title: NMR evidence for gapped spin excitations in metallic carbon nanotubes.
abstract: 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 approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.
authors.affiliation: Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, 91405 Orsay, France.
authors.forename: P M
authors.initials: PM
authors.lastname: Singer
authors.forename: P
authors.initials: P
authors.lastname: Wzietek
authors.forename: H
authors.initials: H
authors.lastname: Alloul
authors.forename: F
authors.initials: F
authors.lastname: Simon
authors.forename: H
authors.initials: H
authors.lastname: Kuzmany