Evaporative Depolarization and Deformation of 3D Imbalanced Fermi Gases

We previously observed phase separation of a spin-imbalanced 3D Fermi gas, with a spatially deformed paired core surrounded by a shell of polarized atoms.\footnote{G. B. Partridge \textit{et al.}, \textit{Science} \textbf{311}, 503 (2006); G. B. Partridge \textit{et al.}, \textit{PRL} \textbf{97}, 190407 (2006).} By studying the dynamics of evaporation, we find that trap depth anisotropy and fast evaporation produces the deformation. The deformed state is remarkably metastable, with a superfluid-normal transition which is extended to a much higher polarization than observed in the MIT and ENS experiments.\footnote{M. Zwierlein \textit{et al.}, \textit{Science} \textbf{311}, 492 (2006); S. Nascimb$\grave{e}$ne \textit{et al.}, \textit{PRL} \textbf{103}, 170402 (2009).} Our findings confirm the evaporative depolarization mechanism suggested previously,\footnote{M. M. Parish \textit{et al.}, \textit{PRA} \textbf{80}, 063605 (2009)} in which preferential evaporation from the center of an elongated trap reduces the chemical potential difference in the central core relative to the surrounding polarized phases.