Birds do it, bees do it. Fermionic particles, however, tend not to get cozy with one another. Getting these otherwise unfriendly atoms to form a special type of association known as a "Cooper pair" would be a physics milestone. In a paper published online 22 July by Science, physicists report the latest evidence for the formation of Cooper pairs of fermions in the laboratory.
Cooper pairs of fermions are thought to be responsible for the bizarre properties of superconductors (such as their total lack of resistance) and of superfluids (such as lack of viscosity and a strange, quantized flow within vortices). Achieving Cooper pairing in fermion condensates would enable scientists to examine the properties of Cooper pairs with unprecedented flexibility and would help unravel the enduring mysteries of the physics behind superconductors.
But creating Cooper pairs of fermions is no simple trick. No two fermions can be in the same quantum state, so, on the face of it, they should never be able to condense. There is a way out, however. Take two fermions and bind them so that their spins--which come in half-integers of 1/2, 3/2, and so on--effectively become an integer, and you get a boson which, in turn, can condense. Recently, scientists have been exploiting this loophole with gusto and accumulating evidence of fermionic Cooper pairs (ScienceNOW, 28 January).
Now, a group led by Rudolf Grimm of the University of Innsbruck, Austria, has announced another line of evidence for Cooper pairing. The researchers irradiate the fermionic condensate with radio waves and, by observing which frequencies are absorbed, determine how many atoms are bound together and how tightly they are bound. The results match what you'd expect with Cooper pairs, Grimm says. Also, as the researchers lower the temperature of the condensate, he adds, "all the completely unpaired particles disappear," which implies they're probing deep into the territory where the pairs form.
"This is great work," says Wolfgang Ketterle, a physicist at the Massachusetts Institute of Technology who won a Nobel Prize for his work with Bose-Einstein condensates. However, he cautions that "this experiment is not proof of superfluidity. It's an important piece of the puzzle, but there are still other pieces missing." He adds that researchers should have a smoking gun--such as observing quantized vortices--before declaring victory.
The Science Express paper