Toward the quantum regime of thermal transport by attowatt calorimetry sensitive thermal conductance measurements
December 18th, 2018 OLIVIER BOURGEOIS Institut Néel, CNRS

With the use of nano-engineered systems, one can alter the fundamental physical properties of materials, and consequently a new sets of applications will emerge. This fact has been illustrated particularly in thermal properties of confined systems. However especially at low temperature or in amorphous structures, the fundamental mechanisms of heat transport in nanowires or membranes is still lacking. When the phonon characteristic lengths like the mean free path or the dominant wavelength, exceed that of the system, the whole thermal transport will be governed by boundary scattering, which goes from fully diffusive (Casimir regime) to a ballistic regime (Landauer model), where specular reflections of phonons become dominant at the surfaces. For instance, in this ballistic limit, it is expected that quantum effects will emerge. The heat transport may be governed by the universal value of thermal conductance.
However, since amorphous materials are used for the fabrication (SiN)1, the phonon transport will be also set by the interaction of phonon with local tunnelling two level systems (TLS) that are present in every glassy materials at low temperature. The purpose of this experimental work is to measure the competition between these two universal behaviours in the thermal phonon transport.
For this objective, we have elaborated a new suspended membrane based sensor for heat capacity and thermal conductance measurements2,3. This nano-calorimeter is designed using a differential geometry adapted to very low temperature (below 100mK). We demonstrate measurements with unprecedented power sensitivity of ~5 Attowatt. It is shown that the phonon transport does not show any quantum of thermal conductance value but is rather dominated by the universal scattering of phonon on the TLS2.

Seminar, December 18, 2018, 12:00. ICFO’s Seminar Room

Hosted by Adrian Bachtold