Second sound is a quantum mechanical phenomenon in which heat transfer

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 occurs by wave

Wave equation

The wave equation is an important second-order linear partial differential equation for the description of waves – as they occur in physics – such as sound waves, light waves and water waves. It arises in fields like acoustics, electromagnetics, and fluid dynamics...

-like motion, rather than by the more usual mechanism of diffusion

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

. Heat takes the place of pressure in normal sound waves. This leads to a very high thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

. It is known as "second sound" because the wave motion of heat is similar to the propagation of sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 in air.

Second sound is observed in liquid helium

Liquid helium

Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...

 (³He as well as ⁴He) and in ⁶Li at temperatures below the lambda point

Lambda point

The Lambda point is the temperature below which normal fluid helium transitions to superfluid helium II. More precisely, there is a lower lambda point at 2.172 K, 0.0497 atm, and an upper one at 1.76 K, 29.8 atm....

. In this state, known as helium II, ⁴He has the highest thermal conductivity of any known material (several hundred times higher than copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

).

Second sound in helium II

At temperatures below the lambda point 2.1768 K

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, ⁴He enters a superfluid

Superfluid

Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...

 state and has almost perfect heat conduction. The helium is in a macroscopic quantum state

Macroscopic quantum state

A macroscopic quantum state is a state of matter in which macroscopic properties, such as mechanical motion, thermal conductivity, electrical conductivity and viscosity, are governed by quantum mechanics as opposed to classical mechanics...

. At temperatures falling toward 0 K the speed of temperature and entropy waves increases. These can be generated and observed in a resonator. At a temperature of 1.8 K the temperature wave propagates at approximately 20 m/s.

Second sound in other media

³He has second sound below 2.5 mK, ⁶Li also close to 0 K. Superfluidity of ⁶Li has been observed at a temperature of 50 nK at MIT in April 2005.. Second sound has also been observed in some dielectric solids, such as Bi and NaF. Documented at Bell Laboratories, New Jersey in 1972, the paper was received 10 April 1972 and published by The American Physical Society in the issue dated 29 May 1972.

"We have observed the transition from ballistic to second sound to diffusive propagation of heat (phonon) pulses in the semimetal bismuth in the temperature range of 1.2 to 4.0 K. The saturated second-sound velocity is found to be independent of orientation and has a a value of (0.78±0.05)×105 cm/sec (1/3√3 times the Debye velocity). The inverse of the normal process phonon lifetime has a value TN-1=4.49×104T4 sec-1. Scattering due to the hole-phonon interaction is observed for ballistic L modes propagating along the C3 axis." - V. Narayanamurti and R. C. Dynes
Bell Laboratories, Murray Hill, New Jersey.