Bertrand Reulet

Superconductivity in Ropes of Single-Walled Carbon Nanotubes

We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a 2 orders of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1 T, or by a dc current greater than 2.5 microA. These features strongly suggest the existence of superconductivity in ropes of SWNT.

Environmental effects in the third moment of voltage fluctuations in a tunnel junction.

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 nonsuperconducting tunnel junctions as a function of voltage bias, for various temperatures and bandwidths up to 1 GHz. The data demonstrate the significant effect of the electromagnetic environment of the sample.

Acoustoelectric Effects in Carbon Nanotubes

We show that it is possible to detect mechanical bending modes on 1µm long ropes of single walled-carbon nanotubes suspended between 2 metallic contacts. This is done by measuring either their dc resistance in a region of strong temperature dependence (in the vicinity of superconducting or metal-insulator transition), or their critical current. The vibrations are excited by a radio-frequency electric field produced by an antenna located in the vicinity of the sample. We analyze the mechanism of detection of the mechanical resonances in terms of heating and phase breaking effects.

Dynamic Response of Isolated Aharonov-Bohm Rings Coupled to an Electromagnetic Resonator

We have measured the flux dependence of both the real and the imaginary conductance of

Diamagnetic Orbital Response of Mesoscopic Silver Rings

\mathrm{GaAs}/\mathrm{GaAlAs}

Quantum transport through carbon nanotubes: Proximity-induced and intrinsic superconductivity

isolated mesoscopic rings at 310 MHz. The rings are coupled to a highly sensitive electromagnetic superconducting microresonator and lead to a perturbation of the resonance frequency and quality factor. This experiment provides a new tool for the investigation of the conductance of mesoscopic systems without the need for invasive probes. The results obtained can be compared with recent theoretical predictions emphasizing the differences between isolated and connected geometries and the relation between ac conductance and persistent currents.

Dynamics of quantum noise in a tunnel junction under ac excitation.

We report measurements of the flux-dependent orbital magnetic susceptibility of an ensemble of 10(5) disconnected silver rings at 217 MHz. Because of the strong spin-orbit scattering rate in silver this experiment is a test of existing theories on ensemble averaged persistent currents. Below 100 mK the rings exhibit a magnetic signal with a flux periodicity of h/2e consistent with averaged persistent currents, whose amplitude is of the order of 0.3 nA. The sign of the oscillations indicates unambiguously diamagnetism in the vicinity of zero magnetic field. This sign is a priori not consistent with theoretical predictions for average persistent currents. We discuss several possible explanations of this result.

Very low shot noise in carbon nanotubes

We report low-temperature transport measurements on suspended single-walled carbon nanotubes ~both individual tubes and ropes!. The technique we have developed, where tubes are soldered on low-resistive metallic contacts across a slit, enables a good characterization of the samples by transmission electron microscopy. It is possible to obtain individual tubes with a room-temperature resistance smaller than 40 k V, which remain metallic down to very low temperatures. When the contact pads are superconducting, nanotubes exhibit proximity-induced superconductivity with surprisingly large values of supercurrent. We have also recently observed intrinsic superconductivity in ropes of single-walled carbon nanotubes connected to normal contacts, when the distance between the normal electrodes is large enough, since otherwise superconductivity is destroyed by ~inverse! proximity effect. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsive interactions at low temperature, and enable investigation of superconductivity in a one-dimensional limit never explored before.

ac electric and magnetic responses of nonconnected Aharonov-Bohm rings

We report the first measurement of the dynamical response of shot noise (measured at frequency omega) of a tunnel junction to an ac excitation at frequency omega0. The experiment is performed in the quantum regime, variant Plancks over 2piomega approximately variant Plancks over 2piomega0>>kBT at very low temperature T=35 mK and high frequency omega0/2pi=6.2 GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.

Full counting statistics of avalanche transport: An experiment

Abstract:We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for the strong shot noise reduction: i) Transport within a rope takes place through few nearly ballistic tubes within a rope and possibly involves non integer effective charges with e*∼ 0.3e. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50).