Halfway Across the BEC-BCS Prairie

Researchers in Colorado have discovered a new form of atomic matter, a fermionic condensate unlike anything seen before. To approach this conceptually-difficult but physics-rich topic, we will proceed in several parts: providing a quantum background, defining the word "degeneracy," summarizing the new atomic state, and finally assessing the advantages of the new state.

1. Quantum background. In exploring the exotic landscape of quantum gases, physicists have lavished much attention on bosonic atoms (atoms whose total spin has an integer value, such as 0 or 1 or 2). In 1995 scientists succeeded in cooling (bosonic) atoms so that in a quantum sense the atoms began to overlap, at which point they really could not be distinguished and had, in effect, become part of a single quantum entity called Bose Einstein condensate(BEC).

Fermions (possessing half-integer spins, such as 1/2 or 3/2 or 9/2), whether elementary particles like electrons and quarks, or whole atoms (and in determining whether an atom is a boson or fermion one has to add up the spins of all its constituent protons,
neutrons, and electrons), do not act like bosons. The Pauli exclusion principle dictates that no two identical fermions may occupy the same quantum state.

Most of chemistry here on Earth and elsewhere is dictated by the simple Pauli rule: electrons fill atomic orbitals in such a way that no two electrons have exactly the same quantum values. Partially filled orbitals determine what kind of chemical affinity that atom will have. Note that fermion atoms are not precluded from interacting in ordinary chemical reactions (the atoms have differing nuclear and electronic internal configurations). But they may not enter into an extensive BEC kind of quantum condensate where the atoms do possess the same quantum attributes.

2. Degeneracy. Pauli is on duty at all times, but he chiefly manifests himself in a quantum setting, such as in the orbitals
within an atom or in the chilled molasses of a microkelvin-level atom trap. In this rarefied realm, bosons can all fall into that
singular BEC state. All having the same energy, these atoms are said to be degenerate.

With fermions, it's quite different. In a quantum setting---whether electrons moving through a crystal or fermion atoms chilled in a trap, fermions are obliged to fill, one by one, all the different possible quantum energy states, starting at the low end. On an energy level diagram, the fermions look as if they were perching on the rungs of a ladder, filling all the rungs singly. (The uppermost rung is called the fermi energy and the temperature that corresponds to that energy is called the fermi temperature.) Commonplace example: the free-roaming electrons in a metal crystal, even at room temperature, are obliged to assume a set of discrete quantum-allowed energies in this way. These electrons are said to constitute a degenerate fermi gas.

In the fermion context, "degenerate" means that the particles fill up the plenum of possible energy states. Creating such a gas of degenerate fermion atoms proved more difficult to make than a degenerate (BEC) gas of boson atoms. In fact, a degenerate fermi gas was first accomplished only in 1999 (Update 447) in an experiment by Deborah Jin and her NIST/JILA colleagues, the same lab where the new results have been performed. By the way, although physicists had long assumed the Pauli principle would apply to atoms (composite objects) as well as to electrons (truly elementary particles), it was only in recent work that this was demonstrated experimentally.

3. New state of matter. Fermions, if you pair them, can become bosons. And in that way, fermions can enter pairwise into a quantum condensate. There are, however, a whole spectrum of pairing mechanisms. At one extreme is the case where the atoms pair strongly, after which they can (as molecules) collapse into a Bose Einstein condensate (BEC).

At the other end of the spectrum the atoms can pair weakly, or more to the point, combine in an unbound but correlated state analogous to the Cooper pairs of electrons that form the essence of quantum currents in superconductors or the pairs of helium-3 atoms that constitute a superfluid. In previous months a number of labs have reported forming condensations of strongly-bound molecules (see Update 663).

Now Deborah Jin and her colleagues Cindy Regal and Marcus Greiner at NIST and the University of Colorado report making great progress in moving across the plain between the BEC and BCS pairing alternatives. The type of pairing can be adjusted by subtly altering the strength of an external magnetic field.

The NIST researchers, who cool potassium-40 atoms to microkelvin temperatures, are at the cross-over region: they are not at the BEC regime because the applied magnetic field would not permit the kind of pairing one needs for a BEC condensate. Also they can affirm that they are not in the BCS regime either because the strength of the interaction among atoms is too strong for the kind of weak Cooper pairing that occurs in superconductivity or helium-3 superfluids.

This new condensed form of atomic matter should not be thought of merely as a way station between the BEC and (weak) BCS pairing alternatives, but as a unique state in its own right. Eric Cornell (also at NIST but not part of Jin's group), who won a Nobel prize for his part in the discovery of BEC, describes the new NIST state as "a dramatic new sort of fermionic condensate, basically Cooper pairing in the strong-field limit."

4. Assessment. One of the goals in pursuing this research is the
chance to form novel types of Cooper pairs or superfluids, and possibly to custom make different kinds of superconductivity. In these cold fermi gases the interactions (and the strength of the pairing) can be adjusted by turning a knob (changing the magnetic field), which is more than you can say about conventional superconductivity, metallic or ceramic.

Here is one hint that this work might lead to warmer, even room temperature, superconductivity: In the new potassium fermionic condensate the ratio of transition temperature (at which condensation of pairs occurs) to fermi temperature is about 1 to 5. In conventional low-temp superconductors the ratio is 1 to 1000 (or even 100,000). Even in high-temp superconductors, the ratio is 1 to 100. (Regal et al., Physical Review Letters, 30 January 2004; additional background in Physics Today, Oct 1999 and Oct 2003.)