Ferromagnetism in armchair graphene nanoribbon heterostructures

Abstract

We study the properties of flat-bands that appear in a heterostructure\ncomposed of strands of different widths of graphene armchair nanoribbons. One\nof the flat-bands is reminiscent of the one that appears in pristine armchair\nnanoribbons and has its origin in a quantum mechanical destructive interference\neffect, dubbed `Wannier orbital states' by Lin et al. in Phys. Rev. B 79,\n035405 (2009). The additional flat-bands found in these heterostructures, some\nreasonably closer to the Fermi level, seem to be generated by a similar\ninterference process. After doing a thorough tight-binding analysis of the band\nstructures of the different kinds of heterostructures, focusing in the\nproperties of the flat-bands, we use Density Functional Theory to study the\npossibility of magnetic ground states when placing, through doping, the Fermi\nenergy close to the different flat-bands. Our DFT results confirmed the\nexpectation that these heterostructures, after being appropriately hole-doped,\ndevelop a ferromagnetic ground state that seems to require, as in the case of\npristine armchair nanoribbons, the presence of a dispersive band crossing the\nflat-band. In addition, we found a remarkable agreement between the\ntight-binding and DFT results for the charge density distribution of the\nso-called Wannier orbital states.\n