An overview of second sound in solid materials

The study of wave-like heat transport in solids (or second sound) received considerable research attention in the 1960s. Surprisingly, its observation remained exclusive to a few materials (e.g., solid He, Bi, and NaF) and for the lower temperature range (T < 15 K) for almost 50 years. Recently, its successful observation at higher temperatures (T > 100 K) in graphite and germanium triggered renewed interest in the field. Developing a clear path to control and exploit such thermal propagation regime has the potential to redefine the strategies to control heat propagation more efficiently. In this talk I will give an overview of the evolution of the field since its discovery in solid materials, with special focus on the latest discoveries. In particular, I will discuss the observation of two different flavors of second sound (drifting and high-frequency second sound), which are found in different experimental conditions. I will discuss the different experimental approaches used for its observation, as well as the theoretical framework used to address its origin and predict its propagation velocity and relaxation time.