Reversal of quantised Hall drifts at non-interacting and interacting topological boundaries

Boundaries between topologically distinct materials give rise to gapless edge modes whose robustness is fundamentally and technologically relevant. Therefore, it is crucial to gain a better understanding of topological edge states, both regarding their transport properties as well as their response to interparticle interactions. Here, we experimentally study quantised Hall drifts in a harmonically confined topological pump of non-interacting and interacting ultracold fermionic atoms. We find that quantised drifts halt and reverse their direction when the atoms reach a critical slope of the confining potential, revealing the presence of a topological boundary. The drift reversal corresponds to a band transfer between a band with Chern number C = +1 and a band with C = -1 via a gapless edge mode, in agreement with the bulk-edge correspondence for non-interacting particles. We establish that a non-zero repulsive Hubbard interaction leads to the emergence of an additional edge in the system, relying on a purely interaction-induced mechanism, in which pairs of fermions are split.