Second sound
As a second sound refers to a quantum mechanical phenomenon in which in superfluid temperature - or entropy - waves can be generated. The heat is not transported here as usual by diffusion , but by temperature pulses with a wave character . The name comes from the fact that the phonons involved , whose movement is in any case wave-like propagation, also appear in density waves during the temperature pulse . Therefore one could also speak of sound in sound . The effect has so far only been observed with 3 He , 4 He and 6 Li , each below the lambda point .
Helium II
At temperatures below 2.1768 K , 4 He has a superfluid state, shows almost ideal heat conduction and is referred to as helium II . At temperatures tending towards absolute temperature zero (0 K), the speed of the temperature or entropy waves, which are present alongside conventional sound waves, also increases. These can best be generated and observed in a resonator . The helium is in a macroscopic quantum state . At a temperature of 1.8 K, a temperature wave moves through the liquid at around 20 m / s .
Other element isotopes
At 3 He the second sound can be observed below 2.5 mK.