Transport and pairing in the unitary Fermi gas

5 problems

• Critical behavior of second-sound damping at the superfluid transition

  Open

  In a superfluid, heat travels as a wave (second sound). Near the transition its damping should grow following a universal law that has not been measured.

• Is there a pairing pseudogap above $T_{\mathrm{c}}$ in the unitary gas?

  Contested

  Atoms may form short-lived pairs above the temperature where they become superfluid, opening a partial energy gap. Experiments disagree on whether this gap exists.

• Origin of excess conductance of a strongly interacting gas through a constriction

  Partially resolved

  When two clouds of strongly interacting fermions are joined by a narrow channel, atoms flow through faster than the quantum limit for independent particles, even above the superfluid temperature. Pairing fluctuations are the leading explanation, but a full quantitative account is missing.

• Universal lower bound on spin diffusion in strongly interacting fermions

  Contested

  When up and down spins are separated, strong collisions slow how fast they mix back. Experiments suggest a floor of about $\hbar/m$, but two kinds of measurement disagree.

• Minimum shear viscosity to entropy ratio of the unitary gas

  Open

  Viscosity divided by entropy measures how close a fluid comes to flowing with no internal friction. A conjectured lower limit is 1/(4 pi) in natural units, and the unitary gas is among the closest fluids to it.