Superfluidity refers to a transition that occurs in the properties of liquid helium at a temperature of 2.17K, called the lambda point of helium. A discontinuity in heat capacity is observed, the density of the liquid is decreased, and a fraction of the liquid helium becomes a superfluid, i.e., a zero viscosity fluid that exhibits an extraordinarily enhanced fluidity through any medium.
In its elemental state, helium is a gas that can occur as two isotopes, helium-4, which is the most common, and helium-3, which occurs only in very small amounts. Helium-4 has a nucleus consisting of two protons and two neutrons surrounded by two electrons, which is the definition of a boson, i.e. a species consisting of even numbers of elementary particles. The helium-3 nucleus however, has only one neutron and is therefore termed a fermion because it consists of an odd number of spin-1/2 particles in its nucleus. The fundamental distinction between bosons and fermions is that the former obey Bose-Einstein statistics, which allows them to condense to the state of lowest possible energy, the ground state, as predicted by Albert Einstein. In 1924 he proposed that bosons could condense in unlimited numbers into a single ground state because they are not subject to the Pauli exclusion principle. A phase transition in which such a phenomenon occurs is referred to as Bose-Einstein condensation. Unlike bosons, fermions such as helium-3 obey Fermi-Dirac statistics, which implies that they should not condense in the lowest energy state. Helium-3 does possess characteristics that allow it to become supefluid, however, this requires a much lower temperature than helium-4. These two forms of helium remains unique in their ability to experience superfluidity because other elements will typically freeze at such temperatures even though they may also be spin-zero atoms.
Superfluidity was first discovered in 1937 in the helium-4 isotope by the Russian physicist Pyotr Leonidovich Kapitza (Nobel Prize in physics, 1978) and it is considered one of the most remarkable breakthroughs of low-temperature physics. Theoretically, it was shown shortly after the discovery by Fritz London and Lev Landau (Nobel Prize in physics, 1962) that the effect was due to a two-fluid state of the helium: one fraction remained a normal liquid and the superfluid fraction consisted of those atoms undergoing Bose-Einstein condensation, thus becoming incapable of contributing to the entropy or heat capacity of the liquid. In 1996, the Nobel prize was conferred to D.M. Lee, D.D. Osheroff, and R.C. Richardson for discovering in the 1970s that the helium-3 isotope can also be made superfluid at a temperature only about two thousandths of a degree above absolute zero (-273.15°C). This achievement greatly surprised the physics community since fermions were not expected to undergo Bose-Einstein condensation. The BCS theory for superconductivity in metals, formulated by John Bardeen, Leon Cooper, and John Robert Schrieffer (Nobel Prize in physics, 1972) provided the explanation. It was proposed that electrons, being fermions since they consist of one particle, must follow Fermi-Dirac statistics just as the helium-3 isotopes do. However in supercooled metals, two electrons can combine as Cooper pairs, which allows them to exhibit boson-like behavior, such as the capacity to undergo Bose-Einstein condensation.
Superfluidity cannot be understood in terms of classical physics. When helium becomes superfluid, its atoms lose all randomness and move in a coordinated manner, which causes the liquid to totally lack friction, thus allowing it to flow out of very small holes, and exhibit other manifestations of non-classical behavior. Understanding the fundamental properties of such a liquid requires an advanced form of quantum physics, and for this reason, liquid helium is often referred to as a quantum liquid. Further research developments have shown that helium-3 has at least three different superfluid phases, one of which occurs when the helium is placed in a magnetic field. As a quantum liquid, helium-3 can thus exhibit a considerably more complicated structure than helium-4.
An interesting application of superfluidity in helium-3 is that it is being used to investigate the formation of the cosmic strings required by string theory. These enormous hypothetical objects, believed to be involved in the birth of galaxies, could have arisen as a consequence of the rapid phase transitions believed to have taken place a fraction of a second after the big bang. Research teams used neutrino-induced nuclear reactions to quickly heat superfluid helium-3, followed by rapid re-cooling. This resulted in the production of balls of vortices that were proposed to correspond to cosmic strings. Helium-4 has no notable applications. Observation of the superfluid state of helium-4 results in further analysis of its unusual properties such as its ability to move with zero viscosity, against gravity, and not be affected by condensation or evaporation. The fact that it also experiences a tremendous increase in heat conductivity may prevent it from having useful applications.
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