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 2.50.3em0exnm were investigated by noninvasive methods. The observed sudden increase in the microwave absorptivity below 120.3em0exK 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 200.3em0exK points to the existence of a weak ferromagnetism probably of a different set of tubes in the heterogeneous sample. Above 200.3em0exK, the microwave loss function apparently originates predominantly from a narrow-band-gap (70.3em0exmeV) one-dimensional conductor.