Superfluidity Fermionic condensates are attained at lower temperatures than Bose–Einstein condensates. Fermionic condensates are a type of
superfluid. As the name suggests, a superfluid possesses fluid properties similar to those possessed by ordinary
liquids and
gases, such as the lack of a definite shape and the ability to flow in response to applied forces. However, superfluids possess some properties that do not appear in ordinary matter. For instance, they can flow at high velocities without dissipating any energy—i.e. zero
viscosity. At lower velocities, energy is dissipated by the formation of
quantized vortices, which act as "holes" in the medium where superfluidity breaks down. Superfluidity was originally discovered in liquid
helium-4 whose atoms are
bosons, not fermions.
Fermionic superfluids It is far more difficult to produce a fermionic superfluid than a bosonic one, because the
Pauli exclusion principle prohibits fermions from occupying the same
quantum state. However, there is a well-known mechanism by which a superfluid may be formed from fermions: That mechanism is the
BCS transition, discovered in 1957 by
J. Bardeen,
L.N. Cooper, and
R. Schrieffer for describing superconductivity. These authors showed that, below a certain temperature, electrons (which are fermions) can pair up to form bound pairs now known as
Cooper pairs. As long as collisions with the ionic lattice of the solid do not supply enough energy to break the Cooper pairs, the electron fluid will be able to flow without dissipation. As a result, it becomes a superfluid, and the material through which it flows a superconductor. The BCS theory was phenomenally successful in describing superconductors. Soon after the publication of the BCS paper, several theorists proposed that a similar phenomenon could occur in fluids made up of fermions other than electrons, such as
helium-3 atoms. These speculations were confirmed in 1971, when experiments performed by
D.D. Osheroff showed that helium-3 becomes a superfluid below 0.0025 K. It was soon verified that the superfluidity of helium-3 arises from a BCS-like mechanism.
Condensates of fermionic atoms When
Eric Cornell and
Carl Wieman produced a Bose–Einstein condensate from
rubidium atoms in 1995, there naturally arose the prospect of creating a similar sort of condensate made from fermionic atoms, which would form a superfluid by the BCS mechanism. However, early calculations indicated that the temperature required for producing Cooper pairing in atoms would be too cold to achieve. In 2001, Murray Holland at
JILA suggested a way of bypassing this difficulty. He speculated that fermionic atoms could be coaxed into pairing up by subjecting them to a strong
magnetic field. In 2003, working on Holland's suggestion,
Deborah Jin at JILA,
Rudolf Grimm at the
University of Innsbruck, and
Wolfgang Ketterle at
MIT managed to coax fermionic atoms into forming molecular bosons, which then underwent Bose–Einstein condensation. However, this was not a true fermionic condensate. On December 16, 2003, Jin managed to produce a condensate out of fermionic atoms for the first time. The experiment involved 500,000
potassium-40 atoms cooled to a temperature of 5×10−8 K, subjected to a time-varying magnetic field. ==Examples==