Space-time Floquet operator: Non-reciprocity and fractional topology of space-time crystals

Abstract: We introduce a space-time Floquet operator, a generalization of the conventional Floquet operator, that captures the long-time behavior of space-time crystals - systems where spatial and temporal periodicities are intrinsically intertwined. Unlike the standard Floquet operator, which describes evolution over a full time period, the space-time Floquet operator evolves the system over a fraction of the period, thereby resolving finer details of its dynamics. Its eigenmode spectrum defines a space-time band structure that unfolds conventional Floquet bands to respect the intertwined crystal symmetry in reciprocal wavevector-frequency space. We relate the topology of these space-time bands to quantized transport phenomena, such as Bloch oscillations and adiabatic charge transport, and uncover a fractional version of the latter. We also demonstrate how nonreciprocal parametric resonances are naturally anticipated by our framework. The approach applies broadly to both classical and quantum systems with space-time symmetry, including non-Hermitian crystals.