Evidence for orbital superfluidity in the P-band of a bipartite optical square lattice

Georg Wirth, Matthias Ölschläger & Andreas Hemmerich

The successful emulation of the Hubbard model in optical lattices has stimulated extensive efforts to extend their scope to also capture more complex, incompletely understood scenarios of many-body physics. A promising approach is to consider higher bands, where the orbital degree of freedom gives rise to a structural diversity that is directly relevant, for example, for the physics of strongly correlated electronic matter. Here we report evidence for the formation of a superfluid in the P-band of a bipartite optical square lattice with S-orbits and P-orbits arranged in a chequerboard pattern. The observed momentum spectra feature cross-dimensional coherence with a lifetime of nearly 20 ms. Depending on the value of a small adjustable anisotropy of the lattice, our findings are explained either by real-valued striped superfluid order parameters with different orientations P_x±P_y, or by a complex-valued P_x±iP_y order parameter, which breaks time-reversal symmetry.

**Groupmeeting by Dan Fine, Feb 9th, 2011**