Fermionic condensate makes its debut
Jan 28, 2004
Physicists in the US have created an elusive state of matter known as a “fermionic condensate” for the first time. Deborah Jin, Markus Greiner and Cindy Regal at the JILA laboratory in Boulder, Colorado, made the condensate from pairs of ultracold fermionic atoms (C Regal et al. 2004 Phys. Rev. Lett. 92 040403)
The breakthrough could help physicists improve their understanding of superconductivity and superfluidity. "The strength of pairing in our fermionic condensate would correspond to a room temperature superconductor when adjusted for mass and density," said Jin at a press conference today. “This makes me optimistic that the fundamental physics we learn through fermionic condensates will eventually help others design more practical superconducting materials."
Atoms behave very differently at temperatures near absolute zero depending on the value of their intrinsic angular momentum or “spin”. Bosons have spins with integer values in units of the Planck constant divided by 2π, while fermions have spins of 1/2, 3/2, 5/2 and so on. Fermions obey the Pauli exclusion principle, which means that they cannot occupy the same quantum state. However, there are no such restrictions on bosons, so they can all collapse into the same quantum ground state. This process, known as Bose-Einstein condensation, is at the heart of superconductivity - the flow of electric current without any resistance.
Since electrons are fermions they must form Cooper pairs - named after Leon Cooper of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity – before they can form a Bose condensate. If this process could be mimicked in a gas of fermionic atoms, it should be possible to learn a great deal more about superconductivity.
Late last year the Boulder team and, independently, a team at Innsbruck managed to form a molecular condensate from a gas of fermionic atoms. The atoms in a molecule are much more strongly bound than those in a Cooper pair. Now, the JILA team has made a condensate from pairs of individual fermionic atoms in a gas. The two fermions are not bound into a molecule but simply move together in a correlated way. Collectively the pair acts as a boson and can therefore undergo condensation.
Jin and co-workers started with a gas of potassium-40 atoms, which are fermions, in an optical trap at a temperature of about 300 nanokelvin. Next, they applied a magnetic field to change the interactions between the atoms and create a “Fesbach resonance” at which the interaction changes from being highly repulsive to become highly attractive. If the value of the magnetic field is carefully controlled, the atoms will form Cooper pairs rather than molecules.
To confirm that they had produced a condensate from pairs of atoms - and not a molecular condensate as in the earlier work – Jin and co-workers actually transformed the pairs into molecules. They did this by applying a second magnetic field - which had exactly the right strength to create molecules - and opening the optical trap at the same time. This allowed them to observe the characteristic shape of a condensate cloud (see figure). According to the JILA team, the change in the magnetic field can cause molecules to form, but the changes are too fast to create a molecular condensate.
“We expect that the fermionic condensates we have observed will exhibit superfluid behaviour,” said Jin. “They represent a novel phase that lies in the crossover between superconductors and Bose-Einstein condensates. This opens up the very exciting potential to study superconductivity and superfluid phenomena under extreme conditions that have never existed before."
About the author
Peter Rodgers is Editor of Physics World. Belle Dumé is Science Writer at PhysicsWeb