Thermal imaging at the nanoscale in quantum materials – 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.