Suppression of tunneling into multiwall carbon nanotubes.

We have studied tunneling of electrons into multiwall carbon nanotubes (NTs) in NT-gold and NT-NT junctions, the latter created by atomic force microscope manipulation. The tunneling conductance goes to zero as the energy (temperature and bias) is reduced, and the functional form is consistent with a power law. The exponents depend upon sample geometry. The relationship between these results and theories for tunneling into ballistic and disordered metals is discussed.