ELECTRICAL-RESISTANCE OF A CARBON NANOTUBE BUNDLE

The first direct electrical resistance measurements performed on a sin gle carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependen ce of the resistance above 2 K, it is shown that some nanotubes exhibi t a semimetallic behavior akin to rolled graphene sheets with a simila r band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In c ontrast to graphite which shows a constant low-temperature resistivity , the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At l ower temperatures, the resistance shows an unexpected drop at a critic al temperature which increases linearly with magnetic field. These str iking features could be related to the unique quasi-one-dimensional st ructure of the carbon nanotubes.