ASYNCHRONOUS LATTICE STRAIN PHYSICS AND CHEMISTRY OF HARD CONDENSED Another important result of the study is the demonstration of the existence MATTER, EARTH SCIENCES of an asynchronous effect (Fig. 3). As shown in Fig. 3, the maximum SYNCHROTRON SOLEIL HIGHLIGHTS 2020 value of the longitudinal and shear strains are not attained at the same time: the unit cell first expands in the direction normal to the sample’s surface within about 20 ps, and the shear strain develops only after, CRISTAL BEAMLINE reaching its maximum value at a delay of about 60 ps after laser excitation (Fig. 3). This asynchronous effect can be explained by the existence of Associated publication different sound velocities in the lattice. While the longitudinal coherent Ultrafast light-induced shear strain acoustic phonons (out-of-plane strain component) propagate at around probed by time-resolved x-ray 5000 m/s, the coherent shear phonons (in-plane strain component) diffraction: Multiferroic BiFeO as a3 propagate only at around 3000 m/s. This leads to a two-step transient case study. deformation of the unit cell (Fig. 3). V. Juvé, R. Gu, S. Gable, T. Maroutian, Finally, the analysis of the strain amplitudes shows that the ultrafast light- G. Vaudel, S. Matzen, N. Chigarev, induced thermal expansion (thermal effect, TE, as depicted by horizontal S. Raetz, V. E. Gusev, M. Viret, orange and blue dashed lines in Fig. 3) can account only partially for A. Jarnac, C. Laulhé, A. A. Maznev, the light-induced strain. The study thus demonstrates that non-thermal B. Dkhil & P. Ruello. phenomena, such as possibly the inverse-piezoelectric effect, are at play during this ultrafast photoexcitation mechanism. Physical Review B., 102(22): art.n° 220303 (2020). FIGURE 3 References [1] C. Thomsen etal. , Phys. Rev. B 34, 4129 (1986). [2] P. Ruello & V. Gusev, Ultrasonics 56, 21 (2015). [3] D. Schick et al., Phys. Rev. Lett. 112, 097602 (2014). [4] H. Wen et al., Phys. Rev. Lett, 110 037601 (2013). [5] M. Lejman et al., Nature Comm. 5, 1-7 (2014). Corresponding author Pascal Ruello Institut des Molécules et Matériaux du Mans, UMR CNRS 6283 Avenue O. Messiaen, Le Mans Université, 72085 Le Mans Cedex, France Pascal.ruello@univ-lemans.fr Captions FIGURE 1: BiFeO3 unit cell anisotropic deformation in presence of both the light-induced shear and longitudinal strains. FIGURE 2: (left) Geometry of the time-resolved X-ray diffraction experiment. (bottom) Non-equivalent temporal dependences of the d201 and d201 interplanar distances as a consequence of photoinduced shear strain. FIGURE 3: (top) Longitudinal (ηL) and shear (Sη) strains as a function of time. (bottom) Sketch of the asynchronous dynamics of the longitudinal and shear strains: a two- step unit cell deformation. 69