An SU(6) Mott insulator of an atomic Fermi gas realized by large-spin Pomeranchuk cooling

The Hubbard model accounts for many of the diverse phenomena observed in solid-state materials, despite incorporating only nearest-neighbour hopping and on-site interactions for correlated electrons. One interesting extension to the model involves enlarging its spin symmetry to SU(N > 2), which describes systems with orbital degeneracy. Here we report a successful formation of the SU(6) symmetric Mott-insulator state with an atomic Fermi gas of ytterbium (Yb-173) atoms in a three-dimensional optical lattice. In addition to the suppression of compressibility and the charge-excitation gap characteristic of a Mott-insulating phase, we reveal that the SU(6) system can achieve lower temperatures than the SU(2) state, owing to differences in the entropy carried by an isolated spin. The mechanism is analogous to Pomeranchuk cooling in solid He-3 and will be helpful for investigating exotic quantum phases of the SU(N) Hubbard system at extremely low temperatures.