Observation of momentum-band topology in PT-symmetric Floquet lattices

Momentum-band topology is a groundbreaking concept in multidisciplinary physics. Unlike the conventional energy-band topology, it defines a distinctive band topology within the energy Brillouin zone. Despite revolutionizing the paradigm of topological band theory, both theoretical and experimental studies of this new concept remain in their infancy. Here, we unveil the momentum-band topology in a PT-symmetric Floquet lattice, where the drive-induced momentum gap can be rigorously characterized by a quantized Berry phase. Experimentally, we synthesize the model using an acoustic cavity-tube structure coupled to custom-designed external circuits. By innovatively reconstructing the Floquet operator from measured time-domain wavefunctions, we extract the system's eigenstates and, for the first time, provide direct bulk evidence of momentum-band topology via momentum-band inversion and topological invariants. This is accompanied by a clear observation of time-localized interface states, thus providing a comprehensive examination of the temporal bulk-boundary correspondence. Our work paves the way for further experimental studies on the burgeoning momentum-gap physics.