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


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 Px±Py, or by a complex-valued Px±i Py order parameter, which breaks time-reversal symmetry.

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Figure 1: Population of excited bands.
Figure 2: Evolution of band populations and momentum distribution.
Figure 3: Tuning of anisotropy and condensate fractions.
Figure 4: Nature of superfluid order.
Figure 5: Anisotropy and P-band energy minima.

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This work was partially supported by DFG (He2334/10-1) and the Excellence Cluster ‘Frontiers in Quantum Photon Science’. We are grateful to C. Morais Smith and L-K. Lim for fruitful discussions.

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Correspondence to Andreas Hemmerich.

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Wirth, G., Ölschläger, M. & Hemmerich, A. Evidence for orbital superfluidity in the P-band of a bipartite optical square lattice. Nature Phys 7, 147–153 (2011).

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