Second sound

Second sound is a phenomenon observed in certain superfluid materials, notably in liquid helium-4 when it is cooled to temperatures near absolute zero. In these states, the normal laws of thermal conduction change, and heat can propagate as a wave, rather than by conventional conduction. In classical materials, heat is transferred by the diffusion of thermal energy, which can be thought of as a statistical flow of phonons (quanta of vibrational energy) and other excitations.