Meydi Ferrier

Transport and elastic scattering times as probes of the nature of impurity scattering in single-layer and bilayer graphene.

Transport and elastic scattering times, tau{tr} and tau{e}, are experimentally determined from the carrier density dependence of the magnetoconductance of monolayer and bilayer graphene. Both times and their dependences on carrier density are found to be very different in the monolayer and the bilayer. However, their ratio tau{tr}/tau{e} is found to be close to 1.8 in the two systems and nearly independent of the carrier density. These measurements give insight on the nature (neutral or charged) and range of the scatterers. Comparison with theoretical predictions suggests that the main scattering mechanism in our samples is due to strong (resonant) scatterers of a range shorter than the Fermi wavelength, likely candidates being vacancies, voids, adatoms or short-range ripples.

Proximity effect and multiple Andreev reflections in few-layer graphene

We have investigated electronic transport of few-layer graphene (FLG) connected to superconducting electrodes. The device is prepared by mechanical exfoliation of graphite. A small mesa of FLG is connected to two tungsten electrodes, separated by 2.5 μm, grown by focused ion beam. Whereas the tungsten electrodes are superconducting below 4 K, the proximity effect in FLG develops below 1 K, and is characterized by a factor 2 differential resistance drop at low bias. We find multiple Andreev reflection peaks at voltages corresponding to submultiples of 2Δ/e (with Δ the superconducting gap of the electrodes), which persist up to fields of a few tesla.

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

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.

Proximity DC squids in the long junction limit

We report the design and measurement of Superconducting/normal/superconducting (SNS) proximity DC squids in the long junction limit, i.e. superconducting loops interrupted by two normal metal wires roughly a micrometer long. Thanks to the clean interface between the metals, at low temperature a large supercurrent flows through the device. The dc squid-like geometry leads to an almost complete periodic modulation of the critical current through the device by a magnetic flux, with a flux periodicity of a flux quantum h/2e through the SNS loop. In addition, we examine the entire field dependence, notably the low and high field dependence of the maximum switching current. In contrast with the well-known Fraunhoffer-type oscillations typical of short wide junctions, we find a monotonous gaussian extinction of the critical current at high field. As shown in [15], this monotonous dependence is typical of long and narrow diffusive junctions. We also find in some cases a puzzling reentrance at low field. In contrast, the temperature dependence of the critical current is well described by the proximity effect theory, as found by Dubos et al. [16] on SNS wires in the long junction limit. The switching current distributions and hysteretic IV curves also suggest interesting dynamics of long SNS junctions with an important role played by the diffusion time across the junction.

Superconducting diamagnetic fluctuations in ropes of carbon nanotubes

We report low-temperature magnetisation measurements on a large number of purified ropes of single wall carbon nanotubes. In spite of a large superparamagnetic contribution due to the small ferromagnetic catalytical particles still present in the sample, at low temperature (

Probing the dynamics of Andreev states in a coherent Normal/Superconducting ring

T<0.5K

Superconductivity in ropes of carbon nanotubes

) and low magnetic field (

Geometry-related magnetic interference patterns in long SNSJosephson junctions

H<80 Oe

Quantum Fluctuations along Symmetry Crossover in a Kondo-Correlated Quantum Dot

), a diamagnetic signal is detectable. This low temperature diamagnetism can be interpreted as the Meissner effect in ropes of carbon nanotubes which have previously been shown to exhibit superconductivity from transport measurements.

Alteration of superconductivity and radial breathing modes in suspended ropes of carbon nanotubes by organic polymer coatings

The supercurrent that establishes between two superconductors connected through a normal N mesoscopic link is carried by quasiparticule states localized within the link, the “Andreev bound states (ABS)”. Whereas the dc properties of this supercurrent in SNS junctions are now well understood, its dynamical properties are still an unresolved issue. In this letter we probe this dynamics by inductively coupling an NS ring to a multimode superconducting resonator, thereby implementing both a phase bias and current detection at high frequency. Whereas at very low temperatures we essentially measure the phase derivative of the supercurrent, at higher temperature we find a surprisingly strong frequency dependence in the current response of the ring: the ABS do not follow adiabatically the phase modulation. This experiment also illustrates a new tool to probe the fundamental time scales of phase coherent systems that are decoupled from macroscopic normal contacts and thermal baths.A long phase coherent normal (N) wire between superconductors (S) is characterized by a dense phase dependent Andreev spectrum . We probe this spectrum in a high frequency phase biased configuration, by coupling an NS ring to a multimode superconducting resonator. We detect a dc flux and frequency dependent response whose dissipative and non dissipative components are related by a simple Debye relaxation law with a characteristic time of the order of the diffusion time through the N part of the ring. The flux dependence exhibits