First principles application of the Guyer criteria and heat pulse simulation for phonon hydrodynamics in fluorides and alkali hydrides

Previous experimental studies have reported the wave-like transport of heat in a small number of crystalline solids. Such phenomena, referred to as 'second sound', is considered evidence for phonon hydrodynamics. In this work, we employ an ab-initio framework to study phonon hydrodynamics in fluorides and alkali hydrides crystals: sodium fluoride (NaF), lithium fluoride (LiF), lithium hydride (LiH), and sodium hydride (NaH). We first use Guyer's criteria as a guide to determine the regime window of phonon hydrodynamics in the temperature-length scale phase space. We then use an efficient numerical phonon BTE solver under the Callaway approximation to simulate practical experimental geometries. Based on these calculations, we observe the ballistic, hydrodynamic and diffusive phonon transport regimes for each of these four materials. Our calculations predict the existence of the second sound in NaF at a temperature of 15 K and a 8.3 mm characteristic length, consistent with a previous experimental observation [Phys. Rev. Lett. 25, 26 (1970) - Second Sound in NaF].