Low-temperature anomaly of microwave absorption and ac susceptibility of single-wall carbon nanotubes: Bulk superconductivity and weak ferromagnetism

The electromagnetic properties of a bulk sample of high quality single-wall carbon nanotubes with a mean diameter of 2.5 nm were investigated by noninvasive methods. The observed sudden increase in the microwave absorptivity below 12 K is taken as indicative of the formation of a superconducting phase of a small part of the sample. The increase in the ac susceptibility peaking at 20 K points to the existence of a weak ferromagnetism probably of a different set of tubes in the heterogeneous sample. Above 20 K, the microwave loss function apparently originates predominantly from a narrow-band-gap (7 meV) one-dimensional conductor.