• Chimie/ou physique
  
• Physique
  
• Chimie
  
• Physique/Matière Condensée/Science des matériaux
  
• Physique/Matière Condensée/Autre
  
• Sciences de l'Homme et Société/Gestion et management
  
• Physique/Matière Condensée/Systèmes désordonnés et réseaux de neurones
  

The low frequency lattice vibrations and relaxations are investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and α-FAPbI3, at the Brillouin zone center using Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. The temperature dependence of the PbX6 lattice modes in the four compounds can be renormalized into universal curves, highlighting a common vibrational dynamics at the cubic to tetragonal transition. In particular, no soft vibration is observed excluding a displacive-like transitional dynamics. The reorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-low frequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to center of mass anharmonic motions and rattling of the molecular cations in the perovskite cavities. Its partially unexpressed critical behavior at the transition points toward the general importance of defects in HOPs preventing the net divergence of order parameter correlations at the critical temperatures.

Polarized optical microscopy and spectroscopy are progressively becoming key methods for the high-throughput characterization of individual carbon nanotubes (CNTs) and other one-dimensional nanostructures, on substrate and in devices. The optical response of CNTs on substrate in cross polarization experiments is usually limited by the polarization conservation of the optical elements in the experimental setup. We developed a theoretical model taking into account the depolarization by the setup and the optical response of the substrate. We show that proper modelization of the experimental data requires to take into account both noncoherent and coherent light depolarization by the optical elements. We also show how the nanotube signal can be decoupled from the complex reflection factor of the antireflection substrate, which is commonly used to enhance the optical contrast. Finally, we describe an experimental protocol to extract the depolarization parameters and the complex nanotube susceptibility, and how it can improve the chirality assignment of individual carbon nanotubes in complex cases.

Brillouin spectroscopy is used to investigate the elastic properties of XAlSiO4 aluminosilicate glasses where X =Li, Na, K, Mg0.5, Ca0.5, Sr0.5, Ba0.5, and Zn0.5. The Brillouin frequency shifts obtained in two different scattering geometries allow the calculation of the refractive index, the two sound velocities and Poisson's ratio.Measurements of the mass density give in turn the elastic moduli and the Debye temperature.We find that the elastic properties scale with the atomic density of the glassy network or the charge-balancing cation field strengthwhile they negatively correlate with the glass transition temperature. Further, Poisson's ratio depends on the nature of the non-framework cations in this glass series.

Sound velocities of vitreous silica are measured under He compression in the pressure range 0-6~GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6~GPa.