Detection of graphene's divergent orbital diamagnetism at the Dirac point.

The electronic properties of graphene have been intensively investigated over the past decade. However, the singular orbital magnetism of undoped graphene, a fundamental signature of the characteristic Berry phase of graphene's electronic wave functions, has been challenging to measure in a single flake. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage­dependent magnetization of a single graphene monolayer encapsulated between boron nitride crystals. The signal exhibits a diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. Our measurements offer a means to monitor Berry phase singularities and explore correlated states generated by the combined effects of Coulomb interactions, strain, or moiré potentials.

Biaxial Strain Transfer in Supported Graphene.

Understanding the mechanism and limits of strain transfer between supported 2D systems and their substrate is a most needed step toward the development of strain engineering at the nanoscale. This includes applications in straintronics, nanoelectromechanical devices, or new nanocomposites. Here, we have studied the limits of biaxial compressive strain transfer among SiO2, diamond, and sapphire substrates and graphene. Using high pressure-which allows maximizing the adhesion between graphene and the substrate on which it is deposited-we show that the relevant parameter governing the graphene mechanical response is not the applied pressure but rather the strain that is transmitted from the substrate. Under these experimental conditions, we also show the existence of a critical biaxial stress beyond which strain transfer become partial and introduce a parameter, α, to characterize strain transfer efficiency. The critical stress and α appear to be dependent on the nature of the substrate. Under ideal biaxial strain transfer conditions, the phonon Raman G-band dependence with strain appears to be linear with a slope of -60 ± 3 cm-1/% down to biaxial strains of -0.9%. This evolution appears to be general for both biaxial compression and tension for different experimental setups, at least in the biaxial strain range -0.9% < ε < 1.8%, thus providing a criterion to validate total biaxial strain transfer hypotheses. These results invite us to cast a new look at mechanical strain experiments on deposited graphene as well as to other 2D layered materials.

Nanofaceting as a stamp for periodic graphene charge carrier modulations.

The exceptional electronic properties of monatomic thin graphene sheets triggered numerous original transport concepts, pushing quantum physics into the realm of device technology for electronics, optoelectronics and thermoelectrics. At the conceptual pivot point is the particular two-dimensional massless Dirac fermion character of graphene charge carriers and its volitional modification by intrinsic or extrinsic means. Here, interfaces between different electronic and structural graphene modifications promise exciting physics and functionality, in particular when fabricated with atomic precision. In this study we show that quasiperiodic modulations of doping levels can be imprinted down to the nanoscale in monolayer graphene sheets. Vicinal copper surfaces allow to alternate graphene carrier densities by several 10(13) carriers per cm(2) along a specific copper high-symmetry direction. The process is triggered by a self-assembled copper faceting process during high-temperature graphene chemical vapor deposition, which defines interfaces between different graphene doping levels at the atomic level.

Hybrid superconductor-semiconductor devices made from self-assembled SiGe nanocrystals on silicon.

The epitaxial growth of germanium on silicon leads to the self-assembly of SiGe nanocrystals by a process that allows the size, composition and position of the nanocrystals to be controlled. This level of control, combined with an inherent compatibility with silicon technology, could prove useful in nanoelectronic applications. Here, we report the confinement of holes in quantum-dot devices made by directly contacting individual SiGe nanocrystals with aluminium electrodes, and the production of hybrid superconductor-semiconductor devices, such as resonant supercurrent transistors, when the quantum dot is strongly coupled to the electrodes. Charge transport measurements on weakly coupled quantum dots reveal discrete energy spectra, with the confined hole states displaying anisotropic gyromagnetic factors and strong spin-orbit coupling with pronounced dependences on gate voltage and magnetic field.

Tunable superconducting phase transition in metal-decorated graphene sheets.

We have produced graphene sheets decorated with a nonpercolating network of nanoscale tin clusters. These metal clusters both efficiently dope the graphene substrate and induce long-range superconducting correlations. We find that despite structural inhomogeneity on mesoscopic length scales (10-100 nm), this material behaves electronically as a homogenous dirty superconductor with a field-effect tuned Berezinskii-Kosterlitz-Thouless transition. Our facile self-assembly method establishes graphene as an ideal tunable substrate for studying induced two-dimensional electronic systems at fixed disorder and our technique can readily be extended to other order parameters such as magnetism.

Tunable graphene dc superconducting quantum interference device.

Graphene exhibits unique electrical properties on account of its reduced dimensionality and "relativistic" band structure. When contacted with two superconducting electrodes, graphene can support Cooper pair transport, resulting in the well-known Josephson effect. We report here the fabrication and operation of a two junction dc superconducting quantum interference device (SQUID) formed by a single graphene sheet contacted with aluminum/palladium electrodes in the geometry of a loop. The supercurrent in this device can be modulated not only via an electrostatic gate but also by an applied magnetic fielda potentially powerful probe of electronic transport in graphene and an ultrasensitive platform for nanomagnetometry.

Gate-tuned high frequency response of carbon nanotube Josephson junctions.

Carbon nanotube Josephson junctions in the open quantum dot limit are fabricated using Pd/Al bilayer electrodes, and exhibit gate-controlled superconducting switching currents. Shapiro voltage steps can be observed under radio frequency current excitations, with a damping of the phase dynamics that strongly depends on the gate voltage. These measurements are described by a standard resistively and capacitively shunted junction model showing that the switching currents from the superconducting to the normal state are close to the critical current of the junction. The effective dynamical capacitance of the nanotube junction is found to be strongly gate dependent, suggesting a diffusive contact of the nanotube.

Raman spectroscopy of free-standing individual semiconducting single-wall carbon nanotubes.

The radial breathing modes and tangential modes have been systematically measured on a large number of individual semiconducting single-wall carbon nanotubes (thin bundles) suspended between plots (free-standing single-wall carbon nanotubes). The strong intensity of the Raman spectra ensures the precision of the experimentally determined line shapes and frequencies of these modes. The diameter dependence of the frequencies of the tangential modes was measured. This dependence is discussed in relation with recent calculations. The present data confirm/contradict some previous interpretations.

Carbon nanotube superconducting quantum interference device.

A superconducting quantum interference device (SQUID) with single-walled carbon nanotube (CNT) Josephson junctions is presented. Quantum confinement in each junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with two lateral electrostatic gates. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting change in the hybridization of the QD states with the superconducting contacts. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of the switching current with magnetic flux is achieved when both QD junctions are in the 'on' or 'off' state. In particular, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled pi-junctions; that is, the sign of the current-phase relation across the CNT junctions can be tuned with a gate voltage. The CNT-SQUIDs are sensitive local magnetometers, which are very promising for the study of magnetization reversal of an individual magnetic particle or molecule placed on one of the two CNT Josephson junctions.