Magnetically mediated hole pairing in fermionic ladders of ultracold atoms

Abstract: Pairing of mobile charge carriers in doped antiferromagnets plays a key role in the emergence of unconventional superconductivity. In these strongly correlated materials, the pairing mechanism is often assumed to be mediated by magnetic correlations, in contrast to phonon-mediated interactions in conventional superconductors. A precise understanding of the underlying mechanism in real materials is, however, still lacking, and has been driving experimental and theoretical research for the past 40 years. Early theoretical studies established the emergence of binding among dopants in ladder systems, where idealised theoretical toy models played an instrumental role in the elucidation of pairing, despite repulsive interactions. Here, we realise this long-standing theoretical prediction and report on the observation of hole pairing due to magnetic correlations in a quantum gas microscope setting. By engineering doped antiferromagnetic ladders with mixed-dimensional couplings we suppress Pauli blocking of holes at short length scales. This results in a drastic increase in binding energy and decrease in pair size, enabling us to observe pairs of holes predominantly occupying the same rung of the ladder. We find a hole-hole binding energy on the order of the superexchange energy, and, upon increased doping, we observe spatial structures in the pair distribution, indicating repulsion between bound hole pairs. By engineering a configuration in which binding is strongly enhanced, we delineate a novel strategy to increase the critical temperature for superconductivity.