Quantum interference can deeply alter the nature of many-body phases of matter1.
In the case of the Hubbard model, Nagaoka proved that introducing a single itinerant
charge can transform a paramagnetic insulator into a ferromagnet through path
interference2–4. However, a microscopic observation of this kinetic magnetism induced
by individually imaged dopants has been so far elusive. Here we demonstrate the
emergence of Nagaoka polarons in a Hubbard system realized with strongly interacting
fermions in a triangular optical lattice5,6. Using quantum gas microscopy, we image
these polarons as extended ferromagnetic bubbles around particle dopants arising
from the local interplay of coherent dopant motion and spin exchange. By contrast,
kinetic frustration due to the triangular geometry promotes antiferromagnetic
polarons around hole dopants7. Our work augurs the exploration of exotic quantum
phases driven by charge motion in strongly correlated systems and over sizes that are
challenging for numerical simulation8–10.
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This content will become publicly available on May 8, 2025
Directly imaging spin polarons in a kinetically frustrated Hubbard system
The emergence of quasiparticles in quantum many-body systems underlies the rich phenomenology in many strongly interacting materials. In the context of doped Mott insulators, magnetic polarons are quasiparticles that usually arise from an interplay between the kinetic energy of doped charge carriers and superexchange spin interactions. However, in kinetically frustrated lattices, itinerant spin polarons—bound states of a dopant and a spin flip—have been theoretically predicted even in the absence of superexchange coupling. Despite their important role in the theory of kinetic magnetism, a microscopic observation of these polarons is lacking. Here we directly image itinerant spin polarons in a triangular-lattice Hubbard system realized with ultracold atoms, revealing enhanced antiferromagnetic correlations in the local environment of a hole dopant. In contrast, around a charge dopant, we find ferromagnetic correlations, a manifestation of the elusive Nagaoka effect. We study the evolution of these correlations with interactions and doping, and use higher-order correlation functions to further elucidate the relative contributions of superexchange and kinetic mechanisms. The robustness of itinerant spin polarons at high temperature paves the way for exploring potential mechanisms for hole pairing and superconductivity in frustrated systems. Furthermore, our work provides microscopic insights into related phenomena in triangular-lattice moiré materials.
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- Award ID(s):
- 2110475
- PAR ID:
- 10532003
- Publisher / Repository:
- Nature
- Date Published:
- Journal Name:
- Nature
- ISSN:
- 1476-4687
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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