Non-Hermitian physics is at the forefront of research, investigating dissipative phenomena observed in various physical systems. In quantum systems, dissipation leads to a complex-valued energy spectrum, marking the onset of non-Hermitian quantum mechanics. Recent advancements in this field have unveiled new types of topological phases that defy conventional understanding. One such example is the non-Hermitian skin effect (NHSE), where all eigenstates collapse towards system boundaries, reflecting a bulk-boundary correspondence shaped by complex spectral topology. Despite extensive exploration, a lingering question remains: do these exotic topological phases endure in the presence of many-body interactions? Previous studies suggest the Pauli exclusion principle may suppress the skin effect, prompting further investigation into the interplay between many-body interactions and topological phenomena in open quantum systems.

In a study published in Communications Physics, a collaborative team led by Prof. PARK Moon Jip from Hanyang University, Dr. KIM Beom Hyun, and Dr. HAN Jae-Ho from the IBS Center for Theoretical Physics of Complex Systems presented a compelling counterexample, reshaping our understanding of non-Hermitian topology in interacting open quantum systems. The researchers have uncovered the collective excitations of doublon-holon pairs vividly exhibit the skin effect, even under strong many-body interaction that is contrary to expectations (see Fig. 1).

The team's findings established the robustness of this effect, unraveling a profound bulk-boundary correspondence mediated by the point gap topology within the many-body energy spectrum. These discoveries not only shed light on the intricate interplay between many-body interactions and topological phases but also underscore the existence of non-Hermitian topological phases in collective excitations of many-body interacting systems. As researchers continue to unravel the mysteries of open quantum systems, these findings pave the way for new frontiers in quantum physics, offering tantalizing prospects for the development of novel materials and technologies harnessing the unique properties of non-Hermitian dynamics.

Figure 1. Schematic diagrams of (a) doublon (doubly occupied site) and holon (empty site) excitations, and (b) their motion in the presence of non-reciprocal hopping. As illustrated in (b), non-reciprocal hopping, denoted by red arrows, results in the rightward (leftward) movement of doublons (holons). The gray arrows in b represent the initial positions of fermion spins prior to hopping. Under open boundary conditions, doublon and holon become localized at opposite edges of a one-dimensional chain. This segregation in localized doublon and holon is responsible for the Non-Hermitian skin effect.