Cecile Naud

Electronic Confinement and Coherence in Patterned Epitaxial Graphene

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.

Electronic Structure of Epitaxial Graphene Layers on SiC: Effect of the Substrate

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.

Multiscale investigation of graphene layers on 6H-SiC(000-1)

In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

Self-assembled single wall carbon nanotube field effect transistors

We report detailed characterization of in-situ wired single wall carbon nanotube (SWNT) field effect transistors (FETs). They were batch processed using a single step technique based on hot filament chemical vapor deposition. Raw samples show an ambipolar field effect. The temperature dependence of the gain confirms the presence of Schottky barriers at the nanotube/metal interface. Moreover the gate dependence exhibits hysteresis at any temperature due to extraction and trapping of charges. Below 30 K, Coulomb blockade occurs at low drain-source bias and partially washes out the influence of the Schottky barriers.

Epitaxial graphene morphologies probed by weak (anti)-localization

We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of silicon carbide, with mobilities ranging from 120 to 12 000 cm2/(V·s). Depending on the growth conditions, we observe anti-localization and/or localization, which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the “pure” mono-layer and bilayer of graphene [MacCann et al., Phys. Rev. Lett. 97, 146805 (2006)].

Electronic properties of epitaxial graphene

It has been known for almost 25 years that high temperature treatment of polar faces of SiC crystals lead to the graphitisation of the surface as a consequence of Si preferential sublimation. The group of Walt de Heer in Atlanta has proposed to use this procedure to obtain macroscopic multilayer graphene samples. This has lead to a renewal of interest for the study of the graphitic layers grown on the SiC surfaces. Due to the polar nature of the material, the 6H(4H)-SiC substrate present two non-equivalent surfaces: the (0001) is known as the Si face, and the (000-1) one is the C face. It turns out that the structure and the properties of the multilayer graphene samples obtained on those two surfaces are rather different. In particular the multilayer graphene samples grown on the C-terminated face presents electronic properties that are rather similar to those of monolayer graphene with very high mobility. This is shown by transport and optical experiments under high magnetic field. This surprising result is explained by the rotational stacking sequence, observed experimentally. which leads to an electronic decoupling as demonstrated by ab initio band structure calculations.

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

We have altered the superconductivity of a suspended rope of single walled carbon nanotubes, by coating it with organic polymers. Upon coating, the normal state resistance of the rope changes by less than 20 percent. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below 300 mK, is gradualy suppressed. We correlate this to the suppression of radial breathing modes, measured with Raman Spectroscopy on suspended Single and Double-walled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in carbon nanotubes.

V-shaped superconducting artificial atom based on two inductively coupled transmons

Circuit quantum electrodynamics systems are typically built from resonators and two-level artificial atoms, but the use of multilevel artificial atoms instead can enable promising applications in quantum technology. Here we present an implementation of a Josephson junction circuit dedicated to operate as a V-shape artificial atom. Based on a concept of two internal degrees of freedom, the device consists of two transmon qubits coupled by an inductance. The Josephson nonlinearity introduces a strong diagonal coupling between the two degrees of freedom that finds applications in quantum nondemolition readout schemes, and in the realization of microwave cross-Kerr media based on superconducting circuits.