Host-guest interaction effect on electron transport in disordered network of nanographite domains

Nanosized graphite (nanographite) is unique carbon material with localized spins originating from the nonbonding $\pi$-electron state (edge-state) on their edge sites instead of the diamagnetic properties of bulk-sized graphite. Activated carbon fibers (ACFs) consist of a three-dimensional disordered network of nanographite metallic domains and have large specific surface areas due to the presence of nanosized pores (nanopores). The electron transport in ACFs can be explained by the Coulomb-gap-type variable hopping between nanographite domains, and it exhibits a large positive magnetoresistance at low temperatures. This magnetoresistance decreases significantly upon magnetic oxygen adsorption, in spite of its insensitivity to nonmagnetic molecular species such as nitrogen, argon, and helium. This result suggests the presence of a strong interaction between the oxygen molecule spin and edge-state spin. The strong effect of oxygen adsorption on magnetotransport is theoretically explained in terms of the interaction between the electric dipole moment of the edge-state $\pi$-electron and electric quadrupole moment of the adsorbed oxygen molecule. Theoretical calculation reproduces the experimentally reported strength of the exchange interaction between the oxygen molecule spin and edge-state spin, in addition to the behavior of the magnetoresistance.

Figure 1

Allowed and forbidden hopping paths depending on the spin orientation between the edge-state spins on adjacent nanographite domains.

Figure 2

Electric dipole-quadrupole interaction between the edge-state spin and oxygen molecule spin. $\mu$, $Q$, ${\mathit{R}}_{\mathrm{Q}\text{−}\mu }$, ${\theta }_Q$, and ${\theta }_{\mu }$ are the electric dipole on the edge-site of a nanographite, electric quadrupole moment of an oxygen molecule, distance vector between the centers of the electric dipole $\mu$ and electric quadrupole $Q$, angle formed by the intersection of the molecular axis of oxygen with vector ${\mathit{R}}_{\mathrm{Q}\text{−}\mu }$, and angle formed by $\mu$ with vector ${\mathit{R}}_{\mathrm{Q}\text{−}\mu }$, respectively.

Figure 3

Antibonding orbital of an oxygen molecule consisting of oxygen atoms A and B. The molecular axis is along the $z$ axis.

Figure 4

Second-order perturbation process induced by Hamiltonian ${\widehat{H}}_{Q\text{−}\mu }$ between an oxygen molecule and zigzag edge-state. The three small arrows (up/down) labeled 1, 2, and 3 denote the electron spins.

Figure 5

Zeeman energies of the edge-state spin (a) without and (b) with the adsorbed oxygen molecules. The effective field acting on the edge-state spin is expressed as $H_{e,\pi }=H+H_{\pi \text{−}\mathrm{Ox}}$, where $H_{\pi \text{−}\mathrm{Ox}}$ is positive when $H$ is negative and vice versa.