Block spin magnetism and metal-insulator transition in a two-dimensional Hubbard model with perfect vacancy superstructure

We study the phase diagram of a square-lattice Hubbard model with a perfect vacancy superstructure. The model can also be defined on a bipartite lattice with each building block consisting of a minimal square. The noninteracting model is exactly solved and a midband gap opens at the Fermi energy in the weak interblock hopping regime. Increasing the Coulomb interaction will develop the Neel antiferromagnetic order with varying block spin moments. The metal-insulator transition with U(MI), which is smaller than the one without vacancies, occurs above themagnetic instability U(M). The emergent intermediate magnetic metal phase develops substantially in the moderate interblock hopping regime. Drastic increases in the ordered moment and the gap magnitude are observed on the verge of a tight-binding band insulator with increasing U. The implications of these results for the recently discovered (A, T1)(y)Fe(2-x)Se(2) compounds are discussed.