Probing boundaries in interacting topological systems*

Boundaries between topologically distinct materials give rise to gapless edge modes whose robustness against perturbations is believed to be technologically relevant.

Therefore, it is crucial to gain a better understanding of topological edge states, especially regarding their response to interparticle interactions.

In our experiment, we study quantised bulk Hall drifts of interacting ultracold fermions in the presence of a harmonic confinement.

We discovered that quantised drifts halt and reverse in the opposite direction at the topological boundary which emerges due to the harmonic confinement.

In the absence of interactions this reflection can be understood as a transfer of atoms between bands with opposite Chern numbers $C = +1$ and $C = -1$ via a gapless edge mode, in agreement with the bulk-edge correspondence.

Interestingly, this reflection can be used to study the edge in an interacting system since a non-zero repulsive Hubbard $U$ leads to the emergence of an additional edge in the system, which is purely interaction-induced.