Fermionic condensate
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A fermionic condensate (or Fermi–Dirac condensate) is a
superfluid phase formed by fermionic particles at low temperatures. It is closely related to the Bose–Einstein condensate, a superfluid phase formed by bosonic atoms under similar conditions. The earliest recognized fermionic condensate described the state of electrons in a superconductor; the physics of other examples including recent work with fermionic atoms is analogous. The first atomic fermionic condensate was created by a team led by Deborah S. Jin using potassium-40 atoms at the University of Colorado Boulder in 2003.[1][2]
Background
Superfluidity
Fermionic condensates are attained at lower temperatures than Bose–Einstein condensates. Fermionic condensates are a type 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
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
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.[a]
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,
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.[2]
Examples
Chiral condensate
A chiral condensate is an example of a fermionic condensate that appears in theories of massless fermions with
chiral symmetry breaking, such as the theory of quarks in Quantum Chromodynamics
.
BCS theory
The
gauge symmetry
of a superconductor, giving rise to the wonderful electromagnetic properties of such states.
QCD
In quantum chromodynamics (QCD) the chiral condensate is also called the quark condensate. This property of the QCD vacuum is partly responsible for giving masses to hadrons (along with other condensates like the gluon condensate).
In an approximate version of QCD, which has vanishing quark masses for N quark
quark matter
in this limit.
This is very similar to the
gauge symmetries are unbroken. Corrections for the masses of the quarks can be incorporated using chiral perturbation theory
.
Helium-3 superfluid
A
superfluid
. These Cooper pairs are substantially larger than the interatomic separation.
See also
Footnotes
- ^ 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.
References
Sources
- Guenault, Tony (2003). Basic superfluids. ISBN 978-0-7484-0892-4.
- "NIST/University of Colorado scientists create new form of matter: A Fermionic condensate" (Press release). University of Colorado. 28 January 2004. Archived from the original on 7 December 2006.
- Rodgers, Peter; Dumé, Bell (January 28, 2004). "Fermionic condensate makes its debut". Physics World. Retrieved 29 Jun 2019.</ref>
- Hägler, Ph. (2010). "Hadron structure from lattice quantum chromodynamics". Physics Reports. 490 (3–5): 49–175. ISSN 0370-1573.
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