Spin-dependent conductance of small graphene flakes

Using {\it ab initio} density-functional theory and quantum transport calculations based on nonequilibrium Green's function formalism we study structural, electronic, and transport properties of small graphene flakes. Rectangular and triangular graphene flakes are stable, having magnetically ordered edge states. We show that a spin-polarized current can be produced in pure, hydrogenated, rectangular graphene flakes by exploiting the spatially separated edge states of the flake using asymmetric, nonmagnetic contacts (1). Sharp discontinuities in the transmission spectra which arise from Fano resonances of localized states in the flake are also predicted. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. Ground state of triangular flakes have non-zero magnetic moments and their conductance are spin polarized. \\ (1) H. Sahin and R. T. Senger, Phys. Rev. B \textbf{78}, 205423 (2008).