Laser-induced coherent acoustical phonons mechanisms in the metal-insulator transition compound NdNiO3: Thermal and nonthermal processes

International audienceCoherent acoustic phonon generation processes on the picosecond time scale were investigated in the metal-insulator transition compound NdNiO3. Time-resolved optical pump-probe methods were employed. While at high temperature thermoelastic process drives the photogeneration of acoustic phonons, an additional contribution can be identified when the transition to the insulating state at

Optical excitation and detection of picosecond acoustic pulses in liquid mercury

International audienceUltrashort optical pulses are used to excite and interferometrically detect picosecond longitudinal-acoustic pulses in thin films of liquid mercury sandwiched between sapphire plates. We show that the shape of the strain pulses in the mercury can be directly measured through ultrafast changes in optical reflectivity. By analyzing consecutive acoustic echoes, we derive the dispersion of the ultrasonic attenuation and sound velocity for this liquid at frequencies up to 10 GHz. Significant effects of structural relaxation are observed and are compared to a simple model that indicates the presence of picosecond relaxation times in mercury

Surface-wave dispersion inversion in an unconsolidated granular medium

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Pre-avalanche structural rearrangements in the bulk of granular medium: Experimental evidence

International audienceAbstract – Granular avalanches are typical threshold-type phenomena implying the loss of stability of granular packings when their slope exceeds a critical angle. Detection of avalanche precursors is important for both basic studies and numerous applications. In this respect, only surface observations are presently available, whereas other known techniques are not suitable to detect internal pre-avalanche rearrangements expected from physical considerations and numerical simulations. Here, we report the first experimental evidence of avalanche precursors and the long-lasting post-avalanche rearrangements in the bulk of 3D granular packings. We use an original nonlinear-acoustic probing methodology which is selectively sensitive to the state of weakest intergrain contacts. This methodology allowed us to clearly detect transient pre-avalanche rearrangements of the weak-contact network. Those rearrangements get stronger and exhibit quasi-periodicity closer to the avalanche triggering, whereas the statistics of the sounding signal amplitude shows clear transition from Gaussian to power law behavior. Copyright c EPLA, 200

Ultrafast electronic dynamics in the metal-insulator transition compound NdNiO3

International audienceVisible ultrafast optical property studies have been performed in the metal-insulator transition compound NdNiO3 from room temperature down to 16 K. A clear slowing down of the electronic relaxation is reported when the system becomes insulating. Two characteristic times of electronic relaxation have been found in the insulating phase while a single one is observed in the metallic state. Moreover, the magnitude of the ultrafast electronic transient reflectivity exhibits a drastic increase in the insulating state. Finally, a hysteresis of the electronic response is evidenced in accordance with electrical resistivity measurements

Elastic waves in phononic granular membranes

International audienceElastic membranes composed of a few number of interacting articles/nanoparticles or grains layers can be encountered in different applications including sensors and filters for the individual molecules. Membranes, which are constructed through spatially periodic assembling of particles, exhibit phononic properties, interesting in view of the possible nondestructive testing of these membranes or in view of their applications to control elastic waves propagation. Some results on the elastic properties of multilayered phononic granular membranes, where each particle possesses up to 3 translational and 3 rotational degrees of freedom, are reported. The specific feature of granular metamaterials is known to be an efficient interaction between the shear and the rotational elastic motions of the grains, which leads to the existence of the mixed shear/rotational elastic modes [Phys. Rev. E 82, 031305 (2010)]. The influence of number of granular layers, of bending and spin rigidity of the inter-grain contacts on the phononic properties of granular membranes is analyzed

Two-dimensional discrete granular phononic crystal for shear wave control

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