Ferromagnetism in the single-orbital Hubbard model, which contains only local Coulomb repulsion and no explicit ferromagnetic exchange interactions, has been extensively studied. However, how the associated fluctuations influence the electronic properties near the transition remains a fundamental issue. Here, by applying the dynamical vertex approximation to single-orbital systems with a partially flat-band dispersion, we demonstrate that finite-correlation-length ferromagnetic fluctuations generate an emergent Luttinger surface and drive a Fermi-surface expansion reminiscent of exchange splitting, even without magnetic order. We further derive an analytical expression that reproduces these effects, clarifying the microscopic origin for fluctuation-driven exchange splitting in correlated electron systems.

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• Electronic structure
• Flat bands
• Magnetic susceptibility
• Strongly correlated systems
• Diagrammatic methods
• Dynamical mean field theory
• Hubbard model