The workshop ""Phonon lifetime from disordered to complex systems: Measurement and Interpretation""  is organized by the Institute of Light and Matter in Lyon on the 19^th and 20^th of December, 2019.

The workshop focuses on the challenge of experimentally resolving and understanding acoustic phonon lifetimes in a variety of materials, from disordered to complex crystalline systems. This challenge is put in perspective with the understanding of thermal transport, for which phonon lifetime represents the key parameter.

It aims at gathering expertises in inelastic techniques (Neutrons, X-rays, pump-probe based techniques) and the theoretical approaches on phonon scattering processes and anharmonicity.

Phonons are the quantized wave of vibrational motion of atoms in a system. When the atomic arrangement is ordered, like in crystals, the periodicity allows for the good definitions of phonons, as plane waves with a defined wavevector and energy. In presence of a strong disordered, breaking down all possible periodicity, this is not possible anymore. Still at long wavelengths collective excitations exist, reminiscent of phonons in a crystal. At these wavelengths the details of the atomic arrangement are invisible to the vibrational waves. We are still in presence of propagating plane waves, which, in disordered systems, have been called Propagons.

When the wavelength becomes comparable with the disorder lengthscale, the wave is strongly scattered, its lifetime dramatically reduced. This corresponds to the Ioffe-Regel limit, beyond which well defined plane waves don’t exist anymore. The vibrational modes here are not propagative but diffusive and are called Diffusons.

Phonons in crystals, propagons and diffusons in disordered systems, are important for understanding sound and heat transport in materials.

Phonons come in two types: acoustic and optic. Acoustic phonons act like sound waves, zipping through a crystal at a fixed speed regardless of their wavelength. They carry heat. In contrast, optic phonons generally have higher energies, move more slowly, and carry little heat. The two types of vibrations are very different: the simplest acoustic phonon consists of all the atoms sloshing back and forth in concert. The simplest optic phonon consists of neighboring atoms oscillating in opposite directions. Determining a phonon's individual properties, especially those of acoustic phonons, is then the basis of any knowledge-driven thermal engineering approach in energy-recovering and production strategies. While phonon energies can be relatively easily measured by inelastic scattering techniques (using X-rays or neutrons), this is not the case for the phonon attenuation, which translates into its intrinsic energy width, whose detection in crystals still remains beyond the current resolution limits of state-of-the art spectrometers.

Phonon attenuation is inversely proportional to its lifetime: the average time between two scattering processes, i.e. the average time over which the phonon is efficient in transporting heat.

It is clearly fundamental to overcome technical limitations for achieving the experimental measurement of phonon lifetime and getting a microscopic understanding of thermal transport.