Hydrodynamic phenomena in thermal transport

A large number of experimental observations incompatible with the
classical Fourier description of thermal transport at the nanometer and
in the picosecond scales has been reported in the last decade. Despite
the theoretical efforts done in the topic, a model able to describe the
gathered data at all length and time scales is still not available.

Two different descriptions have been proposed. Phonon hydrodynamics has
been used as a framework to model thermal transport in materials where
momentum conservation in phonon-phonon collisions is important. For
other situations, a kinetic description based on the propagation of
independent phonons, in what is called quasiballistic description, has
been developed. The fundamental difference between them is in the number
of length or time scales required to describe the observations. While in
the hydrodynamic approach, a single scale is enough, in the
quasiballistic description, the full set of phonon scales is necessary.
For graphene and other 2D materials, the hydrodynamic approach has been
the traditional main stream, while the quasiballistic approach has been
more used for classical bulk semiconductors.

In the last years, some experiments and theoretical descriptions seems
to be challenging this traditional splitting. On the one side, some
predictions of the hydrodynamic regime for 2D materials like the second
sound velocity have put on doubt the standard approach. On the other
side, collective phonon behavior like the use of a single time scale to
describe thermal decay in a silicon substrate or the observation of
second sound in germanium seem to indicate that the hydrodynamic
description could be used in these semiconductors. This could be an
indication that a more unified framework could be proposed.

The talk will cover some of the most recent evidences in the theoretical
and experimental research on thermal transport and we will analyze them
in the framework of the Kinetic/Collective model (KCM), developed to
give a more generalized framework to describe thermal experiments.

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