Unconventional superfluid order in the $F$-band of a bipartite optical square lattice

TL;DR

This paper reports the first observation of bosons condensed into an $F$-band in a bipartite optical lattice, revealing a complex superfluid order with staggered vortical currents that break symmetries.

Contribution

It demonstrates the realization of an unconventional superfluid order in the $F$-band of an optical lattice, including detailed momentum spectra analysis and symmetry-breaking features.

Findings

Observation of boson condensation into the $F$-band

Detection of complex-valued staggered angular momentum order

Agreement between measured Bragg peaks and band calculations

Abstract

We report on the first observation of bosons condensed into the energy minima of an $F$-band of a bipartite square optical lattice. Momentum spectra indicate that a truly complex-valued staggered angular momentum superfluid order is established. The corresponding wave function is composed of alternating local $F_{2x^3-3x} + i F_{2y^3-3y}$-orbits and local $S$-orbits residing in the deep and shallow wells of the lattice, which are arranged as the black and white areas of a checkerboard. A pattern of staggered vortical currents arises, which breaks time reversal symmetry and the translational symmetry of the lattice potential. We have measured the populations of higher order Bragg peaks in the momentum spectra for varying relative depths of the shallow and deep lattice wells and find remarkable agreement with band calculations.