
Physics 2009
Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dotsDOI: 10.1103/PhysRevLett.103.246805 Abstract: When an electron is confined to a triangular atomic thick layer of graphene [15] with zigzag edges, its energy spectrum collapses to a shell of degenerate states at the Fermi level (Dirac point) [69]. The degeneracy is proportional to the edge size and can be made macroscopic. This opens up the possibility to design a strongly correlated electronic system as a function of fractional filling of the zeroenergy shell, in analogy to the fractional quantum Hall effect in a quasitwodimensional electron gas[10], but without the need for a high magnetic field. In this work we show that electronic correlations, beyond the Hubbard model[6,7] and meanfield density functional theory (DFT) [7,8] play a crucial role in determining the nature of the ground state and the excitation spectrum of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zeroenergy states. The interactions are treated by a combination of DFT, tightbinding, HartreeFock and configuration interaction methods (TBHFCI) and include all scattering and exchange terms within second nearest neighbors as well as interaction with metallic gate. We show that a half filled charge neutral shell leads to full spin polarization of the island but this magnetic moment is completely destroyed by the addition of a single electron, in analogy to the effect of skyrmions on the quantum Hall ferromagnet [1114] and spin depolarization in electrostatically defined semiconductor quantum dots[1518]. The depolarization of the ground state is predicted to result in blocking of current through a graphene quantum dot due to spin blockade (SB) [18].
