Second sound

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.

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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.

References

• C.T. Lane, H.A. Fairbank, and W.M. Fairbank, "Second Sound in Liquid Helium II," Phys. Rev. 71, 600 (1947). Retrieved on April 15, 2007.
• Sinyan Shen, Surface Second Sound in Superfluid Helium. PhD Dissertation (1973). http://adsabs.harvard.edu/abs/1973PhDT.......142S
• V. Peshkov, "'Second Sound' in Helium II," J. Phys. (Moscow) 8, 381 (1944)
• U. Piram, "Numerical investigation of second sound in liquid helium," Dipl.-Ing. Dissertation (1991). Retrieved on April 15, 2007.