Magnetic Edge-State Excitons in Zigzag Graphene Nanoribbons

We present first-principles calculations of the optical properties of zigzag-edged graphene nanoribbons (ZGNRs) employing the GW-Bethe-Salpeter equation approach with the spin interaction included. Optical response of the ZGNRs is found to be dominated by magnetic edge-state-derived excitons with large binding energy. The absorption spectrum is composed of a characteristic series of exciton states, providing a possible signature for identifying the ZGNRs. The edge-state excitons are charge-transfer excitations with the excited electron and hole located on opposite edges; they moreover induce a spin transfer across the ribbon, resulting in a photoreduction of the magnetic ordering. These novel characteristics are potentially useful in the applications.

Figure 1

Quasiparticle self-energy corrections and the corresponding band structure of 8-ZGNR. (a) Differences between the quasiparticle energy and the LSDA eigenvalue of an 8-ZGNR. The corrections to the magnetic edge states are encircled by dashed red curves. (b) Quasiparticle band structure of 8-ZGNR. The open-circled electronic bands are those included in the present exciton calculations. The interband transitions are represented by blue arrows.

Figure 2

Optical absorption spectra of 8- and 6-ZGNRs with and without electron-hole interaction. A Gaussian broadening of 0.01 eV is applied.

Figure 3

Exciton wave functions of 8-ZGNR for the exciton states, (a) $D_1^{11}$, (b) $E_1^{11}$, (c) $E_2^{11}$, (d) $E_3^{11}$, and (e) $E_1^{12}$. The electron amplitude squared is plotted given that the hole is fixed on top of a $\pi$-orbital at the position marked with the black spot. The distributions have been averaged along the direction perpendicular to the plane of the ZGNRs.