Magnetically mediated hole pairing in fermionic ladders of ultracold atoms

• Published in: Nature (London) 2023-01, Vol.613 (7944), p.463-467
• Main Authors: Hirthe, Sarah, Chalopin, Thomas, Bourgund, Dominik, Bojović, Petar, Bohrdt, Annabelle, Demler, Eugene, Grusdt, Fabian, Bloch, Immanuel, Hilker, Timon A.
• Format: Article
• Language: English
• Subjects: 639/766/119/1003; 639/766/119/997; 639/766/36/1125; 639/766/483/3926; Antiferromagnetism; Binding energy; Current carriers; Energy; Humanities and Social Sciences; Ladders; multidisciplinary; Physics; Science; Science (multidisciplinary); Superconductivity; Thermodynamics; Ultracold atoms; Unconventional superconductors
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-022-05437-y

Conventional superconductivity emerges from pairing of charge carriers—electrons or holes—mediated by phonons 1 . In many unconventional superconductors, the pairing mechanism is conjectured to be mediated by magnetic correlations 2 , as captured by models of mobile charges in doped antiferromagnets 3 . However, a precise understanding of the underlying mechanism in real materials is still lacking and has been driving experimental and theoretical research for the past 40 years. Early theoretical studies predicted magnetic-mediated pairing of dopants in ladder systems 4 – 8 , in which idealized theoretical toy models explained how pairing can emerge despite repulsive interactions 9 . Here we experimentally observe this long-standing theoretical prediction, reporting hole pairing due to magnetic correlations in a quantum gas of ultracold atoms. By engineering doped antiferromagnetic ladders with mixed-dimensional couplings 10 , we suppress Pauli blocking of holes at short length scales. This results in a marked 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 of 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 strategy to increase the critical temperature for superconductivity. The direct observation of hole pairing in a doped Hubbard model is demonstrated using ultracold atoms in a quantum gas microscope setting by engineering mixed-dimensional fermionic ladders.