Electronic transport in single-wall carbon nanotubes

We have studied the electronic properties of single-wall carbon nanotubes (SWNTs) from room temperature down to 10mK by performing electronic transport spectroscopy measurements on individual single-wall nanotubes, nanotube ropes and untreated bulk SWNT material. Transport in metallic single-wall nanotubes at dilution refrigerator temperatures occurs via the resonant tunneling of electrons through discrete molecular orbitals of the tubes. The discrete energy level splitting in a 0.8μm long nanotube was roughly 1.2meV, which agrees with the 1meV estimate for the energy level splitting. Zeeman splitting of a single particle level was observed when a magnetic field was applied perpendicular to the tube, which yielded g = 1.8 for the electronic g-factor in a nanotube. Due to the small capacitance of the nanotube, the conductance at low bias voltages is suppressed by the Coulomb blockade. Surprisingly, the differential conductance of the tube shows a pattern of single electron charging that suggests the tube behaves as a couple of quantum dots in series. A numerical simulation, based on a model of a tube with three levels, reproduces some prominent features of the data. Electrons in a nanotube occupy 1D subbands. Sharp van Hove singularities in the density of states are expected at the band edges of the 1D modes and were observed as steps in the current-voltage characteristics of a semiconducting nanotube at 77K. By varying an external gate voltage we electrostatically doped the tube and controllably populated its subbands. The IV characteristics of a segment of the nanotube covered in part by an impurity were highly asymmetric. This was in sharp contrast to the symmetric IVs observed for a clean segment of the same tube. A simple band model, in which the impurity acts as a local charge dopant, can account for the diodic behavior of the tube. Early experiments on “mats” of bulk SWNT material revealed that low temperature transport in the bulk occurs via three dimensional (3D) variable range hopping of electrons between localized states. A typical characteristic temperature T0 for the material is 110 K. The bulk samples also exhibited a negative magneto-resistance, which indicates that transport in the mats is partly phase coherent. We used 3D weak localization theory to fit the data and obtained l &phis; = 1500Å for the phase coherence length at 100mK