Acoustic Bound Pairs under Nonreciprocal Two-Body Interactions

The exploration of many-body problems stands at the vanguard of modern physics, presenting profound challenges alongside compelling opportunities. Recently, the non-Hermiticity featured with complex spectra has significantly expanded the landscape of many-body phenomena beyond the Hermitian paradigm. Most existing studies focus on the interplay between non-Hermiticity and many-body interactions, with the latter being Hermitian in isolation. Here, by integrating nonreciprocity, a key mechanism inducing non-Hermiticity, into two-body interactions, we analytically demonstrate how such interactions govern correlated pair and experimentally confirm our prediction in a phononic crystal simulator. Resulting phenomena, including the non-Hermitian inverse skin effect where the particles undergo synchronous motion as a bound pair and accumulate opposite to the dominant hopping direction, as well as the time cluster effect where two initially separate particles tend to cluster and last as a bound pair in the time domain, are unambiguously captured in airborne sound measurements. These findings experimentally unveil the unique phenomena driven by nonreciprocal many-body interactions, opening new pathways for controlled engineering in correlated systems.