Spin transport in a unitary fermi gas - Joseph Thywissen

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. Less well understood is the universality of transport in such a gas. After this introduction, I will focus on recent measurements from our group. We study the spin dynamics of a quantum degenerate Fermi gas of potassium near an s-wave Feshbach resonance. The starting point of our measurements is a transversely spin-polarized 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.