J.W. Ding

Odd-even width effect on persistent current in zigzag hexagonal graphene rings.

The electronic structure and thus the persistent current of zigzag hexagonal graphene rings are investigated within the tight-binding formalism. The flux-dependent energy spectrum is grouped into bands with six levels per band due to inter-valley scattering at the corners of the ring. It is found that the degeneracy at the Fermi level is determined by the even or odd quality of the ring width N. The sample ring becomes metallic at odd N but semiconducting at even N, showing up a strange odd-even width effect. In metallic rings, the persistent current within a flux period is linearly changed with magnetic flux varphi, while it is a sinusoidal periodical function of varphi in semiconducting rings. In addition, with increasing N, the persistent current exponentially decreases (increases) at odd (even) N, but finally falls into the consistence with each other at enough large N, showing that the odd-even effect may be experimentally observable only in narrow rings.

Magnetic response of carbon nanotori: the importance of curvature and disorder

Taking into account both curvature and disorder, we study the magnetic moments of carbon nanotori in the presence of magnetic field B (perpendicular to the torus plane). Due to the intrinsic curvature effect, energy gaps of about 10 meV are obtained in those primary metallic carbon nanotori, which result in a novel magnetic response, depending on its chirality. An armchair carbon nanotorus exhibits a diamagnetic response in the vicinity of zero magnetic field, while the primary metallic zigzag nanotorus presents a much weaker paramagnetic response. This result differs substantially from previous predictions. In the weak-magnetic-field regime, it is found that the low-temperature magnetic moments show up the periodic Aharonov–Bohm (AB) oscillation, with period ΔB≈0/S, 0 = h/e being the quantum flux, S the area enclosed by the torus, while at high temperature, the AB oscillation is destroyed because of the loss of phase coherence. A phase transition from diamagnetic to paramagnetic is observed by increasing the disorder strength.