Interactions in nanokelvin Fermi gases are typically dominated by s-wave interactions, because partial waves with higher angular momentum present a centrifugal barrier of about one millikelvin. The range of the interaction potential between atoms is much smaller than the de Broglie wavelength, but of course interactions are still allowed between non-identical fermions. On the other hand, in a spin-polarised gas, the s-wave interactions are suppressed, and the lowest allowed partial wave is the p-wave. We study such a gas, and furthermore enhance the p-wave interactions using a Feshbach resonance. The gas is studied with rf and momentum spectroscopy.
Surprisingly, we find evidence for a remarkable set of universal relations that connects the spectra to pwave pair correlations and to a thermodynamic identity. These relations centre around a parameter called the “contact”, which had previously only been considered for s-wave interactions. We infer an attractive pwave interaction energy as large as half the Fermi energy, near the Feshbach resonance. We are also able to watch p-wave correlations develop after a quench-like jump of the interaction strength.