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Second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the more usual mechanism of diffusion. This leads to a very high thermal conductivity. It is known as "second sound" because the wave motion of heat is similar to the propagation of sound in air.
Additional recommended knowledge
Second sound is observed in liquid helium (3He as well as 4He) and in 6Li at temperatures below the lambda point. In this state, known as helium II, it has the highest thermal conductivity of any known material (several hundred times higher than copper).
Second sound in Helium II
At temperatures below the lambda point 2.1768 K 4He has a superfluid state, has almost perfect heat conduction, and is called helium II. The helium is in a macroscopic quantum state. At temperatures falling against 0K the speed of temperature and entropy waves increases. These can be generated and observed in a resonator. At a temperature of 1.8K the temperature wave propagates at approximately 20m/s.
Second sound in other media
3He has second sound below 2.5mK, 6Li also close to 0K. Superfluidity of 6Li has been observed at a temperature of 50nK at MIT in April 2005.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Second_sound". A list of authors is available in Wikipedia.|