Scale invariance and viscosity of a two-dimensional Fermi gas
  We investigate the collective excitations of a harmonically trapped two-dimensional Fermi gas from the collisionless (zero sound) to the hydrodynamic (first sound) regime. The breathing mode, which is sensitive to the equation of state, is observed at a frequency two times the dipole mode frequency for a large range of interaction strengths and temperatures, and the amplitude of the breathing mode is undamped. This provides evidence for a dynamical SO(2,1) scaling symmetry of the two-dimensional Fermi gas. Moreover, we investigate the quadrupole mode to measure the shear viscosity of the two-dimensional gas and study its temperature dependence. 
Enrico Vogt, Michael Feld, Bernd Fr\"ohlich, Daniel Pertot, Marco   Koschorreck, Michael K\"ohl
cond-mat.quant-gas