The Second Wave

A second wave has appeared on the horizon of ultracold atom research. Known as the p-wave, it is opening the door to probing rich new physics, including unexplored quantum phase transitions. The first wave of ultracold atom research focused on s-wave pairing between atoms, where the “s” meant the resultant molecules are not rotating. In contrast, p-waves involve higher-order pairing where the atoms do rotate around each other. p-wave studies promise to expand and enhance the understanding of ultracold Fermi gases gained from s-wave-based studies by the Jin group of the crossover from fermionic superfluidity to molecular condensates. With p-waves, the group can attempt to create a superfluid gas that involves higher-order pairing, akin to superfluid liquid helium (³He).

Graduate students John Gaebler and Jayson Stewart and Fellows John Bohn and Debbie Jin recently took an important step towards creating p-wave-paired superfluids with the creation of p-wave pairs of fermionic potassium (⁴⁰K) atoms. The researchers used a p-wave Feshbach resonance to convert the ultracold atoms into p-wave K₂ molecules. The p-wave molecules were intrinsically different from the s-wave molecules used in the crossover studies: The rotation of the p-wave atom pairs created a (centrifugal) energy barrier that would normally prevent molecule formation at ultralow temperatures. However, the researchers were able to use the p-wave Feshbach resonance to encourage atoms to tunnel through the barrier and form “quasi-bound” p-wave molecules.

Once the ⁴⁰K atoms formed molecules inside the energy barrier, Gaebler and his colleagues discovered they could change the magnetic field and alter the molecules’ characteristics. When they decreased the field, a quasi-bound molecule would become a real molecule (a.k.a. a true bound state). Molecules created this way last only a few milliseconds before they disappear (most likely decaying into lower energy states undetectable in the experiment). When the researchers increased the magnetic field, molecules would become quasi-bound molecules behind the energy barrier. These quasi-bound molecules were very short lived because they rapidly tunneled out of the barrier. As they tunneled through the barrier, they converted their binding energy into energy of motion and rapidly flew apart from the original atom cloud, as shown here for two different types of p-wave pairs. The researchers studied the energetic atoms to learn more about the unusual molecules that gave rise to them.

Gaebler says the short lifetime of the p-wave molecules poses a significant challenge to producing fermionic p-wave condensates. However, he and his colleagues continue to investigate the formation of p-wave molecules and their behavior as quasi-bound molecules. They’re currently looking for clues of how to make a condensate and study other interesting many-body effects of p-wave atom pairs.   - Julie Phillips