Second sound
A second sound is called a quantum mechanical phenomenon in which temperature - entropy or waves can be generated in superfluid. The heat transport is not, as usual, by diffusion, but by temperature pulses with wave character. The name comes from the fact that the participating phonons whose movement anyway propagating wave-like, in addition occur when temperature pulse in density waves. Therefore, one could also speak of the sound in the sound. The effect was so far only observed in 3He, 4He, and 6Li, respectively below the lambda point.
Helium II
At temperatures below 2.1768 K 4 He has a superfluid state, is an almost ideal heat transfer and is referred to as helium II. With respect to absolute zero temperature (0 K) aspiring temperatures, the speed of sound waves present in addition to conventional temperature or entropy waves increases. This can best be generated and monitored in a resonator. In this case, the helium is in a macroscopic quantum state. At a temperature of 1.8 K, a temperature wave moves at 20 m / s through the liquid.