Analysis of the low-temperature specific heat of multiwalled carbon nanotubes and carbon nanotube ropes

We analyze specific-heat measurements in the temperature range $1<T<\mathrm{}200\mathrm{}\hspace{0.5em}\mathrm{K}$ for two types of carbon nanotube samples: multiwalled tubes and ropes of single-walled tubes. The multiwalled tube sample has a specific-heat not unlike that of graphite, which we maintain is reasonable given the structural similarities between the two materials. In contrast, the low-temperature specific-heat data for ropes are surprisingly large in magnitude and have surprisingly strong temperature dependence. We present model calculations that highlight the puzzling nature of these results and then suggest explanations.