Abstract:
Understanding heat conduction in non-conventional systems requires revisiting the classical particle-based models of phonon transport. In this seminar, we explore two emerging paradigms in thermal science that challenge conventional views: phonon coherence and phonon hydrodynamics.
In the first part, we examine how thermal phonons, traditionally treated as incoherent particles, can also exhibit wave-like coherence. We present a new theoretical framework for heat conduction that incorporates both phonon lifetime and coherence time, allowing us to model the behavior of phonon wave packets with finite spatial coherence length. This approach is particularly insightful in complex crystals, amorphous materials, and nanostructures, where the dense structure of phonon branches in the dispersion relation makes coherence effects prominent. Our findings reveal that the size of the wave packet plays a critical role in modulating thermal conductivity [1].
In the second part, we turn to phonon hydrodynamics—an intermediate regime between ballistic and diffusive transport. Under specific conditions, where Umklapp scattering is suppressed compared to normal and boundary scattering, phonons can exhibit collective motion reminiscent of fluid flow. We demonstrate this phonon liquid regime experimentally in graphite microribbons, and further exploit it to design a graphite Tesla valve [2]—a passive thermal device that rectifies heat flow in the temperature range where hydrodynamic effects dominate.
Together, these two themes shed light on the dual nature of phonons and open new perspectives for engineering heat transport in quantum and nanoscale systems.