Physicists are getting closer and closer to the holy grail of research into ultracold atomic gases – the observation of superfluidity in a Fermi gas. Research groups in Austria and the US have recently taken a major step towards this goal by creating a Bose-Einstein condensate of bosonic molecules from a gas of fermionic atoms. If the atoms can be persuaded to form weakly-bound pairs, rather than molecules, physicists hope to learn more about the basic mechanisms responsible for superconductivity.
Atoms behave very differently at temperatures near absolute zero depending on the value of their intrinsic angular momentum or spin. Spin is measured in units of the Planck constant divided by 2π. Bosons have spins with integer values in these units, while fermions have spins of 1/2, 3/2, 5/2 and so on. A molecule made of two fermionic atoms will be a boson because it will have an integer value of spin.
All fermions must 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 – to allow them to collapse into a Bose condensate. If physicists were able to mimic this process in a gas of fermionic atoms, it should be possible to learn a great deal more about superconductivity.
Rudolf Grimm and colleagues at the University of Innsbruck in Austria started with a gas of fermionic lithium-6 atoms in an optical trap and cooled them in a magnetic field to produce a condensate containing over 100 000 lithium molecules (S Jochim et al. 2003 Sciencexpress 1093280). The condensate lasted for more than 20 seconds. Meanwhile, Deborah Jin and co-workers at the JILA laboratory in Boulder, Colorado, performed a similar feat with potassium-40 atoms (M Greiner et al. 2003 Nature to be published; arxiv.org/abs/cond-mat/0311172).
The atoms in a molecule are strongly bound together, whereas the particles in a Cooper pair are only weakly bound and can be quite far apart. By using magnetic fields to control the interactions between fermionic atoms – which is difficult to do with electrons – physicists hope to explore a wide range of novel phenomena between these two extremes.
