The second sound in liquid helium is a specific quantum-mechanical phenomenon in liquid helium , as well as in some other substances, consisting in the presence of two speeds of sound for two parts of the energy spectrum of elementary excitations, phonon and roton.
The second sound is the slightly damped fluctuations in temperature and entropy in superfluid helium. The propagation velocity of the second sound is determined from the equations of hydrodynamics of a superfluid liquid in a two-component model . If we neglect the coefficient of thermal expansion anomalously small for helium, then only the temperature and entropy oscillate in the wave of the second sound, and the density and pressure remain constant. The propagation of the second sound is not accompanied by the transfer of matter. In the second sound, heat transfer occurs through a wave-like motion, and not by the more familiar diffusion mechanism. This leads to very high thermal conductivity. The name "second sound" is due to the fact that the movement of heat waves is similar to the propagation of sound in air.
Second Sound in Liquid Helium
The second sound in helium II is observed at temperatures below the lambda point (2.1768 K), when 4 He goes into the superfluid state and has an almost perfect thermal conductivity. When the temperature drops to 0 K, the velocities of the waves of temperature and entropy increase. They can be generated and observed in the resonator. At a temperature of 1.8 K, the temperature wave propagates at a speed of about 20 m / s. The results on the superfluidity of helium in a solid were also announced, but they were questioned in a number of sources [1] .
The second sound can also be interpreted as fluctuations in the concentration of quasiparticles in superfluid helium. In pure 4 He, these are vibrations in the system of rotons and phonons .
The existence of the second sound was predicted theoretically by Landau , the calculated value was 25 m / s. Actually measured - 19.6 m / s [2] .
Second Sound in Other Substances
3 He also has a second sound at a temperature below 2.5 mK, 6 Li is also close to 0 K. Superfluidity of 6 Li was observed at a temperature of 50 nK at the Massachusetts Institute of Technology in April 2005 [3] . A second sound is also observed in some dielectric solids, such as Bi and NaF. In 2019, a second sound was discovered in graphite [4]
Notes
- ↑ Superfluidity of solid helium: can sensation be canceled?
- ↑ Academician P. L. Kapitsa, Properties of liquid helium
- ↑ A new state of matter - a gas with high-temperature superfluidity - was obtained by physicists at the Massachusetts Institute of Technology
- ↑ S. Huberman1, RA Duncan, K. Chen, B. Song, V. Chiloyan, Z. Ding, AA Maznev, G. Chen, KA Nelson Observation of second sound in graphite at temperatures above 100 K // Science , 26 Apr 2019: Vol. 364, Issue 6438, pp. 375-379