Flat bands and magnetism in Fe4GeTe2\mathrm{Fe_4 Ge Te_2} and Fe5GeTe2\mathrm{Fe_5GeTe_2} due to bipartite crystal lattices

Abstract

Fen=4,5GeTe2\mathrm{Fe_{n=4,5}GeTe_2} exhibits quasi-two-dimensional properties as a promising candidate for a near-room-temperature ferromagnet, which has attracted great interest. In this work, we notice that the crystal lattice of Fen=4,5GeTe2\mathrm{Fe_{n=4,5}GeTe_2} can be approximately considered to be stacked by three bipartite crystal lattices. By combining the model Hamiltonians of bipartite crystal lattices and first-principles calculations, we investigate the electronic structure and the magnetism of Fen=4,5GeTe2\mathrm{Fe_{n=4,5}GeTe_2}. We conclude that flat bands near the Fermi level originate from the bipartite crystal lattices and that these flat bands are expected to lead to the itinerant ferromagnetism in Fen=4,5GeTe2\mathrm{Fe_{n=4,5}GeTe_2}. Interestingly, we also find that the magnetic moment of the Fe5 atom in Fe5GeTe2\mathrm{Fe_5 Ge Te_2} is distinct from the other Fe atoms and is sensitive with the Coulomb interaction UU and external pressure. These findings may be helpful to understand the exotic magnetic behavior of Fen=4,5GeTe2\mathrm{Fe_{n=4,5} Ge Te_2}.Comment: 9 pages, 8 figure

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