page_title: Superfluid helium-4
section_title: Properties
hierarchical_section_title: Superfluid helium-4 / Properties
caption_attribution_description: English: Heat capacity of 4HeУкраїнська: Теплоємність рідини 4He при тиску насиченої пари як функція температури. Пік при T = 2,17 K позначає фазовий перехід другого роду.
context_page_description: Superfluid helium-4 is the superfluid form of helium-4, an isotope of the element helium. A superfluid is a state of matter in which matter behaves like a fluid with zero viscosity. The substance, which looks like a normal liquid, flows without friction past any surface, which allows it to continue to circulate over obstructions and through pores in containers which hold it, subject only to its own inertia.
  Known as a major facet in the study of quantum hydrodynamics and macroscopic quantum phenomena, the superfluidity effect was discovered by Pyotr Kapitsa and John F. Allen, and Don Misener in 1937. It has since been described through phenomenological and microscopic theories. The formation of the superfluid is known to be related to the formation of a Bose–Einstein condensate. This is made obvious by the fact that superfluidity occurs in liquid helium-4 at far higher temperatures than it does in helium-3. Each atom of helium-4 is a boson particle, by virtue of its zero spin. Helium-3, however, is a fermion particle, which can form bosons only by pairing with itself at much lower temperatures, in a process similar to the electron pairing in superconductivity.
context_section_description: Figure 1 is the phase diagram of ⁴He. It is a pressure-temperature (p-T) diagram indicating the solid and liquid regions separated by the melting curve (between the liquid and solid state) and the liquid and gas region, separated by the vapor-pressure line. This latter ends in the critical point where the difference between gas and liquid disappears. The diagram shows the remarkable property that ⁴He is liquid even at absolute zero. ⁴He is only solid at pressures above 25 bar.
  Figure 1 also shows the λ-line. This is the line that separates two fluid regions in the phase diagram indicated by He-I and He-II. In the He-I region the helium behaves like a normal fluid; in the He-II region the helium is superfluid.
  The name lambda-line comes from the specific heat – temperature plot which has the shape of the Greek letter λ. See figure 2, which shows a peak at 2.172 K, the so-called λ-point of ⁴He.
  Below the lambda line the liquid can be described by the so-called two-fluid model. It behaves as if it consists of two components: a normal component, which behaves like a normal fluid, and a superfluid component with zero viscosity and zero entropy. The ratios of the respective densities ρₙ/ρ and ρₛ/ρ, with ρₙ (ρₛ) the density of the normal (superfluid) component, and ρ (the total density), depends on temperature and is represented in figure 3. By lowering the temperature, the fraction of the superfluid density increases from zero at Tλ to one at zero kelvins.  Below 1 K the helium is almost completely superfluid.
  It is possible to create density waves of the normal component (and hence of the superfluid component since ρₙ + ρₛ = constant) which are similar to ordinary sound waves. This effect is called second sound. Due to the temperature dependence of ρₙ (figure 3) these waves in ρₙ are also temperature waves.
caption: Fig. 2. Heat capacity of liquid 4He at saturated vapor pressure as function of the temperature. The peak at T=2.17 K marks a (second-order) phase transition.
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