We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T =20 mK. The conductance exhibits a ln T dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis, increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.

The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, Δ ≈ 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast 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 lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes.

A scanning tunneling microscope (STM) has been used to locally expose Langmuir–Blodgett layers of a negative electron beam resist (ω-tricosenoic acid) on top of a thin gold film. The STM operates in a dry nitrogen atmosphere at a voltage difference of about −10 V between the electrochemically etched Pt-Ir tip and the gold surface. After development in ethanol, the unexposed areas of the gold film are removed by argon ion milling. Gold wires with a linewidth down to 15 nm have been prepared. Electrical transport measurements confirm the homogeneity and the one-dimensional metallic character at low temperatures of the gold structures.

The superconducting field (H)-temperature ( T ) phase boundary has been measured in mesoscopic AI samples of different topology: lines, open and filled squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j , has been achieved. Sharp integer and rational matching peaks in the j,(H) curve are observed. The possibility of the "quantum design" of the superconducting critical parameters (H,(T) and j c ( T , H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

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