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.

A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast with the charge state of an atomic or molecular ion, the charge states of such an ""islandÏÏ involve a macroscopic number of conduction electrons of the island. Island charge states bear some resemblance with the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the Ðeld in the cavity. For a normal island, charge states decay irreversibly into charge states of lower energies. However, the ground state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states di †ering by two electrons (i.e. a Cooper pair). We describe an experiment in which this Josephson e †ect involving only one Cooper pair is measured.

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, can be observed in several strongly correlated materials such as heavy fermion compounds or possibly high-temperature superconductors, and is believed to govern many of their fascinating, yet still unexplained properties. In contrast to these bulk materials with very complex electronic structure, artificial nanoscale devices could offer a new and simpler vista to the comprehension of quantum phase transitions. This long-sought possibility is demonstrated by our work in a fullerene molecular junction, where gate voltage induces a crossing of singlet and triplet spin states at zero magnetic field. Electronic tunneling from metallic contacts into the C60 quantum dot provides here the necessary many-body correlations to observe a true quantum critical behavior.

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We investigate the organized formation of strain, ripples, and suspended features in macroscopic graphene sheets transferred onto corrugated substrates made of an ordered array of silica pillars with variable geometries. Depending on the pitch and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lie fully suspended between pillars in a fakir-like fashion over tens of micrometers. With increasing pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to organized domains of parallel ripples linking pillars, eventually leading to suspended tent-like features. Spatially resolved Raman spectroscopy, atomic force microscopy, and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the structural transition between fully suspended and collapsed graphene. For the arrays of high density pillars, graphene membranes stay suspended over macroscopic distances with minimal interaction with the pillars' apexes. It offers a platform to tailor stress in graphene layers and opens perspectives for electron transport and nanomechanical applications.

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.

We have investigated the suppression of single-electron charging effects in metallic single-electron transistors when the conductance of the tunnel junctions becomes larger than the conductance quantum e 2 ͞h. We find that the Coulomb blockade of the conductance is progressively shifted at lower temperatures. The experimental results agree quantitatively with the available 1͞T expansion at high temperature, and qualitatively with the predictions of an effective two-state model at low temperature, which predicts at T 0 a blockade of conductance for all gate voltages.[S0031-9007 (97)03849-0] PACS numbers: 73.23.Hk, 73.20.Jc, 73.40.Gk, 85.30.WxSingle-electron devices consist of small "island" electrodes whose charge is nearly perfectly quantized in units of e, but which can exchange electrons through tunnel junctions. These two seemingly contradictory requirements can be met if the tunnel conductances of the tunnel junctions are much lower than the conductance quantum G K e 2 ͞h. In the recent years, different single-electron devices, such as single-electron transistors [1], turnstiles [2], and pumps [3,4], have been successfully operated, and their behavior is now well understood [5]. However, little is known on single-electron effects when the tunnel conductances are comparable to or greater than G K . In this strong tunneling regime, one expects that quantum fluctuations of the island charges will eventually suppress single-electron effects. Indeed, such a suppression of Coulomb blockade with increasing tunneling strength has been observed in the particular case of tunnel junctions with only a few, well-transmitted channels [6,7]. In this Letter, we investigate the effect of strong tunneling in the case of metallic tunnel junctions with a large number of low-transparency channels.For this purpose, we have measured the zero-voltage conductance of metallic single-electron transistors (SET) with moderate to large conductances. A SET consists of two series-connected tunnel junctions defining one island (see inset of Fig. 1) and of a gate electrode which electrostatically controls the current through the device. We first recall the predicted conductance within the sequential tunneling model (SM), on which our data analysis will be based. This model, relevant for weak tunneling, assumes that the number n of electrons in the island is a good quantum number. It only considers tunnel transitions n ! n 6 1 at the lowest order in perturbation theory, level shifts being neglected [5,8]. The SM predictions for the conductance G of the SET can be expressed using a single function g of reduced parameters:T2 ͒ is the series tunnel conductance of the two junctions, n g C g V g ͞e is the dimensionless gate charge, T is the temperature, and E 0 C e 2 ͞2C S is the bare charging energy of one excess electron on the island, C S C 1 1 C 2 1 C g being the total geometric ca-pacitance of the island. The predictions of the model are summarized in Fig. 1, in the case of a zero-impedance electromagnetic environment for the SET. Finite imped...

When a Josephson junction array is built with hybrid superconductor/metal/superconductor junctions, a quantum phase transition from a superconducting to a two-dimensional (2D) metallic ground state is predicted to happen upon increasing the junction normal state resistance. Owing to its surface-exposed 2D electron gas and its gate-tunable charge carrier density, graphene coupled to superconductors is the ideal platform to study the above-mentioned transition between ground states. Here we show that decorating graphene with a sparse and regular array of superconducting nanodisks enables to continuously gate-tune the quantum superconductor-to-metal transition of the Josephson junction array into a zero-temperature metallic state. The suppression of proximity-induced superconductivity is a direct consequence of the emergence of quantum fluctuations of the superconducting phase of the disks. Under perpendicular magnetic field, the competition between quantum fluctuations and disorder is responsible for the resilience at the lowest temperatures of a superconducting glassy state that persists above the upper critical field. Our results provide the entire phase diagram of the disorder and magnetic field-tuned transition and unveil the fundamental impact of quantum phase fluctuations in 2D superconducting systems.