitted to Appl. Phys. Let

Phonons in the multiferroic langasite Ba_3\_3NbFe_3\_3Si_2\_2O_14\_{14} : evidences for symmetry breaking

The chiral langasite Ba$\_3$NbFe$\_3$Si$\_2$O$\_{14}$ is a multiferroic compound. While its magnetic order below T$\_N$=27 K is now well characterised, its polar order is still controversial. We thus looked at the phonon spectrum and its temperature dependence to unravel possible crystal symmetry breaking. We combined optical measurements (both infrared and Raman spectroscopy) with ab initio calculations and show that signatures of a polar state are clearly present in the phonon spectrum even at room temperature. An additional symmetry lowering occurs below 120~K as seen from emergence of softer phonon modes in the THz range. These results confirm the multiferroic nature of this langasite and open new routes to understand the origin of the polar state

Lattice and spin excitations in multiferroic h-YMnO3

We used Raman and terahertz spectroscopies to investigate lattice and magnetic excitations and their cross-coupling in the hexagonal YMnO3 multiferroic. Two phonon modes are strongly affected by the magnetic order. Magnon excitations have been identified thanks to comparison with neutron measurements and spin wave calculations but no electromagnon has been observed. In addition, we evidenced two additional Raman active peaks. We have compared this observation with the anti-crossing between magnon and acoustic phonon branches measured by neutron. These optical measurements underly the unusual strong spin-phonon coupling

Construction of the Pauli-Villars-regulated Dirac vacuum in electromagnetic fields

Using the Pauli-Villars regularization and arguments from convex analysis, we construct solutions to the classical time-independent Maxwell equations in Dirac's vacuum, in the presence of small external electromagnetic sources. The vacuum is not an empty space, but rather a quantum fluctuating medium which behaves as a nonlinear polarizable material. Its behavior is described by a Dirac equation involving infinitely many particles. The quantum corrections to the usual Maxwell equations are nonlinear and nonlocal. Even if photons are described by a purely classical electromagnetic field, the resulting vacuum polarization coincides to first order with that of full Quantum Electrodynamics.Comment: Final version to appear in Arch. Rat. Mech. Analysi

Phase transition close to room temperature in BiFeO3 thin films

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, displays a reversible temperature-induced phase transition at about 100\circ, thus close to room temperature.Comment: accepted in J. Phys.: Condens. Matter (Fast Track Communication

Non-perturbative embedding of local defects in crystalline materials

We present a new variational model for computing the electronic first-order density matrix of a crystalline material in presence of a local defect. A natural way to obtain variational discretizations of this model is to expand the difference Q between the density matrix of the defective crystal and the density matrix of the perfect crystal, in a basis of precomputed maximally localized Wannier functions of the reference perfect crystal. This approach can be used within any semi-empirical or Density Functional Theory framework.Comment: 13 pages, 4 figure

Potentiel du contrôle ultrasonore d’une plaque masquée par des écrans parallèles

National audienceThe in-service inspection of internal structures of future liquid sodium-cooled fast reactors isan important issue of the CEA. The ultrasonic methods can overcome the sodium opacity,are insensitive to radiation and are adapted to high temperatures. Moreover, they allow tocontrol from outside the circuit sodium; this communication regards this point. Thepropagation of ultrasounds is studied in a plate masked by n plates immersed in liquidsodium to control it. The need to obtain a maximum of propagating energy directed the studytowards Lamb waves in layered solid-fluid structures. The model developed is based on themethod of transfer matrix and leads to the optimization of the pair (angle, frequency). Anexperimental study validates the model on a set of stainless steel plates immersed in water.The possibilities for the control of a plate behind n screens are then analyzed and discussed.L'inspection en service de structures internes des réacteurs à neutrons rapides refroidis ausodium liquide constitue une problématique importante au CEA. Les méthodes ultrasonorespermettent de s’affranchir du caractère opaque du sodium, sont insensibles au rayonnementet sont adaptées aux hautes températures. De plus, elles permettent des contrôles depuisl’extérieur du circuit sodium ; c’est dans ce cadre que s’insère cette communication. Il s’agitd’étudier la propagation des ultrasons dans une plaque masquée par n plaques immergéesen sodium afin de la contrôler. La nécessité de propager le maximum d'énergie a orientél'étude vers les ondes de Lamb dans des structures multicouches solide-fluide. Lamodélisation repose sur la méthode des matrices de transfert et conduit à l’optimisation ducouple (angle, fréquence). Une étude expérimentale valide ce modèle sur un ensemble deplaques d’acier inoxydable immergées dans l’eau. Les possibilités de contrôle d’une plaquederrière n écrans sont analysées et discutées

Three dimensional collective charge excitations in electron-doped cuprate superconductors

High temperature cuprate superconductors consist of stacked CuO2 planes, with primarily two dimensional electronic band structures and magnetic excitations, while superconducting coherence is three dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, believed to be incoherent in the normal state, yet lacking a comprehensive characterization in energy-momentum space. Here, we use resonant inelastic x-ray scattering (RIXS) with polarization analysis to uncover the pure charge character of a recently discovered collective mode in electron-doped cuprates. This mode disperses along both the in- and, importantly, out-of-plane directions, revealing its three dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the CuO2 plane distance rather than the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction drives the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought acoustic plasmon, predicted for layered systems and argued to play a substantial role in mediating high temperature superconductivity.Comment: This is the version of first submission. The revised manuscript according to peer reviews is now accepted by Nature and will be published online on 31st Oct., 201

Genetic diversity and the emergence of ethnic groups in Central Asia

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Structure, electrochemical properties and functionalization of amorphous CN films deposited by femtosecond pulsed laser ablation

Amorphous carbon nitride (a-C:N) material has attracted much attention in research and development. Recently, it has become a more promising electrode material than conventional carbon based electrodes in electrochemical and biosensor applications. Nitrogen containing amorphous carbon (a-C:N) thin films have been synthesized by femtosecond pulsed laser deposition (fs-PLD) coupled with plasma assistance through Direct Current (DC) bias power supply. During the deposition process, various nitrogen pressures (0 to 10 Pa) and DC bias (0 to ¿ 350 V) were used in order to explore a wide range of nitrogen content into the films. The structure and chemical composition of the films have been studied by using Raman spectroscopy, electron energy-loss spectroscopy (EELS) and high-resolution transmission electron microscopy (HRTEM). Increasing the nitrogen pressure or adding a DC bias induced an increase of the N content, up to 21 at.%. Nitrogen content increase induces a higher sp2 character of the film. However DC bias has been found to increase the film structural disorder, which was detrimental to the electrochemical properties. Indeed the electrochemical measurements, investigated by cyclic voltammetry (CV), demonstrated that a-C:N film with moderate nitrogen content (10 at.%) exhibited the best behavior, in terms of reversibility and electron transfer kinetics. Electrochemical grafting from diazonium salts was successfully achieved on this film, with a surface coverage of covalently bonded molecules close to the dense packed monolayer of ferrocene molecules. Such a film may be a promising electrode material in electrochemical detection of electroactive pollutants on bare film, and of biopathogen molecules after surface grafting of the specific affinity receptor.This work is produced with the financial support of the Future Program Lyon Saint-Etienne (PALSE) from the University of Lyon (ANR-11-IDEX-0007), under the “Investissements d'Avenir” program managed by the National Agency Research (ANR)