NMR Evidence for Gapped Spin Excitations in Metallic Carbon Nanotubes

We report on the spin dynamics of ¹³C isotope enriched inner walls in double-wall carbon nanotubes using ¹³C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T₁) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T≳150  K), we find that the T and H dependence of 1/T₁T is consistent with a 1D metallic chain. For T≲150  K we find a significant increase in 1/T₁T with decreasing T, followed by a sharp drop below ≃20  K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2Δ≃40  K (≡3.7  meV) is H independent.