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The fermionic condensate is a superfluid phase formed by fermionic atoms at low temperatures. It is closely related to the Bose-Einstein condensate, a superfluid phase formed by bosonic atoms under similar conditions. Unlike the Bose-Einstein condensates, fermionic condensates are formed using fermions instead of bosons. The fermions form a condensate in a manner analogous to the electrons in a superconductor. The first fermionic condensate was created by Deborah S. Jin in 2003.
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 low velocities without dissipating any energy (i.e. zero viscosity.) At higher 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, in 1938, by Pyotr Kapitsa, John Allen and Don Misener. Superfluidity in helium-4, which occurs at temperatures below 2.17 kelvins (K), has long been understood to result from Bose condensation, the same mechanism that produces the Bose-Einstein condensates. The primary difference between superfluid helium and a Bose-Einstein condensate is that the former is a liquid while the latter is a gas.
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. This is the BCS transition, invented in 1957 by John Bardeen, Leon Cooper and Robert 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.
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 Douglas 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. (The theory of superfluid helium-3 is a little more complicated than the BCS theory of superconductivity. These complications arise because helium atoms repel each other much more strongly than electrons, but the basic idea is the same.)
Creation of the first fermionic condensates
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 the Joint Institute for Laboratory Astrophysics (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 and Rudolf Grimm at University of Innsbruck managed to coax fermionic atoms into forming molecular bosons, which then underwent Bose-Einstein condensation. However, this was not a true fermionic condensate. Later that year, 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. The findings were published in the online edition of Physical Review Letters on January 24 2004.
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