Emile Bevillon

Theoretical and experimental study of the structural, dynamical and dielectric properties of perovskite BaSnO3

The structural, dynamical and dielectric properties of the cubic phase of perovskite barium stannate BaSnO3, a potential candidate as protonic conductor for solid oxide fuel cells, have been investigated by the means of first-principles density functional calculations, and the structural and electrical properties have been explored at low temperature. From density functional perturbative calculations, the phonon modes, the Born effective charges and the dielectric tensor are derived and analyzed, at zero pressure. The phonon band-structure of the cubic phase does not exhibit unstable modes, in good agreement with x-ray diffraction, which shows that BaSnO3 remains perfectly cubic down to 10 K. The dielectric response in BaSnO3 as measured and calculated is lower than in titanate and zirconate perovskites.

Free electron properties of metals under ultrafast laser-induced electron-phonon nonequilibrium: a first-principles study

The dynamics of laser-excited materials is an area of intense research as diagnostics of laser-matter experiments can be discussed by back-tracking the transient properties of the irradiated material. Particularly, the primary phenomena of transient electronic excitation and energy transport are of utmost importance. Irradiating a metal by a short laser pulse (� 100 fs) can lead to a significant rise of the electronic temperature with respect to the ionic lattice as the energy of the laser pulse can be deposited before the material system starts dissipating energy by thermal or mechanical ways. The electronic excitation can affect both electronic and structural properties of the solid, impacting optical coupling, transport and phase transitions. The confinement of the absorbed energy at solid density pushes the matter into an extreme nonequilibrium state and new thermodynamic regimes are triggered. The interplay between the ultrafast excitation and the material response still requires a comprehensive theoretical description for highly excited solid materials including in particular the excitation-dependent band structure evolution as this influence the response to laser action. 1 Recent advances in studying processes induced by short laser pulses have revealed the determinant role of primary excitation events. Their accurate comprehension is necessary to correctly describe ultrafast structural dynamics, 2,3 phase transitions, 4,5 nanostructure formation, 6 ablation dynamics, 7,8 or strong shock propagation. 9 In such nonequilibrium conditions, conduction electrons participating to energy exchange are expected to evolve in time, depending on the excitation degree. 10 They largely determine the material transient properties and transformation paths. In this context, they are a crucial parameter required to describe complex ultrafast phenomena involving relaxation of excited states. Particularly, pure electronic effects (population and band distribution) determining transient coefficients before structural transitions set in are of interest and we will follow excitation influence in the form of nonequilib

How dopant size influences the protonic energy landscape in BaSn1−xMxO3−x/2 (M = Ga, Sc, In, Y, Gd, La)

The energy landscape of the protonic defect is investigated in acceptor-doped barium stannate using density-functional calculations. Several trivalent dopants are studied (Ga, Sc, In, Y, Gd, La), covering a wide range of ionic radii. All the dopants are found attractive with respect to the proton, with (negative) association energies varying from −0.40 to −0.07 eV. A radius rc1 is defined to separate the “small” dopants that induce tensile stress from the “large” ones that induce compressive stress in the host matrix (rc1 ≈ 0.72 A). The protonic energy surface exhibits a non-trivial evolution with ionic radius of the dopant: for low dopant radii, the most stable protonic site is the oxygen first-neighbor of the dopant, while for high dopant radii, the most stable position is obtained when the proton is bonded to an oxygen second-neighbor of the dopant. This evolution of the protonic energy surface with dopant ionic radius is smooth and the transition takes place between In and Y, i.e. for a critical radius rc2 between 0.80 and 0.90 A (rc2 > rc1 significantly). The dopant–proton association energy exhibits a minimal value ≈−0.07 eV (weakest attraction) at this transition, i.e. in the case of yttrium, for which the first-neighbor and second-neighbor positions are almost degenerate. Other dopants, smaller or larger, are more attractive to protons. The present study gives useful information about the modification of the trapping effect according to the dopant ionic size.

PREPARATION AND CHARACTERIZATION OF IN-SUBSTITUTED BaSnO3 COMPOUNDS

In-substituted BaSnO3 (BaSn1-xInxO3-δ, x = 0.125, 0.25 and 0.50) compounds were prepared by a gel polymerization method. Their microstructure, hydration energy and electrical properties were investigated. Single phases were confirmed by X-ray diffraction analysis over the whole range of substitution, and were identified as cubic perovskite-like structure. The grain size and conductivities of samples measured under dry and wet atmospheres increase significantly with indium content as such, 1–2 μm for BaSn0.875In0.125O3-δ(x = 0.125) and 20 μm for BaSn0.50In0.50O3-δ(x = 0.50). The highest proton conductivity at 600°C, σ = 9.5 × 10-4 S⋅cm-1, is obtained for BaSn0.5In0.5O2.75(x = 0.50).

Nonequilibrium optical response of metals irradiated by ultrafast laser pulses (Conference Presentation)

Ultrafast light coupling with metal surfaces shows strong potential for nanostructuring applications relying on the capacity to localize light energy on the nanoscale. Controlling light confinement requires to understand the transient variation of the optical response during ultrafast irradiation. The fundamental approach we propose based on ab initio calculations allows elucidating the influence of electron-phonon nonequilibrium on optical properties. This results from the investigation of the primary processes responsible for the optical change during laser-solid interaction. Calculations are carried out in the framework of the density functional theory associated to quantum molecular dynamics. Our results shed light on the intricate role of electronic structure modifications and possible optical transitions, driving the laser energy absorption into the material. The revealed key processes based on Fermi smearing on an evolving density of states are of paramount interest for controlling laser energy deposition, surface plasmon excitation and subsequent surface nanostructuring. The calculations predict the possibility of an ultrafast laser-driven plasmonic switch on a typically non-plasmonic material (W), confirmed by pump-probe ellipsometric measurements [1]. The consequence of our results is far reaching as they propose also a route for achieving the highest energy confinement under ultrashort laser irradiation. [1] E. Bevillon, J.P. Colombier, V. Recoules, H. Zhang, C. Li, R. Stoian, “Ultrafast switching of surface plasmonic conditions in nonplasmonic metals”, Physical Review B 93 (16), 165416 (2016).

Modeling 2D and 3D periodic nanostructuring of materials with ultrafast laser pulses (Conference Presentation)

Generation of periodic arrangements of matter on materials irradiated by laser fields of uniform and isotropic energy distribution is a key issue in controlling laser structuring processes below the diffractive limit. Using three-dimensional finite-difference time-domain methods, we evaluate energy deposition patterns below a materials rough surface [1] and in bulk dielectric materials containing randomly distributed nano-inhomogeneities [2]. We show that both surface and volume patterns can be attributed to spatially ordered electromagnetic solutions of linear and nonlinear Maxwell equations. In particular, simulations revealed that anisotropic energy deposition results from the coherent superposition of the incident and the inhomogeneity-scattered light waves. Transient electronic response is also analyzed by kinetic equations of free electron excitation/relaxation processes for dielectrics and by ab initio calculations for metals. They show that for nonplasmonic metals, ultrafast carrier excitation can drastically affect electronic structures, driving a transient surface plasmonic state with high consequences for optical resonances generation [3]. Comparing condition formations of 2D laser-induced periodic surface structures (LIPSS) and 3D self-organized nanogratings, we will discuss the role of collective scattering of nanoroughness and the feedback-driven growth of the nanostructures. [1] H. Zhang, J.P. Colombier, C. Li, N. Faure, G. Cheng, and R. Stoian, Physical Review B 92, 174109 (2015). [2] A. Rudenko, J.P. Colombier, and T.E. Itina, Physical Review B 93 (7), 075427 (2016). [3] E. Bévillon, J.P. Colombier, V. Recoules, H. Zhang, C. Li and R. Stoian, Physical Review B 93 (16), 165416 (2016).