Metallic single-electron transistor without traditional tunnel barriers

We report on a new type of single-electron transistor (SET) comprising two highly resistive Cr thin-film strips (~ 1um long) connecting a 1 um-long Al island to two Al outer electrodes. These resistors replace small-area oxide tunnel junctions of traditional SETs. Our transistor with a total asymptotic resistance of 110 kOhm showed a very sharp Coulomb blockade and reproducible, deep and strictly e-periodic gate modulation in wide ranges of bias currents I and gate voltages V_g. In the Coulomb blockade region (|V| < 0.5 mV), we observed a strong suppression of the cotunneling current allowing appreciable modulation curves V-V_g to be measured at currents I as low as 100 fA. The noise figure of our SET was found to be similar to that of typical Al/AlOx/Al single-electron transistors.Comment: 5 pages incl. 4 fig

Variable electrostatic transformer: controllable coupling of two charge qubits

We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling. We also discuss how to construct a phase gate making use of the controllable coupling.Comment: final version, to appear in Phys. Rev. Let

Adapting the rice crop to hotter environments: Current and future activities at IRRI

Future climate scenarios are claiming for an increase in global temperature of 2 to 4°C by 2100 in the rice production areas in Asia One of the mandates of the International Rice Research Institute is to predict to what extent the different rice growing areas will be affected, tu analyze consequences on rice production and to provide adaptive strategies. A regional assessment of vulnerability to heat has been conducted by lRRl scientists on rice cropping areas by linking ORYZA2000 with Geographic Information System (GIS). The establishment of a spatio-temporal geo-statistical framework will soon allow identifying regions of risks of heat induced sterility, for which the threshold panicle temperature commonly ranges from 35 to 38°C with respect to the variety. To face this major issue, lRRl scientists are conducting multi-location testing of promising varieties and developing new genetic materials by screening donors from gene bank accessions. Some heat tolerance breeding populations have been developed and dispatched for hotspot screening, and 4 QTL mapping populations have been developed for polymorphism characterization. In addition, anthers of 3 lines contrasted for heat induced sterility were extracted, and some candidate genes are currently being sequenced and will be targeted for transformation. Donors for earlier time of the day of anthesis are investigated for heat induced sterility avoidance: 42 lines among 4000 from the lRRl gene bank accessions appeared to have peaked by 9am and were sent for testing in 5 Asian countries. An integrated phenotyping study for earlier time of the day of anthesis, heat tolerance to sterility and heat tolerance to chalkiness during grain filling, is actually conducted on a set of 212 contrasted accessions in the phytotron. Indeed, lRRl scientists demonstrated under plant temperatures higher than 30°C that genotypes that did not adapt to high temperature produced chalky grains whereas those that sacrificed part of their sink size maintained high quality grains. Similarly, such temperature regimes affect plant growth processes also at earlier stages like for leaf elongation rate. In the case of addressing confounding effects of climatic factors, the correlation observed during the last 15 years in the lRRl farm between the increase in night time temperature from 22 to 24°C and the reduction in grain yield is now confronted with additional data collected in a contrasted night temperature setup in the field. In collaboration with lRRl, scientists from Cirad and NIAES are collecting data in various field environments to quantify panicle temperature and predict its variation with regard to weather conditions, crop architecture and plant cooling ability. At the same time, lRRl scientists are developing the energy balance and exchange routines of OR YZA2000 and adding routines for canopy temperature and spikelet sterility. Considering rice is often grown in humid environments and soon under doubling air [C02], additional routines addressing interactions between temperature, humidity and [C02] will be developed by lRRl collaborators and included into crop models. Such cumulated efforts from rice scientists are necessary to face the challenges of future climate scenarios and make the rice production systems more resilient. (Texte intégral

Mesoscopic quantum transport: Resonant tunneling in the presence of strong Coulomb interaction

Coulomb blockade phenomena and quantum fluctuations are studied in mesoscopic metallic tunnel junctions with high charging energies. If the resistance of the barriers is large compared to the quantum resistance, transport can be described by sequential tunneling. Here we study the influence of quantum fluctuations. They are important when the resistance is small or the temperature very low. A real-time approach is developed which allows the diagrammatic classification of ``inelastic resonant tunneling'' processes where different electrons tunnel coherently back and forth between the leads and the metallic island. With the help of a nonperturbative resummation technique we evaluate the spectral density which describes the charge excitations of the system. From it physical quantities of interest like current and average charge can be deduced. Our main conclusions are: An energy renormalization leads to a logarithmic temperature dependence of the renormalized system parameters. A finite lifetime broadening can change the classical picture drastically. It gives rise to a strong flattening of the Coulomb oscillations for low resistances, but in the Coulomb blockade regime inelastic electron cotunneling persists. The temperature where these effects are important are accessible in experiments.Comment: 24 pages + 23 figures (available by fax or conventional mail, upon request) tfp-1994-1

Smearing of Coulomb Blockade by Resonant Tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant impurity level. We show that the strong energy dependence of the transmission coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is sufficiently narrow, the Coulomb staircase shows very sharp steps even if the transmission through the impurity at the Fermi energy is perfect. This is in contrast to the naive expectation that perfect transmission should completely smear charging effects.Comment: 4 pages, 3 figure

Cotunneling at resonance for the single-electron transistor

We study electron transport through a small metallic island in the perturbative regime. Using a recently developed diagrammatic technique, we calculate the occupation of the island as well as the conductance through the transistor in forth order in the tunneling matrix elements, a process referred to as cotunneling. Our formulation does not require the introduction of a cut-off. At resonance we find significant modifications of previous theories and good agreement with recent experiments.Comment: 5 pages, Revtex, 5 eps-figure

Parity Effect in Ground State Energies of Ultrasmall Superconducting Grains

We study the superconductivity in small grains in the regime when the quantum level spacing $\delta\varepsilon$ is comparable to the gap $\Delta$. As $\delta\varepsilon$ is increased, the system crosses over from superconducting to normal state. This crossover is studied by calculating the dependence of the ground state energy of a grain on the parity of the number of electrons. The states with odd numbers of particles carry an additional energy $\Delta_P$, which shows non-monotonic dependence on $\delta\varepsilon$. Our predictions can be tested experimentally by studying the parity-induced alternation of Coulomb blockade peak spacings in grains of different sizes.Comment: 4 pages, revtex, multicol.st

Parity effect and spontaneous currents in superconducting nanorings

New physical effects emerge from an interplay between the electron parity number and persistent currents in superconducting nanorings. An odd electron, being added to the ring, produces a countercurrent which may substantially modify the ground state properties of the system. In superconducting nanorings with an embedded normal metal layer a novel ``$\pi /N$-junction'' state can occur for the odd number of electrons. Changing this number from even to odd yields spontaneous supercurrent in the ground state of such rings without any externally applied magnetic flux. Further peculiar features of the parity effect are expected in structures with resonant electron transport across the weak link.Comment: 9 pages, 5 figures. Invited talk at the International Conference "Frontiers of Quantum and Mesoscopic Thermodynamics", Prague, July 200

Strong Charge Fluctuations in the Single-Electron Box: A Quantum Monte Carlo Analysis

We study strong electron tunneling in the single-electron box, a small metallic island coupled to an electrode by a tunnel junction, by means of quantum Monte Carlo simulations. We obtain results, at arbitrary tunneling strength, for the free energy of this system and the average charge on the island as a function of an external bias voltage. In much of the parameter range an extrapolation to the ground state is possible. Our results for the effective charging energy for strong tunneling are compared to earlier -- in part controversial -- theoretical predictions and Monte Carlo simulations

Resonant tunneling through a macroscopic charge state in a superconducting SET transistor

We predict theoretically and observe in experiment that the differential conductance of a superconducting SET transistor exhibits a peak which is a complete analogue in a macroscopic system of a standard resonant tunneling peak associated with tunneling through a single quantum state. In particular, in a symmetric transistor, the peak height is universal and equal to $e^2/2\pi \hbar$. Away from the resonance we clearly observe the co-tunneling current which in contrast to the normal-metal transistor varies linearly with the bias voltage.Comment: 11 pages, 3 figures, Fig. 1 available upon request from the first autho