Experimentalists have a remarkable degree of control over neutral ultracold Fermi gases. Simply by changing the ambient magnetic field, we can tune the interaction strength, near a so-called Feshbach resonance. I will explain how this works, and discuss the \"unitary\" limit of resonant s-wave interactions. The divergence of the s-wave scattering length leaves a scale-invariant system. It is well understood that thermodynamics are \"universal\" in this regime, i.e., that all unitary Fermi gases have the same equation of state, no matter what the origin of the inter-particle interaction.
Still under debate are transport properties, although they too should be universal. We have recently measured spin transport in a unitary Fermi gas. The starting point of our measurements is a transversely spin-polarised gas, where each atom is in a superposition of the lowest two Zeeman eigenstates. In the presence of an external field gradient, a spin texture develops across the cloud, which drives diffusive spin currents. The slow diffusive relaxation reveals strong scattering in the unitary gas, such that the inferred value of the transport lifetime is near a conjectured quantum limit.
Coffee and Refreshments will be available in SSC 1511.