Ferromagnetism in armchair graphene nanoribbon heterostructures

We study the properties of flat bands that appear in a heterostructure composed of strands of different widths of graphene armchair nanoribbons. One of the flat bands is reminiscent of the one that appears in pristine armchair nanoribbons and has its origin in a quantum mechanical destructive interference effect, dubbed ``Wannier orbital states'' by Lin et al. in Phys. Rev. B 79, 035405 (2009). The additional flat bands found in these heterostructures, some reasonably closer to the Fermi level, seem to be generated by a similar interference process. After doing a thorough tight-binding analysis of the band structures of the different kinds of heterostructures, focusing on the properties of the flat bands, we use density functional theory to study the possibility of magnetic ground states when placing, through doping, the Fermi energy close to the different flat bands. Our DFT results confirmed the expectation that these heterostructures, after being appropriately hole doped, develop a ferromagnetic ground state that seems to require, as in the case of pristine armchair nanoribbons, the presence of a dispersive band crossing the flat band. In addition, we found a remarkable agreement between the tight-binding and DFT results for the charge density distribution of the so-called Wannier orbital states.