Altermagnetic phase transition in a Lieb metal

We analyze the phase transition between a symmetric metallic parent state and itinerant altermagnetic order. The underlying mechanism we reveal in our microscopic model of electrons on a Lieb lattice does not involve orbital ordering, but derives from sublattice interference.

💡 Research Summary

In this work the authors present a microscopic realization of an altermagnetic (AM) phase transition in a Lieb lattice metal. Starting from a single‑orbital Hubbard model on the Lieb lattice, they incorporate nearest‑neighbor hopping t between the A site and the B/C sites and next‑nearest‑neighbor hopping t′ between B and C, together with a possible onsite energy detuning µ_A−µ_B,C. The non‑interacting band structure features a flat band at the Fermi level and a van Hove singularity (VHS) at the M point. Crucially, the eigenstates at the Fermi surface are strongly sublattice‑polarized: away from the zone corners the states reside almost entirely on the B or C sublattice, while the A sublattice contributes negligibly. This sublattice interference (SI) is the key ingredient that shapes the effective interaction once electron‑electron repulsion is turned on.

To treat the many‑body problem without bias, the authors employ the functional renormalization group (FRG) in its truncated‑unity (TU‑FRG) formulation. During the RG flow high‑energy modes are integrated out, and the momentum‑dependent four‑point vertex is renormalized. Because the SI imprints a strong momentum dependence on the bare interaction, the flow rapidly enhances particle‑hole channels with momentum transfer q = Γ. The leading instability is identified as a d‑wave spin‑Pomeranchuk (ℓ = 2) order, characterized by an order parameter Δ(k)=Δ_M σ_z