The complex ac conductance G(ω<sub>0</sub>) of a system measures the dynamical response of the current to a small
voltage excitation at frequency ω<sub>0</sub>. It cannot in general be deduced from the only knowledge of the dc I(V )
characteristics. Similarly, we investigate the <i>dynamical</i> response of current noise to an ac excitation, i.e. the
in-phase and out-of-phase response of current noise density S(ω) measured at frequency ω. We present a detailed
calculation of this new response function χ<sub>ω0</sub> (ω), that we name <i>noise susceptibility</i>, at arbitrary frequencies for
a coherent conductor in the scattering matrix formalism. We exemplify the relevance of our calculation by the
measurement of the noise susceptibility of a tunnel junction in the quantum regime &barh;ω ~ &barh;ω<sub>0</sub>> >k<sub>B</sub>T, which is
in remarkable agreement with our theory.
We present the first measurements of the third moment of the voltage fluctuations in a conductor. This technique can provide new and complementary information on the electronic transport in conducting systems. The measurement was performed on non-superconducting tunnel junctions as a function of voltage bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate the significant effect of the electromagnetic environment of the sample.
Carbon nanotubes may constitute the ultimate conducting wires for nano-electronics, with their diameters as small as a few tens of atoms and their length of order one micrometer. Because of the particular band structure of graphite, nanotubes have at most two conducting channels, which makes them a one dimensional conductor with very exotic properties. Experimental investigations have indeed shown non conventional features, such as non-ohmic behavior, superconductivity and an ability to carry a huge current density.
We have carried out shot noise measurements on nanotubes which are suspended between metallic electrodes. One consequence of the suspended geometry is a very low 1/f noise, thereby enabling the extraction of shot noise. In bundles of nanotubes, we find a reduction of shot noise by more than a factor 100 compared to the full noise 2.e.I expected for uncorrelated electrons. A low noise is also found in an isolated single wall nanotube.
In a simple non-interacting-electron picture, such a low shot noise implies that the electrical conduction through a bundle of nanotubes is concentrated in a few ballistic tubes. Another interpretation however would be that a substantial fraction of the tubes conduct with a strong reduction of the effective charge (more than a factor 50) due to electron-electron interaction.