Phonography

The aim of this project is to study an intriguing behavior of the phonon flow which appears when phonons interact strongly between each other and behave like a viscous fluid. This regime of transport is neither diffusive nor ballistic, and is often referred to as the hydrodynamic regime. To study it, I will build an accurate probe to measure locally the temperature at the surface of solids. This probe will work in a cryogenic environment (from a few tens of milliKelvin to a few tens of Kelvin) so as to be able to observe this phenomenon which usually appears around 1-10 K. This probe is a superconducting quantum interference device (SQUID) deposited at the end of a sharp tip and has already proven to be one of the most sensitive thermometers in a cryogenic environment. By navigating this tip above a surface, one can reconstruct spatial temperature profiles and gain access to local sources of dissipation. One key signature of this hydrodynamic regime is the emergence of a Poiseuille phonon flow, which will create a parabolic temperature profile in the direction perpendicular to the heat flow. The SQUID-on-tip (SOT) is the perfect tool to study this parabolic temperature profile. Moreover, by developing a new fabrication method, I will also unlock temperature and magnetic field ranges never attained before, where the current state of the art SOT are not working. The more long term and general goal will be to build a formidable microscopy platform that will enable to characterize nano-devices in a very unique way. Indeed, from the great versatility of the SOT technique, it is possible to study down to a few tens of nanometers both the magnetic and thermal properties of crystals but also nano-devices. As dissipation and thermal transport are fundamental properties that govern the dynamic of many nano-devices it is expected that this tool will interest a lot of scientists in the Parisian area but also around the world and lead to many fruitful collaborations.