Evidence for an atomic chiral superfluid with topological excitations

Author(s): Xiao-Qiong Wang [sup.1] [sup.2] , Guang-Quan Luo [sup.1] [sup.2] , Jin-Yu Liu [sup.1] [sup.2] , W. Vincent Liu [sup.2] [sup.3] [sup.4] [sup.5] , Andreas Hemmerich [sup.6] , Zhi-Fang Xu [...]
Topological superfluidity is an important concept in electronic materials as well as ultracold atomic gases.sup.1. However, although progress has been made by hybridizing superconductors with topological substrates, the search for a material--natural or artificial--that intrinsically exhibits topological superfluidity has been ongoing since the discovery of the superfluid .sup.3He-A phase.sup.2. Here we report evidence for a globally chiral atomic superfluid, induced by interaction-driven time-reversal symmetry breaking in the second Bloch band of an optical lattice with hexagonal boron nitride geometry. This realizes a long-lived Bose-Einstein condensate of .sup.87Rb atoms beyond present limits to orbitally featureless scenarios in the lowest Bloch band. Time-of-flight and band mapping measurements reveal that the local phases and orbital rotations of atoms are spontaneously ordered into a vortex array, showing evidence of the emergence of global angular momentum across the entire lattice. A phenomenological effective model is used to capture the dynamics of Bogoliubov quasi-particle excitations above the ground state, which are shown to exhibit a topological band structure. The observed bosonic phase is expected to exhibit phenomena that are conceptually distinct from, but related to, the quantum anomalous Hall effect.sup.3-7 in electronic condensed matter. A globally chiral atomic superfluid is induced by time-reversal symmetry breaking in an optical lattice and exhibits global angular momentum, which is expected to lead to topological excitations and the demonstration of a topological superfluid.