We study the emergence of altermagnetism from repulsive interactions for electrons on the Lieb lattice as a model of quasi-2D oxychalcogenides with the so-called “$\mathrm{anti}\text{−}{\mathrm{CuO}}_2$” lattice structure. A comprehensive study of the Lieb lattice Hubbard model, using unrestricted Hartree-Fock and exact diagonalization techniques, establishes the presence of a spin-$\frac{1}{2}$ altermagnetic Mott insulating ground state for average electron densities of 2 and 4 per unit cell. Both phases show the characteristic spin splitting in the electron bands as well as in the magnon bands, as indicated by solutions of an effective spin-$\frac{1}{2}$ Heisenberg model that we construct. We also provide evidence for altermagnetic metal formation in the electron- and hole-doped Mott state, giving rise to Fermi surfaces with $d_{x^2-y^2}$-wave spin splitting and quasi-one-dimensional characteristics.

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• Quantum phase transitions
• Altermagnets
• Exact diagonalization
• Hartree-Fock methods
• Hubbard model