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Double-exchange ferromagnetism of fermionic atoms in a p-orbital hexagonal lattice

Haoran Sun1, Erhai Zhao2, Youjin Deng1, and W. Vincent Liu3,*

  • *Contact author: wvliu@pitt.edu
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Phys. Rev. Research 8, 033173 – Published 11 August, 2026

DOI: https://doi.org/10.1103/tspx-7mly

Abstract

A broad class of correlated quantum materials features strong Hund’s coupling, yet cold-atom quantum simulators have so far focused primarily on single-orbital Fermi-Hubbard systems near a Mott insulator. Here, we show that repulsively interacting fermions loaded into the p bands of a hexagonal lattice offer a unique platform to study the interplay of “Hundness” and “Mottness.” Our theory predicts that the orbital degrees of freedom, despite geometric frustration, produce a rich phase diagram featuring a competing itinerant ferromagnetic (FM) metal and a spin-1 antiferromagnetic (AFM) insulator. While FM order is known to emerge at low fillings near the flat-band limit, we show that this itinerant ferromagnetism is remarkably robust, persisting to stronger interactions and higher fillings well beyond the flat-band regime. More importantly, we identify that near half filling, FM is stabilized by a different mechanism: the double exchange with spins aligning to avoid Hund-rule penalties at the expense of the kinetic energy of Dirac fermions. The competition between the kinetic and double-exchange energies leads to, within our approximations, a first-order transition between FM and AFM orders in the chemical potential-interaction strength plane. Our analysis suggests that p-orbital Fermi gases—as multiorbital systems with coexisting localized and itinerant spins—reveal a rich competition between correlated “Hund metals” and other unconventional states driven by quantum fluctuations.

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Phys. Rev. Research 8, 033173– Published 11 August, 2026
  • Received 7 February 2026
  • Revised 17 May 2026
  • Accepted 22 June 2026
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