Quantum phase diagram of the spin-12 Heisenberg antiferromagnet on the square-kagome lattice: a tensor network study

We assess the quantum phase diagram of the spin-1/2 Heisenberg antiferromagnetic model on the square-kagome lattice upon varying the two symmetry inequivalent nearest-neighbor couplings, J₁ on squares and J on triangles. Employing large-scale tensor network simulations based on infinite projected entangled pair states, we find four distinct valence bond crystal (VBC) states and a ferrimagnetically ordered region. Starting from the limit of weakly interacting squares for small J/J₁ where a plaquette cross-dimer VBC with long-range singlets is stabilized, we show that with increasing J/J₁ it transitions to a VBC with resonances over hexagons, the so-called loop-six VBC, which persists across the isotropic point. Interestingly, a generalized version of the pinwheel VBC, earlier reported to be a closely competing state for the isotropic model is energetically stabilized in a sliver of parameter space right beyond the isotropic point. For further increases in J/J₁, a decorated loop-six VBC occupies an appreciable region of parameter space before transitioning into an imperfect ferrimagnet which finally evolves into a Lieb ferrimagnet. Our characterization of the underlying phases and phase transitions is based on a careful analysis of the energy, magnetization, spin-spin correlations, and bond entanglement entropy.