Speaker
Description
Zigzag graphene nanoribbons are narrow strips of graphene with parallel zigzag edge termination that sparked research interest in recent years due to their distinctive electronic and magnetic properties. They exhibit partially flat edge bands around the Fermi level that host edge-localized states. These edge states give rise to a robust magnetic order. At half-filling, the edges are known to host electrons with opposite spin orientation (AF order). Upon increasing the chemical potential, the magnetic order is reported to transition from the AF order to ferromagnetic inter-edge coupling with the same spin polarization on both edges (F order), and back to the AF order. These transitions can be attributed to the filling of edge bands alone.
We perform self-consistent mean-field theory calculations to determine the magnetic order of the ground state at various chemical potentials and ribbon widths. In particular, when we tune the chemical potential to align with higher energy subbands, referred to as bulk bands, we predict additional magnetic order phase transitions in sufficiently wide zigzag ribbons. Their appearance is linked to an interplay between the spectral overlap of edge and bulk bands and the fact that the AF order bands are spin-degenerate while the F order bands split into two spin subbands.
At half-filling, we perform Hybrid Monte Carlo (HMC) simulations to go beyond mean-field calculations. Work is underway in an attempt to verify the self-consistent theory results at finite chemical potential, focusing on narrow ribbons and moderate chemical potential, where the first AF-to-F order phase transition is expected to occur.