Scientists worldwide are keen to understand the topological phases of matter because of their unique characteristics. Those characteristics often result in exotic surface or boundary modes, whose existence is rooted in the non-trivial topological properties of the underlying system. In particular, the robustness of these properties makes them interesting for applications.
A periodic driving technique has been used to imitate the physics of undriven topological solid-state systems. The properties of driven topological methods, however, transcend those of their static counterparts.
Using a BEC of 39K loaded into a periodically-modulated optical honeycomb lattice, scientists could produce such a time-dependent topological system.
For specific modulation parameters, the system is in a so-called anomalous Floquet regime, where the Chern numbers of all bulk bands are equal to zero. At the same time, chiral edge modes exist in all quasienergy gaps. These non-trivial topological properties stem from the non-trivial winding of the quasienergy spectrum and cannot occur in undriven systems.
In a new study, an international team led by physicists from the Ludwig-Maximilians Universitaet (LMU) in Munich realized a novel genuine time-dependent topological system with ultracold atoms in periodically-driven optical honeycomb lattices.
Scientists combined energy gap and local Hall deflection measurements, and the full set of topological invariants describing the time-dependent system was determined experimentally for the first time. The existence of chiral edge modes could be revealed even in geometry with smooth boundaries.
Because of its unique characteristics, especially in the presence of disorder, the anomalous Floquet phase promises the realization of interacting, periodically-driven systems that may support many-body-localized bulk, but thermalizing edge modes. This intriguing non-equilibrium many-body phase may prove resilient to conventional Floquet heating.
- Karen Wintersperger et al. Realization of an anomalous Floquet topological system with ultracold atoms, Nature Physics (2020). DOI: 10.1038/s41567-020-0949-y