We have studied systematically basic transport properties of multiwalled carbon nanotubes in a relatively unexplored diameter range, corresponding to tubes just slightly larger than single-or double-walled tubes to tubes up to 17 nm in diameter. We find in all the smaller tubes a gap in the transport data which increases with decreasing tube diameter. Within the gap region of several tubes, negative differential resistance was observed at small or moderate biases and at bias values that scaled inversely with the tube diameter. For this latter type of behavior of the conductance, we tentatively propose interlayer resonant tunneling as the cause.

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Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin–orbit coupling (SOC) manifested as the weak anti-localization effect. The enhanced SOC is believed to result from a combination of inversion symmetry breaking at the grain boundary interfaces along with antisymmetric confinement potential between grains, inducing a Rashba-type SOC. From a pronounced zero bias peak in the differential conductance, we demonstrate signatures of a triplet component believed to result from spin mixing caused by tunneling of singlet Cooper pairs through such Rashba-SOC grain boundary junctions.

Ultrathin layers of polypyrrole (PPy) were electrochemically grown between microelectrodes on a Si/SiO(2) substrate. Conducting nanolayers of PPy are directly grown onto ultrathin discontinuous gold (Au) film between the microelectrodes, with thicknesses in the range 10-100 nm. The system therefore forms a novel (PPy/Au) nanocomposite conductor. Atomic force microscopy (AFM) imaging and conductivity measurements indicate that at all thicknesses a relatively uniform film is formed but with significant roughness that reflects the roughness of the metallic island layer. In PPy/Au films with thickness ∼10 nm, the small barriers around the gold islands dominate the conduction, and as the film thickness increases to 100 nm the intrinsic conductivity of highly doped PPy dominates the charge transport.

We report on transport features in heavily boron doped nanocrystalline diamond (BNCD) films which are not seen in conventional (s-wave) granular superconductors. Observations include an anomalous resistance peak near to the superconducting transition temperature as well as a strong zero bias conductance peak in the current-voltage spectra. The effect of finite bias current on the evolution of the resistance peak is systematically investigated in this system. The shape of the resistance-temperature curves near the critical temperature is seen to be strongly influenced by both magnetic field and bias current. As the bias current is lowered the resistance peak becomes more pronounced whereas when the magnetic field is varied the peak shifts towards lower temperatures, the resistance upturn shows a quadratic temperature dependence as expected for a Kondo transition. We find that a number of transport features such as resistance peak height, zero bias conduction peak height and width as well as magnetoresistance peaks scale according to a power law dependence. We interpret these features as a result of a charge-Kondo effect where hole dopants act as degenerate Kondo impurities by opening additional pseudo-spin scattering channels. I.