The Phonon Quantum of Thermal Conductance—Are Simulations and Measurements Estimating the Same Quantity?

The quantum of thermal conductance (QTC) sets a fundamental limit to the amount of heat, entropy, and information that can flow through an individual quantum channel. Such upper bound has been demonstrated when electrons and photons are the main heat carriers. However, when heat is carried mostly by phonons, differences between theory and experiments obscure the demonstration of the phonon QTC. In this talk, we will present our calculations of phonon transport in suspended structures similar to those used in measurements of the phonon QTC. We will show that part of the disagreement between theory and experiment arises from the use of macroscopic concepts to analyze the measured data. Our calculated temperature profiles, in the wave ballistic nonequilibrium regime, at sub-kelvin operating temperatures, show that the conductance value extracted from an experiment depends not only on the thermal properties of the device of interest, but also on the thermal properties and geometrical characteristics of the measuring platform. Our results suggest the need for experimental validation of diffusive transport assumptions in measuring devices operating at sub-kelvin temperatures. This presentation is in memory of Dr. Natalio Mingo and his invaluable contributions to the thermal transport community.