An Overview of Second Sound in Solid Materials

The study of heat transport beyond Fourier’s regime has attracted renewed interest in recent years. Great efforts have been performed to unravel the physical properties of thermal waves, as well as the experimental conditions that are necessary for their observation. Applications based on such concepts have been envisioned and discussed extensively already in many recent publications[1–3].The spatio-temporal propagation of the temperature field in the form of waves is known as “second sound,” a term that was adopted in analogy to “first sound” (or simply “sound”, i.e. mechanical lattice vibrations). First and Second Sound are both described by a similar equation where the variables have a different physical meaning, i.e. pressure and temperature, respectively[4].Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experimental observation was previously restricted to a small number of materials (solid He, Bi, NaF, SrTiO3, and graphite), usually in rather narrow temperature windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying temperature field. High-frequency second sound is demonstrated in bulk natural Ge between 7 kelvin and room temperature by studying the phase lag of the thermal response under a harmonic high frequency external thermal excitation, and addressing the relaxation time and the propagation velocity of the heat waves. In this talk, I will present a comprehensive overview on the historical observation of second sound, with special emphasis on the appearance of different flavors of thermal waves depending on the thermal excitation conditions. In particular, I will discuss the recent experimental observation of high frequency second sound[5]