Persistent currents in carbon nanotube based rings

Carbon nanotubes are micrometer-long hollow cylinders with nanometer scale radii, and electronic properties strongly depending on their geometrical helicity. As promising tools for building up nanoscale electronic devices, nanotubes also allow one to challenge the well established common theories of mesoscopic physics. Recently, many works of quantum transport in these systems have revealed puzzling properties resulting from the mixing between their nanometer and micrometer combined length scales. Nanotubes are very particular in the sense that they lie in between small molecular systems such as benzene-type ring molecules, and mesoscopic systems such as metallic or semiconducting wires. In the former entities, basic electronic conduction properties are entirely monitored by the highest occupied–lowest unoccupied molecular orbital ~HOMOLUMO! gap and discrete features of the spectrum, whereas quantum wires may manifest, apart from ballistic transport, band conduction, allowing quantum interference phenomena whenever the full coherence of the wave function is maintained over a reasonable scale. In metallic single wall nanotubes, at the charge neutrality point there exists some evidence of a Luttinger liquid behavior, namely, that electron-electron interactions are strong enough to deviate the electronic status from Fermi liquid. Besides, a recent work shows that the tube-tube interaction between different nanotube layers is able to induce a transition from the Luttinger liquid to a strongly correlated Fermi system or a Fermi liquid. However, other results on scanning tunneling spectroscopy are fully interpreted in terms of the independent electrons model indicating the importance of screening effects. Rings of single-walled carbon nanotubes have been synthesized experimentally. Depending on the circumference length of the ring, a transition from n-type semiconducting to metallic behavior was observed experimentally ~from ;60 to ;1 mm), and further discussed theoretically. Magnetotransport experiments on rings have