We have measured the differential conductance of individual multiwall carbon nanotubes. Coulomb blockade and energy level quantization are observed. The electron levels are nearly fourfold degenerate (including spin) and their evolution in magnetic field (Zeeman splitting) agrees with a g factor of 2. In zero magnetic field the sequential filling of states evolves with spin S according to S = 0-->1/2-->0.... A Kondo enhancement of the conductance is observed when the number of electrons on the tube is odd.
The low‐temperature electrical conductivity σ(T) of uncompensated insulating Si : P with P concentration N just below the metal–insulator transition (MIT), i.e. 30×1018 cm–3 ≲ N ≲ 3.5 × 1018 cm–3, was measured between 0.05 and 5 K. With decreasing N, σ(T) shows a crossover from Mott variable‐range hopping (VRH) to Efros‐Shklovskii VRH. The data on the insulating side can be described by the universal phenomenological scaling function proposed by Aharony et al. From the N dependence of the Mott temperature TM a correlation‐length exponent ν = 1.1 is obtained, compatible with the conductivity exponent μ ≈ 1.3 for metallic samples. Indeed, the data on both sides of the MIT can be combined to yield dynamic scaling of σ(N, T). Upon lowering N on the insulating side further, a change from Efros‐Shklovskii VRH to simple activated conduction is observed near N ≈ 2.7 × 1018 cm–3. This is attributed to the activation from the lower to the upper Hubbard band, as inferred from a sign change in the thermoelectric power and the absence of such a feature in compensated Si:(P, B).
We use a simultaneous flow of ethylene and hydrogen gases to grow single-wall
carbon nanotubes by chemical vapour deposition. Strong coupling to the gate is
inferred from transport measurements for both metallic and semiconducting
tubes. At low temperatures, our samples act as single-electron transistors where
the transport mechanism is mainly governed by Coulomb blockade. The
measurements reveal very rich quantized energy level spectra spanning from the
valence to the conduction band. The Coulomb diamonds have similar
addition energies on both sides of the semiconducting gap. Signatures of the
subband population have been observed at intermediate temperature.
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