Direct signatures of d-level hybridization and dimerization in magnetic adatom chains on a superconductor

Magnetic adatom chains on superconductors provide a platform to explore correlated spin states and emergent quantum phases. Using low-temperature scanning tunneling spectroscopy, we study the distance-dependent interaction between Fe atoms on 2H-NbSe₂. While single atoms exhibit four Yu-Shiba-Rusinov states and partially occupied d levels consistent with a S = 2 spin state, the spin is quenched when two Fe atoms reside in nearest-neighbor lattice sites, where the d levels of the atoms hybridize. The non-magnetic dimer configuration is stable in that dimerization persists in chains with weak interactions among the dimers. Thus, the spin-state quenching has important implications also for Fe chains. While even-numbered chains are stable and non-magnetic, odd-numbered chains host a single magnetic atom at one of the chain's ends, with its position being switchable by voltage pulses. Our findings emphasize the role of interatomic coupling in shaping quantum ground states.